what happens if the resistance in series with the shunt field of a DC motor is increased in value

1. Sparking at the trailing edges of the brushes in a DC machine is usually prevented by fitting

 

(a) copper gauze brushes (b) a compensating winding

(c) a compound winding (d) commutating (or inter-) poles

2.A series-excited field on a DC machine is connected

(a) so that it carries the full machine current

(b) directly across the armature terminals

(c) in series with the shunt field

(d) in parallel with the armature

3.A DC machine with compound field windings is one that

(a) has no speed or EMF variation due to field adjustment

(b) has both shunt and series fields

(c) has field windings, made of special material

(d) is never used.

4. If the resistance in series with the shunt field of a DC motor is increased in value, the motor will

(a) slow down (b) stop

(c) overheat (d) speed up

5. A starter is necessary for a DC motor because it

(a) will not rotate without one

(b) prevents the operator from receiving electric shocks (c) will draw excessive current if one were not used

(d) adds to the cost of the machine

6.A new DC generator which will not self-excite can be induced to do so by

(a) ‘flashing’ the field (b) increasing the speed

(c) using a permanent magnet (d) hitting it

find the cost of your paper

Sketch the load characteristics, terminal voltage/load current, of the following types of DC generator

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Calculate the synchronous speed of a 10-pole three-phase machine connected to a 50 Hz supply.

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