ocd048 university of bolton rak academic centre … · out torque) that the motor can deliver. (4...

OCD048

UNIVERSITY OF BOLTON

RAK ACADEMIC CENTRE

BENG(HONS) ELECTRICAL AND ELECTRONIC

ENGINEERING

SEMESTER TWO EXAMINATION 2017/2018

ELECTRICAL MACHINES AND POWER

ELECTRONIC DRIVES

MODULE NO: EEE6001

Date: Thursday 31 May 2018 Time: 10:00 – 12:00

INSTRUCTIONS TO CANDIDATES: There are 5 questions. Answer any 4 questions. All questions carry equal marks. Marks for parts of questions are shown

in brackets.

of 13 University of Bolton RAK Academic Centre BEng(Hons) Electrical & Electronic Engineering Semester 2 Examinations 2017/18 Electrical Machines And Power Electronic Drives Module No. EEE6001

Q1

(a) The Open Circuit and Short Circuit test data are given below for a single

phase, 5 kVA, 200V/400V and 50Hz transformer.

Open Circuit test from Low Voltage side: 200V 1.25A 150W

Short Circuit test from High Voltage side: 20V 12.5A 175W

(i) Draw the equivalent circuit of the transformer referred to both Low

Voltage and High Voltage sides

(6 marks)

(ii) Calculate the efficiency of the transformer at 75% loading with load power

factor = 0.7. Find the load or kVA at which the transformer is to be

operated for maximum efficiency and also calculate the value of

maximum efficiency.

(4 marks)

(b) Instrument transformers are used in AC system for the measurement of large

voltage and current, Justify the statement. If a potential transformer 110K/110

V and a current transformer of 60/5 A are used to measure voltage and

current of a transmission line, and voltmeter indicates 100 V and ammeter

indicates 2 A, what is the voltage and current in the transmission line?

(5 marks)

(c)

(i) Compare and contrast the working of two winding transformer and auto

transformer. (5 marks)

(ii) If an ideal autotransformer has a coil with total number of turns NCD = 200

between terminals C & D and It has got one tapping at A such that NAC =

100 and another tapping at B such that NBA = 50, Calculate currents in

various parts of the circuit and show their directions when 400 V supply is

connected across AC and two resistive loads of 60Ω & 40Ω are

connected across BC and DC respectively


(5 marks)

Total 25 marks

Please turn the page

Q2.

(a) A 230 V dc machine has an emf constant 212.21 V s/ Wb radK ´

and 0.278 aR . The field is separately excited and produces 0.01 Wb per

pole, 36 AaI for all parts of the problem.

(i) Find the speed and the total power developed by the armature if the

machine is operated as a dc motor with rated terminal voltage.

Determine the speed of the motor if an external resistance 1 xR is

added in series with the armature circuit. Also calculate the rotational

losses at the point of operation if the motor is known to be supplying 10

HP to a coupled mechanical load.

(5 marks)

(ii) Find the speed and the total power developed by the armature if the

machine is operated as a dc generator with rated terminal voltage.

(5 marks)

(b) A dc shunt generator has armature resistance Ra = 0.1 Ω, shunt field winding

resistance Rfw = 80 Ω, and Nf = 1200 turns per pole. The rated power, voltage

and speed of armature are 12 kW, 100 V and 1000 rpm respectively. The rated

field current is 1 ampere. The magnetization characteristic at 1000 rpm is shown

in Figure. (From Figure 1: Generated EMF Eg= 98 V at If= 0.94 A, Eg=112V at

If=1.4 A, Eg=113.18V at If=1.45 A, Eg=99.4V at If=0.99 A)


Figure 1- Magnetization characteristics

Q2 continued over the page...

Q2 continued

(i) Neglect the armature reaction effect. Determine the terminal voltage at full

load. Also calculate the value of the field circuit control resistance (Rfc) required

to make terminal voltage Vt = 100 V at full load condition. (5 marks)

(ii) Consider that armature reaction at full load is equivalent to 0.06 field amperes.

Find the full-load terminal voltage and also the field current required to make

the terminal voltage Vt = 100 V at full load condition (5 marks)

(iii) The dc machine is provided with a series winding, Rsr = 0.01 Ω so that it can

operate as a cumulatively compound dc machine. The short shunt machine is

required to provide a terminal voltage of 100 V at no load as well as at full load.

If the shunt field winding has 1200 turns per pole along with field control

resistance of 20Ω, how many series turns per pole are required to obtain zero

voltage regulation?

(5 marks)

Total 25 marks Q3.

(a) (i) Analyse the principle of operation of three phase induction motors with

necessary diagrams. (5 marks)

(ii) If a 3φ, 460 V, 100 hp, 60 Hz, four-pole induction machine delivers rated

output power at a slip of 0.05, determine the synchronous speed, motor


speed, frequency of rotor circuit, slip rpm and speed of rotor field relative

to rotor circuit. Also find the rotor induced voltage at the operating speed, if

the stator-to-rotor turns ratio is 1:0.5. (5 marks)

(iii) Single phase induction motors are not self-starting, Justify the statement.

(5 marks)

(b) Following data is taken from no-load, locked rotor, and DC tests of a 3-phase,

wye connected 40 HP, 60 Hz, 460 V, induction motor with a rated current of

57.8 A. Assume motor is NEMA Design B.

Q3 continued over the page...

Q3 continued

No-load test: Supply frequency = 60 Hz, Line voltage = 460 V, Line current

= 32.7 A and Input power = 4664.4 W

Blocked-rotor test: Frequency = 15 Hz, Line voltage = 36.2 V, Line current

= 58 A and Input power = 2573.4 W

DC Test: Vdc = 12 V, Idc = 59 A.

(i) Determine Total core, friction and windage losses. Find the motor

parameters and draw the approximate equivalent circuit.

(7 marks)

(ii) Calculate the parameters (Vth, Rth, Xth) for the Thevenin equivalent

circuit

(3 marks)

Total 25 marks

Q4.

(a) A 3φ, 5 kVA, 208 V, four-pole, 60 Hz, star-connected synchronous machine has

negligible stator winding resistance and a synchronous reactance of 8 ohms per

phase at rated terminal voltage. The machine is first operated as a generator.

(i) Determine the excitation voltage and the power angle when the machine is

delivering rated kVA at 0.8 PF lagging and draw the phasor diagrams

(4 marks)


(ii) With the field current as in part 1, the prime mover power is slowly increased.

What are the corresponding values of the stator (or armature) current, power

factor, and reactive power at this maximum power transfer condition?

(2 marks)

(iii)If the machine is operated as a synchronous motor and the field excitation is

adjusted so that the power factor is unity when the machine draws 3 kW from

the supply. Find the excitation voltage and the power angle. Also draw the

phasor diagram for this condition. If the field excitation is held constant and

the shaft load is slowly increased, determine the maximum torque (i.e., pull-

out torque) that the motor can deliver. (4 marks)

Q4 continued over the page…

Q4 continued

(b) Analyse construction, principle of operation and applications of Universal motor.

(5 marks)

(c) A 100 KVA, 3000 V, 50 Hz, 3 phase star connected alternator has effective

armature resistance of 0.2 Ω. The field current of 40A produces short circuit

current of 200 A and an open circuit emf of 1040 V (line value). Calculate the full

load voltage regulation at 0.8 power factor lagging, 0.8 power factor leading and

unity power factor. Also draw appropriate phasor diagrams.

(10 marks)

Total 25 marks

Q5.

(a) A single-phase full converter is used to control the speed of a 5 hp, 110 V,

1200 rpm, separately excited dc motor. The converter is connected to a

single-phase 120 V, 60 Hz supply. The armature resistance and circuit

inductances are Ra = 0.4 Ω, La = 5 mH. The motor voltage constant is KΦ =

0.09 V/rpm.


(i) Rectifier (or motoring) operation: The dc machine operates as a motor,

runs at 1000 rpm, and carries an armature current of 30 amperes.

Assume that motor current is ripple-free. Determine firing angle α, power

to the motor and supply power factor. (5 marks)

(ii) Inverter operation (regenerating action): The polarity of the motor back

emf Ea is reversed, say by reversing the field excitation. Determine firing

angle to keep the motor current at 30 amperes and power fed back to the

supply when the speed is 1000 rpm.

(5 marks)

Q5 continued over the page…

Q5 continued

(b) A three-phase ac voltage controller is used to start and control the speed of a

3φ, 100 hp, 460 V, four-pole induction motor driving a centrifugal pump. At

full-load output the power factor of the motor is 0.85 and the efficiency is 80

percent. The motor current is sinusoidal. The controller and motor are

connected in delta. Calculate the control range of the firing angle α and also

determine the rms current rating and peak voltage rating of the thyristor.

(5 marks)

(c) With the aid of necessary diagrams , analyse the following operational circuits

(i) DC choppers (5 marks)

(ii) Inverters (5 marks)

Total 25 marks

END OF QUESTIONS


Please turn the Page for the EQUATION SHEET


EQUATION SHEET

DC GENERATORS:

Self Excited DC Generator

Shunt generator:

Compound generator:

(+ for cumulative compound generator, - for differential compound generator)

Short Shunt:

Efficiency of machine as generator:


DC MOTORS:

Self Excited DC motor:

Series motor:

Shunt motor:

Losses:

Efficiency of machine as motor:


TRANSFORMERS:

Transformer on no-load:

Transformer on Load:

Secondary winding reffered to primary:

Primary winding referred to secondary:

Equivalent circuit of transformer refferd to primary:

Voltage regulation of transformer:

Efficiency of transformer:

Open circuit test:


Short circuit test:

Efficiency from OC test and SC test:

INDUCTION MOTOR:

Three phase Induction Motor:

Torque Equation:

Losses:


Equivalent circuit reffered to stator:

Thevenin’s Equivalent circuit:

SYNCHRONOUS MACHINES:

Three phase AC voltage controllers

Single phase full converter:

End of Paper

ocd048 university of bolton rak academic centre … · out torque) that the motor can deliver. (4...

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