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# AC motor speed controller

Discussion in 'General Electronics Chat' started by jerseyguy1996, Feb 27, 2008.

1. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
10
Is it possible to control the speed of an AC motor that plugs into a wall outlet or is the speed really just a function of the frequency of the alternating current?

2. ### techroomt Senior Member

May 19, 2004
198
1
some motors speed can be controlled, and yes.

3. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
10
So would the process be to vary the frequency of the alternating current? If so how is that typically accomplished or is there a resource that would describe it in detail?

4. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
10
Okay a quick google search and I think I get the idea. Convert the AC current to DC current and then create your own alternating current out of the DC current at whatever frequency you want. It looks like you have to have a corresponding change in voltage as you lower or raise the frequency but it looks like you can use PWM to handle that. Am I sort of on the right track? The end result of this exercise is to control the speed of two powerheads (pumps) in a fish tank to create random currents to simulate wave action. I'm hoping to use an arduino to control all of this switching. Does this sound doable (assuming it was someone who knew a lot more about electronics than myself)?

5. ### jpanhalt Expert

Jan 18, 2008
7,670
1,874
There are several types of single-phase "AC motors." Some, like the universal motor wth brushes are relatively easy to control. The capacitor-start and split-phase can be very difficult to control, and when controlled, they lose a great part of their torque. The aquarium pump is possibly a synchronous motor, in which case you should be able to control it with frequency, not pwm.

The place to start is to determine what type of AC motor you have. Search on synchronous, split-phase. and capacitor start motor to see if any match what you have. John

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
I don't know that most acquarium pump manufacturers would publish precisely what the technology is behind their motors; only their voltage/Hz requirements along with power consumption.

Motors are generally designed to operate most efficiently at a narrow frequency range. If you deviate very far from that, you should be prepared for the motor to offer little in the way of either torque or reliability.

You could use 1/2 of an H-bridge to generate a square wave version of what your pump motor is used to. However, this is not exactly a project that I would recommend for an admitted electrical novice to undertake. If you foul up the timing between the high and low side of the H-bridge, you'll wind up with a "shoot-through" condition, otherwise known as a dead short across the power supply. This is not a happy situation when you're talking 117V potentials. Loud bangs. Smoke. Lots of smoke.

I suggest looking at unipolar stepper motors. They are fairly low-torque, but most operate at comparatively low voltages. You might still hear bangs and make smoke, but they won't be so loud or dense. You won't have to worry about electrocuting your fish, either. Once you get the basics down, they are VERY easy to control with microcontrollers - and you don't have to spend much money doing it, either.

7. ### jpanhalt Expert

Jan 18, 2008
7,670
1,874
The first question is still to find out what type of motor the OP has. I have seen some aquarium "motors" that were not motors at all in the usual sense. They were simply vibrating diaphrams with a check valve. They can be controlled with back pressure, but I suspect they would be quite difficult to control electronically.

One easy question may be to determine whether the device hums or buzzes, like diaphram pumps tend to do. John

8. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
10
The pump is just a small submersible water pump that circulates water in the tank. The shaft just has a cylindrical permanent magnet attached to it and I believe it spins due to the alternating current switching the poles of the electromagnet coils that surround it. I have two of them in the tank that supply a fairly static current. I want the current to be variable therefore I need to be able to slow one pump down and speed up the other one. I don't want to alternate on and off with the pumps because they tend to start fairly roughly and I understand their lives are shortened dramatically by frequent on/off cycles. They also don't always start off in the same direction and a modification that I am going to make to the pumps impeller would require that it always turn in the same direction.

9. ### jpanhalt Expert

Jan 18, 2008
7,670
1,874
I suspect it is a brushless motor with a magnet on its shaft inside the the plastic casing. That magnet couples with the magnet/impeller through the plastic. So-called stir plates used in laboratories work on a similar principle and are variable speed. The advantage is that the motor unit can be very well sealed as there is no rotating shaft to seal.

I have to leave right now, but will look into the magnetic stirrers I have to look for clues of what you might do. It may be as simple as a light dimmer/triac control.

John

10. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
10
I don't believe anything rotates inside the sealed plastic casing. I believe the magnetic field generated by the coils is "felt" by the permanent shaft magnet through the plastic casing. I will take one apart and photograph it tonight to show what I mean. I think the bottom line will be varying the frequency of the alternating current. I read somewhere that if you lower the frequency you have to also lower the voltage to avoid burning up the motor. I am not sure why it would burn up the motor by lowering the frequency and keeping the voltage steady but that is what it said.

11. ### jpanhalt Expert

Jan 18, 2008
7,670
1,874
It looks like your motor has no capacitor (as in capacitor start, etc.) and no brushes. That eliminates a lot of types. My guess is that it is a shaded-pole motor or synchronous motor. See:

A good shaded-pole motor (not the synchronous type described in the first Wikipedia link) may be so quiet as to not have noticeable vibrations. They are cheap, low power, quiet, and are often used in fans and the like. The magnetic stirrer I mentioned uses a shaded-pole motor. The single-phase synchronous type is very low power and is used in things like electric clocks. It is also very quiet. The synchronous motors are such low power, they might not be able even to twirl the magnet through something as viscous as water. I am guessing you have a shaded pole motor.

If you have a shaded pole motor, it is relatively easy to control. Here are two links. They look about the same, but the one with KB in it is a datasheet from KB drives. I am too lazy to get the direct link. That link discusses the controllers and how they work. It looks like they are about \$20. A light dimmer switch might also work. If the prices are close, I would go with the KB drive, as that is what they are designed for.

http://www.electricmotorwarehouse.com/PDF/kb_electronics.pdf
http://www.electricmotorwarehouse.com/kbwc.htm

For a full discussion of how single-phase motors work see:

and
http://en.wikipedia.org/wiki/Electric_motor

John

12. ### jpanhalt Expert

Jan 18, 2008
7,670
1,874
It occurred to me that a modern pump might actually function like that. In other words, it takes the incoming single-phase AC and converts to to what is effectively three phase. If that is the case, I don't think you can control it through the mains line.

I would then go for a mechanical solution. You might be able to change the impeller to reduce output, which would be more or less a permanent change, or put a restrict or on the outlet or inlet to reduce flow.

Please let us know what you find. John

Edit: If it can be controlled in that case (three-phase), it will be by variable frequency.

13. ### rherber1 Member

Jan 6, 2008
20
1

Are you perhaps using the wrong pump for your application or do you feel you have to use the pumps you have simply because they are to hand?

I think that you will not be successful in trying to control the flow rate by varying the speed of the pumps you have and you may be better off selling them and getting something more suitable.

At least do some research and ask some questions of people who have experience in this area.

For example have a look at the info here http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=4609

14. ### jerseyguy1996 Thread Starter Active Member

Feb 2, 2008
206
10

The link that you gave me is the pump that I have. Actually I bought them from Drsfosterandsmith.com. I have 3 of them. They are constant speed and I want to be able to vary the speed from low to high. The link in the previous post
http://www.electricmotorwarehouse.co...lectronics.pdf seem to be very close if not exactly what I need but I want to place that in my circuit and allow the microcontroller to vary the speed in a very random fashion to create random currents in the tank similar to wave action on a reef.

15. ### kennethxu New Member

Sep 6, 2008
1
0
Hi jerseyguy1996, did you ever get it working?

Thanks!