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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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