Variable Speed Control for Induction Motor

[Mike OMelia]

I found this:

http://www.reliance.com/prodserv/motgen/b7093.htm

But there has to be something up... I have found no controllers outer there (single phase)

By the way, my bandsaw and table saw both have the centrifugal switch, and it only kicks out at slow speed, well after the time I have switched it off. I never knew what that was till now... thanks.

So how do we go about testing this? Anybody have an unused induction motor sitting around?

Mike

[Dan Friedrichs]

I think with 90VDC motors (they are common) you need some kind of PWM (pulse width modulation) to control them. Incidentally, the value 90 has something to do with how a rectifier converts AC to DC, but I cannot remember.

Mike

Depending on the type, you might. A brushless motor's speed is based on how quickly it is electronicly commutated. A brushed motor's speed is voltage/current dependant, though.

I'm not sure about 90. The DC value of a rectified AC should be 2*sqrt(2)*Vrms/pi. That's about 98 for 110Vrms_AC.

[David Christopher]

Mike, here are some pictures of the motor.....it will be perfect for buffing..I have a arbor for a buffing wheel also.......like I said I was going to use it for my old lathe but I bought a new lathe and I allready have a buffer..... if you are intrested PM me and I will give you details

[David Christopher]

sorry, I forgot the pics

[Mike OMelia]

Wow, thats a big motor. Could not read data on name plate.

That is probably bigger than I need (I think)

Mike

[David Christopher]

Mike, I think its 3/4 HP... looks pretty big in the pic....its about 3 1/2 inches by 12 inches

[Rick Christopherson]

I'm not sure where you're coming from here. I certainly understand how the capacitor causes a phase shift and why it's necessary to get the motor started. But let's look further. What size capacitor do you use? If you use one that's too small, the current in the starting coil will be small, which will result in a low flux at the phase shift angle, giving a low starting torque.
If you use too large a capacitor, the current flow in the starting coil will exceed the capacity of the starting coil and you'll burn it out.

No. You are still thinking like an RF engineer. The size of the capacitor is not the primary dictater in the current, but it is dictating how far out of phase the current is. If you don't shift the current in the start circuit, you don't get a start. If you don't shift it enough, you don't get a start. It doesn't matter if you gave the starting circuit full line power--if the phase shift is not present, the motor will not start and you will smoke the start winding.

If all you needed to do was limit the amperage in the start circuit, you could do that resistively. The function is phase shift, and I can't seem to stress that quite enough.

Well, perhaps you need to tell me how you think motors operate. I've discussed my understanding of the theory of operation in pretty good detail - time for you to do the same.

I know what you are saying here, however, I could convince a layman with a few sentences, but to re-educate an engineer would take volumes. I don't have time to do that. That is what text books are for. I don't mean that in a disrespectful way, but we both know how deep the discussion would have to delve in order to accomplish that task.

[Mike Henderson]

No. You are still thinking like an RF engineer. The size of the capacitor is not the primary dictater in the current, but it is dictating how far out of phase the current is. If you don't shift the current in the start circuit, you don't get a start. If you don't shift it enough, you don't get a start. It doesn't matter if you gave the starting circuit full line power--if the phase shift is not present, the motor will not start and you will smoke the start winding.

If all you needed to do was limit the amperage in the start circuit, you could do that resistively. The function is phase shift, and I can't seem to stress that quite enough.
I know what you are saying here, however, I could convince a layman with a few sentences, but to re-educate an engineer would take volumes. I don't have time to do that. That is what text books are for. I don't mean that in a disrespectful way, but we both know how deep the discussion would have to delve in order to accomplish that task.

I can assure you, Rick, that I understand the need to have a phase shift in the starter circuit.

And I have motor theory textbooks (modern editions, not my old college ones) which I've reviewed recently because of the discussions on this forum (do you really think I could have remembered all this stuff from 40+ years ago?). I am quite willing to stand by the accuracy of what I've posted. If you, or someone else, can post a good valid argument against what I've posted, I'll change my mind. But the argument has to be stated in terms of the theory of operation of an induction motor and not just old wives tales.

Mike

[Roy Bennett]

No one has brought up the effect of varible frequency on the power produced by the motor. As the speed changes, the power available is reduced (proportional to the speed OR to the square of the speed?).

This might be OK in a fan application, since the power required varies as the square of the speed.

[Mike Henderson]

No one has brought up the effect of varible frequency on the power produced by the motor. As the speed changes, the power available is reduced (proportional to the speed OR to the square of the speed?).

This might be OK in a fan application, since the power required varies as the square of the speed.

Very good point, Roy. Thanks for pointing that out.

Most VFDs maintain constant torque below the rated Hz but since HP is torque times RPM, the HP produced falls as the speed goes down. So for a 60Hz motor, the torque will be constant at all frequencies 60Hz and below. If he uses a 2-pole motor, at all speeds below 3450RPM, if a four pole motor, all speeds below 1725RPM.

For this application, buffing, I don't think that will be a problem. As long as he has good torque on the buffing wheel, I think he'll be able to use it well.

And to answer your question, HP is directly related to RPM, not to the square of the RPM. See here for the equations, about half way down, under the heading of "Electrical Horsepower".

Mike

[Chip Lindley]

The JET JSB-10H is offered with a 1725 RPM motor. I would think this is plenty slow for woodworking sanding/buffing! On this Jet, the motor shafts ARE ALSO the arbors. You are limited to this ONE speed.

IF slower RPMs are demanded, this could be achieved at much less cost than trying to re-engineer a brand new machine. There are 1140rpm motors available on eBay or CraigsList for pennies compared to the new Jet machine's price. Use the arbors of these motors to mount your sanding/ buffing wheels to achieve the results you desire.

IF several speeds are needed, you could make your own double arbor machine by mounting the shaft between pillow block bearings, with a step-pulley (as from an old drill press) inbetween. Mount the opposing step pulley on the motor shaft. Change the belt to vary speed.

[Loren Hedahl]

Another possibility would be to take an old circular saw (a worm drive or hypoid drive Makita would be best), remove or cut away the guarding, make a mounting setup and vary the speed with one of those inexpensive 'router speed control boxes' like you get from Harbor Freight.

Unless you were using it a huge number of hours, I'd expect it would hold up just fine. It would tend to insult the ears without earplugs, but some price must be paid for cheapness and cobbling something together.

I bought a nice low speed bench grinder from Woodcraft ten-fifteen years ago. Cost a bunch then, but I've long ago forgotten the price. It usually has a stone on one end and a wire brush on the other. In case I want to use it for something else, there are a couple of wrenches in a drawer in the bench below it as well as a variety of grinding and polishing wheels. It is a machine of many purposes -- one of my better investments in tools.