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?
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.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
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 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.
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.
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
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