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EO315 1 / 6
SCHOOL OF ENGINEERING
MODULAR HONOURS DEGREE
SEMESTER 1
LEVEL 3
2004/2005
ELECTRICAL MACHINES AND DRIVES
Examiner: Dr P.A. Howson
Attempt FOUR questions.
Time allowed 2 hours.
Total number of questions = 6
All questions carry equal marks
The figures in brackets indicate the relative weightings of parts of a question
Special requirement: None
EO315 2 / 6
1) There are three basic methods of stopping and / or reversing a d.c motor. One
method is counter current braking (Plugging).
a) State two other basic methods of stopping and / or reversing a d.c shunt motor.
(4)
b) Describe in detail, if necessary with the aid of diagrams, any two of the methods
in the context of a d.c shunt motor.
(6)
c) A 300 V d.c shunt motor draws 40 A, while supplying the rated load at a speed of
100 rad / s. The armature resistance is 1 ohm and the field winding resistance
250 ohm.
Determine the external resistance that must be inserted in series with the armature
so that the armature current does not exceed 140% of the rated value when the
motor is counter current braked.
(10)
2)
a) Explain with aid of schematic diagrams the Ward-Leonard method of speed
control.
(10)
b) A mine lift is powered by a Ward-Leonard system. The two d.c machines are 84%
efficient and the a.c machine 75% efficient. The lift attached to the system is
travelling vertically upwards at a constant speed of 2 m/s. If the power from the three
supply system into the a.c machine is measured as 15 kW. Calculate the total
weight of the lift, its load and the supporting cable.
(5)
c) On the return journey down the lift, load and cable is reduced to 0.2 metric tons and
the lift is travelling at a constant speed of 5 m/s. Calculate the power from the a.c
machine that is returned to the supply.
Assume no losses in the winding gear and the force of gravity is 9.8 m/s2.
(5)
EO315 3 / 6
3) Figure Q3 shows the per phase equivalent circuit, referred to the stator, of a 410 V,
50 Hz, two pole star connected, wound rotor three phase induction motor.
X1 = 1.0 
R1 = 0.5 
Vp
X2 = 0.4 
Xm =25
Vth
R2 = 0.5
Figure Q3
It can be shown that torque T;
3(Vth) 2
T 


 sync  Rth 
R2
s
2

R2 
2
   Xth  X 2 
s 

the maximum slip S;
S max 
R2
Rth 2   Xth  X 2
2
and the maximum torque ;
Tmax 
3(Vth) 2
2
2 sync  Rth  Rth 2   Xth  X 2 


a) What is the maximum torque of the motor?
(4)
b) At what speed and slip does maximum torque occur?
(4)
c) If the rotor resistance is doubled, what will be the value of slip at which
maximum torque will occur?
(4)
d) Calculate a suitable value of resistance that could be added into the rotor circuit
that will cause the motor to develop maximum torque at start up.
(8)
EO315 4 / 6
4)
Power Thyristors and MOSFET’s semiconductor devices are important components of
Modern machine drive systems.
a) Sketch a schematic diagram of a Power MOSFET and give a basic description of its
Operation.
(5)
b) Power MOSFET’s are largely immune against secondary breakdown effects. Explain
the causes of secondary breakdown and why MOSFET’s are largely immune against
this phenomenon.
(3)
c) State the basic characteristics of a Thyristor and using a NPN and PNP transistor
Model explain how the device can be locked into a conductive state by application
of a trigger pulse to the gate.
(5)
d) Sketch a commonly used circuit composed of two Thyristors, that is capable of both
Switching load current on and off. Briefly explain the operation of the circuit.
(7)
EO315 5 / 6
5)
a) Sketch a simple six diode full wave three phase rectifier circuit and explains its
operation. Indicate on the diagram what input and output waveforms you would
expect to see.
(5)
b) Re-sketch the circuit of “a” showing how you would convert the circuit to operate
as a half wave controlled three phase rectifier.
(5)
c) Frequency changing circuits are a common method of obtaining speed control of
induction
Draw a approximate graph of torque v rotor speed of a four pole three phase 50 Hz
induction motor, Showing the variation of torque v rotor speed in terms of at least 4
different frequencies, two below synchronous speed and two above it.
(5)
d) The basic equation for the flux in an induction motor core can be found from
Faraday’s equation.
vt   N 
d
dt
Where V is the stator applied voltage ? the magnetic flux.
Use this equation to show why it is necessary to derate the applied voltage when the
frequency of the supply is reduced to achieve speed control of the induction motor.
(5)
EO315 6 / 6
6)
a) Discuss the essential principles of operation of a stepper motor and give at least three
applications of a stepper motor.
(8)
b) The two armatures on a 4 Pole hybrid stepper motor each have 50 salient poles (teeth).
Calculate
i) the angle between two successive teeth on each armature
(4)
ii) the angle between one tooth on one armature and the next tooth on the other
armature.
c) Why is viscous damping employed in stepper motors?
(4)
(4)