8/13/2019 Em1 Assignment # 02

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I I I S e m . B . E . ( E & E )

E L E C T R I C A L M A C H I N E R Y - I

ASSIGNMENT # 02

ASSIGNMENT DATE: 25 – 10 – 2013 LAST DATE FOR SUBMISSION: 12 – 11 – 2013

NOTE: Answer the following questions in a long note book only.Submission in sheets will not be accepted

1. The open-circuit characteristic of a 4 pole, 250 V dc shunt generator has a lap connected

armature with 610 conductors and operates at 750 rpm. Determine (a) critical field circuitresistance (b) critical speed for field circuit resistance of 80 (c) residual flux per pole.

If 0 0.5 1.0 2.0 3.0 4.0 5.0

V 10 50 100 175 220 245 262

2. A 4 pole, DC generator has a lap connected armature consisting of 32 slots with 6

conductors per slot. Determine the useful flux per pole required to generate an emf of 500volts when the armature is driven at 1200 rpm.

3. A 10 kW, 230 V, 8 pole DC series generator has armature and series field resistances of0.08 Ω and 0.05 Ω respectively. The flux per pole is 15 mWb. The armature has 250

conductors wave wound conductors. Determine the speed at which the machine is to bedriven. Assume a voltage drop of 1.5 volts per brush and neglect the effect of armature

reaction.

4. A 6 pole, DC short shunt compound generator supplies a load current of 100 A at aterminal voltage of 400 V. Its armature, shunt and series field resistances are 0.5 Ω, 250

Ω and 0.3 Ω respectively. Its armature consists of 48 slots with 8 conductors per slotwave connected. At this load brushes are to be shifted by 20 mech. to account for

armature reaction effect. Calculate the demagnetizing and cross magnetising armaturemmf per pole.

5. With a neat sketch, explain the process of commutation in DC generators.

6. Draw connection diagrams of different types of dc motors and explain the speed-torque

characteristic of each type of motor.

7. With neat connection diagrams, explain the armature and field flux control methods ofDC shunt motor.

The torque of a dc shunt motor varies as the cube of the speed. For a certain speed, thearmature current is 50 A. Calculate the additional resistance to be connected in series

with the armature circuit so as to reduce the speed by 50 % of the original speed.

Consider the armature resistance to be 0.4 and flux to remain constant.

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8. With a neat connection diagram, explain Swinburne’s Test for DC shunt motor.A 500 V dc shunt motor takes 8 A on no-load. The armature resistance is 0.2 and shunt

field resistance is 250 , when measured at room temperature. Neglecting the increase in

resistance due to temperature rise and assuming stray load losses = 1.2 % of power

output, determine the motor efficiency when drawing a line current of 80 A at 500 V.

9. With a neat connection diagram, explain Hopkinson’s Test.The Hopkinson’s test was used to test two shunt motors. The supply current was 15 A at

200 V excluding field currents. The generator output current was 85 A. The field currents

for motor and generator were 2.5 A and 3 A respectively. The armature resistance of eachmachine is 0.05 Ω. Determine the efficiency of each machine.

10. With a neat connection diagram, explain Retardation Test. What are the methods toeliminate/determine the moment of inertia of rotor?

A retardation test is made on a separately excited dc machine operating as a motor. Theinduced voltage falls from 220 V to 190 V in 30 seconds on opening the armature circuit

and 5 seconds on suddenly changing the armature connection from supply to a loadresistance taking an average current of 12 A. Determine the efficiency of the machine

when operating as a motor and drawing a current of 22 A from a supply of 250 V. Theresistance of armature is 0.5 and that of the field winding is 250

Prof. MOHAN KUMAR SCourse Coordinator