Two speed 3 phase induction motor

[Gassan]

Today I had to uninstall some large evaporator fans from the chilling compartments of a fishing boat. The fans were run by three phase induction motors. To my surprise, some of the motors were strangely wired. There were two connections of four-core cables. Each cable had three phases and a ground wire.

First, I assumed that the two connections were used for star delta configuration. However, when I saw the nameplate of one of the motors, I got a satisfying clue. The internal configuration of these motors allowed them to run on two different numbers of poles in order to provide speed variation. With four poles, it spins faster and with eight poles, it spins slower. That's my best attempt in interpreting the nameplate.

Can you have look at the name plate and confirm if my interpretation is correct?

[Andy Watson]

Yes, you are probably correct. Try searching for Dahlander connection.

[soldar]

Dahlander is one type of pole changing motor and it works by changing the way the windings are connected and can only double/half the number of poles.

There are other pole-changing motors which have different windings for each number of poles and these can have say, 4, 6 and 8 poles.

The problem with the speed changing is the sudden stress on the motor but if it has separate windings then one winding can be connected while the other is still working. Suppose it is working with 2 poles at high speed. If we suddenly switch the stresses will be very high but if we connect the 4 pole winding then it will work as a brake/generator and the motor will slow down and then we can disconnect the other winding so the transition is smoother and can be made even smoother with the use of resistors.

[tautech]

My 3 phase lathe uses the same principle for 1440 or 2880 rpm operation.
It has a mechanical multi position multi blade switch to change from one speed to another and also permits reverse in both speed ranges.....so a 4 position 3 pole mechanical switch.
Main ON/OFF and emergency STOP is via an ordinary contactor.

[soldar]

I believe in a lathe the stress while changing speed would be very low as the speed is changed with no load.

I remember reading an article years ago about pole changing motors explaining how they were used in elevators and other machinery that would change speed under heavy load and how stressful it was for the motor so they came up with the multi-winding motors which could change speed more smoothly. I remember they were described as "two motors wound on one stator" and it seemed a fitting description. If you think about it you could put two motors on the same shaft and switch from one to the other.

[tautech]

I believe in a lathe the stress while changing speed would be very low as the speed is changed with no load.

Exactly.
I'll upshift the motor while running but always power it down when down shifting.

They are becoming a thing of the past as good VFD's are not expensive and offer infinite speed adjustment.