Low base speed motors
For applications requiring operation with relatively slow base speed motors (10 or more poles), the limited number of slot combinations between the stator and rotor can lead to an acoustic resonant condition, and the associated very high levels of narrow band noise.
Many motor manufacturers will offer special windings using four pole or six pole motors with a reduced nominal frequency to avoid this phenomenon.
For a European 400 V electricity supply network, a requirement to
deliver full output at, for example 600 r/min, could be achieved with a 10 pole motor design, wound for 400 V at 50 Hz, or with a six pole motor wound for 400 V at 30 Hz.
Equally, because many motors are wound 400 V Delta/690 V Star (Wye), it is possible to provide full flux to a six pole design by connecting the motor in star and feeding with 400 V at 29 Hz base.