Replacement Motor

Hi All,

I've just bought a Super 7 on eBay which is currently single phase. I have been used to an ML7 with a 3ph dual voltage motor running off a VFD and rather than put that back to single phase I want to buy a new motor and convert the new one to 3ph.

I think this means that I need a 4 pole motor running at 1425rpm which I haven't seen on eBay over the past fortnight. What I have seen (and has just been re-listed) is a 1hp dual voltage 2 pole motor running at 2860 rpm.

So my question to 'the panel' is - Does it matter - 2 pole or 4 pole

- 1450 or 2860 - if I intend to run it via a VFD?

My thoughts are that I could program the VFD to provide a maximum of

1425 rpm (or there-abouts) but I have no idea whether that would reduce or increase the potential torque, or cause any other problem that is beyond my wit to imagine!

Your collective wisdom would be most welcome.


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I'm not a true expert, but my gut reaction is that you should go with a motor which natively operates at the correct speed. A couple of factors spring to mind:

  • Torque is limited by the frame size of the motor. A 1 hp @ 1425 rpm motor will be physically larger than a 1 hp @ 2850 rpm motor. If you run the 2850 rpm motor at 1425 rpm using a VFD, you won't get 1 hp.
  • When you reduce the frequency of the AC supply, you also have to reduce the voltage. The motor is an inductive load, and so its impedance is lower at a lower frequency, so you have to reduce the voltage to maintain the same current (a higher current would overheat the motor). The reduction in voltage might also have something to do with reduced back EMF, but at this time of night I can't quite get my head round it. Anyway, the result of reducing the voltage is that you thus reduce the power the motor can draw from the supply.

Best wishes,


Reply to
Christopher Tidy

from "Christopher Tidy"

Thanks for your input Chris - I understand most of it, even at this time of night!

The frame size of the motor on eBay is D80 which is what I think the native Myford motors are, so that might be OK.

Bearing your comments in mind I now think that it might be better to simply change (reduce) the pully size - with a suitable adjustment to the motor position - thereby retaining both the native motor speed _and_ the spindle speed of the S7.


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