EET426 Special Electric

Machines
Module 1
 Permanent Magnet DC Motors
 When permanent magnet is used to create magnetic field in a DC motor, the
motor is referred as permanent magnet DC motor or PMDC motor.
 We can replace electromagnet with permanent magnet, which result in higher
efficiency, less space requirement and better cooling.
 PMDC are best solutions to motion control and power transmission
applications.
 The magnets are radially magnetized and are mounted on the inner periphery
of the cylindrical steel stator.
 It has a commutator segments and brushes, similar to DC motor.
Construction
 The field poles of this motor are essentially made of permanent magnet. A
PMDC motor mainly consists of two parts. • Stator • Rotor
Stator:-
 Stator is a steel cylinder and the magnets are mounted in the inner periphery of this
cylinder.
 The steel cylindrical stator also serves as low reluctance return path for the magnetic
flux.
 The permanent magnets are mounted in such a way that the N-pole and S-pole of
each magnet are alternatively faced towards armature.
 Although field coil is not required in permanent magnet dc motor, sometimes they
are used along with permanent magnet. This is because if permanent magnets lose
their strength, these lost magnetic strengths can be compensated by field excitation
through these field coils.
Rotor
 The rotor of PMDC motor is similar to the armature of DC Motors. It is made of number of
silicon steel sheets to reduce eddy current loss.
 Consists of core, windings, commutator and brushes. There are three types of armature
structures available:
1. Slotted Armature
2. Slotless Armature
3. Moving Coil
Slotted armature
 Slotted armature is made up of silicon sheet steel or carbon sheet steel which are punched
together or mounted on the shaft.
 The armature has slots on its periphery. Armature conductors are placed on this, slots and
properly connected to form armature windings.
 A core having many slots is usually desirable, because the greater the number of slots, the less
the cogging torque and electromagnetic noise.
Slotless armature
 Conductors are fixed on the outer periphery of the core.
 There is no slots.
 Effective air gap length is larger.
 Alnico or rare permanent magnets are used.
 Advantage of this construction is the reduction in torque ripples.
Moving coil Armature
 Iron core is replaced by non-magnetic core which is usually made of glass fibre.
 Advantages of low inertia and no iron los in the armature.
 The commutator and brushes are very small and made up of metals like gold,
silver platinum etc.
 Small sized commutator and brushes helps in stable commutation.
 Types of Permanent Magnets used in Electrical Machines
Alnico
 Commonly used in high temperature applications because of higher thermal stability.
 Alnicos has a low coercive magnetizing intensity and high residual flux density.
 Demagnetization is high.
 It is used where low current and high voltage is required.
Ceramic or Ferrite
 It is the cheapest possible permanent magnet that can be use in PMDC motor.
 It is suitable for moderate temperature.
 They have linear demagnetization characteristics in the second quadrant of BH curve.
 It’s performance is good upto 100◦C.
Samarium Cobalt (Sn2Co17)
 They are suitable for operation up to 200◦C.
 They have high coercive force.
 It has high energy density and linear demagnetization characteristics.
 But it is expensive due to an inadequate supply of samarium.
Neodymium Iron Boron
 They are comparatively the latest member in the family of permanent magnets.
 It has the highest energy density and very good coercivity.
 The Neodymium iron boron is cheaper as compared to Samarium cobalt.
 They can withstand higher temperature.
 They are widely used for making permanent magnets.
 Principle of Operation
⮚ The working of PMDC motor is similar to conventional DC motor.
⮚ Whenever a current carrying conductor is placed in magnetic field, the
conductor experiences a electromagnetic force.
⮚ The direction of force is given by Flemings left hand rule.
⮚ Magnitude of force is given by F = BIl
where, B- flux density
I-current through the conductor
L – length of the conductor.

⮚ When dc supply is given to armature placed in magnetic field, torque will

acts on armature and it starts to rotate.
⮚ When the armature starts rotating, a back emf is produced in armature
windings.
⮚ The direction of back emf is opposite to applied voltage.
 Performance curves of PMDC Motors

 The torque – speed characteristic is almost straight line which indicates the
suitability of PMDC motors for servo applications.
 Linear relationship exists between input current and torque.
 The PMDC motors have better efficiency and is much higher than conventional
DC motors.
Advantages

• Smaller size
• High efficiency compared to conventional DC motor
• Less costly
• No need of field excitation arrangement
Disadvantages
• Above base speed operation is not possible in PMDC motor.
• Armature reaction is more in PMDC motor due to presence of
permanent magnet in stator.
• Risk of demagnetization of the poles which may be caused by
large armature current.
Applications

• Toys
• Drilling machine
• Electric toothbrush
• Gear motors
• Conveyors
 Brush less DC Motor (BLDC)
 Main problem associated with PMDC motor is the commutation with the help of brushes.
 BLDC motor eliminates this problem.
 There is no mechanical commutator and brushes.
 Since the commutator and brushes are absent, maintenance is very less.
 The armature is stationary(stator) and PM field system is mounted on the rotor.
 The commutation is achieved by using semi-conductor switches (electronic
commutation) instead of mechanical commutator.
 BLDC motor = Synchronous motor with permanent magnet rotor and supplied current
from a DC source through an inverter.
 Compared with PMDC motors, BLDC motors have higher efficiency, smaller size and
better cooling.
 Construction of PMBLDC Motor

Figure : Construction Details of BLDC Motor

 The field poles of this motor are essentially made of permanent

magnet which is the rotor.
 A BLDC motor mainly consists of two parts:
• Stator (Armature)
• Rotor (Permanent magnet)
Stator (Armature)
 Stator of a BLDC motor is made up of silicon steel.
 Steel is laminated to reduce eddy current loss.
 Armature core has slots to house windings .
 These windings can be arranged in either star or delta.
 However, most BLDC motors have three phase star connected stator.
 Armature winding is connected to DC source through a semiconducting
switching circuit (electronic commutator).
Rotor
 BLDC motor incorporate a permanent magnet in the rotor.
 Its function is to produce the required magnetic field.
 Permanent magnets offer very high flux density.
 Ferrite magnets are traditionally used to make permanent magnets.
 Permanent magnet rotates and the armature remains static.
 The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate
south and north poles depending on the application requirement.
 More the number of rotor poles lesser will be torque ripple.
Rotor
 On the basis of rotor construction , BLDC motor is generally classified into two.
 Surface mounted PM rotor and Interior PM rotor
 If the magnets are mounted on the surface of the rotor , it is known as surface
mounted permanent magnet rotor.
 If the magnets are inside the rotor in slots , then it known as interior mounted
permanent magnet rotor.
 Position Sensor (Hall Sensor)
 Brushless DC motor requires an electronic commutator to rotate the rotor.
 In order to rotate the motor, the windings of the stator must be energized in a
sequence.
 The position of the rotor (i.e. the North and South poles of the rotor) must be known
to precisely energize a particular set of stator windings.
 The hall effect sensor provides information about the position of the rotor at any
instant to the controller which sends suitable signals to the electronic commutator.
 Thus, for the estimation of the rotor position, the motor is equipped with three hall
sensors. These hall sensors are placed every 120◦.
 Each sensor generates Low (0) and High (1) signals whenever the rotor poles passes
near to it.
Classification
Principle of Operation of PMBLDC Motor

Figure : Drive Circuit of PMBLDC

 In the case of a BLDC motor, the armature is stationary, and the permanent
magnet is rotating.
 Stator windings of a BLDC motor are connected to a voltage fed inverter
(VSI) circuit .
 The inverter converts DC voltage into variable frequency voltage.
 The motor is provided with a position sensor, which provides necessary
signals for switching the appropriate power switches of the inverter.
 The switching is done in such a way that all the three phases conduct at all
time (180◦ conduction mode). The switching interval is 60◦.
• When supply is switched ON, current flows through the armature winding
whose distribution depends on rotor position and the switching ON of devices
at various instants.
• Due to the interaction between current and magnetic field, rotor experiences
torque, and if this torque is greater than the load torque, rotor starts moving.
• BLDC motor is thus a self-starting motor.
• As the rotor moves, a relative velocity exists between the rotor and the stator.
This results in the emf induction in armature winding whose direction is
opposite to the applied voltage.
• Due to this back emf, a momentary change in torque and speed will occur.
• These changes are very fast and the performance is not affected.
 Electronic Commutator
 The most important advantage of BLDC motor is the use of electronic commutator
in the place of mechanical commutator. The electronic commutator is used in
PMBLDC, to transfer current to the armature.
 In PMBLDC motor, the function of commutator and brushes are performed by,
power semiconductor switches.
 The phase windings of PMBLDC motors are energized using these power
semiconductor switches. Thus it is also called electronically commutated motor.
 It is necessary to have stationary armature and rotating field system for
implementing electronic commutator. As the field (PM) is rotating it is necessary to
supply DC voltage to the tapping points of the armature winding.
 To accomplish this, each tapping on the armature winding is connected to the
junction of two semiconducting switches.
Control of BLDC Motor
Comparison between Trapezoidal and Sinusoidal Motors
Comparison between Electronic and Mechanical Commutator
Comparison –BLDC with Conventional DC Motor
Advantages of BLDC Motors
 Maintenance is less due to the absence of brushes and commutator
 Sparking associated with brushes are eliminated
 Higher reliability
 Low weight
 Long life
 Reduction in noise
 Low radio frequency interference
 Can run at high speed
 Higher efficiency
Disadvantages

 High cost
 Electronic controller is expensive
 Need of additional sensors
 Low starting torque
 Requires complex drive circuit
 No flexibility in control due to the absence in field winding
Applications
 Electric vehicles
 Textile industries
 Variable speed fans.
 Robotics.
 Automotive systems
 Biomedical engineering .
 Used in food mixers and portable vacuum cleaners