DEVELOPMENT OF PWM BASED SPEED CONTROLLER FOR DC MOTOR

1
INTRODUCTION

1.1 INTRODUCTION OF SPEED CONTROLLER

The purpose of a motor speed controller is to take a signal representing the demanded
speed, and to drive a motor at that speed. The controller may or may not actually measure the
speed of the motor. If it does, it is called a Feedback Speed Controller or Closed Loop Speed
Controller, if not it is called an Open Loop Speed Controller. Feedback speed control is better,
but more complicated, and may not be required for a simple robot design.
Motors come in a variety of forms, and the speed controller's motor drive output will be
different dependent on these forms. The speed controller presented here is designed to drive a
simple cheap starter motor from a car, which can be purchased from any scrap yard. These
motors are generally series wound, which means to reverse them, they must be altered slightly.

Figure 1.1 Block Diagram Of The Speed Controller

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1.2 THEORY OF DC MOTOR SPEED CONTROL

The speed of a DC motor is directly proportional to the supply voltage, so if we reduce

the supply voltage from 12 Volts to 6 Volts, the motor will run at half the speed. How can this be
achieved when the battery is fixed at 12 Volts?

The speed controller works by varying the average voltage sent to the motor. It could do
this by simply adjusting the voltage sent to the motor, but this is quite inefficient to do. A better
way is to switch the motor's supply on and off very quickly. If the switching is fast enough, the
motor doesn't notice it, it only notices the average effect.

When you watch a film in the cinema, or the television, what you are actually seeing is a
series of fixed pictures, which change rapidly enough that your eyes just see the average effect -
movement. Your brain fills in the gaps to give an average effect.

Now imagine a light bulb with a switch. When you close the switch, the bulb goes on and
is at full brightness, say 100 Watts. When you open the switch it goes off (0 Watts). Now if you
close the switch for a fraction of a second, and then open it for the same amount of time, the
filament won't have time to cool down and heat up, and you will just get an average glow of 50
Watts. This is how lamp dimmers work, and the same principle is used by speed controllers to
drive a motor. When the switch is closed, the motor sees 12 Volts, and when it is open it sees 0
Volts. If the switch is open for the same amount of time as it is closed, the motor will see an
average of 6 Volts, and will run more slowly accordingly.

As the amount of time that the voltage is on increases compared with the amount of time
that it is off, the average speed of the motor increases.

This on-off switching is performed by power MOSFETs. A MOSFET (Metal-Oxide-

Semiconductor Field Effect Transistor) is a device that can turn very large currents on and off
under the control of a low signal level voltage.

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 DEVELOPMENT OF PWM BASED SPEED CONTROLLER FOR DC MOTOR

The time that it takes a motor to speed up and slow down under switching conditions is
dependent on the inertia of the rotor (basically how heavy it is), and how- much friction and load
torque there is. The graph below shows the speed of a motor that is being turned on and off fairly
slowly:

Figure 1.2 Speed V/S Time Characteristic

Here,The average speed is around 150, although it varies quite a bit. If the supply voltage
is switched fast enough, it won’t have time to change speed much, and the speed will be quite
steady. This is the principle of switch mode speed control. Thus the speed is set by PWM – Pulse
Width Modulation.

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 DEVELOPMENT OF PWM BASED SPEED CONTROLLER FOR DC MOTOR

1.3 INTRODUCTION OF PULSE WIDTH MODULATION

PWM, or Pulse Width Modulation, is a method of controlling the amount of power to a

load without having to dissipate any power in the load driver.

Imagine a 10W light bulb load supplied from a battery. In this case the battery supplies
10W of power, and the light bulb converts this 10W into light and heat. No power is lost
anywhere else in the circuit. If we wanted to dim the light bulb, so it only absorbed 5W of power,
we could place a resistor in series which absorbed 5W, and then the light bulb could absorb the
other 5W. This would work, but the power dissipated in the resistor not only makes it get very
hot, but is wasted. The battery is still supplying 10W.

An alternative way is to switch the light bulb on and off very quickly so that it is only on
for half of the time. Then the average power taken by the light bulb is still only 5W, and the
average power supplied by the battery is only supplying 5W also. If we wanted the bulb to take
6W, we could leave the switch on for a little longer than the time it was off, then a little more
average power will be delivered to the bulb.

This on-off switching is called PWM. The amount of power delivered to the load is
proportional to the percentage of time that the load is switched on.

The PWM signals can be generated in a number of ways. There are several methods:

 1. Analogue method
 2. Digital method
 3. Discrete IC
 4. Onboard microcontroller

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 DEVELOPMENT OF PWM BASED SPEED CONTROLLER FOR DC MOTOR

1.3.1 Analog Method

Figure 1.3 A Block Diagram Of An Analogue PWM Generator

We will now go through each of these stages and work out how to implement them.

 The Comparator

We are starting at the output because this is the easy bit. The diagram below shows how
comparing a ramping waveform with a DC level produces the PWM waveform that we require.
The higher the DC level is, the wider the PWM pulses are. The DC level is the 'demand signal'.

The DC signal can range between the minimum and maximum voltages of the triangle
wave.

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When the triangle waveform voltage is greater than the DC level, the output of the op-
amp swings high, and when it is lower, the output swings low.

Pulse width modulation (PWM) is a powerful technique for controlling analog circuits
with a processor's digital outputs. PWM is employed in a wide variety of applications, ranging
from measurement and communications to power control and conversion

 Analog Electronics

An analog signal has a continuously varying value, with infinite resolution in both time
and magnitude. A nine-volt battery is an example of an analog device, in that its output voltage is
not precisely 9V, changes over time, and can take any real-numbered value. Similarly, the
amount of current drawn from a battery is not limited to a finite set of possible values. Analog
signals are distinguishable from digital signals because the latter always take values only from a
finite set of predetermined possibilities, such as the set {0V, 5V}

Analog voltages and currents can be used to control things directly, like the volume of a
car radio. In a simple analog radio, a knob is connected to a variable resistor. As you turn the
knob, the resistance goes up or down. As that happens, the current flowing through the resistor
increases or decreases. This changes the amount of current driving the speakers, thus increasing
or decreasing the volume. An analog circuit is one, like the radio, whose output is linearly
proportional to its input.

As intuitive and simple as analog control may seem, it is not always economically
attractive or otherwise practical. For one thing, analog circuits tend to drift over time and can,
therefore, be very difficult to tune. Precision analog circuits, which solve that problem, can be
very large, heavy (just think of older home stereo equipment), and expensive. Analog circuits
can also get very hot; the power dissipated is proportional to the voltage across the active
elements multiplied by the current through them. Analog circuitry can also be sensitive to noise.
Because of its infinite resolution, any perturbation or noise on an analog signal necessarily
changes the current value.

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1.3.2 Digital Control

By controlling analog circuits digitally, system costs and power consumption can be
drastically reduced. What's more, many microcontrollers and DSPs already include on-chip
PWM controllers, making implementation easy
In a nutshell, PWM is a way of digitally encoding analog signal levels. Through the use
of high-resolution counters, the duty cycle of a square wave is modulated to encode a specific
analog signal level. The PWM signal is still digital because, at any given instant of time, the full
DC supply is either fully on or fully off. The voltage or current source is supplied to the analog
load by means of a repeating series of on and off pulses. The on-time is the time during which
the DC supply is applied to the load, and the off-time is the periods during which that supply is
switched off. Given a sufficient bandwidth, any analog value can be encoded with PWM.

Figure 1.4 PWM Signals for Varying Duty Cycles

Figure 1.4 shows three different PWM signals. Figure 1.4a shows a PWM output at a
20% duty cycle. That is, the signal is on for 10% of the period and off the other 90%. Figures
1.4b and 1.4c shows PWM outputs at 50% and 90% duty cycles, respectively. These three PWM

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 DEVELOPMENT OF PWM BASED SPEED CONTROLLER FOR DC MOTOR

outputs encode three different analog signal values, at 10%, 50%, and 90% of the full strength.
If, for example, the supply is 9V and the duty cycle is 10%, a 0.9V analog signal results

Figure 1.5. A Simple PWM Circuit

Figure 1.5 shows a simple circuit that could be driven using PWM. In the figure, a 9 V
battery powers an incandescent light bulb. If we closed the switch connecting the battery and
lamp for 50 ms, the bulb would receive 9 V during that interval. If we then opened the switch for
the next 50 ms, the bulb would receive 0 V. If we repeat this cycle 10 times a second, the bulb
will be lit as though it were connected to a 4.5 V battery (50% of 9 V). We say that the duty
cycle is 50% and the modulating frequency is 10 Hz
Most loads, inductive and capacitive alike, require a much higher modulating frequency
than 10 Hz. Imagine that our lamp was switched on for five seconds, then off for five seconds,
then on again. The duty cycle would still be 50%, but the bulb would appear brightly lit for the
first five seconds and off for the next. In order for the bulb to see a voltage of 4.5 volts, the cycle
period must be short relative to the load's response time to a change in the switch state. To
achieve the desired effect of a dimmer (but always lit) lamp, it is necessary to increase the
modulating frequency. The same is true in other applications of PWM. Common modulating
frequencies range from 1 kHz to 200 KHZ

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1.4 INTRODUCTION OF MOSFET

The MOSFET has an extremely high input gate resistance and as such a easily damaged
by static electricity if not carefully protected. MOSFET's are ideal for use as electronic switches
or common-source amplifiers as their power consumption is very small. Typical applications for
MOSFET's are in Microprocessors, Memories, Calculators and Logic Gates etc. Also, notice that
the broken lines within the symbol indicates a normally "OFF" Enhancement type showing that
"NO" current can flow through the channel when zero gate voltage is applied and a continuous
line within the symbol indicates a normally "ON" Depletion type showing that current "CAN"
flow through the channel with zero gate voltage. For P-Channel types the symbols are exactly the
same for both types except that the arrow points outwards.

This can be summarized in the following switching table.

MOSFET type Vgs = +ve Vgs = 0 Vgs = -ve

N-Channel Depletion ON ON OFF
N-Channel Enhancement ON OFF OFF
P-Channel Depletion OFF ON ON
P-Channel Enhancement OFF OFF ON

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2
PWM WAVE GENERATOR

2.1 SQUARE WAVE GENERATOR USING OP-AMP

In contrast to sine wave oscillators, square wave outputs are generated when the Op-Amp
is forced to operate in the saturated region. That is the output of the Op-Amp is forced to swing
repetitively between positive saturation +Vsat and negative saturation –Vsat resulting in the
square wave output. One such circuit is shown in figure. This square wave generator is also
called a free running or astable multi-vibrator. The output of the Op-Amp in this circuit will be in
positive or in the negative saturation, depending on whether the differential voltage Vid is
negative or positive, respectively.

Figure 2.1 Square Wave Generator

Assume that the voltage across capacitor c is zero volt at the instant the dc supply voltage
+Vcc and –Vee are applied. This means that the voltage at the inverting terminal is zero initially.
At the same instant, however, the voltage V1 at the non inverting terminal is a very small finite

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value that is function of the output offset voltage Voot and value of resistors R1 and R2. Thus
the differential input voltage Vid is equal to the voltage V1 at the non inverting terminal. For
example suppose that the output offset voltage Voot is positive and that, therefore, voltage V1 is
also positive. Since initially the capacitor C acts as a short circuit, the gain of the Op-Amp is
very large (A); hence V1 drives the output of the Op-Amp to its positive saturation +Vsat. With
the output voltage of the Op-Amp at +Vsat, the capacitor C starts charging toward +Vsat through
resistor R. However, as soon as the voltage V2 across capacitor C is slightly more positive then
V1, the output of the Op-Amp is forced to switch to a negative saturation, -Vsat. With the Op-
Amp’s output voltage at negative saturation, -Vsat, the voltage V1 across R1 is also negative,
since
V1 = (R1 / (R1 + R2))*(-Vsat)
Thus the net differential voltage Vid = (V1-V2) is negative which holds the output of the
Op-Amp in negative saturation. The output remains in negative saturation until the capacitor C
discharge and then recharge to a negative voltage slightly higher then –V1. Now, as soon as the
capacitor’s voltage becomes more negative then -V1, the net differential voltage Vid becomes
positive and hence drives the output of Op Amp back to its positive saturation +Vsat. This
completes one cycle with output at +Vsat, voltage V1 at the non inverting input is
V1 = (R1 / (R1 + R2))*(+Vsat)
The time period T of the output waveform is given by
T = 2RC *[ln((2R1+R2) / R2)]
Fo = 1 / [2RC * ln((2R1+R2) / R2)]
This equation indicates that the frequency of the output Fo is not only a function of RC
time constant but also of the relationship between R1 and R2. It also shows that the smaller RC
time constant, the higher the output frequency Fo, and vice versa. As with sine wave oscillators,
the highest frequency generated by the square wave generator is also set by slew rate of the Op-
Amp. An attempt to operate the circuit at relatively higher frequency causes the oscillator’s
output to become triangular. In practice, each inverting and non inverting terminal needs a series
resistance Rs to prevent excessive differential current flow because the inputs of the op-amps are
subjected to large differential voltages. The resistance Rs used should be 100Kohm or higher. A

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reduced peak – to – peak output voltage swing can be obtain in square wave generator of the
figure by using back to back zeners at the output terminal.
2.2 TRIANGULAR WAVE GENERATOR USING OP-AMP

The Op-Amp triangular-wave generator is another example of a relaxation oscillator. We

know that the integrator output waveform will be triangular if the input to it is a square-wave. It
means that a triangular-wave generator can be formed by simply cascading an integrator and a
square-wave generator, as illustrated in figure. This circuit needs a dual Op-Amp, two capacitors,
and at least five resistors. The rectangular-wave output of the square-wave generator drives the
integrator which produces a triangular output waveform. The rectangular-wave swings between
+Vsat and -Vsat with a time period determined from equation. The triangular-waveform has the
same period and frequency as the square-waveform. Peak to-peak value of output triangular-
waveform can be obtained from the following equation.

Figure 2.2 Triangular Wave Generator

The input of integrator A2 is a square wave and its output is a triangular waveform, the
output of integrator will be triangular wave only when  R4 C2 > T/ 2 where T is the period of
square wave. As a general rule, R4C2 should be equal to T. It may also be necessary to shunt the
capacitor C2 with resistance R5 = 10 R4 and connect an offset volt compensating network at the
non-inverting (+) input terminal of Op-Amp A2 so as to obtain a stable triangular wave. Since the
frequency of the triangular-wave generator like any other oscillator, is limited by the Op-Amp

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slew-rate, a high slew rate op-amp, like LM 301, should be used for the generation of relatively
higher frequency waveforms.

3
SIMULATION AND ANALYSIS OF PWM CONTROLLER

3.1 SIMULATION OF SQUARE WAVE GENERATOR

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Figure 3.1 Simulation of Square Wave Generator

3.2 SIMULATION OF TRIANGULAR WAVE GENERATOR

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Figure 3.2 Simulation of Triangular Wave Generator

3.3 SIMULATION OF PWM CONTROLLER FOR DC MOTOR

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Figure 3.3 Simulation of PWM Controller for Dc Motor

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4
FABRICATION OF PWM CONTROLLER FOR DC MOTOR

4.1 WORKING OF PWM CONTROLLER

The PWM circuit requires a steadily running oscillator to operate. U1a and U1d form a
square/triangle waveform generator with a frequency of around 400 Hz.

Figure 4.1 PWM Controller for DC Motor

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U1c is used to generate a 4.5 Volt reference current which is used as a virtual ground for
the oscillator, this is necessary to allow the oscillator to run off of a single supply instead of a +/-
voltage dual supply.

U1b is wired in a comparator configuration and is the part of the circuit that generates the
variable pulse width. U1 pin 6 receives a variable voltage from the R6, VR1, R7 voltage ladder.
This is compared to the triangle waveform from U1-14. When the waveform is above the pin 6
voltage, U1 produces a high output. Conversely, when the waveform is below the pin 6 voltage,
U1 produces a low output. By varying the pin 6 voltage, the on/off points are moved up and
down the triangle wave, producing a variable pulse width. Resistors R6 and R7 are used to set
the end points of the VR1 control, the values shown allow the control to have a full on and a full
off setting within the travel of the potentiometer. These part values may be varied to change the
behavior of the potentiometer.

Finally, Q1 is the power switch, it receives the modulated pulse width voltage on the gate
terminal and switches the load current on and off through the Source-Drain current path. When
Q1 is on, it provides a ground path for the load, when Q1 is off, the load's ground is floating.
Care should be taken to insure that the load terminals are not grounded or a short will occur. The
load will have the supply voltage on the positive side at all times. LED1 gives a variable
brightness response to the pulse width. Capacitor C3 smoothes out the switching waveform and
removes some RFI, Diode D1 is a flywheel diode that shorts out the reverse voltage kick from
inductive motor loads.

In the 18 Volt modes, regulator U2 converts the 18 Volt supply to 9 Volts for running the
pwm circuit, Q1 switches the 18 Volt load to ground just like it does for the 9 Volt load. See the
schematic for instructions on wiring the circuit for 9 Volts or 18 Volts. When running loads of 1
amp or less, no heat sink is needed on Q1, if you plan to switch more current, a heat sink with
thermal greas is necessary. Q1 may be replaced with a higher current device, suitable upgrades
include the IRFZ34N, IRFZ44N, or IRFZ48N. All of the current handling devices, switch S1,
fuse F1, and the wiring between the FET, power supply, and load should be rated to handle the
maximum load current.

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4.2 PART LIST

U1: LM324N quad op-amp

U2: 78L12 12 volt regulator
Q1: IRF521 N channel MOSFET
D1: 1N4004 silicon diode
LED1 Red LED
C1: 0.01uF ceramic disc capacitor, 25V
C2-C5: 0.1uF ceramic disk capacitor, 50V
R1-R4: 100K 1/4W resistor
R5: 47K 1/4W resistor
R6-R7: 3.3K 1/4W resistor
R8: 2.7K 1/4W resistor
R9: 470 ohm 1/4W resistor
VR1: 10K linear potentiometer
F1: 3 Amp, 28V DC fast blow fuse
S1: Toggle switch, 5 Amps

4.3 SPECIFICATION

PWM Frequency: 400 Hz

Current Rating: 3 A with an IRF521 FET, >10A with an IRFZ34N FET and heat sink
PWM circuit current: 1.5 ma @ 9V with no LED and no load
Operating Voltage: 9V or 18V depending on the configuration

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4.4 ADVANTAGE OF PWM CONTROLLER

One additional advantage of pulse width modulation is that the pulses reach the full
supply voltage and will produce more torque in a motor by being able to overcome the internal
motor resistances more easily. Finally, in a PWM circuit, common small potentiometers may be
used to control a wide variety of loads whereas large and expensive high power variable resistors
are needed for resistive controllers.

4.5 DISADVANTAGE OF PWM CONTROLLER

The main Disadvantages of PWM circuits are the added complexity and the possibility of
generating radio frequency interference (RFI). RFI may be minimized by locating the controller
near the load, using short leads, and in some cases, using additional filtering on the power supply
leads. This circuit has some RFI bypassing and produced minimal interference with an AM radio
that was located under a foot away. If additional filtering is needed, a car radio line choke may
be placed in series with the DC power input, be sure not to exceed the current rating of the
choke. The majority of the RFI will come from the high current path involving the power source,
the load, and the switching FET, Q1.

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4.6 USE OF PWM CONTROLLER

This circuit will work as a DC lamp dimmer, small motor controller, and even as a small
heater controller. It would make a great speed control for a solar powered electric train. The
circuit has been tried with a 5 Amp electric motor using and IRFZ34N FET and worked ok, D1
may need to be replaced with a faster and higher current diode with some motors. The circuit
should work in applications such as a bicycle motor drive system, if you experiment with this, be
sure to include an easily accessible emergency power disconnect switch in case the FET shorts
out and leaves the circuit full-on.

Wire the circuit for 12 Volts or 24 Volts as per the schematic, connect the battery to the
input terminals, and connect the load to the output terminals, be sure not to ground either output
terminal or anything connected to the output terminals such as a motor case. Turn the
potentiometer knob back and forth, the load should show variable speed or light.

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CONCLUSION

Although, AC drives employing PWM converters is the most commonly used technique to
control the speed of induction motor, DC drives offers precise and wider range of speeds above
and below the rated speed Thus, dc motor speed controller described here is a general
purpose device that can control DC devices which draw up to a few ampere of current. In this
method smooth speed control and high starting torque of motor is achieved by not allowing any
voltage drop compared to resistive methods. Thus these simple techniques give us much suitable
speed operation to have speed variation above or below rated speed

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REFRENCES

URL:

I. homepages.which.net/~paul.hills/Circuits/PwmGenerators/PwmGenerators.html/
II. www.solorb.com/elect/solarcirc/pwm1/
III. www.netrino.com/Embedded-Systems/How-To/PWM-Pulse-Width-Modulation
IV. electricalandelectronics.org/2008/09/29/principle-of-operation-of-depletion-type-mosfet/
V. www.electronics-tutorials.ws/transistor/tran_6.html
VI. www.circuitstoday.com/triangular-wave-generator
VII. homepages.which.net/~paul.hills/SpeedControl/SpeedControllers.html

BOOKS:

1. M.H. RASHID, POWER ELECTRONICS; PEARSON EDUCATION, 2002

2. M.D. SINGH & K.B. KHANCHNDANI, POWER ELECTRONICS; TATA
MCGRAW HILL, 2007
3. RAMAKANT A. GAYAKWAD, OP-APM AND LINIEAR INTEGRATED
CIRCUITS; PRENTICE HALL OF INDIA, 2003

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