Electrical Machines Mock

8/2/2019 Electrical Machines Mock

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GTHS KUMBO_Electrical Department_Mock exam......March.2012 1

PART ONE : TECHNOLOGY

1. Explain why the series field DC motor rotates at high speed at no load when it

is supplied at his nominal voltage.

2. For each of the following electrical machines, cite a magnetic material which is

used in the manufacturing of the magnetic circuit: a) transformer; b) DC

machine stator, c) permanent magnet DC motor.

3. Give the role of the isoptherme in an AC electrical machine.

4. Give the role of the excitation resistance in a DC motor.

5. Give the role of auxiliary commutation poles in a DC motor.

6. Give the differences between an asynchronous squirrel cage motor and an

asynchronous wound winding motor.

7. Name the test which is conducted in order to determine iron and friction losses

in a DC motor.

PART TWO: ELECTROTECHNOLOGY

Exercise 1: DC shunt excited generator

A DC generator has the following magnetisation characteristic at 1200 rpm.

Field current If 1 2 4 6 8 10

Generated emf 192 312 468 566 626 660

1.1 Draw the magnetisation curves:

a) at 1200 rpm;

b) at 1000 rpm.

Take as scale: 20mm = 100V; 10mm = 1A.

REPUBLIC OF CAMEROONPeace Work Fatherland

GTHS KUMBO/ ELECT DPT

MOCK EXAMINATIONS

Class: F36

Option: Electrotechnology

Duration: 03H

Coefficient: 3

Written paper

ELECTRICAL MACHINES

No document is allowed except the one given tothe candidates b the examiners.


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The field resistance is 60 and the armature resistance is 0.5

1.2 If the generator is shunt excited and driven at 1000 rpm, determine:

a) The voltage at which it will build up on open circuit.

b) The value of the critical resistance of the shunt field circuit at 1000rpm.

c) The terminal potential difference and the load current for a load resistance of

30 .

Exercise 2: Three phase alternator

A 26 poles star connected three phase alternator delivers a line voltage of 5000V at a

frequency of 50Hz. The internal characteristics is assumed to have the equation

E0 = 10.5i. Where EO is the line emf and i the field current. The resistance of the

stator winding is 5.4 . A short circuit test gave the following results: i = 3.34A;

Isc = 2000A.

Compute:

1. The rotor speed.

2. The synchronous reactance per phase.

3. Neglecting stator resistance, find the value of the field current i for a load

current I = 300A at 0.9 power factor lagging.

4. The stator resistance being 136m and total mechanical losses 400W;

calculate the efficiency for the same load as in question 3 above.

Exercise 3: Three phase induction motor.

The characteristics of a wound rotor tetrapolar (04 poles) three phase induction motor

are as follows:

Normal voltage of a winding: 380V;

Resistance between two stator terminals: r = 0.2

Both the stator and the rotor are delta connected.

Rotor losses are neglected.

A normal load test carried out on this motor yielded the following results:

Current absorbed: I = 40A;

Stator iron losses = 0.5kW:

Nominal speed N = 1450 rpm;

Mechanical losses = 400W;


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Power measured with the two wattmeter method gave: P1 = 15kW, P2 = 6kW.

Rotor iron losses are negligible.

1. Draw the circuit for the load test described above showing clearly the

apparatus for measuring current and power.

2. Determine the synchronous speed and deduce the slip.

3. Calculate the power absorbed and deduce the power factor of the motor.

4. Calculate the resistance of one stator winding and deduce the stator joule

losses.

5. Calculate the power transmitted to the rotor and the rotor joule losses.

6. Calculate the efficiency of the motor.

SUBJECT MASTER: NGOUNE Jean-Paul,

PLET Electrotechnics, GTHS KUMBO.

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Electrical Machines Mock