Quote:
Originally Posted by bomelia
Well, Wes, that was very informative. I am following a thread at Sawmill Creek (woodworking forum) that is discussing this very issue. Nobody can agree on anything. It sounds like you are saying that "typical" shop induction motors (table saw, grinders, etc) can be controlled electronically (speed wise). Now, from what I have been able to discern is that it is true for three phase motors, but not single phase. Also, the single phase motors in our shops have those capacitors for starting. If the motor slows, then there is an electro-mechanical system that will engage the starting capacitors (vary the phase so the the motor speeds back up).
So what would you say to this? And do you know of a device that can be bought reasonably) to control induction motor speed?
Also, I have read that the induction motor cooling design is optimal at the design speed, thus, external cooling should be supplied when slowing them down (mostly talking about 3 phase motors).
Mike
|
Mike,
I think
"that 'typical' shop induction motors (table saw, grinders, etc) can (Wikipedia) be controlled electronically (speed wise)." But it wouldn't be a practical conversion, especially for single phase. Even the 60 Hz U.S. motors run better and cooler at the 60 they are designed for rather than the close 50 HZ European.
I think it would usually be better to buy a variable speed induction control/motor package deal and add it to the equipment. The big advantage is no brushes and convenient mounting compared to some AC-DC transplant. The cooling is also taken into account, as you say. However,
here is an example of 3-phase add-on controllers.
The
control purespeed mentions is designed for AC-DC brush-type motors only. You can achieve the same AC-DC thing by adapting a heavy duty incandescent lamp dimmer to vary voltage. If his works on an induction powered fan for him, it is probably running pretty inefficient and hot by "slipping" (nearly stalling) a lot. This would be the same principle that causes standard Air Conditioner induction motors to burn out during a low voltage brown-out. When single speed induction motors can't run at very near their rated rpm (and track the irrevocable 60 Hz rotating field), they draw (and waste) tremendous amperage trying to stay in phase. That is why they must never be undersized and are quite large and heavy for their HP compared to, say a circular saw AC-DC motor.
Wes
...