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EO315 1 / 5
SCHOOL OF ENGINEERING
MODULAR HONOURS DEGREE
LEVEL 3
SEMESTER 2
2005/2006
ELECTRICAL MACHINES AND DRIVES
Examiner: Dr. P.A. Howson
Attempt FOUR questions only.
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 requirements: None
EO315 2 /5
1) State two common applications of regenerative braking.
a) State two basic methods of stopping and/or reversing a d.c. shunt motor.
(2)
(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 500 V d.c. shunt motor draws 40 A, while supplying the rated load at a speed
of 200 rad/s. The armature resistance is 1 ohm and the field winding resistance
is 200 ohms. 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.
(8)
2) a) State two basic methods of controlling the speed of a d.c. shunt motor, other
than the pulse-width-modulated armature-current chopper system or the WardLeonard method.
(2)
b) Explain, with aid of schematic diagrams, the Ward-Leonard method of speed
control.
(8)
c) A mine lift is powered by a Ward-Leonard system. The two d.c. machines are
80% efficient and the a.c. machine is 73% efficient. The lift attached to the
system is travelling vertically upwards at a constant speed of 4 m/s. If the
power from the three-phase supply system into the a.c. machine is measured as
30 kW, calculate the total weight of the lift, its load and the supporting cable.
(5)
d) On the return journey down, the weight of the lift, load and cable is reduced to
0.5 metric tons and the lift travels at a constant speed of 4 m/s. Calculate the
power from the a.c. machine that is returned to the supply.
Assume there are no losses in the winding gear and that the acceleration due to
gravity is 9.8 m/s2.
(5)
EO315 3 /5
3) a) State two methods of controlling the speed of a three-phase wound-rotor
induction motor.
(4)
b) 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= 1.2 
X2=0.564
R1=0.541
Vp
Vth
Xm = 27.3
R2 = 0.432
Figure 1
X1, R1, X2, R2 and Xm are the stator reactance, stator resistance, rotor
reactance, rotor resistance and stator magnetising reactance respectively. Vp and
Vth are the phase voltage and the Thevenin voltage respectively.
The slip (S) at which maximum torque occurs is given by:
S max 
R2
Rth 2   Xth  X 2
2
The torque () at this speed is given by:
 max 
3(Vth) 2
2
2sync  Rth  Rth 2   Xth  X 2  


QUESTION 3 CONTINUES ON PAGE 4/5
EO315 4 /5
where 2sync is the synchronous speed, Rth and Xth are the Thevenin resistance
and reactance respectively.
i) What is the maximum torque of the motor?
(6)
ii) At what speed and slip does this occur?
(6)
iii) If the rotor resistance is increased by 0.5 ohm, what will be the value of slip
at which maximum torque will occur?
(4)
4) a) Explain, with the aid of sketches, typical torque/slip characteristics of a threephase induction motor, including the effects of adding resistance into the rotor
circuit.
(6)
b) What effect will adding resistance to the rotor circuit have on the efficiency of
the motor?
(2)
c) A typical two-pole, 60 Hz induction motor supplies 25 kW to a load at a speed
of 3550 r.p.m.
i) What is the motor’s slip?
(2)
ii) What is the torque produced by the motor (in N.m) under these conditions?
(2)
iii) How much power will be supplied by the motor when the torque is doubled?
(4)
iv) 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 an
induction motor and indicating on the sketch any assumptions you have
made.
(4)
EO315 5 /5
5) Power thyristors and MOSFET semiconductor devices are important components of
modern machine-drive systems.
a) Sketch a schematic diagram of the semiconductor structure of a power
MOSFET and give a basic description of its operation.
(5)
b) Power MOSFETs are largely immune from secondary breakdown effects.
Explain the causes of secondary breakdown and why MOSFETs are largely
immune from this phenomenon.
c) Discuss the advantages of thyristors compared to power MOSFETs.
(3)
(5)
d) Sketch a commonly used circuit composed of two thyristors that is capable of
switching load current on and off. Explain briefly the operation of the circuit.
(7)
6) A stator-fed variable-frequency drive is a common method of controlling the speed
of a three-phase caged induction motor. However, where a wound-rotor induction
motor is used, it is possible to control the speed from the rotor.
a) Sketch a schematic diagram of a rotor-based speed-control system using a
variable-voltage battery and explain briefly how it functions.
(6)
b) Sketch a schematic diagram of a wound-rotor induction-motor speed-control
system using a load resistor and chopper. Explain briefly how it works.
(6)
c) A wound-rotor three-phase induction motor is controlled by a load resistor and a
chopper speed-control method. The motor is rated at 35 kW, 1170 r.p.m.,
460 V, 60 Hz. The open-circuit rotor line voltage is 400 V and the load resistor
is 0.5 ohm. If the chopper frequency is 200 Hz, calculate the chopper on-time
so that the motor develops a torque of 250 Nm at 900 r.p.m.
(8)