VFD for a single phase motor
VFD for a single phase motor
(OP)
Hi guys,
I've got a 250W single phase motor with a start capacitor and I'd like to hook up a VFD (variable frequency drive) to it. Is something like this at all possible? If not what else can I use to control the speed of such a motor?
Regards,
Tom
I've got a 250W single phase motor with a start capacitor and I'd like to hook up a VFD (variable frequency drive) to it. Is something like this at all possible? If not what else can I use to control the speed of such a motor?
Regards,
Tom





RE: VFD for a single phase motor
Bill
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"Why not the best?"
Jimmy Carter
RE: VFD for a single phase motor
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RE: VFD for a single phase motor
thought you might suggest this. The main problem is however that the motor is attached to a pump and the whole unit looks like this:
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so if I get a new motor I'll have to figure out a way to connect the pump and the motor.
Thanks for the replies none-the-less.
Regards,
Tom
RE: VFD for a single phase motor
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: VFD for a single phase motor
The problem with the capacitor is that it becomes less and less optimal when leaving design frequency. But, on the other hand, it doesn't need to be optimal when speed and torque goes down, which is valid for a fan. Not sure about your pump. But, if torque is speed squared, it will probably work.
But, again, as Jeff says, do not expect to start at zero speed. You will probably need to start at, or near, nominal frequency. If that is a no-no, then you are probably stuck with a three-phase motor and a mechanical job.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD for a single phase motor
thanks for all the tips.
I'd like to start at whatever speed the user wants, so it might be a bit of a problem.
Do you think it's possible to use a DC motor instead and control it via PWM, without a VFD?
Regards,
Tom
RE: VFD for a single phase motor
Gunnar,
The other major issue on a Cap start 1 phase motor and a VFD is that there will be a centrifugal switch that shorts the caps out at 90% speed (or thereabouts) and as you slow the motor down the switch disengages, placing the caps back in the circuit when they are no loner needed. Worse yet is if the user selects a speed where the switch cycles on and of repeatedly. They are not built to have much hysteresis. You end up destroying the caps, the transistors or both.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: VFD for a single phase motor
I imagine this type of motor would work as Gunnar said. The initial start would have to be at or near rated speed to activate the centrifugal switch. After that the speed could be varied as long as it was kept above the reset speed of the centrifugal switch. Given the characteristics of centrifugal fans, it could work well for air handling units. Protection may be a challenge. I imagine the system on slow speed and a failing bearing slows the motor so that the centrifugal switch cuts in. At low voltage and frequency the start winding and capacitors will not be very effective. but on the other hand small motors are often cheaper to replace than to repair.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: VFD for a single phase motor
Good luck
RE: VFD for a single phase motor
Shaded Pole are not capacitor start, and PSC are considered separate because the caps are in the circuit all the time, not switched in and out during starting.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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