DC motor and generator

Discussion in 'Homework Help' started by PG1995, May 18, 2012.

1. PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Hi

Please have a look on the attachment. The working of electric motor is quite similar to a rectangular coil in a magnetic field. We have permanent magnet poles on the left and right of the coil; there are two poles with North pole on the left and the South on the right. When the coil is parallel to the field, then the force on the coil is maximum and the force is effectively zero when it's perpendicular to the field. If the DC current is kept constant, then the coil will keep on rotating on some constant speed, V. Now I come to the question. It is said that increasing the number of poles reduces the speed at which the motor rotates (but increasing the number of coils will increase its overall speed). Suppose we have more two magnet poles at the top and bottom of the page; with North pole on the top. Now we have four magnet poles in total. I can't understand how this configuration will result in reduction of the previous speed, V, when there were only two magnet poles. Could you please help me with it? Thank you.

Regards
PG

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2. steveb Senior Member

Jul 3, 2008
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469
In the DC current case, what happens to the forces and rotating torque when the coil has rotated 180 degrees?

3. PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Hi

When the coil is vertical to the field, it makes 0 degree angle and the brushes are then lined up with the commutator gaps thus no current flows. Then, when the coil has made 90 degrees rotation counterclockwise, then the force is maximum hence maximum torque. But when the coil moves over 90 degrees, the forces start decreasing, and are almost zero when the angle is 180 degrees and the coil is once again vertical to the field. I hope I have it correct. Thank you.

Regards
PG

4. PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Hi

I think I have made some progress and could visualize how using four poles would reduce the speed of the coil. Will share it in some time. If you are in process of writing a reply, then please do share it. Thanks a lot.

Regards
PG

5. PG1995 Thread Starter Active Member

Apr 15, 2011
753
5
Hi

As you already know the answer, so I can take some time to share it with you!

The speed of DC generator would get affected if we had use four magnet poles instead of two.

1: Would the speed of a DC motor be affected by number of the poles even when the rotor is a permanent magnet and the poles are electromagnets? I believe it would be affected. So, the only way to increase the speed is to use large current or use more coils?

2: But does this also hold true for AC motor and generator?

Thank you for the help.

Regards
PG

6. amilton542 Active Member

Nov 13, 2010
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