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View Full Version : 2 Speed three phase motor replacement with vfd on a lathe.



lamestllama
29th Sep 2018, 04:40 PM
The original motor was a 2/4 pole motor so what is best to replace it with if requiring the same range as before? 4 pole motor (and run it at twice rated speed sometimes) or 2 pole motor (and run it as half rated speed sometimes)?

Michael G
29th Sep 2018, 04:54 PM
If you run a (nominal) 50Hz motor at 25Hz, you will have less torque and need additional cooling as the fan on the back (if it has one) will not supply enough cooling for the motor. On that basis, I would go for 4 pole and run it faster some times. My lathe will run to 2200rpm but I must admit that most times I don't run much over 1000 to 1500rpm. At least you will have the low down torque, handy for screw cutting and other demanding tasks.

Michael

BobL
29th Sep 2018, 06:19 PM
If you install a motor that has 2x the original motor power and use a 4 pole motor and a V/F VFD you should have at least the original power between 25 and 125 Hz.
That's at least a 5:1 speed range.

At 125Hz a 2 pole is doing ~7200 RPM - which is a bit much for most two pole motors.
At the same frequency a 4 pole is only doing 3600 RPM which is well within spec for most motor bearings and rotor safety

If you also use a vector drive VFD you should have at least the original power between 12.5 and 125 Hz.
That's 10:1 speed range.
It's unusually to need extra cooling at 25 Hz but you will need it for 12.5Hz.