Consider a force ‘F’ Newton’s acting tangentially on the shaft of radius ‘R’. If the shaft due to this turning moment (F X R) starts rotating at N rpm then.

Work supplied to the shaft/sec = F. Distance moved/sec.

= F.2π.R.N/60 Nm/sec

or, Power (P) = F.R.2π.N/60 Nm/sec or watt.

But F.R = Max.Torque (T) in N-m

i.e. P = 2π.N.T_{max}/60 watts