# Alternator Field Reversed

1. Jan 6, 2017

### jaus tail

1. The problem statement, all variables and given/known data
A three phase alternator has a phase sequence of RYB for its three output voltages. In case the field current is reversed, the phase sequence will become

(A) RBY
(B) RYB
(C) YRB
(D) none of the above.

2. Relevant equations

3. The attempt at a solution
I think the answer should be A. As in the sequence of generator voltage must change, as we're reversing field. But books says the sequence won't change.

Even for motor it says that if we reverse field direction then motor rotation won't reverse. Can someone help as to why won't the direction or sequence change?

2. Jan 6, 2017

### Staff: Mentor

First, can you explain what purpose does the field current serve in an AC generator?

3. Jan 6, 2017

### jaus tail

Field current produces the flux. As the poles rotate the flux also rotates and sweeps past the conductors on the armature, and this rotating magnetic field induces emf.

I get it for alternator that the sequence mustn't change. The diagram below helped. That only for an instant the polarity of R, Y, B will reverse but the sequence won't change.

But for motor the torque I'm not sure, as the locking between field and armature flux is broken and field is reversed.

4. Jan 6, 2017

### Staff: Mentor

I don't believe the question is asking you to describe the wild excursions before equilibrium is eventually restored when a fast spinning generator is subjected to a sudden field reversal. I think the question simply involves the resulting sequence when a generator's field winding has been connected to the wrong polarity DC before start-up.

5. Jan 6, 2017

### jaus tail

Hmm... the before start up part confused me. As like it wasn't mentioned before starting. Thanks. Drawing the diagram above helped.

6. Jan 6, 2017

### jaus tail

Hi,
I'm sorry but I'm confused about winding. It's wrong i guess. Shouldn't it be like this?

like above winding R Y B location and this new winding R Y B location. I understood in case of alternator the sequence won't change. But I'm not able to conclude what will happen in case of motor. Cause motor isn't self starting. So if I just change field nothing will happen. But what happens if I change field while motor is running? Maybe motor will experience slight jerk like for 45 degrees in above picture and then eventually go rotate in same direction.

7. Jan 7, 2017

### Staff: Mentor

What you are describing is a synchronous motor briefly falling out of sync. If this happens under load, then I could only guess at the outcome: it may just momentarily hiccup, or it may be brought chaotically to a halt.