Whenever a current carrying conductor placed in a magnetic field, a mechanical force will be acts upon that conductor and the conductor tends to rotate, if it is arranged freely to rotate. This rotation is due to the turning or twisting force acted on that conductor. This turning or twisting movement of a force about an axis is called torque ‘T’.
T = force * radius Newton-meter.
Work done per revolution = force * distance covered in one revolution.
∴ Work done per revolution = force * 2πr.
Work done per second = force * 2πr N (r.p.s)
Work done per second = 2π N T (äT = F * r)
So power developed in metric horsepower is equal to force 2πNT/735.5 hp.