Inverters and Pole-Change Motors for Variable Speed Control

Yes it can and you no longer need to be able to change poles on the motor. Hard wire the motor to the highest speed configuration (lowest number of poles) and use the VFD to control the speed.

That is correct...and with the right material you can make loads of "nice stuff"...

note:Important the safety issues that may come up

Yes you can connect a motor on inverter output as desired if the motor is a 3 phase suitable for inverter operation. Normally, the 2:1 ratio windings are constant torque, so connect to the high speed or low speed depending on the maximum speed wanted. If you want very low speed you may be better to connect to the low speed 'winding'. If you check the nameplate and you have another winding (as the 6 pole connection, check its power to see if its in ratio or offers better torque.

Depending on you application this may help.

Really, your question isn't esay to answer as you do not say what you have in mind to do with it, low speed, high speed, high torque etc - so if you could outline what you're trying to achieve with parts you have available it would help others to understand better and offer more pertinent and relevant advice. If you can be more specific with your information it would enable a more specific and better reply, I think.

Thanks.

Actually, I am looking at constant power kind of speed combination, mean as the speed increase, the torque will decrease porportionally. for example, i need the motor to output 100Nm @ 500rpm, 50Nm @ 1000rpm and 25Nm @ 2000rpm.

Just a spark in my brain, I am thinking of NOT to use a gearbox and instead a inverter to do this.

What I learnt is that for inverter fitted motor, the frequency control is often limited to maximum 100Hz~120Hz, after which the torque will drop too low to work. Which mean I can't really get a 4x increase on speed while still maintain a good level of torque. One way is to reduce the low frequency to about 25Hz but then motor will need to oversize.

So I am thinking of using two speed pole change motor with the inverter to achieve a greater speed range, if I am using 4, 8 pole motor with inverter working at 50Hz~100Hz, I should be able to get the speed and torque range combination I want. Not sure how this impact on the motor size but just a thought.

most 2:1 ratio pole change don't offer more torque as they are a tap change or similar so please check. If you go for a true 2 speed (like 2 windings in one frame) this will help (but will be in a bigger frame size) but you could also consider increasing the power to do the same. This can benefit in two ways:

1. More nominal torque available for low speed work - if you need inertia too, maybe add a heavy pulley etc

2. The peak pullout torque will be better on the larger power motor giving better high frequency torque availability.

If your supply is 380 / 400V you can reconnect a standard motor to 230-50Hx and use at 400V 87Hz from 400V inverter and then this can be used at 180 - 200Hz for higher speed if its a standard low power 4 pole.

If this is a home machine application another simple good way is to use a pulley reverse change so the belt can change straight over on the same centres.

Hope this may help.