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Old 03-31-2008, 06:44 AM   #1
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Simple VFD for 300watt Induction motor


Hi All! I have just bought a small scrollsaw with an induction motor which I want to run at variable speeds.
It seemed to me that a 12/240vac inverter would be the shot, if I could get into where the 50/60cps frequency control part was? I see very simple circuits that have a pair of free running high power transisters running back to back on 12 volts and feeding into a 12<-120 or 240v transformer. I have the feeling that these square wave jobs would do part of the trick with a large capacitor accross the output to clip off the sharp corners, but they look as if they would run at several hundred cps or more and only be good for universal series motors. How can I vary the frequency from say 20-60cps on a low cost inverter?? Thanks Guys. CaseyJ.

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Old 03-31-2008, 09:31 PM   #2
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Simple VFD for 300watt Induction motor


I am NOT an electronics guy. I understood some of what you were talking about. I'm not sure if this will help or not, but they do have micro VFD's. This isn't something that is inserted into the circuit the way you describe, but it would allow you to run it in the frequencies you describe. It does not however, meet with the low cost part of your problem. There may be smaller units out there somewhere.

http://www.driveswarehouse.com/Drive...00-002MFU.html

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Old 03-31-2008, 09:38 PM   #3
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Simple VFD for 300watt Induction motor


Single phase motors generally do not like their speed controlled. The reason being there's always some sort of starting system involved. Usually a centrifugal switch that controls a different winding. If the speed is lowered too much, this switch will kick in and out constantly, destroying itself in a short time. If you could build an single phase VFD, it'd only be good for about 70% to 100% speed.

The inverters you're talking about are ferro-resonant. Their frequency is controlled by the natural resonance that occurs between the reactance of the transformer and the capacitance of a capacitor. The capacitor would be easy to change, the transformer would not.

If you put a capacitor across the output of such an inverter, it'll likely interfere with the above mentioned resonance to the point the that it simply can't work. If you put a capacitor across the output of a modified sine wave or true sine wave inverter, it'll burn up the output transistors.

There are small VFDs that will accept single phase input, but they drive 3 phase motors. If you connect one to a single phase motor, it'll think the motor is defective, and shut itself down.

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Old 03-31-2008, 09:40 PM   #4
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Simple VFD for 300watt Induction motor


Thank God you're here. I knew you'd have this one.
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Old 03-31-2008, 11:24 PM   #5
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Simple VFD for 300watt Induction motor


Thanks, Goose. You're pretty good as well. You've got a good handle on how to do it right the first time.

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Old 03-31-2008, 11:40 PM   #6
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Simple VFD for 300watt Induction motor


To Goose & Micromind, Thanks you guys. Its not really the price of $179 bucks that is the problem its the fact that Kiwis like me have to grow long fingernails to hang on down here underneath the globe, and postage costs are astronomical. All understood on the starting problems and
I am lead to believe that by starting on a low speed and then winding up there is not so much a problem with a singlephase motor especially if you can start on light or zero load. Its the bit about the extra capacitors that would have thrown me. Thanks again, CaseyJ.
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Old 04-01-2008, 06:30 PM   #7
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Simple VFD for 300watt Induction motor


Sorry to come back for another bite on this but I was a bit vague in my reply last night because I wanted to make doubly sure that it was a permanent split capacitor motor
and so does not have the start run switch to cause problems. The design for a VFD that I had my eye on stipulated a rewound microwave oven transformer and a 68mfarad capacitor. I now see the reason for stipulating the size of capacitor, Thanks for that Micromind!! The question now becomes how to have a variable capacitor around that range, seeing as I can't vary the inductance. I guess that I will have to end up with a rotary switch and 3 or 4 different caps which will allow me to have only 3 or 4 different speeds, but thats fine by me. Thanks you Guys. CaseyJ.

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