Variable Speed Control of an Induction Motor
 

Single phase, 120 volt. Many shop tools use these motors (grinders, buffers, saws, etc.)

Is it possible to control their speed?

Mike

Mike,

I would say yes since I have two variable speed 120v electric drills from Sears and they have lasted a long time with no problems. One even has a torque setting on it that can be changed to apply more or less torque at any speed.

Ron :)

some motors don't like to be slowed down they burn up quickly, but i think Mcmaster-carr sells motor controls to try it, what do you want to control?

Buffing arbor. But I have found a solution.

Mike

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Good you found a solution, Mike. I apologise if you are already familiar with the following. But...

FYI, there are induction motors that are variable speed now. I'd guess that it really got going about 15 years ago. Electric drill motors are still mostly AC-DC brush-type motors, not induction motors.

As a refresher, the way all electric motors work is by creating a temporary magnet across the armature (drive shaft) and placing it in the field of another set of temporary magnets that is set just right to pull it with one pole ahead of it's rotation while pushing it with another opposite right behind it. The magnets must always, in any motor, be chasing each other around, but never catching each other. If something went wrong, and the magnets ever caught each other (it sometimes does), the motor would lock up and hum.

AC-DC motors cause the magnets to always be "chasing" each other by constantly switching the direction of current (flow of electrons) just in time. They do it by constantly interrupting the current with the brushes to different contacts (commutator), essentially a rotary switch. Brush motors work with alternating wall current (AC) or battery direct current (DC). They vary the speed by dropping the voltage pressure and thereby weakening the magnetic fields. Ever have one turning just the right speed and having it slow way down when you put a load on it? Ha. Simply increase the voltage to reflect the new load and it's OK. Voltage increases the torque.

When the AC current alternates, it does so in all the magnets at once, so the poles always retain the same opposite/matching relationship. AC is like water in a garden hose going back and forth just a few inches but never flowing completely out one end or the other by flowing one direction as in DC. The power can be harnessed similar to a foot going back and forth on a bicycle pedal but yet turning the crank.

AC Induction motors are different. They only run on AC. Most common induction motors are wound with the outer field spacing to be placed around the field to match the cycles of AC they will experience and run at a certain rpm. Essentially, the magnetic field "appears" to rotate with the pulses of AC change. In Europe, this might be 50 Hz (cycles-per-second) but in the US it is 60 Hz. Usually the same motors will run on either, but they will run slower in Europe. The term induction comes from there being no electrical connection to the inner armature. At a certain predetermined rpm, the outer field times out just right to induce electrons to flow in the armature, which does have insulated copper wire strategically placed in it to capture such. So the armature is now magnetic also. Most US induction motors run at either 3450 or 1725 rpm because of this and exactly 60 cycles fed to them.

Now remember that the same motor in 50 HZ Europe runs slower. So the trick is to vary the cycles per second on one of these motors, and voila! Slow down the "chase". Variable speed. No brushes to wear out like they do in electric drills (and old locomotive traction motors).

This variable speed induction phenomena only really became practical as advances in power transistors came about. I know you are a musician. Remember the old transistorized audio amps and how they easily burnt out? Well, they don't so much anymore. The power transistors used in audio amps just happen to be the same type that work well at 60 Hz. Well, why not? 60 Hz is an audio frequency, is it not?

So here is what happens. An electronic assembly called an inverter takes 60 Hz AC and converts it to DC. Then it takes the DC and converts it back to AC ...but at a custom variable frequency that is carefully real-time computed to work. This allows an induction motor to run at any desired speed. And torque can be increased by increasing voltage which increases amperage. Magnetic torque is directly related to amperage.

This has been a boon to railroading. All locomotives used to use brush type AC-DC motors along with the maintainence heavy job of changing out the brushes quite often. Each locomotive axle has it's own motor (most have six) and it was common to "cut-out" at least one deadbeat on a 200 mile trip. Consider that we can look at our meter load guage and see that each motor (of six) is being fed up to 1500 amps at several hundred volts. No wonder they fried.

But back in the early '90's, a company called Siemans sent some German Electrical Engineers over here to test some new AC induction locomotives. I mean literally, to North Dakota and Montana. They work great. Huge inverters. Think of the audio amp power possibility going to waste here.

And now we are seeing the technology applied to cars. This is exactly what the Toyota hybrid Prius does. No brushes. The most reliable car in the world, bar none, in 2007 (Consumer Reports). Only now have purely electrical driven cars, and trucks, become more practical.

Some of us reading this have seen the History Channel story regarding the fight between Edison with his proposed public DC power grid and that of Westinghouse who wanted to use AC. Westinghouse wanted to use AC because two windings close to one another act as a transformer to boost voltage pressure. And why this? Because high voltage can be forced along a smaller wire just like high pressure water can be forced through a smaller hose. AC made the grid transfer more practical. Edison lost, in spite of even electrocuting an elephant to show how dangerous AC was. Must have been quite the scorching public sight, with the elephant emptying his ...well. Siemens wasn't done.

Now, Siemens has come up with even more powerful inverters. So powerful that they can be used to invert huge wattages of AC into high voltage DC. And why, you ask? Because high wattage AC loses power (and DC doesn't) because of it's rising and collapsing field around the wire. This can be seen by taking a fluorescent bulb up on a high volt tower and seeing it light up with no connection. Edison would be proud.

The now you know the rest of the story.

Wes

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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

Hey this is cheap
 

Because my tool knowledge doesn't go beyond Harbor Freight, I just buy stuff from there and try it out. I bought a router speed control and plugged it into a 3 speed floor fan and it works great. Of course, it may blow up or turn into a flame thrower at any minute....you pays your money and takes your chances.

http://www.harborfreight.com/cpi/cta...emnumber=43060

Harbor Freight has a router speed control for $19.99. I use it on a three speed floor fan. Works great.

You cannot use that on induction motors. Only "universal" motors which are AC brushed motors (like your AC drill, etc)

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

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It was Nikola Tesla that favored AC. Tesla experimented with wireless power transmission too while he lived in Colorado Springs.

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War of Currents :)

Wes

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It was Edison.