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Old 08-21-2011, 01:09 AM   #1
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AC Motor Speed Control


I was wanting to modify an inverter to operate at a slower frequency to get a motor started under load. Is there a cheap way to soft start an ac motor or a way to adjust the frequency of my inverter? I was thinking that a small resistor tied to the timing capacitor would do it but that seems too simple. Any thoughts are appreciated.

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Old 08-21-2011, 01:58 AM   #2
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AC Motor Speed Control


You're trying to make a variable frequency drive (VFD). These don't work well with single-phase motors at all, mostly because of the startup problem. Single phase motors need to get rotating in a hurry, and they need to do it at the rated frequency. This is because they rely on capacitors and/or auxiliary windings to create the "appearance" of a third phase in the motor: an out-of-phase magnetic field around the rotor at a different angle than the field from the main stator windings. This trick doesn't work if you mess with the frequency too much.

Just as importantly, a variable frequency drive must also vary the voltage. Motors rely on inductive reactance to limit current, and lowering frequency lowers reactance, so voltage must also be lowered to compensate for this. VFD's do this with pulse width modulation.

An inverter contains all the hardware necessary to accomplish this for one phase, but it would be a real hacking job to make it operate that way. You would need to build a new microcontroller-based driver for the H-bridge that controls the output of the inverter. It would be easier to start from scratch. And it still won't work suitably because single phase motors just don't work with VFD's.

If you want to slowly ramp up the speed of a motor, the easiest solution is to use a 3-phase motor and a commercial VFD. For motors under 5HP, you can get a VFD with single-phase input and 3-phase output. It's not worth trying to build one. They are very complicated devices with highly nuanced and largely empirical design details to prevent the power electronics from failing spectacularly. Trust me, motor drives are right up there with high power radio transmitters and solid state Tesla coils when it comes to design difficulty.

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Old 08-21-2011, 03:19 AM   #3
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AC Motor Speed Control


Yes, I see what you mean. I had come across some small frequency drives and their output was 3 phase and they were relatively inexpensive for what I was wanting.
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Old 08-21-2011, 08:26 PM   #4
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AC Motor Speed Control


Quote:
Originally Posted by FredPaul View Post
I was wanting to modify an inverter to operate at a slower frequency to get a motor started under load. Is there a cheap way to soft start an ac motor or a way to adjust the frequency of my inverter? I was thinking that a small resistor tied to the timing capacitor would do it but that seems too simple. Any thoughts are appreciated.
If you try to limit the start up current,
With a load on the motor,
this could cause motor to stall,
Not good for the motor.
But if the load is only light,
you might get away with it.

Use a lamp in series with the motor,
And a relay wired across the lamp,
The size of the filter cap in the coil supply
will dictate the time delay.

May be not legal.
So use care.
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Old 08-21-2011, 10:18 PM   #5
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AC Motor Speed Control


If you need to soft-start a single phase motor, to reduce starting torque or current; reduce voltage, not frequency.

You could use a buck-boost transformer with a double-throw relay to take it out of the circuit, as this will hold the reduced voltage constant regardless of speed of the motor.
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Old 08-22-2011, 12:40 AM   #6
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AC Motor Speed Control


Mircomind's idea will work fine as long it will stay on buck/boost transformer for short while during start up once you get it running you will need to switch it direct line supply { bypass the B/B } othewise a timer will control the relay to switch over to direct.

But if need to adjust the speed pretty frequent then the triphase motour with VSD will work the best they can run much lower speed than single phase motour before it will stall out { about 10 to 1 speed ratio }

Merci.
Marc

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