Why does the Dual Speed Dahlander motor need a bridge in the fast speed configuration?

Question

On the nameplate of the motor is a Delta/Wye-Wye configuration, meaning you supply the ones (U1, V1, W1) and leave the twos open to achieve the slow speed. And for fast speed you supply the twos and bridge the ones... That's for a constant torque single winding pole changing motor. Surprisingly, today I connected this motor without bridging anything and it ran on both speeds just fine.

So the question is why do we need to bridge the ones in the fast speed? Does this also have an effect on the output torque in the fast speed?

Any information on this will be appreciated.

Answer 1

Bridging U1, V1 and W1 reduces the voltage across each winding to 86.6% of the voltage without bridging. That reduces the load torque that the motor can drive, but that is compensated by doubling the number of poles. The motor may seem to operate satisfactorily, but the motor will probably draw more current for any given load. The design is probably designed to be most efficient with the high speed connection. Reducing the voltage per winding at the lower speed may be necessary to keep the motor operating current and temperature within safe limits with a less efficient winding configuration and and the reduced cooling air flow due to the lower rotor speed. Whatever the detailed design reasons are, the motor should be connected as the nameplate indicates.

Answer 2

Each section of the winding is connected to a number of coils which each span half the full pole pitch (as you would calculate it to be in the high speed mode). With current going from say L1 to L3 through the 1U-2U then 2U-1W, that will be configured to give all the poles with the same polarity, forcing the creation of the consequent poles between to give the doubled effective poles. In the high speed connection, those same two legs have 2U-1W in antiparallel with 1U-2U, so now the first leg will give 'north' poles, and the other 'south' poles, so you have half the number of poles compared to the low speed (with some non-contributing poles in between). The coils in each leg are likely interspersed so that there will be pairs opposite each other.

With the shorting connections at the star point the current through the two halves of the phase will be more or less the same, but without, the currents will be out of phase by 60 degrees with each other. The fundamental is still going to be at the right phase angle, but reduced by 13%. This will make the motor less efficient, since there's some current that isn't generating useful torque.