Code No: 310205

III B-Tech I-Semester Regular Examinations

November - 2003
Set No: 3

ELECTROMECHANICS - III

(Electrical and Electronics Engineering)

[Time:3 hours]
Max. Marks: 80

Answer any FIVE questions

ALL questions carry equal marks

1. Describe how the armature windings are arranged are arranged in synchronous generators. What is the effect of distribution factor on the performance of the generator?

2. Explain the effect of armature reaction on terminal voltage of an alternator at i) u.p.f ii) zero p.f. load. Draw the relevant phasor diagrams. What is leakage reactance?

3. A 1000KVA, 11000V 3-phase star-connected alternator has an effective resistance f 2Ω/phase. The characteristics on open-circuit and with full load current at zero P.f and the open-circuit core losses are:

Field current (A)                     40   50      110      140      180 

OC terminal voltage (V)            --   7000   12500   13750    15000

Core loss (KW)                       --     7.5     16.6      22.4     33.5
  
Saturation curve zero p.f (V)     0      --     8500   10500    12400

Deduce by the Z.P.f method

a) The percentage regulation for full load at a Lagging 0.8 p.f. Find also
b) The efficiency at this load, given that the field current has resistance of 0.5Ω and that the mechanical and additional losses amount to 10KW

4. Two single phase alternator are connected in parallel and the excitation of each machine is such as to generate an open-circuit emf of 3500V.The stator winding of each machine has a synchronous reactance of 30ohms and negligible resistance. If there is a phase displacement of 40 electrical degree between the emf's. Calculate

a) The current circulating between the two machines.
b) The terminal voltage and
c) The power supplied from one machine to the other.

5. A 3 ph synchronous motor is designed for a terminal voltage of 3300V and its synchronous impedance is 0.25 + j2.00 Ω / ph. The excitation is adjustable to a value which corresponds to an open circuit terminal voltage of 3500V. Determine the current and p.f. from an output of 750kw

6. a) What is meant by hunting in a synchronous motor?

b) Explain V curves and inverted V curves

7. a) Describe cross field theory as applied to single phase induction motor

b) The following data pertains to a 50Hz single phase motor.

Supply Voltage     :  110V        Rated output  :125W

Rated speed        :  1410rpm    Total copper loss at full load: 25w

Rotational losses  : 25W

Calculate the full load efficiency and the rotor copper loss caused by the backward field. Neglect stator losses.

8. a) Compare the performance characteristics of a.c. Series motor when it is connected across i) a.c. Supply and ii) D.c. supply.

b) Describe the construction and principle of operation of a single phase shaded pole motor with a neat diagram. Give its industrial applications.