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CHAPTER 1

INTRODUCTION

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Electrical drives in industry

The first electrical machine was invented in the first half of the 19th century.
Since then more than a century has passed. During this period, continuous
improvements have been developed for each application area of electrical machines
and as a result, electric motors are nowadays a part of our everyday life. Many
different motor types have been developed in modern industry for hundreds of various
purposes.

Electric motors

There are three classical motors among all of the existing motors on the
market: the Direct Current with commutators (wound field) and two Alternative
Current motors the synchronous and the asynchronous motors. Fig.1.1 shows the
topology of the electric motors now available on the market. These motors, when
properly controlled, produce constant instantaneous torque (very little torque ripple)
and operate from pure DC or AC sinewave supplies. Large motors are always chosen
from one of the classical types like DC commutator motor (with wound field), AC
induction or synchronous motor. This is because of the fact and the need of high
efficiency and efficient utilization of materials and the need for smooth, ripple-free
torque.

Fig. 1.1. Electric Motor Topology.

Brushed DC motor

DC machine was the first practical device which converts electrical power into
the mechanical and vice versa in its generator mode. For a long time the DC motor
was the only motor type available to convert electrical power into mechanical power,

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and due to its straightforward operating characteristics and simple and stable control,
it is still being used to some extent in speed-controlled applications.

Principles of DC Brush Motors

The DC motor works on the principle of that when a current carrying

conductor placed in a magnetic field, it experiences a mechanical force (twisting
force) called Torque The simplest example of the DC motor is shown in Fig 1.2 In
this case, the stationary magnetic flux is produced by poles and when voltage is
applied on the brushes, current flows through the armature and generates force f
which forms torque and causes the armature to rotate.

Fig.1.2 Elementary DC Motor

How DC Motor works?

The well-known DC brush motor, like any other rotating machine, has a stator
and rotor (as shown in Fig. 1.3). On the stator (stationary part), there is a magnetic
field which can be provided either by permanent magnets or by excited field windings
on the stator poles. On the rotor, the main components are the armature winding,
armature core, a mechanical switch called commutator which rotates, and a rotor
shaft. The commutator segments are insulated from one another and from the clamp
holding them. In addition to that brushes, the stationary external components of the
rotor, together with the commutator act not only as rotary contacts between the coils
of the rotating armature and the stationary external circuit, but also as a switch to
commutate the current to the external DC circuit so that it remains unidirectional even
though the individual coil voltages are alternating.

The maximum torque is produced when the magnetic field of the stator and
the rotor are perpendicular to each other (can be seen in Figs. 1.4 through 1.6,
hypothetically). The commutator makes it possible for the rotor and stator magnetic
fields to always be perpendicular. The commutation thus plays a very important part
in the operation of the DC brush motor. It causes the current through the loop to
reverse at the instant when unlike poles are facing each other. This causes a reversal

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in the polarity of the field, changing attractive magnetic force into a repulsive one
causing the loop to continue to rotate.

Fig. 1.3. Basic model of a DC brush motor.

When applying a voltage at the brushes, current flows through two of the
coils. This current interacts with the magnetic field of the permanent magnet and
produces torque. This torque causes to move. When the motor moves the brushes will
switch to a different coil automatically causing the rotor to turn further. If the voltage
(armature) is increased it will turn faster and if the magnetic field of permanent
magnet is higher then it will produce more torque.

Since the back-EMF generated in the coil is short-circuited by the brush, a

large current flows causing sparking at the interface of the commutator and the
brushes, as well as causing heating and the production of braking torque. In order to
minimize this problem, commutation is carried out in the magnetic field crossover
region. Even after taking these measures, because of the distortion of the effective
magnetic flux due to the armature reaction, some back-EMF is still generated in the
coils in the magnetic filed crossover region. It is desirable to minimize the crossover
region in order to maximize the utilization of the motor.

In general DC motors, the applied voltage (EMF) is never going to be greater

than back-EMF. The difference between the applied EMF (voltage) and back-EMF is
always such that current can flow in the conductor and produce motion.

Fundamental operation of the DC motor is explained from Figs. 1.4 through

1.6 as follows:
The direction of current flow from the DC voltage source in the figures is
based on electron theory in which current flows from the negative terminal of a source
of electricity to the positive terminal. On the contrary, the older convention supposes
that current flows from positive terminal of a source of electricity through to the
negative terminal.

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KARTHIK Kongu Engineering College

The coils pole pair positioning, shown in the figures, is decided by using
Flemings left hand rule for generators. If a coil resides in a magnetic field and the
current and rotation of the coil are known, then direction of the magnetic field for the
coil can be found easily by using Flemings left hand rule. This rule states that if the
thumb and the first and middle fingers of the left hand are perpendicular to one
another, with the first and middle fingers pointing in the flux direction and the thumb
pointing in the direction of motion of the conductor, the middle finger will point in the
direction in which the current flows.

Fig. 1.4. Fundamental operation of a DC brush motor (Step 1).

With the loop in Fig. 1.4, the current flowing through the coil makes the top of
the loop a north pole and the underside a south pole. This is found by applying the
left-hand rule under the assumption of the back-EMF (direction) is opposite of the
direction of current flow which is provided by DC voltage source.

Fig. 1.5. Fundamental operation of a DC brush motor (Step 2).

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The magnetic poles of the loop will be repelled by the like poles and attracted
by the corresponding opposite poles of the field. The coil will therefore rotate
clockwise, attempting to bring the unlike poles together.

When the loop has rotated through 90 degrees, shown in Fig. 1.5,
commutation takes place, and the current through the loop reverses its direction. As a
result, the magnetic field generated by the loop is reversed. Now, like poles face each
other which means that they repel each other and the loop continues to rotate in an
attempt to bring unlike poles together.

Fig. 1.6 shows the loop position after being rotated 180 degrees from Fig. 1.5.
Now the situation is the same as when the loop was back in the position shown in Fig.
1.5. Commutation takes place once again, and the loop continues to rotate. In this very
basic DC brush motor example, two commutator segments used with one coil loop for
simplicity. Having a small number of commutator segments in DC brush motor
causes torque ripples. As the number of segments increases, the torque fluctuation
produced by commutation is greatly reduced. In a practical machine, for example, one
might have as many as 60 segments, and the variation of the load angle between stator
magnetic flux and rotor flux would only vary 3 degrees, with a fluctuation of less
than 1 percent. Thus, the DC brush motor can produce a nearly constant torque.

Fig. 1.6. Fundamental operation of a DC brush motor (Step 3).

Trends in DC Brush Motors:

DC motors have been the most widespread choice for use in high performance
systems. The reasons for widespread expansion of the DC motor in many types of
industrial drive applications are good control characteristics (speed versus voltage and
speed versus torque), good performance and high efficiency. In spite of rapid
development of the lower cost motors, advantages associated with inherent stability
and relatively simple control of DC machine, are indisputable.

The main reason for their popularity is the ability to control their torque and
flux easily and independently. In DC brush machines, the field excitation that
provides the magnetizing current is occasionally provided by an external source, in
which case the machine is said to be separately excited. In particular, the separately

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excited DC motor has been used mainly for applications where there was a
requirement for fast response and four-quadrant operation with high performance near
zero speed.
Generally in DC brush motors, flux is controlled by manipulating field
winding current and torque by changing the armature winding current. The trade-off
is less rugged motor construction, which requires frequent maintenance and an
eventual replacement of the brushes and commutators. It also precludes the use of a
DC motor in hazardous environments where sparking is not permitted. Moreover,
there is a potential drop called contact potential difference, associated with this
arrangement, and is usually in the range of 1-1.5 V, leading to a drop in the effective
input voltage.

The speed of the motor is controlled by controlling the armature voltage, and
the torque by the armature current, that is, the flux and the torque can easily be
controlled separately. This is the main principle on which all the modern AC control
methods nowadays rely. The first DC motors were controlled with some chopper
technology, such as the pulse width modulation (PWM). Network-connected thyristor
bridges were mainly used in higher power range, typically in a variety of applications
such as in printing and paper industry, passenger lifts, and any kinds of drives
subjected to high transient loading, such as in rolling mills. Chopper technology was
mainly used in the lower power range, such as in machine tool applications.
Development in permanent magnet materials introduced a permanent magnet DC
(PMDC) motor, in which the stator excitation coil was replaced by permanent
magnets. Some advantages in using permanent magnet excitation were decreased
copper losses, higher power density, and a smaller torque ripple at low speeds. Using
permanent magnet material in the magnetic circuit causes a low armature inductance
and hence a low armature reaction. An extremely linear speed-torque characteristic of
the motor, which result from the permanent magnet-provided constant field flux at all
speeds, makes the control of the PMDC very straightforward; the speed of the motor
is controlled by simply adjusting the armature DC voltage. PMDC machines were,
however, limited to the lower power range due to the absence of the proper magnets
until the 1980s. Typical applications of PMDC were low-voltage battery powered
applications, such as machine tools, automotive auxiliary drive applications, and solar
powered applications. Above the 10 kW range, the separately excited DC motor was
the only solution, as it provided high dynamic performance especially when fully
compensated.

In separately excited DC motors torque and thus also the speed can be
controlled by armature current by adjusting the armature voltage. The field can be
controlled by adjusting the excitation current of the separate magnetizing winding.
The flux and torque are therefore separately controllable. On this basic principle was
built thyristor control for higher-power rating of drives used for printing and paper
industry and chopper control for lower-power applications.

Problems in DC Motors:

The characteristics of the DC motors are not ideal for applications with strict
requirements for reliability, service interval and noises due to the mechanical and
electrical limits set by the motor commutator. In addition, the carbon brushes require
regular service.

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Although the separately excited DC motor suits extremely well to servo

applications thanks to its dynamic performance, the major problem is the mechanical
commutator and the carbon brushes, which require regular maintenance. The
commutator also degrades the overloading capability of the machine. Good
commutation, which means armature current reversal in a single armature coil without
sparking at the brushes, is extremely important to prevent the premature brush failure.
There is a physical limit to the speed and to the power, at which the current can be
commutated. If the limit is excessively exceeded, a ring of heavy sparks runs around
the commutator circumference. If the commutation limit has to be exceeded, several
armatures on a single shaft are required, which makes the drive more complex and
expensive. It is in other view that the drawback of the DC motors is the fact that while
the rotation speed increases, the voltage between commutator segments also increases
and in combination with high armature current, a voltage breakdown between
adjacent commutator segments will result in brush fire or also known as brush
flashover. It will rapidly destroy the brushes and the commutator and it should be
avoided since it damages the commutator and brush gear which reduces the life
expectancy of the motor. Thus operational area of the motor is bounded by different
factors. Despite these disadvantages numerous applications require drives based on
brushed DC motors. In spite of the good control characteristics enumerated drawbacks
of DC motors are inconvenient for producing reliable high performance belt drives.

There are still numerous applications, in which the most demanding motion
control is realized with a brushed DC motor, because when properly maintained, the
DC motor has a dynamic performance equal to the modern vector controlled AC drive
with a notably simpler control. Therefore it is not surprising that even today, in the
literature, the word servo motor often refers to a brushed DC motor.

AC Motors and Their Trends

Unlike DC brush motors, AC motors such as Permanent Magnet AC motors

(PMSM, and BLDC motors), and Induction Motors (IM) are more rugged meaning
that they have lower weight and inertia than DC motors. The main advantage of AC
motors over DC motors is that they do not require an electrical connection between
the stationary and rotating parts of the motor. Therefore, they do not need any
mechanical commutator and brush, leading to the fact that they are maintenance free
motors. They also have higher efficiency than DC motors and a high overload
capability.

All of the advantages listed above label AC motors as being more robust, quite
cheaper, and less prone to failure at high speeds. Furthermore, they can work in
explosive or corrosive environments because they dont produce sparks.

All the advantages outlined above show that AC motors are the perfect choice
for electrical to mechanical conversion. Usually mechanical energy is required not at
a constant speeds but variable speeds. Variable speed control for AC drives is not a
trivial matter. The only way of producing variable speeds AC drives is by supplying
the motor with a variable amplitude and frequency three phase source.

Variable frequency changes the motor speed because the rotor speed depends
on the speed of the stator magnetic field which rotates at the same frequency of the

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applied voltage. For example, the higher the frequency of the applied voltage the
higher the speed. A variable voltage is required because as the motor impedance
reduces at low frequencies the current has to be limited by means of reducing the
supply voltage.

Before the days of power electronics and advanced control techniques, such as vector
control and direct torque control AC motors have traditionally been unsuitable for
variable speed applications. This is due to the torque and flux within the motor being
coupled, which means that any change in one will affect the other.

Trends in Induction motors

The AC squirrel-cage induction motors are the largest group of all electrical
drives in the industry. It has been estimated that they are used in 70-80% of all
industrial drive applications, especially in fixed-speed applications such as pump or
fan drives. The benefits of induction motor are undisputable: simple construction, low
cost compared to other motors, simple maintenance, high efficiency and satisfactory
characteristics at the high speeds. With appropriate power electronics converters
induction motors are used in wide power range from kW to MW levels.

A common structure of induction motor is represented in Fig.1.7. Stator

rotation field induces an electromotive force in the short-circuited rotor winding. Due
to the induced voltage and short-circuited winding, current occurs in the rotor and
electromagnetic torque is produced.

Fig.1.7. Common structure of Induction machine

Induction motor was invented in the end of 19th century. Its theory is well-
known and power electronic converter technology provides appropriate variable-
voltage/current, variable-frequency supply for efficient and stable variable-speed
control. Thus, it is possible to obtain a dynamic performance in all respects better than
which could be obtained with a phase-controlled DC drive combination. The
significant characteristic of the induction motor is a slip caused by the rotor lagging
the rotating stator magnetic field. Rotor copper losses are directly proportional to the
slip. For example, the rotor copper losses in 4 kW motor are approximately 4.7% of

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KARTHIK Kongu Engineering College

the nominal power if the efficiency of 85% is supposed. The slip has also impact
when a high dynamic performance is needed, since it derates the transient response of
the motor.

The main disadvantage of the induction motor for the servo control systems is
the nonlinear speed versus torque and speed versus control voltage characteristics.
Therefore such motors are inconvenient for the implementation of the belt-drives.

In the early times, very limited speed control of induction motors was
achieved by switching the three-stator windings from delta connection to star
connection, allowing the voltage at the motor windings to be reduced. If a motor has
more than three stator windings, then pole changing is possible, but only allows for
certain discrete speeds. Moreover, a motor with several stator windings is more
expensive than a conventional three phase motor. This speed control method is costly
and inefficient.

Another alternative way of speed control is achieved by using wound rotor

induction motor, where the rotor winding ends are connected to slip rings. This type
of motor however, negates the natural advantages of conventional induction motors
and it also introduces additional losses by connecting some impedance in series with
the stator windings of the induction motor. This results in very poor performance.

At the time the above mentioned methods were being used for induction motor
speed control, DC brush motors were already being used for adjustable speed drives
with good speed and torque performance.
The goal was to achieve an adjustable speed drive with good speed
characteristics compared to the DC brush motor. Even after discovering of the AC
asynchronous motor, also named induction motor, in 1883 by Tesla, more than six
decades later of invention of DC brush motors, capability of adjustable speed drives
for induction motors is not as easy as DC brush motors.

Speed control for DC motors is easy to achieve. The speed is controlled by

applied voltage; e.g. the higher the voltage the higher the speed. Torque is controlled
by armature current; e.g. the higher the current the higher the torque. In addition, DC
brush motor drives are not only permitted four quadrant operations but also provided
with wide power ranges.

Recent advances in the development of fast semiconductor switches and cost-

effective DSPs and micro-processors have opened a new era for the adjustable speed
drive. These developments have helped the field of motor drives by shifting
complicated hardware control structures onto software based advanced control
algorithms. The result is a considerable improvement in cost while providing better
performance of the overall drive system. The emergence of effective control
techniques such as vector and direct torque control, via DSPs and microprocessors
allow independent control of torque and flux in an AC motor, resulting in
achievement of linear torque characteristics resembling those of DC motors.

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Trends in PERMANENT MAGNET (PM) MOTORS:

The advent of modern permanent magnets (PM) with significant amount of energy
density led to the evolution of dc machines with PM Field excitation in the 1950s.
Replacing electromagnets with PM eliminated the need of using windings and
external energy source. Therefore, compact dc machines were introduced. PM
excitation also replaced the dc field excitation of the synchronous machines. In late
1950s, the availability of switching power devices led to the development of inverters.
This achievement enabled the replacement of the mechanical commutator with an
electronic commutator. Therefore permanent magnet synchronous and brushless dc
machines were developed. By removing the mechanical commutator, the armature of
the dc machine can be on the stator side. This enables better cooling and higher
voltages to be achieved. In this configuration, PM poles used as excitation field are in
the rotor side. From structural point of view, permanent magnet machines are the
inside out of dc machines with the field and armature interchanged from the stator to
the rotor and rotor to stator respectively.

Permanent magnet machines present a unique set of opportunities to the drive

designer. Combining high efficiency with high power density makes them widely
appealing. Permanent magnet machines are synchronous machines without auxiliary
rotor windings. Therefore accompanying power electronics drive is essential for their
operation. These motors obtain life long field excitation from permanent magnets.
The absence of rotor electrical circuit makes their analysis simple. Since there are no
windings on the rotor, electrical losses in the rotor are minimal.

PERMANENT MAGNET MATERIALS:

A magnet is any object that exhibits an external magnetic field. However, this
does not necessarily make it a permanent magnet as it also includes electromagnets
made of current carrying wire. A permanent magnet is a material that when inserted
into a strong magnetic field will not only begin to exhibit a magnetic field of its own,
but also continue to exhibit a magnetic field once removed from the original field.
This field would allow the magnet to exert force (ability to attract or repel) on other
magnetic materials. The exhibited magnetic field would then be continuous without
weakening provided the material is not subjected to a change in environment
(temperature, demagnetizing field, etc.). The ability to continue exhibiting a field
while withstanding different environments helps to define the capabilities and types of
applications in which a magnet can be successfully used.

The properties of the permanent magnet material will affect directly the
performance of the motor and proper knowledge is required for the selection of the
materials. It also plays a vital role in the design of a permanent magnet motors.

Until the late 1930s, permanent magnets were predominantly steel

compositions with low energy product and coercivity. Rather than utilizing permanent
magnets, loudspeakers used an interaction of electromagnetic fields and motors were
of the induction type. The invention of ALNICO allowed product size reduction
through the use of permanent magnets in place of induction coils. Approximately
every 12 years thereafter, a new magnetic material was discovered. Figure 1.8 shows
how the maximum energy product has increased. It also illustrates that materials with

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KARTHIK Kongu Engineering College

lower energy product, specifically ferrite, can be commercially successful. Introduced

in 1961, ferrite remains the largest selling permanent magnet material on a weight
basis primarily because of its relatively low price.

Figure 1.8 Developments of Permanent Magnet Materials

New materials have not obviated older ones: each has advantages and
disadvantages. ALNICO, though magnetically weaker than rare earth magnets, is
much more temperature stable. Applications requiring stability over wide temperature
ranges still rely on alnico. But the newer materials (Ferrite, Samarium Cobalt, and
Neodymium-Iron-Boron) all have a very important characteristic, a square second
quadrant intrinsic curve, which allows use in applications which were not possible
before.

There are several classes of modern commercialized magnets, each based on

their material composition. Within each class is a family of grades with their own
magnetic properties. These general classes are:

Hardened steel - Earliest manufactured magnets are made from steel

Aluminium Nickel & Cobalt alloys (ALNICO)
Strontium Ferrite (or) Barium Ferrite (Ferrite)
Samarium Cobalt (SmCo) - First generation rare earth magnets
Neodymium Iron-Boron (Nd-Fe-B) - Second Generation rare earth
Magnets used in recent days.

The earliest manufactured magnet materials were hardened steel. Magnets

made from steel were easily magnetized. However, they could hold very low energy
and it was easy to demagnetize. In recent years other magnet materials such as
Aluminum Nickel and Cobalt alloys (ALNICO), Strontium Ferrite or Barium Ferrite
(Ferrite), Samarium Cobalt (First generation rare earth magnet) (SmCo) and

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Neodymium Iron-Boron (Second generation rare earth magnet) (Nd-Fe-B) have been
developed and used for making permanent magnets.

Nd-Fe-B and SmCo are collectively known as Rare Earth magnets because
they are both composed of materials from the Rare Earth group of elements.
Neodymium Iron Boron (general composition Nd2Fe14B, often abbreviated to Nd-Fe-
B) is the most recent commercial addition to the family of modern magnet materials.
At room temperatures, Nd-Fe-B magnets exhibit the highest properties of all magnet
materials. Samarium Cobalt is manufactured in two compositions: Sm1Co5 and
Sm2Co17 - often referred to as the SmCo 1:5 or SmCo 2:17 types. 2:17 types, with
higher Hci values, offer greater inherent stability than the 1:5 types. Ceramic, also
known as Ferrite, magnets (general composition BaFe2O3 or SrFe2O3) have been
commercialized since the 1950s and continue to be extensively used today due to their
low cost. A special form of Ceramic magnet is "Flexible" material, made by bonding
Ceramic powder in a flexible binder. Alnico magnets (general composition Al-Ni-Co)
were commercialized in the 1930s and are still extensively used today.

These materials span a range of properties that accommodate a wide variety of

application requirements. The following is intended to give a broad but practical
overview of factors that must be considered in selecting the proper material, grade,
shape, and size of magnet for a specific application. The Table1.1 below shows
typical values of the key characteristics for selected grades of various materials for
comparison.

Table 1.1 Magnet Material Comparisons

Material Grade Br Hc Hci BHmax Tmax
(Deg C)*
Nd-Fe-B 39H 12,800 12,300 21,000 40 150
SmCo 26 10,500 9,200 10,000 26 300
Nd-Fe-B B10N 6,800 5,780 10,300 10 150
Alnico 5 12,500 640 640 5.5 540
* Tmax (maximum practical operating temperature) is for reference only. The maximum
practical operating temperature of any magnet is dependent on the circuit the magnet is
operating in.

The rare earth magnets are categorized into two classes: Samarium Cobalt
(SmCo) magnets and Neodymium Iron Boride (Nd-Fe-B) magnets. SmCo magnets
have higher flux density levels but they are very expensive. Nd-Fe-B magnets are the
most common rare earth magnets used in motors these days. A flux density versus
magnetizing field for these magnets is illustrated in figure 1.9.

Property of Permanent Magnet

The property of permanent magnet and the selection of pertinent materials are
crucial in the design of permanent magnet machine. Barium and strontium ferrites are
broadly used as permanent magnets. Low cost and huge supply of raw material are
two major advantages of ferrite. They can be easily produced and their process is
adopted for high volume as well as moderately high service temperature. The magnet
has a practically linear demagnetization curve but has a low remanance. Therefore,
the machine has a high volume as well as weight. The Cobalt-Samarium magnet is

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built of iron, Nickel, Cobalt, and rare-earth Samarium. High remanance, high energy
density, and linear demagnetization characteristic are among its advantages. Although
the material is quite expensive due to insufficient supply of Samarium, the service
temperature can be as high as 300 C and the temperature stability is very satisfactory.
The Neodymium-iron-boron (Nd-Fe-B) magnet has the highest energy density,
highest remanance, and very good coercivity. The disadvantages are low service
temperature, and susceptibility to oxidation if it is not protected by coating. In
addition, the temperature stability is lower than that of a CoSm magnet. Although the
material is expensive compared to ferrite, the machine weight is reduced due to its
higher energy density magnets. Nowadays, Nd-Fe-B magnets are being used in
different applications.

Figure 1.9 Flux Density versus Magnetizing Field of Permanent Magnetic

Materials

Design Considerations

Basic problems of permanent magnet design revolve around estimating the

distribution of magnetic flux in a magnetic circuit, which may include permanent
magnets, air gaps, high permeability conduction elements, and electrical currents.
Exact solutions of magnetic fields require complex analysis of many factors, although
approximate solutions are possible based on certain simplifying assumptions.

In most cases, the higher remanence with higher coercivity in a permanent

magnet is desired by motor designers. The alnico magnet provides a fairly high
remanence flux density but a low coercive force. When the coercive force is low and
two opposing magnetic poles are in proximity of each other, the magnetic poles can
weaken each other and there is a possibility of permanent demagnetization by the
opposing field.

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KARTHIK Kongu Engineering College

Unlike an alnico magnet, the ferrite magnet has a low flux density, but a high
coercive force. It is possible to magnetize the ferrite magnet across its width as a
result of this high coercive force. Ferrite magnets are most widely used in electric
motors because their material and production costs are low. The cost of a typical
ferrite magnet material at this time is about 6-8 times lower than the Nd-Fe-B.
Another measurement is an output power per unit cost of active material. It is
predicted that the output power per unit cost is about 4 times lower for ferrite magnet
motor compared to the Nd-Fe-B magnet motor. Delco Remy uses the ferrite and Nd-
Fe-B magnets for different starter motor applications. Rare-earth magnets have both
high magnetic remanence, and high coercive force. Since the initial cost is high, these
permanent magnets are used in applications such as high performance and high-
energy density motor applications. For a given volume, the flux density is twice that
of the ferrite, leading to a larger torque production. Nd-Fe-B magnetic materials are
superior to any other magnetic material now on the market.

The only disadvantage of using an Nd-Fe-B magnet, as opposed to a Sm Co

magnet, is that the high energy density Nd-Fe-B permanent magnet has a maximum
operating temperature of 100 to 150 degrees C, as compared to 200-300 degrees C for
Sm Co, alnico, and ferrite. However obtaining an optimum magnet design often
involves experience and tradeoffs.

DEVELOPMENT IN PERMANENT MAGNET MOTORS

The permanent magnet machine is highly coveted for its high power density
and high efficiency. This is mainly due to the high energy density NdFeB and SmCo
magnets, which are commercially available today. In other words, advancements in
high-energy permanent magnet materials and magnet manufacturing technologies
enabled the manufacturing of high power density and high efficiency permanent
magnet motors at a reasonable cost. Also, the availability of fast switching high power
semiconductor devices with low on-state voltage drop such as MOSFETs and IGBTs.
Ever increasing high-speed microprocessors/digital signal processors have contributed
to permanent magnet electric motors. While the cost for semiconductors and the
permanent magnets is still high at the present time, trends for cost reduction are
continuing and encouraging.

There are two types of permanent magnet motors: brush and brushless.
Todays vehicle applications almost exclusively use brush type permanent magnet
motors.

Brushed Type Permanent Magnet Motor

The brush permanent magnet motors have four general characteristics that
cause them to be useful for vehicle application:
1) Desirable torque versus speed,
2) Simple control of torque and speed,
3) High electromagnetic power density and
4) Inverters are not required.

Nevertheless, there are six general characteristics that detract from more wide
applications in the automotive industry:

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1) Friction between the brushes and the commutator,

2) Brushes and commutators require maintenance,
3) Current is supplied to the armature through the brushes and commutator,
4) Brushes and commutators are open and produce sparking,
5) Cooling of a DC motor is difficult and
6) Switching of large currents is required for control of DC motors.

The brushless motors are becoming stronger candidates over traditional brush
type motors for the following reasons: higher efficiency, higher power density, better
heat dissipation, and increased motor life. In addition, brushless motors experience no
losses due to brush friction and they deliver higher torque compared to a brushed type
motor of equal size and weight.

Brushless Type Permanent Magnet Motor

Electronically commutated, brushless permanent magnet motors are however,

becoming prime movers in vehicle propulsion, industrial drives, and actuators as a
result of improvements in permanent magnet materials, advances in the power
electronic devices, and power integrated circuits in the last two decades. Not only
have there been gradual improvements in Alnico and Ferrite (ceramic) alloys, but the
rapid development of rare-earth magnets, such as samarium-cobalt (SmCo) and
neodymium-boron-iron (Nd-Fe-B) around 1980, have provided designers with a
significant increase in available field strength. This new high density, brushless,
permanent magnet motor system provides a very high torque to inertia ratio. Figure
1.10 shows radial and axial field permanent magnet motors.

Figure1.10. Schematics of radial field and axial field

permanent magnet motors

For an axial flux BLDC motor, as the name suggests, the magnetic field that
interplays between the stator and the rotor crosses the air gap in the axial direction.
Whereas in radial flux BLDC motor the magnetic field crosses the air gap in the radial
direction.

Permanent Magnet Synchronous Machines (PMSM)

Permanent magnet synchronous machines emerged into servo drives since the
second part of the 20th century and, nowadays, this motor type is widely spread in
industry, especially in windmill generators and propulsion motors. Main
disadvantages of early permanent magnet motors were the risk of demagnetization of
magnets by high stator currents during starting and the restricted maximum allowable

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KARTHIK Kongu Engineering College

temperature. The development of high quality permanent magnet materials during the
1970s overcame these problems. PMSMs mechanical and electrical characteristics,
especially the inductances, are highly dependent on the rotor structure. PMSMs can be
realized either with embedded or surface magnets on the rotor. As the relative
permeability of modern magnet materials is close to unity, the effective air gap
becomes large with a surface magnet construction. As a result, the direct axis
inductance of the machine remains low, which improves the overloading capability of
the motor but at the cost of the reduced field weakening range. Another advantage of
such a motor configuration is low inertia.

Permanent magnet synchronous motors are very good alternatives for belt
drives due to very good control characteristics and the absence of the mechanical
commutator. The drawback of PMSM is their relatively higher costs compared to the
other types of motors.

Trends in Permanent Magnet Brushless DC Machines (PMBLDC):

First electric servo systems were all DC drives, since the DC motor was for a
long time the only motor type capable of high dynamic performance. Regardless of its
certain drawbacks namely a complex construction (especially with a fully
compensated machine), lower efficiency than in the AC machines, and high cost the
torque control of the DC motor has nevertheless always been very straightforward, as
the air gap flux and the torque can be controlled separately. However, the DC
machine requires regular maintenance due to its carbon brushes. Its overloading
capability is also poor due to the mechanical commutator, which is not capable of
commutating high currents. Further, its rotor construction limits the maximum speed,
and if high-speed operation is required, some special measures must be adopted to
ensure adequate mechanical ruggedness. Starting in the 1960s, development on
permanent magnet (PM) materials, power switches, and microprocessors made it
possible to utilize also AC in the speed control of electrical machines. This way, it
was possible to get rid of the brushes by electronically commutating the current from
one phase to another.

The first brushless machines were three-phase PM machines with rectangular-

shaped stator voltage, although the back-EMF waveform ranged from sinusoidal to
trapezoidal. Since BLDC motor has a trapezoidal back EMF, to have a smooth torque
this motor should be fed by rectangular current pulses that are aligned with the flat
portions of the back EMF waveform and with the same polarity. Although the
direction of the current of these machines varied, their control was yet quite similar to
the control of DC machines. BLDC motors have received wide attention as their
performance can be superior to conventional DC commutator motor and induction
motors.

Nowadays, these machines are often referred to as permanent magnet DC

motors (PMDC) or brushless DC motors (BLDC), although the use of terminology is
still very confusing and depends largely on the source.

Also the recent progress in the area of magnetic materials and power
semiconductors has improved the performance of brushless dc motors (BLDCM)
significantly. Its advantages are high power/weight ratio, high torque/current ratio,

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KARTHIK Kongu Engineering College

high dynamics-flexible control for variable speed operation, no maintenance without

mechanical commutator (brushless-type), etc. These features have made brushless dc
motors one of the best choices to replace conventional brush-type dc motors in
demanding servo and robotics applications.

Trends in Stepper Motor:

Before the discovery of the rotating magnetic field by Tesla in 1887, the first
reluctance motor was similar to the doubly salient synchronous reluctance motor,
nowadays known as the stepper motors and later with the invention of power
converters lead to the development of switched reluctance motor.

A Stepper motor is marvel in simplicity. It has no brushes, or contacts.

Basically it's a synchronous motor with the magnetic field electronically switched to
rotate the armature magnet around.

A stepper motor, also called stepping motor, pulse motor or digital motor, is
an electromechanical device which rotates a discrete step angle when energized
electrically. Stepper motors are synchronous motors in which rotors positions depend
directly on driving signal. Rotary moment is defined by magnetic energy and is
proportional to the tooth number of the rotor. Stepping motors can be viewed as
electric motors without commutators. Typically, all windings in a stepper motor are
part of the stator, and a rotor is either a permanent magnet or, in the case of variable
reluctance motors, a toothed block of some magnetically soft material or a hybrid of
both. The main difference between the stepping motor and a general motor is that the
stepping motor only powered by a fixed driving voltage does not rotate. A stepping
motor exhibits excellent functions such as accurate driving, rapid stopping, rapid
starting and the like. The stepper motor provides controllable speed or position in
response to input step pulses commonly applied from an appropriate control circuit.

For some applications, there is a choice between using servomotors and

stepping motors. Both types of motors offer similar opportunities for precise
positioning, but they differ in a number of ways. Servomotors require analog feedback
control systems of some type. Typically, this involves a potentiometer to provide
feedback about the rotor position, and some mix of circuitry to drive a current through
the motor inversely proportional to the difference between the desired position and the
current position. In making a choice between steppers and servos, a number of issues
must be considered; which of these will matter depends on the application. For
example, the repeatability of positioning done with a stepping motor depends on the
geometry of the motor rotor, while the repeatability of positioning done with a
servomotor generally depends on the stability of the potentiometer and other analog
components in the feedback circuit.

Stepper motors generally have two phases, but three, four and five-phase
motors also exist. A two phase stepping motor may described as comprising at least
first and second coils perpendicularly oriented with respect to each other which are
alternately driven with currents of opposite polarities. In a stepper motor having two
phases, a drive circuit may cause electrical current to flow through the stator windings
(phases) in accordance with the sine/cosine law. A two-phase stepping motor is
mainly employed for use requiring a medium accuracy; while a three-phase stepping

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motor excellent in cost performance is employed for use requiring high-accuracy, low
vibration and low noise. The three-phase machine comprises a cylindrical permanent
magnet type rotor formed with multiple magnets in a cylindrical shape, or a hybrid
type rotor having a permanent magnet held between two magnetic plates formed with
multiple pole teeth, and a stator formed with pole teeth opposite the rotor surface. The
structure of the stepping motor can be classified into a VR type (variable reluctance or
variable magnetic resistant), a PM type (permanent magnet), and a hybrid (HB) type
that combines the above two. A variable reluctance type stepping motor is driven by
the attractive force between rotors that form teeth of the motor and stators of the
magnetic poles. A permanent magnet type stepping motor is driven by the attractive
force and repulsive force between a rotor formed by a permanent magnet having
alternatively arranged N poles and S poles and stators of the magnetic poles. The
hybrid stepping motor has a structure combining those of the VR type and PM type.

The permanent magnet stepping motor allows easy control of forward and
reverse running operation and its physical size can be easily reduced depending on
required driving torque. Use of a permanent magnet is widely diversified and in the
use, a permanent magnet maintains an important position as a constituent element of
an electronic apparatus, particularly, a small-sized motor, particularly as a rotor
magnet. Permanent magnet stepper motors are currently used in a wide variety of
apparatus including cameras, printers and scanners. Their ability to effect discrete and
precise movement makes them the preferred choice for driving mechanical elements
in this type of equipment. Hybrid stepping motors have been well known as actuators
appropriated for highly accurate positioning movements. The hybrid stepping motors
are widely used in various machine tools, e.g. with fully automated production lines,
as well as computer related instruments including printers, plotters, facsimile
machines, and disk drive units.

Stepping motors can be used in simple open-loop control systems; these are
generally adequate for systems that operate at low accelerations with static loads, but
closed loop control may be essential for high accelerations, particularly if they
involve variable loads. If a stepper in an open-loop control system is overtorqued, all
knowledge of rotor position is lost and the system must be reinitialized.

Special machine is a broad term used for many aspects. Inconsequence with
the available literatures regarding stepper motor, a detailed and student friendly book
for other permanent magnet and reluctance motor drives is lacking. Henceforth this
book focuses on the aforesaid topics and a fine balance has been struck between
theory and practice.

Trends in Synchronous Reluctance Motor (synchrel):

Recently, synchronous reluctance motors (Synchrel or SynRM) have been

considered as a possible alternative motor drive for high performance applications.
Synchronous reluctance motors (SynRM) which is a family member of brushless AC
machines

The reluctance motor is arguably the simplest synchronous motor of all, the
rotor consisting of a set of iron laminations shaped so that it tends to align itself with
the field produced by the stator. The stator winding is identical to that of a three-phase

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KARTHIK Kongu Engineering College

induction motor. However, the rotor is different in that it contains saliency (a

preferred path for the flux); this is the feature which tends to align the rotor with the
rotating magnetic field, making it a synchronous machine. The practical need to start
the motor means that a form of starting cage also needs to be incorporated into the
rotor design and the motor is started as an induction motor, the reluctance torque then
pulling in the rotor to run synchronously in much the same way as for a permanent
magnet rotor.

Reluctance motors may be used on both fixed-frequency (mains) supplies and

inverter supplies. These motors tend to be one frame size larger than a similarly rated
induction motor and have low power factor (perhaps as low as 0.4) and poor pull in
performance. As a result of these limitations their industrial use has not been
widespread except for some special applications such as textile machines where large
numbers of reluctance motors may be connected to a single bulk inverter and
maintains synchronism. Even in this application, as the cost of inverters has reduced,
bulk inverters are infrequently used and the reluctance motor is now rarely seen.

Trends in Switched Reluctance Motor:

Switched Reluctance Machines (SRMs) are receiving significant attention

from industries in the last decade. They are extremely inexpensive, reliable and weigh
less than other machines of comparable power outputs.

Permanent magnet motors and induction motors are the most commonly used
in drive motors for HEV and EV as well as other electric motor applications. The
Permanent magnet motors has some advantages such as its simple design and ease of
control without any position sensors, which makes the development-time short.
Another advantage of PM motor is its high full-load efficiency due to free field
excitation from the permanent magnets which is important for low power
applications. Though the Permanent magnet motors has gained in the market instead
of the SRM, it has some disadvantages when compared to the SRM which are the
price of the magnets, the difficult assembly of the magnets and weak no-load
efficiency. The field from the magnets are almost constant and not reduced at no-load,
which means that there is no way to reduce the iron-losses when the torque demand is
low.

Recently there have been increased activities in switched reluctance motor

technology due to the high performance, fault tolerance operation, simplicity of
construction, and better cost-effectiveness than, rare earth permanent magnet motors.

There are four reasons that support the switched reluctance motor technology
as another widely developed motor type:

1. Economical yet powerful computational computers and its software

2. High frequency power electronic devices such as MOSFETs and IGBTs with
affordable cost
3. Better understanding of the switched reluctance technology
4. Integrated design of motor and electronics capability.

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KARTHIK Kongu Engineering College

The most important property of a Switched Reluctance Motor is the salient

poles on both the moveable part (rotor) and the stationary part (stator).

The concept of a switched reluctance machine is very simple. The Switched

Reluctance Motor is a singly-excited motor with salient poles on both the stator and
the rotor. It consists of stator and rotor poles, made of laminated steel with high
magnetic permeability. Only the stator carries windings and the rotor has neither
windings nor magnets and is built up from a stack of steel laminations. One stator
phase consists of two series- connected windings on diametrically opposite poles.
Only the stator poles are excited by coils. Fig.1.11 shows a typical configuration is the
SRM with 8 stator and 6 rotor poles, a so called 8/6 SRM. With 8 coils on the stator,
4 phases are created with the corresponding coils in parallel. A sequence of anti-
clockwise excitations of the different phases results in a clockwise rotation of the
rotor due to a positive torque generation.

Fig.1.11 C.S. of a (8/6) SRM

The Switched Reluctance Motors show promise as potentially low cost

electromechanical energy conversion devices because of their simple mechanical
construction. The advantages of a Switched Reluctance Motor are the production cost,
efficiency and the torque/speed characteristics.

In conclusion, the switched reluctance motor has several good features that
make it attractive for a range of variable automotive application. These include:
1. High efficiency over a wide range of torque and speed..
2. High torque capabilities at the low operational speeds.
3. Simple and rugged rotor construction.
4. Fault tolerant, four-quadrant operation.
5. Suitable for extreme condition operation.
6. The wide use of switched reluctance motor technology requires that
improvements need to be made on noise and torque ripple.

Current Controlled Inverter

The motor is fed form a voltage source inverter with current control. The
control is performed by regulating the flow of current through the stator of the motor.
Current controllers are used to generate gate signals for the inverter. Proper selection
of the inverter devices and selection of the control technique will guarantee the
efficiency of the drive.

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KARTHIK Kongu Engineering College

Inverter

Voltage Source Inverters are devices that convert a DC voltage to AC voltage

of variable frequency and magnitude. They are very commonly used in adjustable
speed drives and are characterized by a well defined switched voltage wave form in
the terminals. Figure1.12 shows a voltage source inverter. The AC voltage frequency
can be variable or constant depending on the application.

Figure 1.12 Voltage Source Inverter Connected to a Motor

Three phase inverters consist of six power switches connected as shown in

figure 1.12 to a DC voltage source. The inverter switches must be carefully selected
based on the requirements of operation, ratings and the application. There are several
devices available today and these are thyristors, bipolar junction transistors (BJTs),
MOS field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs)
and gate turn off thyristors (GTOs).

The devices lists with their respective power switching capabilities are shown
in table1.2. MOSFETs and IGBTs are preferred by industry because of the MOS
gating permits high power gain and control advantages. While MOSFET is considered
a universal power device for low power and low voltage applications, IGBT has wide
acceptance for motor drives and other application in the low and medium power
range. The power devices when used in motor drives applications require an inductive
motor current path provided by anti-parallel diodes when the switch is turned off.
Inverters with anti-parallel diodes are shown in figure 1.13.

Table 1.2 Devices Power and Switching Capabilities

Device Power capability Switching speed
BJT Medium Medium
GTO High Low
IGBT Medium Medium
MOSFET Low High
THYRISTOR High Low

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Introduction to IGBTs

Prior to the development of the IGBTs (insulated gate bipolar transistor),

power MOSFETs were used in medium or low voltage applications which require fast
switching, whereas bipolar power transistors, thyristors and GTOs were used in
medium to high voltage applications which require high current conduction. A power
MOSFET allows for simple gate control circuit design and has excellent fast
switching capability. On the other hand, at 200V or higher, it has the disadvantage of
rapidly increasing on-resistance as the breakdown voltage increases. The bipolar
power transistor has excellent on-state characteristics due to the low forward voltage
drop, but its base control circuit is complex, and fast switching operation is difficult as
compared with the MOSFET. The IGBT developed in the early 1980s has the
combined advantages of the above two devices. It has a MOS gate input structure,
which has a simple gate control circuit design and is capable of fast switching up to
100kHz. Additionally, because the IGBT output has a bipolar transistor structure, its
current conduction capability is superior to a bipolar power transistor. Based upon
these excellent characteristics, the IGBT has been extensively used in applications
exceeding 300V voltage as an alternative to power MOSFETs and bipolar power
transistors. Its area of application continues to increase. The IGBT is becoming more
modular as its use increases in applications that require higher current conduction
capability.

Figure 1.13 Inverter with IGBTs and Anti-parallel Diodes

Fig. 1.14: Physical structure of an IGBT

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The insulated gate bipolar transistor (IGBT) combines the positive attributes
of BJTs and MOSFETs. BJTs have lower conduction losses in the on-state, especially
in devices with larger blocking voltages, but have longer switching times, especially
at turn-off while MOSFETs can be turned on and off much faster, but their on-state
conduction losses are larger, especially in devices rated for higher blocking voltages.
Hence, IGBTs have lower on-state voltage drop with high blocking voltage
capabilities in addition to fast switching speeds. IGBTs have a vertical structure as
shown in Fig.1.14. This structure is quite similar to that of the vertical diffused
MOSFET except for the presence of the p+ layer that forms the drain of the IGBT.
This layer forms a pn junction (labeled J1 in the figure), which injects minority
carriers into what would appear to be the drain drift region of the vertical MOSFET.
The gate and source of the IGBT are laid out in an interdigitated geometry similar to
that used for the vertical MOSFET.

The symbolic representation and the equivalent circuit of an IGBT are shown
in Figure 1.15.

Figure 1.15 IGBT Symbolic representation and its Equivalent Circuit

Turn-on Operation

When the device is in the forward blocking mode, and if the positive gate bias
(threshold voltage), which is enough to invert the surface of P-base region under the
gate, is applied, then an n-type channel forms and current begins to flow. At this time
the anode-cathode voltage must be above 0.7V (potential barrier) so that it can
forward bias the P+ substrate / N- drift junction (J1). The electron current, which flows
from the N+ emitter via the channel to the N- drift region, is the base drive current of
the vertical PNP transistor. It induces the injection of hole current from the P+ region
to the N- base region. The conductivity modulation improves because of this high
level injection of the minority carrier (hole). This increases the conductivity of the
drift region by a factor varying from ten to hundred. This conductivity modulation
enables IGBTs to be used in high voltage applications by significantly reducing the
drift region resistance. There are two kinds of currents flowing into the emitter
electrode. One is the electron current (MOS current) flowing through the channel, and
the other is the hole current (bipolar current) flowing through the P+ body / N- drift
junction (J2).

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KARTHIK Kongu Engineering College

Turn-off Operation

The gate must be shorted to the emitter or a negative bias must be applied to
the gate. When the gate voltage falls below the threshold voltage, the inversion layer
cannot be maintained, and the supply of electrons into the N- drift region is blocked,
at which point, the turn-off process begins. However, the turn-off cannot be quickly
completed due to the high concentration minority carrier injected into the N- drift
region during forward conduction. First, the collector current rapidly decreases due to
the termination of the electron current through the channel, and then the collector
current gradually reduces, as the minority carrier density decays due to
recombination.

Advantages, Disadvantages and Characteristic Comparison with BJT and

MOSFET:
Advantages
(1) High forward conduction current density and low on-state voltage drop:
The IGBT has a very low on-state voltage drop due to conductivity modulation and
has superior on-state current density compared to the power MOSFET and bipolar
transistor. So a smaller chip size is possible and the cost can be reduced.
(2) Low driving power and a simple drive circuit due to the input MOS gate structure:
The IGBT can easily be controlled as compared to current controlled devices
(thyristor, BJT) in high voltage and high current applications.
(3) Wide SOA:
With respect to output characteristics, the IGBT has superior current conduction
capability compared with the bipolar transistor. It also has excellent forward and
reverse blocking capabilities.

Disadvantages
(1) Switching speed (less than 100 kHz) is inferior to that of the power MOSFETs,
but it is superior to that of the BJT. The collector current tailing due to the minority
carrier causes the turn-off speed to be slow.
(2) There is the possibility of latch-up due to the internal PNPN thyristor structure.

The Characteristics Comparison with a BJT and a MOSFET

Table 1.3 The IGBT Characteristics Comparison with BJT, MOSFET

Features BJT MOSFET IGBT
Drive Method Current Voltage Voltage
Drive Circuit Complex Simple Simple
Input Impedance Low High High
Drive Power High Low Low
Switching Speed Slow (s) Fast (ns) Middle
Operating Frequency Low (less than Fast (less than Middle
100kHz) 1MHz)
S.O.A. Narrow Wide Wide
Saturation Voltage Low High Low

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Multi-phase Machines

Power electronic converters are being utilized for variable speed drives. The
power rating of the converter should meet the required level for the machine and
driven load. However, the converter ratings can not be increased over a certain range
due to the limitation on the power rating of semiconductor devices. One solution to
this problem is using multi-level inverter where switches of reduced rating are
employed to develop high power level converters. The advent of inverter-fed motor
drives also removed the limits of the number of motor phases. This fact made it
possible to design machine with more than three phases and brought about the
increasing investigation and applications of multi-phase motor drives. Multi-phase
machines can be used as an alternative to multi-level converters. In multi-phase
machines, by dividing the required power between multiple phases, more than the
conventional three, higher power levels can be obtained and power electronic
converters with limited power range can be used to drive the multi-phase machine.

However whether it is better to use multi-phase machines vs. multi-level

converters is debatable and in fact it is extremely application dependent. Insulation
level is one of the limiting factors that can prohibit the use of high voltage systems.
Therefore, multiphase machines that employ converters operating at lower voltage
level are preferred.

Multi-phase motor drives posses many advantages over the traditional three-
phase motor drives such as reducing the amplitude and increasing the frequency of
torque pulsation, reducing the stator current per phase without increasing the voltage
per phase and increasing the reliability and power density.

The first speed-controlled drive was introduced over 100 years ago by Harry
Ward Leonard in his paper Volts versus ohms speed regulation of electric motors.
The rotating rectifier consisted of a grid-supplied induction machine that rotated a DC
generator. By adjusting the magnetization of the DC generator, controllable DC
voltage was available for the speed control of a DC motor. Although three machines
were required, it was at the time the only possibility to realize a speed controlled
drive. When the transistors and first micro-processors were introduced, chopper
technologies such as the PWM enabled the accurate speed control of DC machines.
Brushless DC motors with permanent magnets in the rotor were also introduced in the
early 1960s, but since there were not powerful enough PM materials available yet,
their power range was limited typically below 10 kW. Typical applications for
brushless DC motors were small machine tools, tape recorders, and robotics. For
higher-power speed-controlled applications, brushed DC motor was for a long time
the only solution. Until the early 1980s, when high energy density NdFeB magnets
were introduced, it was possible to get rid of the brushes also at the higher power
range up to hundreds of kWs by using a brushless DC motor. Later on, the
introduction of the fieldoriented control for machines made it possible to use AC
machines in demanding speed-controlled applications. First, the speed-controlled AC
drives were induction motor drives, but as the vector control for PMSMs was
introduced in the early 1990s, they soon started to gain ground from the DC motors
and have dominated in the motion control industry ever since. The trend in the motion
control nowadays is clearly towards the brushless AC machines with sinusoidal

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KARTHIK Kongu Engineering College

excitation, which, in practice, means that a permanent magnet synchronous motor or

an induction motor must be used.

Induction motors have always been a minority in the motion control, and they
are mainly used in applications, where the field weakening can be utilized to avoid the
over-sizing of the drive, which would be the case with PMSMs. The principle of using
the differences of reluctances to produce the torque has been known for over 160
years. Before the discovery of the rotating magnetic field by Tesla in 1887, the first
reluctance motor was similar to the doubly salient synchronous reluctance motor,
nowadays known as the switched reluctance motor. Though the first SRM was built in
1838, it did not find widespread use until the late 1970s. This was due to the
difficulty in controlling the machine. Since the 1960s, with the advent of power
electronics and high power semiconductor switches, control of the SRM has become
much easier and there has been a renewed interest in SRM drives. However it can be
concluded that the brushed DC motors dominated the speed controlled drives in the
1960s and 70s, and the brushless DC motors in the 1980s. Since the early 1990s,
PMSMs have dominated the motion control industry to the present, and, according to
the current trend, there seems to be no end for that.

With this briefing let us imbibe the advanced brushless permanent magnet and
Reluctance motors in detail in the forthcoming chapters.

The following table 1.4 lists the main benefits and drawbacks of the motor
types discussed above.

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KARTHIK Kongu Engineering College

Table 1.4 Benefits and drawbacks of different motors in Industries

Brushed DC IM PMSM BLDC motor Synchrel motor SRM motor
motor
Benefits Good Excellent Smooth torque High power Improved High reliability
controllability dynamics with possible density and saliency ratio High torque-to-
Linear torque proper control High efficiency torque-to-inertia More efficient at inertia ratio.
current curve High speed High ratio low speed than an High starting
Low torque Operation possible torque/volume Good heat induction machine torque
ripple Less expensive High pull-out Dissipation line-start feature Less expensive
and simple torque possible good was no longer
construction Good heat overloading required with the
Durable dissipation capability invention of
Several good transistor voltage
Suppliers overloading inverter
available capability
Drawbacks Low reliability Complicated Expensive Expensive Lower power Torque ripple
Requires control Danger of Torque ripple factor Contribute severe
maintenance Always lagging demagnetization Danger of Lower pull-out acoustic noise
Low overloading power factor of the magnets demagnetization torque High friction and
capability Low efficiency Poor field of the magnets Lower efficiency windage losses
Low heat with lighter loads weakening Poor field
dissipation weakening

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