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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I just bought a 1800rpm/180v permanent magnet motor. I was going to use it in reverse as a generator. I decided to play with it and slowly powered it with a DC source. As I increased the voltage it ran faster.
I am wondering, though, how it knows to spin the correct way when it starts. Are there two sets of winds for a DC motor?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Unfortunately, carbonRod's link describes a permanent magnet AC machine, which can be run as intended as an AC generator, or if controlled by semiconductor switching, a stepper/synchrnous motor.
A DC motor will have brushes and commutator such that the rotor is always energised in the same direction with repsect to the permananent magnetic field, so will always turn in the same direction. There should be any number of Physics 101 diagrams out there to explain the principle.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yeah, it's basically down to the direction of the current (determined by the arrangement of brushes, windings and commutator), the arrangement of the magnets, and Fleming's right hand rule.
DC motors have from three to hundreds of armature windings, that are turned on and off by the commutator as needed. AFAIK, they are actually all connected in series into one big winding, and the commutator segments are taps on it. The smallest motors with three windings and three segments are wired the same as a three-phase delta.
If you like, you can think of it as a synchronous permanent magnet motor turned inside out and powered by a mechanical inverter. :P
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
Most mid-sized DC generators (overpriced) I have seen still require output conditioning as there is a short line drop out during the cycle.
A generator with solid state rectifiers (still retain a good range of operation, please read the pdf) are generally going to last and cost less per watt in the long run.
What about Neil Fleming's Left/Right hand rule... Knowing the Ampere/current balance and Watt balance is far more useful for students.
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permanent magnet motors
