variable speed ac motors

It was not you I was addressing overall, that statement seemed to be misleading so I used it. The responses after learning that it was single phase surprised me. As you mentioned most people have 120vac fans with speed control in their homes. IF you have been in industry/commercial work for a few years then you have worked with a variac at some point.

The responses talked about drives and there not being single phase drives. He has a DC motor with controller board and pot. An AC motor can use a simple speed control device or a variac depending on the application.

Simple is OK, i am simple so like it that way. Then again simple as was stated did not offer all the information and the information offered was not totally correct.

I was kind of just asking in general, but the aplications I have are centrifical pumps and a packaging turn table. The turn table someone put a pot on which just makes the motor pulse.

What about something like this? http://www.grainger.com/Grainger/searchresults.jsp?from=categoryListing&action=Go!&search_type=Category&QueryString=AC+Speed+Controls&segment=Motors+%26+Power+Transmission&family=Supplies&subcat=AC+Speed+Controls&xi=xi

In general with any motor application torque will be the key, the motor must be able to deliver the necessary torque when starting. In many cases it easier to use DC motors and controllers because the necessary curves are provided....sometimes they are the cheapest solution too. Then again VFD's and inverter rated motors can be obtained in the 1HP range for less than $500 total so its open which one to use.

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As Ron said, the key here is torque. Your turn table application with a pot apparently requries more torque than can be delivered with a simple voltage speed control method. When you try to control a singlephase AC motor with a voltage only method output torque is going to fall off exponentially - with ideal torque being proportional to the square of the voltage (1/2 the voltage is 1/4 the torque) but in actuality, it is never "ideal". Motor efficiency degrades as well, compounding the torque fall off.

Thus, the applicaiton and the motor must be carefully matched. The motor/fan blade pairing on variable speed over head ceiling fans for example, have been engineered to provide an acceptable trade off on fan load -vs- motor torque to provide a simple and inexpensive consumer solution.

The centrifugal pump will most likely have problems as well. A centrifugal pump requires a surprising amount of torque. Most pump/motor pairings have the motor fully loaded for maximum efficiency. Intuitively it may not seem that way because at first glance the motor is just turning a impeller in a fluid. How hard can that be? But it goes back to the most basic of physics equations, Force = mass * acceleration. That centrifugal pump must accelerate a mass (fluid) and that takes force (read torque from the motor). Don't expect good results by using a var voltage controller such as those from Grainger on a pump motor. The cheapest way to control pumping speed on a small pump is with a valve. Most larger industrial pumps use variable frequency drives on three phase motors to control pumping speed.

i have to change the speed of a single phase water pump 0.75 kw 220 vac with permanent split capacitor (20 microfarads, 450 volts). can i use a variac or do i need an ac drive to do it?

For the original poster, it sounds to me like cheap is the key element in your question. In that case, DC controller and permanent magnet motor rules and will continue to rule for some years yet. This will be true up thru probably 2 maybe 3 horsepower.

Now, if performance and precision is more important than it sounds, you may want to consider AC but again, if cheap rules, then you need an AC drive with a single phase 120 or 240V input and a three phase 230V motor for the output to the load. AC is not as cheap but is getting down there in cost and offers some performance benefits as well. As to single phase AC, simply rule them out. It won't be cheap and it won't have performance or precision either. Not a good deal.

This is a 2 year old resurrected thread

Go back Kataeb and read post 14, split phase and cap start motors can not use controllers.

You did not mention the drive aspect i.e. whether gear box, chain, or belt. The simple solution may be changing the gear box, sprockets or pulleys.

It all depends on the application and needs, provide more details of the application and more information may be provided.

Monophase pump speed control

The pump is used to provide water on a wet wipes machine, and its speed should change proportionally with the increase of the production speed (low precision is acceptable). This water pump has the following specs: 0.75 kw , 220 vac , 1 phase , with permanent split capacitor (20 microfarads, 450 volts).

Is there any type of electronic or electric device capable of changing the speed of this monophase pump? Can I use a variac to do it?

Thanks

ready961 said:

Thanks guys. The motors I have are 1 phase. Do they make variable speed drives for 1 phase? Everything I've found is 3 phase. Im not looking for anything fancy just need to control the speed, and of course the cheaper the better.

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potentiometer will help you

you mean that i should supply the pump using a power potentiometer, thus reducing the voltage which will reduce the speed as well? no possible damage to the pump or capacitor in this case?

I Will Reiterate

This is a resurrected thread, what the original poster wanted and what Kataeb wants are entirely different.

As I mentioned, that motor you have can not use a controller or variac. It depends on what you need to do, if just change to slower/faster speed then change gearbox, sprocket, or pulleys.

If you need variable speed then replace motor and use VFD.

Just for general knowledge, what will happen to this motor if we use a controller or variac to supply it? (motor specs: 0.75 kw , 220 vac , 1 phase , with permanent split capacitor (20 microfarads, 450 volts)).



On the other hand, the motor shaft is directly mechanically coupled to the pump, so I think the best solution, as Rsdoran mentioned, will be to replace this motor by a squirrel cage 3-phase motor supplied by an ac drive.



Thanks guys

Kataeb said:

Just for general knowledge, what will happen to this motor if we use a controller or variac to supply it? (motor specs: 0.75 kw , 220 vac , 1 phase , with permanent split capacitor (20 microfarads, 450 volts)).



On the other hand, the motor shaft is directly mechanically coupled to the pump, so I think the best solution, as Rsdoran mentioned, will be to replace this motor by a squirrel cage 3-phase motor supplied by an ac drive.



Thanks guys

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The cap will burn up the controller very quickly. It will not work.

Here are a couple of links that might help you guys wanting to contol the speed of a single-phase, ac motor.

http://www.freescale.com/

At the Freescale site, look under VIEW APPLICATIONS -> MOTOR CONTROL. Then click on the AC motor. Finally 1-PHASE AC INDUCTION MOTOR.

http://www.fairchildsemi.com/

At the Fairchild site search for AN-3006. This is an application note by Fairchild. I've built the circuit on page 3 with success. The speed of the motor was controlled with a pot. connected to the 555 timer.

Again, this might not work for your application, but there is some good information here.