Thread: Variable speed motors

Mal, as far as I know my motor is not an induction type but I will be contacting CMG tomorrow to check this. I will also ask about burning out the start windings and the capacitors.

Originally Posted by Jim Carroll

You are not able to put variable speed on any standard single phase motor as you can burn out the start windings and capacitors.

If you are running the motor at low speed for an extended period then you should use a lower pulley setting so the motor is running at a higher speed to run cooler.

Make the pulley speed work for you not the motor

Some folks add a separate constant running blower or fan to keep a constant flow of air over, if a sealed motor, or through, if an open motor. This keeps the motor cool while running slowly.

Having a variable speed controller and 4 or 5 step pulley is the best of both worlds.

I have a 20 inch swing Woodfast that was converted to variable speed with a DC drive and motor.

I can put a huge chunk of timber roughed with a chain saw on it and start barely moving until I knock the corners off, then speed up with the dial, and then change to a higher pulley ratio. Changing the belt from step to step is easy on my machine.

Originally Posted by Paul39

Some folks add a separate constant running blower or fan to keep a constant flow of air over, if a sealed motor, or through, if an open motor. This keeps the motor cool while running slowly.

I saw this on one setup and thought it had merits till I had a chat with the owner and he indicated it had pluses and minuses.

Plus cool in summer as there was a constant breeze

Minus in winter as there was a cold breeze

Minus all the time as it stirred up the dust so made sanding a chore.

• My current motor IS an induction motor
• My Electrickery expert cousin warns against the unit from Jaycar for reasons already put forward by Jim, Mal and Paul
• The good folk at CMG in Crestmead elaborated on the above info and said
• An older 3 phase motor will not suit a VFD unit as the wiring will not have the required insulation. Any motor will need to be less than 10 years old.

I am now awaitng some prices before deciding which way to go.

MAny,many thanx to all for the input and warnings..

Originally Posted by artme

• An older 3 phase motor will not suit a VFD unit as the wiring will not have the required insulation. Any motor will need to be less than 10 years old.

I would take the above advice with a 5 pound block of salt.



I believe the concern about the insulation is because of heat build up. Almost everything designed to run on 120 / 240 volts is insulated to 600 volts. As long as you are using a pulley to get low speed and / or not running below half speed on the motor, there will be enough air circulation to keep it cool.


There are many folks on the Practical Machinist site that put VFDs on 50 year old lathes, mills, scrapers, etc. with original motors with no trouble.


Metal lathes will also be used at max HP for hours, hogging off strips of smoking hot steel.


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<hr style="color:#FFFFFF; background-color:#FFFFFF" size="1"> vernon, another 2 cents worth....

yes, older motors did not have the new fangled "vfd" iinsulation varnish. but where this IS a REAL problem is running 230v rated old motors on a 460v power source; ie., a shop may have 460v 3ph and a guy may say, hey, I can put a 460v vfd on this old 230v motor and just limit the output to the motor in the settings to 230vac rms max.... I have seen SOME of these applications last from 2 minutes to 30 minutes before popping the motor insulation; others do survive well. But you can see why: the old 230v motors used 600v insulation! the 460 into the vfd makes a 650vdc bus that gets chopped up and sent to the motor as a pwm that is effectively 0-230vac rms, BUT the 650v bus makes 650*2=1300v spikes! WAY above the poor old motors ratings. Do that and your Gorton motor likely will die in some number of minutes.

But you ain't doing that! u r feeding 230v in, making 320vdc bus, chopping it up, then sending that out; 2x320=640v max spikes! ur insulation is rated 600v. ya, possibly a tad over but you know what? In all the vfds we have put on old motors like this we have never had one fail due to the insulation that we know of! Hundreds done. So altho the engineering lit says it is a problem, u gotta temper it with the rest of the story.....

that said, if it were MY BABY I would buy a properly rated 3% impedance reactor (choke) and put that in series with the vfd and motor! NOW those poor spikes will go drop to like 300-400v spikes by the time they hit your motor and all is as new!

THAT done, I would also have NO QUALMS with doing the vfd route since I am 100% confident you have 2-3 competent motor repair shops close by who can rebuild it with new material better than it ever was, quickly and not too expensively....

You might ask around a machine shop locally for an opinion. I think you folks call them engineer shops.

Good luck with your adventures.

Thanx Paul. Very interesting.

Will further investigate.

Getting more interesting

Originally Posted by Jim Carroll

I saw this on one setup and thought it had merits till I had a chat with the owner and he indicated it had pluses and minuses.

Plus cool in summer as there was a constant breeze

Minus in winter as there was a cold breeze

Minus all the time as it stirred up the dust so made sanding a chore.

Good points.

Originally Posted by artme

• An older 3 phase motor will not suit a VFD unit as the wiring will not have the required insulation. Any motor will need to be less than 10 years old.

Another useful bit of info.

Paul39 - it is possible that we may have different electric motor standards applying here in Australia, now and in the past. Your thoughtful input from the US context may or may not apply here.

Originally Posted by artme

[LIST]
An older 3 phase motor will not suit a VFD unit as the wiring will not have the required insulation. Any motor will need to be less than 10 years old.

I agree with Paul, it sounds like the electrician is chasing a bit of business. If anything the older motors are more robust and better insulated than newer ones. What can happen is the insulation can break down over time but a megger check can easily sort this out. The 3 phase motor on my Hercus must be more than 35 years old and tested out just fine on a megger.

Originally Posted by NeilS

Another useful bit of info.

Paul39 - it is possible that we may have different electric motor standards applying here in Australia, now and in the past. Your thoughtful input from the US context may or may not apply here.

Very possible.

The US common household voltage has been a progressive 110, 115, to 120 volts currently.

I would think the 220 - 240 volt standard in AU would bring equal or better insulation than the 600v US standard.

Originally Posted by Paul39

I would think the 220 - 240 volt standard in AU would bring equal or better insulation than the 600v US standard.

Well according to BobL, you would be right about that. And, the man with the Megger would know...