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PAPER CODE NO. EXAMINER: Prof S. Taylor
ELEC 331 DEPARTMENT: EE&E TEL. NO: 4517

FIRST SEMESTER EXAMINATIONS 2019/20

Drives

TIME ALLOWED: 2 Hours

INSTRUCTIONS TO CANDIDATES

The numbers in the right hand margin represent an approximate guide to the marks available
for that question (or part of a question). Total marks available are 100.

μ0 = 4π x 10-7 H m-1

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1. a) A magnetic circuit is formed using an iron core with relative permeability 1000
and single air gap, and has the following parameters:
cross-sectional area: AC =1.6 × 10-3 m2; mean core length lC = 0.8 m;
air gap length g = 2 x 10-3 m.
A coil of 100 turns is wound on the iron core.

i) Calculate the reluctance of the core RC and that of the gap RG
neglecting the effects of fringing fields at the air gap and leakage flux
from the coil.

4
ii) A current of i = 2 A now flows in the coil. Calculate the total flux, the flux
linkage and the coil inductance L.
4
iii) Design a new coil and choose an operating current to double the flux
linkage while keeping the coil inductance below 12 mH.
6

b)

c)
What are the advantages of synchronous drives? With the aid of a diagram,
explain the operation of a synchronous drive and give two applications where
such drives are commonly used.

A 3600 rpm, 60 Hz, 13. 8 kV synchronous generator has a synchronous reactance
of 20 Ω. The generator is operating at rated voltage and speed with the per phase
excitation voltage E 0=11.5kV and the torque angle δ = 15 °. Calculate stator
current, power factor, and total output power. What design changes could you
make to reduce the synchronous reactance? How would these changes affect the
stability of the generator?

10

12

d)

e)
With the aid of a diagram explain how the torque is produced in a Switched
Reluctance Motor. What design features make this motor an attractive
option. Give two applications where such drives are commonly used.

With reference to one of the common types of molecular motor explain
how chemical energy is converted into motion. For the nanoscale motor
that you have chosen discuss which features it has in common with a
macroscale motor.

8

6

Total 50
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2 A three-phase star connected, 6-pole 7.5 kW induction drive operates at 220 V and
60 Hz and has the following constants in ohms per phase, referred to the stator:

R1 = 0.294; R2’ = 0.144

X1 = 0.503; X2’ = 0.209, XM = 13.25

where R1 represents the stator resistance, X1 represents the stator leakage reactance,
R2’ represents the rotor resistance, X2’ represents the stator leakage reactance and
XM the magnetising reactance.

Draw an equivalent circuit for this drive.

The total friction, windage and core losses may be assumed to be constant at 403 W

For a slip of 2% calculate the speed, the output torque and power.

Determine the maximum torque and the slip at maximum torque and sketch the
Torque-speed curve for this drive

How could the design of the machine be altered to reduce the speed at which
maximum torque is obtained?

6

14

5

Total
25

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3. a) What are the advantages and disadvantages of permanent magnet direct current
(PMDC) drives? Give two applications of PMDC drives

For one of the applications you have chosen, explain what design variables may be
adjusted to enhance the operation.

A permanent magnet DC motor is known to have an armature resistance of 1.03 Ω.
When it is operated from a DC source of 50 V, the speed is 2100 rpm and the
current drawn from the source is 1.25 A. Calculate:
The speed – voltage constant for the motor,
The no- load rotational losses of the motor,
The power output and torque of the motor when it is operating at 1700 rpm from a
48 V power source.

6

4

15

Total
25
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