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SCHOOL OF ENGINEERING
MODULAR HONOURS DEGREE
Level 3
January 2007
ELECTRICAL MACHINES AND DRIVES
Examiner: Dr P.A. Howson
Answer only Four questions.
Special requirement, none
Time allowed Two Hour hours.
1. There are three basic methods of stopping and / or reversing a d.c motor. One
method is counter current braking (Plugging).
i.
State two other basic methods of stopping and / or reversing a d.c shunt motor. (4)
ii.
Describe in detail, if nessicary with the aid of diagrams, any two of the methods
in the context of a d.c shunt motor.
(6)
iii. A 400 V d.c shunt motor draws 40 A, while supplying the rated load at a speed of 100
rad / s. The armature resistance is 2 ohm and the field winding resistance 300 ohm.
Determine the external resistance that must be inserted in series with the armature
so that the armature current does not exceed 150% of the rated value when the motor
is counter current braked.
(10)
2.
i.
ii.
State two basic methods of controlling the speed of a d.c shunt motor, other than pulse
width modulated armature current chopper system or the Ward-Leonard method.
(5)
Explain with aid of diagrams the Ward-Leonard method of speed control.
(5)
iii 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 phase supply system
into the a.c machine is measured as 20 kW. Calculate the total weight of the lift, its load
and the supporting cable.
iv
(5)
On the return journey down the lift, load and cable is reduced to 0.5 metric tons and
the lift is travelling at a constant speed of 10 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)
3.
i.
State two methods of controlling the speed of a three phase induction motor.
ii.
Figure 1 shows the per phase equivalent circuit, referred to the stator, of a 460V, 60 Hz, four pole
star connected, wound rotor three phase induction motor.
X1= 1.1 
X2=0.464
R1=0.641
Vp
(4)
Xm=26.3
Vth
R2=0.332
Figure 1
Where, X1, R1, X2, R2 and Xm are the stator reactance, stator resistance , rotor resistance,
rotor reactance and stator magnitising reactance respectively. Vp and Vth are the phase voltage
and Thevenin voltage respectively.
Given the slip (S) where maximum torque occurs is given by;
S max 
R2
Rth 2   Xth  X 2
2
And the torque () at this speed is given by;
 max 
3(Vth) 2
2 sync  Rth  Rth 2   Xth  X 2
2

where 2 is the synchronous speed, Rth and Xth are the Thevenin resistance and reactance
respectively.
a. What is the maximum torque of the motor.
(6)
b. At what speed and slip does this occur.
(6)
c.
If the rotor resistance is doubled, what will be the value of slip at which maximum
torque will occur.
(4)
4.
i.
Explain with the aid of sketches of typical Torque / slip characteristics of a three phase
induction motor, the effects of adding resistance into the rotor circuit.
(6)
ii. What effect will adding resistance to the rotor circuit have on the efficiency of the motor. (2)
iii. A typical two pole, 50 Hz induction motor supplies 15 kW to a load at a speed of 2950 r.p.m.
a. What is the motor’s slip.
(2)
b. What is the torque produced by the motor in N m under these conditions.
(2)
c.
(8)
How much power will be supplied by the motor when the torque is doubled.
What will the operating speed of the motor be if the torque is doubled. Justify your answer
by sketching a typical Torque / slip characteristic of a induction motor and indicating on the
sketch any assumptions you have made.
5.
Power Thyristors and MOSFET’s semiconductor devices are important components of
Modern machine drive systems.
c) Sketch a schematic diagram of a Power MOSFET and give a basic description of its
Operation.
(5)
c) 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)
6.
The thyristor is a switching device is commonly found in electronic machine drive circuits.
i.
State two other semiconductor switching devices that can be used in place of the thyristor
and list their advantages and disadvantages compared to the thyristor.
ii.
(4)
Figure 2 shows a typical thyristor switching circuit capable of switching current on and off
through the load.
Load
R
C
Vdc
Th1
Th2
Figure 2
ii.
Redraw the circuit and explain how it works, indicating current directions.
(6)
iii. 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.
(6)
iv. Re sketch the circuit of iii showing how you would convert the circuit to operate as a half
wave controlled three phase rectifier.
(4)