AC motor speed controller

[Gene Howe]

Is there such a thing as a speed controller for a 1/2 hp 7amp ac capacitor start motor?
Thanks everyone.

[Steve Baumgartner]

I think this would require a variable frequency drive, since an induction motor's speed is determined by the number of poles and line frequency.

[Dan Friedrichs]

As Steve said, you could use a VFD, but those start around $100.

[Rod Sheridan]

You also can't use them on single phase motors because of the centrifugal starting switch.........Rod,

[Mike Henderson]

You also can't use them on single phase motors because of the centrifugal starting switch.........Rod,
Well, you can use them on single phase motors, with certain operational constraints. You need to start the motor with a setting of standard line frequency (60Hz in the US), or pretty close to standard line frequency, and it would be best if you kept the speed above the speed where the centrifugal switch will close (which is pretty slow on most motors). You can usually hear the switch click in so you can note the speed where it closes and just stay above that speed. It won't hurt the motor to have the starting coil energized (that is, the motor will operate quite well with the starting coil energized) but some starting coils are not wound with wire which will tolerate continuous operation. So to be safe, don't let the motor go too slow.

Mike

[Rod Sheridan]

Hi Mike, yes I agree with all your points, I was just too lazy to write them all down.

That said I have a Steel City variable speed grinder and it seems to be a split phase motor with a simple phase angle fired speed controller, simple and innexpensive although it doesn't have a wide speed range...........Rod.

[Gene Howe]

Thanks all for your help.

[george wilson]

I have an old Oliver lathe,1 phase. I'd like to put a VFD on it. Maybe I can use your plan,Mike,to do it.

[Mike Henderson]

Gene sent me a note via PM which I'm going to try to answer here so that others can offer their knowledge.

+++++++++++++++++++++++++++++++++++++
I'm guessing that a VFD has settings that will allow me to select 60HZ? This is a 1725 RPM motor that will drive an 8' sanding belt. I'd probably be OK @ 8 or 900 rpm. Do you think this is achievable? I have another motor that needs brushes. When I take to the motor guy, could he rework it to spin slower? Thanks much for your help!! Gene
+++++++++++++++++++++++++++++++++++++++++++++

The VFD probably has an indicator that tells you the RPM rather than the frequency. In that case, set the RPM to the rated RPM of the motor. So your 1725 RPM motor will run that speed at 60Hz. I don't know if the centrifugal switch will cut in at 800 RPM, only a test will tell. If you have something which will measure RPM, connect it to your motor, power it up and turn the switch off and let it coast down. Note the RPM when you hear the switch cut in. That will tell you the lowest speed you can run.

If this motor is just a standard induction motor with a standard mounting, you can purchase an equivalent 3 phase motor (same HP and same mounting) and use that with the VFD. That will allow you to go essentially as slow as you want to, except for cooling (which is another discussion). You can find 3 phase motors on eBay for good prices because most people can't use them.

Your other motor - the one with brushes - is probably a universal motor and you can buy one of those "router speed controllers" for that motor. Universal motors are different from induction motors and the speed control is different.

Mike

[Dick Strauss]

Interesting discussion...

How do make the connections from VFD (with three hot wires) to a 110v or 220v motor with one or two hot wires?

[Mike Henderson]

Interesting discussion...

How do make the connections from VFD (with three hot wires) to a 110v or 220v motor with one or two hot wires?
I don't know enough about VFDs which is why I wanted to continue the discussion.

From a theoretical point of view, on a 3 phase VFD operating at 240V, there's 240V between each set of two wires, so you could just use any two wires from the output of the VFD to run a single phase motor. Someone who knows VFDs better can comment it there's any reason why that won't work.

My comments are based on my knowledge of electrical power and not on my knowledge of VFDs (which is sadly lacking). (Well, I know how VFDs work from a theoretical point of view but not how they're implemented in the real world.)

Mike

[Mike Henderson]

Let me add one more thing about using a VFD on any induction motor - single phase or three phase. The problem is cooling when running at low speed under load. The motor will develop the same torque at the lower speed so it will use the same power and generate the same heat. But because of the lower RPM, the cooling fan on the motor will be moving a lot less air. So if you're going to use a standard induction motor (not one designed to be run on a VFD) you'll have to make separate provisions for cooling. Usually, putting a decent size fan aimed at the motor is sufficient.

Mike

[Also note that HP is essentially RPM times torque (times a factor) so the HP decreases as the speed decreases, even though the torque remains constant.]

[Dick Strauss]

Mike,
I'm not an expert (and I definitely didn't stay a Holday Inn last night)!!!

I was under the impression that a std 220V motor relies on both 110V legs being 180* out of phase along with cap(s) to generate some slight change in phase angle to get/keep the motor turning directionally.

I thought that a 220V 3ph has 3 legs 120* out of phase from each other. Since the legs are 120* apart (electrically speaking), the motor doesn't need a cap to generate a directional pulse because it is supplied by two corresponding legs that favors rotation in one direction.

With a VFD, I assume it generates 3 legs 120* apart meant to feed 3-ph motors. If you are only supplying two legs 120* apart to a std 220V motor, wouldn't the motor lack normal power since the amplitude of the sine wave is much lower than normal at the 180* phase timing the motor normally receives?

I agree....even with a 3ph motor when run by a VFD, cooling can definitely be an issue (especially with TEFC motors) because the fan is also spinning slower thus moving less air to keep the motor cool. You want a motor designed for a VFD if possible to avoid future issues.

Hopefully we'll see a few responses from folks with lots more knowledge than I have to help answer the questions posed here.

[Dick Strauss]

I have it from a few knowledgable people that Mike is correct...you can get a full 240V by using two legs off of a three phase system. I don't know how a VFD will react to having only two legs connected (I think it will show an error).