# GTU previous year question papers -BE- Sem-Vth -Electrical Machines-II -May/June -2012

**GTU previous year question papers**

**GUJARAT TECHNOLOGICAL UNIVERSITY**

**BE- V****th ****SEMESTER–EXAMINATION – MAY/JUNE – 2012**

**Subject code: 150802 **

**Subject Name: Electrical Machine-II**

**Instructions:**

**1. Attempt all questions.**

**2. Make suitable assumptions wherever necessary.**

**3. Figures to the right indicate full marks.**

**Q.1 (a) **Discuss the difference between single layer and double layer winding.

Under what conditions are dummy coils needed in a wave winding.

**(b) **What are the advantages of Hopkinson’s test over Swinburne’s test and

what are its limitations.** **

**(c) **A 6-pole,DC armature with 36 slots and 2 coil-sides/slot is to be wave –

wound(double-layer).Calculate the back-pitch, front –pitch and

commutator-pitch.How many dummy coils, if any, are required?** **

**Q.2 (a) **Explain the basic purpose of a tertiary winding in transformer. To what

additional use it can be put? Explain Scott connection with neat diagram

for 3 phase transformer.** **

**(b) **A 50kVA,2200/110 V transformer when tested gave the following results:

OC test, measurements on the LV side:400W,10A,110V

SC test, measurements on the HV side:808W,20.5A,90V

Compute all the parameters of the equivalent circuit referred to the HV

and LV sides of the transformer.** **

**OR**

**(b) **A 500kVA ,11/0.43kV,3-phase delta/star-connected transformer has the

rated load an HV copper-loss of 2.5kW and an LV loss of 2kW.The total

leakage reactance is 0.06pu.Find the ohmic values of the equivalent

resistance and leakage reactance on the delta side.

**Q.3 (a) **Describe briefly the methods of controlling speed of an Induction motor. ** **

**(b) **A 3-phase, 400V, star-connected induction motor has a star-connected

rotor with a stator to rotor turn ratio of 6.5.The rotor resistance and

standstill reactance per phase are 0.05 ohm and 0.25 ohm respectively.

What should be the value of external resistance per phase to be inserted in

the rotor circuit to obtain maximum torque at starting and what will be

rotor starting current with this resistance.** **

**OR**

**Q.3 (a) **Why are large squirrel cage IM’s not stared direct on line? How they are

normally started?** **

**(b) **A 3-phase,400V induction motor gave the following test readings:

No load:400V,1250W,9A,Short circuit:150V.4kW,38A

Draw the circle diagram.

If the normal rating is 14.9kW,find the circle diagram, the full –load value

of current,pf and slip.** **

**Q.4 (a) **Under what conditions does the voltage regulation of a synchronous

machine becomes negative? Explain with a phasor diagram.** **

**(b) **Explain power developed by a synchronous motor with neat diagram. ** **

**OR**

**Q.4 (a) **Discuss the effect of excitation on Armature current and Power factor

with neat diagram (synchronous motor).

** **

**(b) **Explain construction of V curves with neat diagram (synchronous motor). ** **

**Q.5 (a) **Explain various types of stepper motors .State the advantages of

permanent magnet brushless DC machine.** **

**(b) **Explain briefly the action of a transformer with schematic diagram. ** **

**OR**

**Q.5 (a) **State one advantage and one disadvantage in the application of each of the

three basic types of DC motors.** **

**(b) **A 3-phase induction motor has a 4-pole,star-connected stator winding and

runs on 50Hz with 400V between lines. The rotor resistance and standstill

reactance per phase are 0.4 ohm and 3.6 ohm respectively. The effective

ratio of rotor to stator turns is 0.67.Calculate:(1)the gross torque at 4%

slip,(2) the gross mechanical power at 4% slip,(3)maximum

torque,(4)speed at maximum torque, and(5) maximum mechanical

power(gross).Neglect stator impedance.