speed control for cap. motor

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Old 03-27-04, 07:23 PM
fabman
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speed control for cap. motor

i have a 1/4 hp AC capacitor motor that i want to control the the speed. i know there are speed controllers for DC motors, but are there ones for your standard AC electric motors?
 
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Old 03-27-04, 07:50 PM
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I know they make them for a/c motors, but not sure about cap started ones.
 
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Old 03-27-04, 08:47 PM
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I see the "AC motor variable speed" question on here from time to time. The short answer is NO you can't, in general, control the speed of an AC motor. This answer is, however, a little misleading. The proper answer is you can't CHEAPLY control the speed of an AC motor. AC motors work by the rotating part, that part connected to the output shaft, following the rotating magnetic field set up by the stationary part of the AC motor. Now, the rotating part will only run at the frequency of the input AC voltage. In the US it's 60 cycles per second(Hz), in europe it's 50 cycles per second(Hz). There are some AC motors that will run at two or three switch selectable descrete (or defined) fixed speeds. Motors like this can usually be found in fan units that can be run a low, medium, an high speeds. I've seen such motors for sale in the Graingers catalog and we used to use many of them here at work in air conditioning fan coil units. The other way to get an AC motor to run at a variable speed it to hook it up to a variable speed drive. An AC drive first converts the 60 cycle AC input voltage to DC then reconverts the DC to a VARIABLE frequency AC signal that finally runs the motor. Drives like that are usually very expensive devices and their use isn't usually practical for an 1/4 hp AC motor because the cost would be many times the cost of the motor itself. If you need to run an AC motor at a variable speed it's best to just figure on using a DC motor with a cheap variable voltage DC supply.
 
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Old 03-27-04, 08:58 PM
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Guess it just depends, I've been running a small A/C motor off a standard dimmer switch for years.

You can buy a device they sell as a 'speed control' for a router for $20 from most of the tool houses. It just has a standard plug on it, so you could conceivably plug whatever you wanted into it as long as it was within the power rating. I think most routers are in the 1-2 hp range, and they are just single speed a/c motors..
 
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Old 03-28-04, 03:16 AM
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They do make what is called a universal motor. It's basically a DC motor run on AC. You can vary the speed of such a motor but as you vary the speed you also vary the torque. If you need full torque at lower speeds you won't get it like you do with some DC motors. Torque drops to near zero as speed is reduced. The universal motor has bushes and a commutator just like a DC motor but the motor core is built differently to reduce the losses from the AC power source. The motor in question here is a capacitor start true AC motor and you will have a difficult time trying to get it to run at anything other that it's design speed.
 

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Old 03-28-04, 04:41 AM
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Some 'split-capacitor' type motors used in applications like fans and blowers can also be cheaply speed controlled. Lutron makes items which look like light dimmers, but are intended as fan speed controls.

It really, really depends on the type of motor and what sort of mechanical load is on the motor. If you want to run some sort of machine load on a variable speed AC motor, you pretty much need to control the AC supply frequency...you can do this for as little as a couple 100 dollars. But with something like a 'fan' load and a motor designed for high slip operation, adjusting the voltage will do a reasonable job.

Care to share the details of your system?

-Jon
 
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Old 03-29-04, 04:47 AM
fabman
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thanks everyone for the information.

what i will be driving will need the torque for sure. so ill just shop around for a DC motor for the application.

thanks again,


mike
 
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Old 03-29-04, 09:39 AM
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Depending on exactly what you need, look at an old electric treadmill. I just took the DC motor out of one. 90v, 8 amps, plenty of tourque and speed controllable. Not sure yet what I'll do with it, maybe make some kind of animated display for christmas or something.
 
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