When DC Generator fails to build up the voltage, what are the reasons?
Answer: No residual magnetism, reversal of field connections and the resistance in the load circuit may be greater than critical resistance
What is field flashing?
Answer: In case when the dc machine fails to build up the voltage due to lack of residual magnetism, filed coils are connected to the dc source (battery) for short while for magnetizing the filed poles. Application of external source of direct current to the field of the dc machine is called field flashing
Why do we use starter for dc machine?
Answer: At the time of starting, no back emf is offered by the dc machine, hence huge starting currents flow in to the machine which damage the armature, in order to avoid the damage starter is employed. By using starter armature current is limited during starting the machine by providing external resistance and while starting this external resistance is removed in steps
Factor that has to be considered while choosing the resistor?
Answer: While choosing the resistor for connecting in filed or armature circuit care should be taken that the resistor current rating is more than the full load current rating of the circuit. For example, if rated field current is about 2Amps. Then resistor connecting in the field circuit should be chosen in such a manner that the current rating of the resistor is greater than 2Amps.
What are the different losses in dc machine?
Answer:
- Armature and field copper loss
- Iron and magnetic losses (hysteresis and eddy current loss)
- Mechanical losses
- Stray-load losses
Drawbacks of Brake test?
Answer: Some of the drawbacks of the brake test are:
- This test is performed with small motors only
- Internal losses cannot be determined
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