# Shaft speed of a motor at 80% efficiency

1. Aug 6, 2011

### NEGATIVE_40

1. The problem statement, all variables and given/known data

A motor produces 1000kW of power, running at 1500rpm. What is the running speed if it runs at 80% power?

2. Relevant equations

None that I know of. You can find the torque by the below equation (but again, I don't know how to deal with the 80%)

$$P = \frac{Tn}{9549}$$
where P is power (kW), T is torque (N.m), n is rotational speed in rpm.

3. The attempt at a solution
Is it just a proportion type thing?

i.e

80% power means 0.8*1000 kW = 800 kW
so then
0.8*1500rpm = 1200 rpm ?