# Efficiency of an electric motor

Tags:
1. Feb 24, 2016

### JayCAquino#4

The motor runs on dc current. I understand efficiency is useful work/work input. It is attached to the pulley. I think efficiency will remain constant as long as voltage of mains and vertical height remain constant. Am I wrong? Other people claim efficiency increases with load

2. Feb 24, 2016

### Staff: Mentor

Usually, efficiency increases with load in an electric motor. Many motor manufacturers will provide spec sheets showing the efficiency vs load.

3. Feb 24, 2016

### JayCAquino#4

Using the formula useful work/total work input. I use mg(delta)h/ivt and this suggests efficiency is constant because when mass increases so does time. But how come when you take into account torque efficiency increases with load?

4. Feb 24, 2016

### Staff: Mentor

The formula for work input doesn't tell you anything about efficiency. Work input and output are mostly independent of each other, being driven by different factors.

5. Feb 24, 2016

### JayCAquino#4

Oh okay so I should use power instead of work. Thanks for the help. Much appreciated

6. Feb 24, 2016

### Staff: Mentor

Right: work input is a combination of mechanical and electrical inefficiencies, which are functions of different things.

7. Feb 24, 2016

### billy_joule

What is efficiency when the load is zero? I.e. the motor is spinning freely at it's no load rpm? What about when the load is very large and the motor is stalled so not rotating?

8. Feb 25, 2016

### JayCAquino#4

What equation do I use to calculate my power input if the motor is connected to a shaft which is lifting a load off the ground. I know the rpm and the radius I just don't know how to put it all together

9. Feb 25, 2016

### JayCAquino#4

I don't know how to use rpm to calculate power

10. Feb 25, 2016

### billy_joule

P = IV (electrical power)
P = Fv (mechanical power)

Efficiency:
η = P out / P in

It's P = Tω

My questions were somewhat rhetorical. The answer to both is 0% as there is no work being done so output power is 0 W.