4. What will be the effect on direction
of rotation of a D.C. motor if the
supply terminals are reversed ?
Answers
Answer:
The direction of eletromagnetic torque developed by armature depends upon the direction of flux or magnetic field and the direction of flow of current in armature conductors. If either of the two is reversed the direction of torque developed and direction of rotation will be reversed. When both (the direction of field as well as that of armature current) are reversed the direction of rotation will not change. So in case of DC shunt motor, if the supply terminals are interchanged, there is no change in direction of rotation.
Answer:
If the motor has permanent magnets in it’s field it will immediately reverse. If it is a series wound motor with an electromagnetic field it will continue to run in the rotational direction it was running in. A long time ago I worked with a DC system that had an electrically excited field rather than permanent magnets. This was a large motor, maybe 3 to 5 horsepower. Only the armature voltage was reversed, the ffield voltage was kept at the same polarity. This was not a series wired motor and it had a specialized controller that ran it.
Continual reversal of the motor in this system about every minute or so did what was required of it but it wore the commutator very badly and very quickly. Mechanically, at the point of reversal on the commutator produced deep pits in the copper of the commutator and the motors (we had more than one system like this) The motors needed replacement about every six months or so, and they had to sent to their manufacturer for rebuilding. They were a specialized motor and could not be rebuilt locally.