How Frequency Dependence on Motor Rating

The frequency affects the speed of many motors. The speed affects the cooling, because many motors have a fan fitted. So the cooling affects the rating, Guv. It's as simple as that.

yes u're right but let me reframe it by asking like...how & why does frequency affects speed of motors..i know lower the freq. , lesser wil be the speed..is it because of magnetisation and demagnetisation of core .?.if freq. increases it'll tend to magn. & demag. (as ac alternates,lets talk about induction motor) it more frequently and with that flux will be more..so more torque??? m not sure can someone add to it or correct me plz..

AC makes the magnetic field rotate (or appear to rotate) at an angular frequency proportional to the AC frequency, winding overlap, and iron pole structure.

http://en.wikipedia.org/wiki/Rotating_magnetic_field

Motor ratings are usually specified at one frequency. Motor specs are usually de-rated with any change in the base frequency.

Assuming AC motors. You never specified but it was a good assumption.

Frequency determines speed of AC induction and synchronous motors thusly:

Srpm = 120 x F / P

Srpm = synchronous revolutions per minute.

120   = constant

F       = supply frequency (in cycles/sec)

P       = number of motor winding poles

Induction motors then have what is called "slip"of between 2 and 5% typically.

Torque is a function of the ratio of volts and hertz. Motor power is a function of speed and torque, so even if you have the same torque from two motors at different frequencies, the power (HP or kW) will vary by the difference in frequencies.

No impact...For example..you use 50Hz motor in 50Hz system. If it 60Hz, then use 60Hz motors.