Cochin University of Science and Techology (CUST) 2006 B.Tech Electrical and Electronics Engineering ELECTRICAL MACHINES1 - Question Paper

the 2nd machine is 610V. compute the bus-bar voltage and output of every
machine.

5. a) elaborate the 3 important characteristics of a dc motor and discuss the
characteristics of a shunt motor and series motor?

b) The speed of a 50HP (37.3KW) series motor working on 500V supply is750rpm at
full load and with 90% efficiency. If the load torque is made 350Nm and 5ohm
resistance is connected in series with a machine compute the speed at which will
run. presume the magnetic circuit to be un-saturated and the armature and the field
ressitance to be 0.5ohm.
OR

6. a) discuss the necessity of starter in a dc motor. With a neat sketch, discuss the
working of a 4-point starter. elaborate the advantages with 4-point starter over 3-
pointstater?

b) Briefly discuss the Hopkinson’s test on a pair of shunt motors. In such a test on
250V machines, the line current was 50A and the motor current 400A not including
the field currents of 6A &5A. The armature resistance of every machine was
0.015ohm. compute the efficiency of every machine.

7. a) Draw an equivalent circuit for a single- phase 2 winding transformer with referred
to primary winding. Also show how an equivalent circuit is useful in the analysis of
performance of a transformer.

b) The subsequent data referred to a single- phase transformer. Turns ratio is
19.5:1.R1=25 ohm; x1=100 ohm; R2=0.06 ohm; X2=0.25 ohm. No load current
=1.25A leading the flux by 30degree. The secondary delivers 200A at a terminal
voltage of 500V and a power factor of 0.8 lagging. Determine by the aid of phasor
diagram, the primary applied voltage, the primary power factor and the efficiency.

OR

8. a) elaborate the various losses in a transformer? Derive the conditions for maximum
efficiency of the transformer.

b) The primary and secondary windings of a 40KVA, 6600/250V single-phase
transformer have resistances of 10ohm and 0.02ohm respectively. The total leakage
reactance is 35ohm as referred to primary winding. obtain full load regulation at a
power factor of 0.8 lagging.

9. a) define the back-to-back test for separation of losses in 2 identical transformers.

b) Consider a 4KVA, 200/400V single phase transformer supplying full load current at
0.8 lagging power factor. The OC and SC test outcomes are as follows.
OC test: 200V, 0.8A, 70W (on LV side)

SC test: 20V, 10A, 60W (on HV side)

compute the efficiency, secondary voltage and current in the primary at the above
load. compute the load at unity power factor corresponding to maximum
efficiency.

OR

10. a) provide an account on: 1) open delta or V-V connection
2) scott connection or T-T connection

b) 2 transformers A&B are connected in parallel to a load of ( 2+j5) ohm. The
impedance in secondary are ZA=(0.15+j0.5) ohm and ZH=(0.1+j0.6) ohm. Their
no load terminal voltages are EA=207V and EB=205V.
obtain the power output and power factor of every transformer.

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Earning:   Approval pending.