EE 3033: ELECTRICAL

MACHINES & DRIVES III

ASSIGNMENT 1

NAME : B.P.M.Karunarathna

INDEX NO : 130275A

DATE OF SUB : 27/06/2017

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 CONTENT

1. Introduction 03

2. Single Quadrant Thyristor DC Motor Drive 05

2.1 Single Quadrant -Phase Controlled Rectifier (using thyristors) DC Motor
Drive 05
2.1.1 Single Phase Half Wave Converter Drives 05
2.1.1.1 Operation & Control 06
2.1.2 Single Phase Semi Converter Drives 07
2.1.2.1 Operation & Control 07
2.2 Single Quadrant Chopper-fed (using thyristors) DC Motor Drive 08
2.2.1 Operation and Control 08

3. Four Quadrant Thyristor DC Motor Drive 10

3.1 Four Quadrant Phase Controlled Rectifier (using thyristors) DC Motor Drives
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3.1.1 Single Phase Duel Converter Drive 10
3.1.1.1 Operation and Control 10
3.1.2 Three Phase Duel Converter Drive 12
3.2 Four Quadrant Chopper-fed (using thyristors) DC Motor Drive 13
3.2.1 Operation and Control 13
3.2.2 Bipolar Switching Scheme 15
3.2.3 Unipolar Switching Scheme 16
4. Applications 18
5. References 18

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 1. Introduction

DC drive is an electrical drive which uses DC motors as the prime mover. DC motors
have been available from more than 100 years. DC drives have some advantages such as easy
control and decouple control of torque and flux. But it has several limitations over AC drives
such as bulkiness, need of regular maintenance, expensiveness, speed limits and sparking. AC
dives have some attractive advantages. They are lighter, compact, less expensive, required less
maintenance and capable of providing high speed. Therefore AC drives are used widely in
modern industry (about 85%). But still DC drives are used in several applications in electric
vehicles, electric trains, in paper processing mills and textile industry. Before semiconductor
power electronic converters were introduced (before 1950s), AC motors were used for fixed
speed applications while DC motors were used in variable speed applications. With the
introduction of semiconductor devices in 1960s, AC motors are also used in variable speed
applications. But their application is limited to medium performance applications. Still DC
motors have the highest performance of speed controlling. AC motors also used in high
performance applications with the introduction of vector controlled drives.

DC motors are controlled by power electronic devices. Basically two types of DC drives
are available called as controlled rectifier-fed (thyristor-fed) DC drives and chopper-fed DC
drives. With thyristors, not only the AC supply can be converted to a DC output but also the
output voltage can be varied by changing the firing angle of the thyristor. So variable DC output
voltage can be directly obtained from AC source by controlled rectifier fed DC drives. Chopper
is an electronic switching circuit which converts the constant DC input voltage into a variable
DC output voltage. It is done by switching the supply ON and OFF. To obtain the DC constant
voltage from AC supply, diode rectifier converter (un-controlled rectifier converter) can be
used.

According to the direction of the torque and the speed of the motor, it can be operate in
one of the four quadrants called as forward motoring, forward braking, reverse motoring and
reverse braking.

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 Fig.1. Four quadrant operation

Regenerative and non-regenerative DC drives are available in industry. Non-regenerative

drives are the most commonly used and the most conventional type which can only be able
to control motor speed and the torque in one direction (1st quadrant). Reverse motoring (3rd
quadrant) can be achieved by reversing the polarity of the voltage of the controller. In both
these cases, torque and the speed are in the same direction. Regenerative drives are capable
of controlling not only the speed and the direction of the rotation but also the direction of
the torque. (i.e the motor can operate in all 4 quadrants). “Regenerative” means that the
mechanical energy of the motor and the load can be converted into electrical energy which
is returned to the AC supply during the braking. In this quadrant, motor operate as a
generator. The back EMF generated in the armature is greater than the average voltage
applied to the motor. Thus the armature current flows towards the source.

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 2. Single Quadrant Thyristor DC Motor Drives

Single quadrant operation of DC drives with thyristors can be done by either controlled
rectifier-fed (thyristor-fed) DC drives or chopper controlled DC drives. Thyristor can not only
rectify the AC input voltage but also vary the DC output voltage by changing the firing angle.
Therefore it can directly convert the AC input voltage into variable DC output voltage. This is
called as the controlled rectifier fed DC drive. Also first AC input voltage can be converted
into DC constant voltage with un-controlled rectifiers (such as diodes) and then can be
converted into a variable DC output voltage using switching the constant DC supply ON and
OFF by semiconductor devices such as thyristors, power BJTs, MOSFETS and IGBTs.
Therefore with a switching pulse for the gate of the thyristor, it can be used in chopper fed DC
motor drives (switch mode converters). So single quadrant operation with thyristors can be
achieved from chopper converters also.

2.1 Single Quadrant Phase Controlled Rectifier (using thyristors) DC

Motor Drive

Single quadrant operation can be achieved in two ways using thyristors. They are
“Single phase half wave converter drives” and “single phase semi converter drives”. In half
wave convertor, only one thyristor is used while in semi convertor, two thyristors and two
diodes are used.

2.1.1 Single Phase Half Wave Converter Drives

Fig. 2. Single phase half wave converter drive

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2.1.1.1 Operation & Control

Separately excited DC motor is fed through a single phase half wave converter as shown
in the Fig.2. Variable Dc output can be obtained by varying the trigger angle (firing angle) of
the thyristor. During the positive half cycle, thyristor is forward biased and input supply can be
turned on by applying a suitable gate pulse for the gate. During the negative half cycle, armature
current flows through the freewheeling diode. Thus the current through the thyristor becomes
zero and thyristor is reversed biased cause to stop the conduction. This process repeats and DC
motor receives positive average voltage which is low because the thyristor controls only the
positive half cycle.

Fig.3.Waveforms

For this convertor, average value of the output voltage is given by;

𝑉𝑚
𝑉𝑜 = (1 + 𝑐𝑜𝑠𝛼) ; for (0 < α < π) where Vm = is the peak value of input line voltage.
2𝜋

This average voltage is set to be higher than the back EMF of the armature. Therefore the
DC motor operates in the 1st quadrant. This type of converters are used in around 0.5 kW DC
motors.

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2.1.2 Single Phase Semi Converter Drives

Fig.4. Single phase semi converter drive

2.1.2.1 Operation & Control

In semi converter drive, two thyristors and two diodes are used. The armature voltage
can’t be negative because the diodes cannot have a positive potential difference in their
terminals. Therefore this converter can’t regenerate. When thyristor T11 switch ON, current
flow through T11 and D12 complete the circuit. When the T11 is off (during the negative half
cycle), D11 & D12 act as a freewheeling diodes and current circulates through them and the
motor. When T12 thyristor is fired in the negative half cycle, current flow though the T12 and
D11 complete the cycle. So in this converter, even in the negative half cycle of the supply, has
a positive DC output. So it has a higher positive DC average output (two times) than the half
wave convertor drive.

Fig.5. Waveforms of Output Voltage

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The diodes which use for this converter should be ultra-high speed diodes to protect the circuit
from undesirable voltages.

The average output voltage is given by;

𝑉𝑚
𝑉𝑜 = (1 + 𝑐𝑜𝑠𝛼); for (0 < α < π) where Vm = is the peak value of input line voltage.
𝜋

This semi converter DC drives can be used in applications up to 15kW DC motors.

2.2 Single Quadrant Chopper-fed (using thyristors) DC Motor Drive

2.2.1 Operation and Control

Generally chopper converter drives have high efficiency and fast response than phase
controlled rectifier converters. The main difference is, here the input to the converter is a
constant DC output from another AC to DC un-controlled converter. It has a semiconductor
device such as a thyristor, BJT, MOSFET or IGBT which operate as a fast switch. When the
switch is ON, total input DC supply is applied to the load (DC motor) and when the switch is
OFF applied voltage is zero.

Fig.6. Single quadrant hard switching DC motor drive

PWM scheme can be used to derive the switching signals for the thyristor. Using a carrier
signal (generally a saw tooth signal) and the reference signal can be compared using a
comparator. Ts is switching period and the fs is the switching frequency. Duty factor is the ratio
between the ON time of the switch to the period (cycle time) and given by D.

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 Fig.7. Switching signal for thyristor

Fig.8. Output voltage waveform

Average output voltage for the DC motor is given by;

𝑉0,𝑎𝑣𝑔 = 𝐷𝑉𝑑 , where D is the duty ratio and Vd is the supply voltage (DC).

So variable DC output voltage can be obtained by varying the duty factor of the switching
signal. Variation of D can be done by adjusting the control voltage (Vcontrol) of the PWM
scheme as shown in the Fig 7. For the motor to be operated in the continuous conduction mode,
average output current (armature current) should be positive.

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 3. Four Quadrant Thyristor DC Motor Drives

Four quadrant operation of DC drives with thyristors can also be done by either
controlled rectifier-fed (thyristor-fed) DC drives or chopper controlled DC drives. In controlled
rectifier-fed converters can be either single phase of three phase. Generally three phase
controlled rectifier-fed converters are used in high power applications up to megawatt power
levels. Armature current is mostly continuous, so three phase drive’s performance is better than
a single phase drive.

3.1 Four Quadrant Phase Controlled Rectifier (using thyristors) DC

Motor Drive

3.1.1 Single Phase Duel Converter Drive

Fig.9. Single phase duel converter drive

3.1.1.1 Operation and Control

In the single phase duel converter drive, two single phase full wave converters are
connected in back to back connection. If the left side converter operates to supply the positive
output voltage (+Vo) for the motor, then the other convertor provides the negative output
voltage (-Vo). Operation in the 1st and 2nd quadrants (forward motoring and forward braking)
is provided by the converter which produces the positive output voltage. Operation in the 3rd
and 4th quadrants (reverse motoring and reverse braking) is provided by the other converter.
Two converters have different firing angles. In one converter all 4 thyristors have the same

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firing angle. If the firing angle of the left side converter is α1 and of the other converter is α2;
then the relationship between two firing angles is:

𝛼1 + 𝛼2 = 𝜋

Fig. 10. Waveforms of single phase duel converter

If output voltage of one converter is given by;

2𝑉𝑚
𝑉𝑜1 = 𝑐𝑜𝑠𝛼1 ; for (0 < α1 < π) where Vm is the line to line peak input voltage
𝜋

Similarly, output voltage of the other converter is;

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 2𝑉𝑚
𝑉𝑜2 = 𝑐𝑜𝑠𝛼2 ; for (0 < α2 < π) and α2 = π – α1 where Vm is the line to line peak input
𝜋

voltage.

Inverting operation occurs by reversing the current in the armature rather by reversing
the motor counter electromotive force which requires a reversal of the field. Rapid response of
reversing the motor can be achieved by this converter because it doesn’t requires a field
reversal.

This single phase drive can be used in applications up to about 15 kW.

3.1.2 Three Phase Duel Converter Drive

Fig.11. Three phase duel converter Drive

Same as the single phase duel converter, in a three phase duel converter drive, two fully
controlled three phase converters are connected in back to back connection. This has twelve
thyristor switches. This three phase drives are used in very high power applications up to
megawatt level.

Output voltage is given by;

3𝑉𝑚 3𝑉𝑚
𝑉𝑜1 = cos 𝛼1 & 𝑉𝑜2 = cos 𝛼12 where;
𝜋 𝜋

Vm: line to line peak input voltage

α1, α2 are firing angles of converter 1 and converter 2 and

α1 + α2 = π
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3.2 Four Quadrant Chopper-fed (using thyristors) DC Motor Drive

Fig.12. Four quadrant chopper drive

T1, T2, T3 & T4 are switching devices may be thyristors, MOSFETs, BJTs or IGBTs.
For this case, they are considered to be thyristors and their switching signals as S1, S2, S3 & S4
respectively. According to the switching patterns, two control modes are available named as
“Bipolar voltage control” and “Unipolar voltage control”. Operation in all 4 quadrants means
that output voltage (Vo) and armature current (Ia) can be controlled in magnitude and polarity.
Therefore the power flow can be in either direction. Left side of the converter (i.e T 1,D1 and
T4,D4) is named as “Leg A” and other part as “Leg B”. Both switches in each leg are
alternatively switched (i.e. when T1 is ON, T4 should be OFF and vice versa).

3.2.1 Operation and Control

 Positive armature current (Ia) (Positive torque)

Fig.13. Operation in 1st and 4th quadrants

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 When T1 and T2 are ON, Vo = VD (input DC voltage). Therefore current through the
armature increases and motor operates in the 1st quadrant. (forward motoring) If T1 is OFF,
then current freewheels through T2 and D4, V0 become zero and current decreases. Finally when
T2 is OFF, current flow through D4 and D3 causes current to decrease and output voltage to be
(-VD). Therefore energy returns to the supply, hence 4th quadrant operation (reverse braking)
of motor achieved. Note that during this operation, armature current flows in the same
direction. (positive direction). Thus torque is positive.

 Negative Armature Current (Ia) (Negative torque)

Fig.14. Operation in 2nd and 3rd quadrants

When T3 and T4 are ON, voltage across the motor becomes (-VD) and current increases in
the negative direction. This is the operation of motor in 3rd quadrant (Reverse motoring). When
T3 is turned OFF, current circulates through T4 and D2. Magnitude of current decreases. When
the T4 is also turned OFF, current flows through D2 and D1 causes to output voltage to be (+
VD) and current to be decreased. Therefore energy returns to the supply, hence 2nd quadrant
operation (forward braking) of motor achieved. Note that during this operation, armature
current flows in the reverse direction. (negative direction). Thus torque is negative.

Depending on the relationship between the two switching signals, 4 quadrant choppers have
two switching schemes called bipolar switching and unipolar switching.

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3.2.2 Bipolar Switching Scheme

In bipolar voltage control scheme, switching signals are derived from pulse width
modulation (PWM) modulator.

Fig.15. PWM modulator for bipolar voltage control

Reference signal and a sawtooth signal is used to derive the switching signal. See Fig.7.
This switching signal is fed to the switch S1 and S2, and the inverted signal is fed to the S3 and
S4. In bipolar switching scheme, output voltage varies between (+VD) and (-VD).

Fig.16. Bipolar switching PWM

Output voltage frequency equal to the frequency of the carrier signal (sawtooth signal).
Average voltage across the motor in bipolar voltage is given by;

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𝑉𝑜,𝑎𝑣𝑔 = (2𝐷 − 1)𝑉𝐷 Where,

D = duty factor

VD = DC input voltage

3.2.3 Unipolar Switching Scheme

This switching scheme treats the two legs of the inverter separately. Practically in 4
quadrant drives, unipolar voltage control is used because it has several advantages over bipolar
voltage control. Unipolar scheme gives better output voltage waveform with less ripples, lower
current ripples and better frequency response.

Fig.17. Unipolar PWM modulator

Switching signals for the leg A and leg B of the chopper drive can be generated from a
unipolar PWM modulator which uses two comparators to compare the carrier signal (sawtooth
signal) and the control signal (reference signal) as shown in Fig 17. Sa signal is fed to the leg
A and Sb signal is fed to the leg B.

 S1 = Sa and S4 = inverted Sa
 S3 = Sb and S2 = inverted Sb

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 Fig.18. Unipolar switching PWM

Output voltage frequency equal to two times of the frequency of carrier signal (sawtooth
signal). Average voltage across the motor in bipolar voltage is given by;

𝑉𝑜,𝑎𝑣𝑔 = (2𝐷 − 1)𝑉𝐷 Where,

D = duty factor

VD = DC input voltage

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 4. Applications

Single quadrant drives are used in commonly used conventional speed drives because
it can only operate in the motoring operation. They are inexpensive and used in domestic
applications such as sewing machines, power tools, toys and etc. single quadrant drives are
commonly used in pumps and fans.

Specially chopper fed DC otor drives are used in applications such as trolley cars,
marine hoists, forklift trucks and mine haulers.

Four quadrant DC motor drives Applications

 Battery operated vehicles

 Lifts and cranes
 Engine test loading systems
 Electric traction systems
 Spindle and tool drives in machine tools
 Auxiliary drives in robotic systems
 Position control systems
 Electrical trains

5. References

 S.N.Manius, AC-DC & DC-DC Converters for DC Motor Drives , Proceedings of the
2013 International Conference on Electronics and Communication Systems
 Mrs. Shimi S.L , Solid state control of DC drive
 Dr. Ungku Anisa Ungku Amirulddin, Chopper Controlled Dc drives
 Rohit Gupta, Ruchika Lamba, Subhransu Padhee, Thyristor Based Speed Control
Techniques of DC Motor:A Comparative Analysis,

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