When more poles are added to an AC synchronous motor, the speed is decreased according to:RPM = 120 x f / n where n is the (even) number of poles and f is the frequency in Hz.I can find plenty of sources stating this effect, but nowhere can I find an explanation on why (ie: the physical mechanism), explicitly anyways, the number of poles slows down the speed. Also, where the 120 constant derived from and what are it's units (or is it non-dimensional)?Thanks
basically it is because you only rotate the amount the poles are separated for each cycle. double the poles, you rotate 1/2 has slow. the motor will run smoother, though.
During one cycle of the power waveform, a fixed point on the rotor moves past one pair of poles. Frequency, hertz, is the same as cycles per second. Speed is revolutions per minute. You need to multiply by 60 to convert from cycles per second (units 1/s) to revolutions per minute (units 1/m). You also need to multiply by 2 because there are 2 poles in a pair of poles. Therefore 120 = 60 minutes per second X 2 poles per pair.
