Variable Speed Control for Induction Motor

[Mike OMelia]

OK, full disclosure: I am interested in a Jet Sander/Buffer (JET JSB-10H ) but mostly for buffing. Problem is, 3450 RPMs is way too high for woodworking (buffing). The motor is capable of 115/230 and is a standard induction motor. single phase. Is there a reasonable method for speed control? (not overly expensive)

Or, should I keep looking? Seriously, I want RPMs below 1000, and 600 RPMs would be awesome.

Mike

[Glenn Roberts]

Having a quick look at them they say that they are available in 2 speeds ( eg: 4 pole and 2 pole) either way there is no easy way to alter the speed of a single phase induction motor. If it was 3 phase then a VSD could be used.(assuming correct type of motor). So a 3 phase bench grinder might be an option??

For buffing I have made a similar unit from an old 'weight loss machine motor' They looked similar to the buffing unit (double ended) but used a flat belt ( that vibrated). these units run at around 900 rpm on 50hz (in Australia) and the slower speed is ideal.

[Mike OMelia]

You know, what is really needed is a Reeves Drive (like that on my Powermatic Drill Press)... CVT. I wonder if anyone sells the internals. Maybe buy a scrapped powermatic.

Mike

[Mike Henderson]

Mike sent me a PM last night asking about this. I'm posting my reply here so that others can see what I said and offer suggestions and corrections.

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It is possible to control the speed of a single phase 115V induction motor by controlling the frequency of the power input to the motor. That said, there's some issues.

First, you'll have to find a VFD that will work with single phase output, and will output 115V. Talk to the VFD suppliers and they'll tell you whether their VFD can be used with single phase output, and whether the output can be set to 115V. If your motor is dual voltage you may have better success finding a VFD with 220V single phase output.

Second is startup. A single phase motor uses a starting circuit, which is usually (for the motors we woodworkers use) a separate coil in the stator and a capacitor. If you try to start the motor at too low a frequency, the impedance of the capacitor will be too high and not enough current will flow in the starting circuit to start the motor. You can avoid this problem by always starting the motor at 50 or 60Hz.

Also, after starting, if you run the motor to a low speed (low frequency input), the centrifugal starting switch will close, engaging the starting circuit. I doubt if that will be a problem because the impedance of the capacitor will be very high at low frequency so you won't have excessive current in the stating coil.

That's about it. Many VFDs provide for reversing but the method of reversing a single phase motor is different from a 3-phase motor so you probably won't be able to reverse. Ask the VFD people about that.

Good luck!

Let us know how you do.

Mike

[Rick Christopherson]

It is possible to control the speed of a single phase 115V induction motor by controlling the frequency of the power input to the motor.

I would not recommend using a VFD on a standard single phase motor, and that is also why you would be pretty hard pressed to even find a VFD rated to do this. The capacitor is large enough that it is not going to act as a very good low frequency filter. It is there to provide a phase shift, not filter frequency. If you don't notice that the start circuit has reengaged, you are going to fry the motor.

[Mike Henderson]

I would not recommend using a VFD on a standard single phase motor, and that is also why you would be pretty hard pressed to even find a VFD rated to do this. The capacitor is large enough that it is not going to act as a very good low frequency filter. It is there to provide a phase shift, not filter frequency. If you don't notice that the start circuit has reengaged, you are going to fry the motor.

I'm not sure I understand your comment, Rick. The starting capacitor will be out of the circuit when the motor is up to speed.

When you slow the motor down enough to re-engage the centrifugal switch, the input frequency will be low enough that the impedance of the capacitor will be quite high and it will not pass a lot of current through the starting coil. This will limit the heat in the starting coil.

Also, note that many modern single phase motors leave a capacitor in the starting circuit (called the running capacitor) during normal operation. That capacitor is smaller than the one used for starting to limit the current in the starting coil (at rated Hz). If a low starting torque is acceptable, the motor may be designed without a centrifugal switch and just start with the "running" capacitor. Fan motors are an example. [Added note: the reason the designers use a running cap is that by using the starting coil to produce flux during normal operation, they can save a bit of copper in the main stator coil. If they can start the motor with the running cap, they save the starting cap, the centrifugal switch and a bit of copper in the main stator coil.]

Since he's using the motor as a buffer, which would start without load, and the motor had both a starting and running cap, he could always disconnect the starting cap and just use it with the running cap if the motor got hot because of excessive current in the starting coil. Or, he could replace the starting cap with a smaller value, which would reduce the starting torque but also reduce the current through the starting coil.

I don't see any theoretical reason why you couldn't speed control a single phase motor with a single phase VFD. I have read about this being done but I don't remember where at this time.

Mike

[One additional thing I forgot to address, but Rick's post caused me to remember it, is heat. Standard induction motors are designed for cooling at the rated speed. TEFC motors, especially, will not have enough cooling at lower speeds because the fan will not be moving fast enough to blow sufficient air over the motor. While buffing is not a heavy load, and is intermittent, you could overheat the motor. A small external fan blowing on the motor would probably be enough to keep it cool. This problem is common to all standard induction motors (even three phase ones) operated on a VFD.]

[Thom Sturgill]

How about this (PSI VS lathe motor & controller) and this (mandrel) and build your own?

[Mike OMelia]

Tom, that is a VERY good idea. I did look at that lathe motor kit (googled, found it in the woodturners forum). But for some reason I did not read too deeply. I see it is a 1/2 hp motor which is ideal for buffing delicate wood (guitars). And the mandrel is almost perfect, shaft length may be too short) but I did not read everything yet. Of course, Grizzly sells a specific buffing mandrel which is ideal. About $100. Now all I need is an rpm sensor. I have been reading how folks use bicycle sensors for this, but that seems a bit cheesy to me. And, it is not entirely necessary.

Mike, others, this is an intersting topic, please do not let my discovery here shut it down. I am still interested in the concept.

Mike

[Curt Harms]

We use a B & D electric lawn mower. I've replaced the brushes a few times and poked around with a VOM. It appears to me that the motor runs on 90 Volt D.C. and there's a little device that appears to be a rectifier. Would it be practical to adapt one of these motors to a variable speed role? It seems to have torque similar to a 3-4 h.p. gas engine.

[Mike OMelia]

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

[David Christopher]

Mike, I have a DC motor that allready has speed controller and forward, reverse switch and a pulley I bought to put on my old lathe but bought a new lathe insted....if you want I can send a picture when I get home

[Mike Henderson]

I never looked much at DC motors but the speed of a DC motor is controlled by the voltage (and thus the current) into the rotor. At least as far as I remember.

Mike

[Rick Christopherson]

The starting capacitor will be out of the circuit when the motor is up to speed. When you slow the motor down enough to re-engage the centrifugal switch, the input frequency will be low enough that the impedance of the capacitor will be quite high and it will not pass a lot of current through the starting coil. This will limit the heat in the starting coil.

The dropout rpm of the start circuit is not that far below the full rpm of the motor. You will not have a significant drop in frequency before the start circuit reengages.

Also, note that many modern single phase motors leave a capacitor in the starting circuit (called the running capacitor) during normal operation. That capacitor is smaller than the one used for starting to limit the current in the starting coil (at rated Hz).

The purpose of capacitors on a motor are NOT to limit current via an RLC circuit. They are there to provide a phase shift, which is necessary to get the motor spinning. The start capacitor is the equivalent of a static phase converter on a 3-phase motor. Think about it--if the capacitor was being used as a filter, then the start cap would be smaller than the run cap.

If a low starting torque is acceptable, the motor may be designed without a centrifugal switch and just start with the "running" capacitor. Fan motors are an example. [Added note: the reason the designers use a running cap is that by using the starting coil to produce flux during normal operation, they can save a bit of copper in the main stator coil. If they can start the motor with the running cap, they save the starting cap, the centrifugal switch and a bit of copper in the main stator coil.

Aaahhh, I think you need to bone up on your motor types a little bit. You are confusing different types of motors.

Or, he could replace the starting cap with a smaller value, which would reduce the starting torque but also reduce the current through the starting coil.

No, that would increase the starting current because it would reduce the phase shift and delay the startup time. You are thinking like an RF EE, but that is not how motors operate.

[Chris Padilla]

You are thinking like an RF EE, but that is not how motors operate.

Guilty here!

[Mike Henderson]

The dropout rpm of the start circuit is not that far below the full rpm of the motor. You will not have a significant drop in frequency before the start circuit reengages.

I don't have any data on the re-engagement speed of the centrifugal switch, but I bet it's quite a bit lower speed. Several reasons:
1. Once the rotor starts moving in one direction, you get a positive "feedback" situation where the magnetic vector in that direction is enhanced and the counter rotating vector is reduced. So you don't have to do a lot to get the motor started, just get it moving in one direction. Think about when you test a motor with a failed starting capacitor - you just give it a spin with your fingers and turn the power on. That small spin is enough to get the motor started.
2. When you have a motor that spins down over time, such as a grinder, you can hear the centrifugal switch click in (re-engage) as the motor slows down. All the motors I've listened to had to slow down quite a bit before I heard that click, probably at least half speed.

The purpose of capacitors on a motor are NOT to limit current via an RLC circuit. They are there to provide a phase shift, which is necessary to get the motor spinning. The start capacitor is the equivalent of a static phase converter on a 3-phase motor. Think about it--if the capacitor was being used as a filter, then the start cap would be smaller than the run cap.

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.
So once you know the maximum value of the capacitor, you can reduce that value and you'll reduce the current in the starting coil, and thus the starting torque. As long as the starting torque is greater than the load and the frictional losses, the motor will start.

Aaahhh, I think you need to bone up on your motor types a little bit. You are confusing different types of motors.

I need more detail on how I'm confusing different types of motors before I can reply to your comment here.

No, that would increase the starting current because it would reduce the phase shift and delay the startup time. You are thinking like an RF EE, but that is not how motors operate.

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.
But let me expand on this item. Torque is related to flux density and flux density is related to current. If you reduce the size of the starting cap, you'll reduce the current in the starting coil, and reduce the starting torque. A smaller capacitor will also result in a lower phase angle which will also reduce starting torque. If the motor is not under load, such as a buffing motor, the reduced torque will be more than sufficient to start the motor. It may take 2 seconds to get the motor up to speed, instead of 1.5 seconds, and that could generate a bit more heat. But the thermal inertia of the motor is so great that the extra heat (if it exists) would not matter.