Thread: variable speed on single phase motor!

Most of those on You Tube use a light dimmer switch.

This is basically a kludge way of controlling motor speed especially under a loaded situation and if you are not careful you can over heat and cook both the dimmer switch and the motor.

If you read the comments associated with this method of control you will see all the problems associated with using dimmers.

Can you really control an induction motor with a light dimmer? Dimmers typically just chop the waveform without affecting the frequency, while induction motors are effectively synchronous motors whose RPM corresponds to the line frequency, hence you need to reduce the line frequency if you want to slow such motors. I guess you could use a dimmer to increase the slip, but it sounds like a lot of heat and not a lot of torque or load regulation.

Originally Posted by RustyArc

Can you really control an induction motor with a light dimmer? Dimmers typically just chop the waveform without affecting the frequency, while induction motors are effectively synchronous motors whose RPM corresponds to the line frequency, hence you need to reduce the line frequency if you want to slow such motors. I guess you could use a dimmer to increase the slip, but it sounds like a lot of heat and not a lot of torque or load regulation.

I assumed the OP was talking Induction motors but he could have been meaning Universal motors which can be controlled OK with a light dimmer.

No it was an induction motor. So how can speed be adjusted if not by (variable) resistors?

Originally Posted by yourmomm

No it was an induction motor. So how can speed be adjusted if not by (variable) resistors?

It can't be adjusted, even using variable resistors, unless the input frequency is changed, and for that you need a frequency converter that outputs variable frequency single phase called a CCV (Cycloconverter) or one of its more modern equivalents. These gizmos can usually only produce frequencies equal to or less than the input frequency.

If this was easy and cheap lots of folks would do this, instead it works out to be cheaper and easier, and it offers a lot more than a CCV, to buy a used 3 phase motor and budget 1 to 3 phase VFD.

You can also run a single phase motor off a normal 1->3ph VFD by putting the motor across two of the phase outputs.

The "Future Wave" pool pump speed controller does just that, using a cheap "LS" brand VFD.

Originally Posted by RustyArc

You can also run a single phase motor off a normal 1->3ph VFD by putting the motor across two of the phase outputs.

The "Future Wave" pool pump speed controller does just that, using a cheap "LS" brand VFD.

Interesting - I'd though that VFD's wouldn't like that?

Originally Posted by BobL

Interesting - I'd though that VFD's wouldn't like that?

Ridgy didge VFD's would probably not like to do it as I suspect that they would be monitoring individual phases and looking for significant imbalance, as you can't really use external protection that would unload the VFD between it and the motor without risking blowing the VFD.

But, if someone set there mind to it, they could design something similar to a VFD with a single phase output. Silicon Chip designed a kitset unit a few years ago that was intended to VFD single phase and 3 phase motors, but it comes with a caveat about being suitable for induction motors not incorporating a centrifugal switch for the start winding.

There are some cap run motors (no start cap) out there but they generally have poor starting torque compared to cap start motors, are rare and the SC kit is in the $250-300 bracket for a box of bits to be assembled, tested and trouble shot.

Incidently one the the reported uses for the unit is slowing pool pumps down to reduce consumption, as suggested by Rusty Arc, so pool pumps may be an area to investigate for single phase motors that will varispeed.

There was also a single phase VFD project and circuit in Silicone Chip a year or two ago. RayG was looking into it at the time. I think it turned out to be more expensive to build than to buy one.... Can't find the thread at the moment = someone here will recall it and use the right words to search for it...

Originally Posted by BobL

Interesting - I'd though that VFD's wouldn't like that?

Everyone kind of thinks that, but the Future Wave unit showed it was possible, with the caveat, as mentioned, that it wasn't suited to motors with centrifugal start switches.

Silicon Chip first reviewed the Future Wave unit, revealing some of its secrets (ie. it was a cheap VFD in an enclosure) and later did their own project, which was, as mentioned, a single -> 3ph drive where you used 2 of the phases to run your pool pump.