Efficiency of a DC Motor

Hi I am trying to calculate the efficiency of my DC motor. I know that efficiency is:

Power Out / Power In * 100

I have been told by my teacher that Power In is calculated by the following formula,

Power In = Power * Time

And Power Out is;

Power Out = Mass * Gravity * Height.

I think that there is an error here in what he has given us. First of all I believe that we need to calculate work in and work out for efficiency. To do so we would need torque and RPM calculations for Work Out? And i'm not sure about Work In. Also I believe these equations he has given us have contradictory units and hence are incorrect.

Could anyone tell me how I would calculate efficiency and what equations would I need for In and Out values. And if it was torque and RPM what equation would I use, using these two to calculate the Out value.

Thanks,

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256bits
Gold Member
Power In = Power * Time

Well, that doesn't seem right , does it.

With SI units,
Power is measured in watts, which is J/s ( Joules/second ).
Joules is a unit of energy.
Power = Energy / Time

Power Out = Mass * Gravity * Height.
That also is to annoying.

The terms multiplied on the right hand side look amazingly the same as gravitational potential energy, which is measured in joules.

In both cases, your notes incorrectly equate power to energy.

Work, by the way, in a measurement of energy.

( Can you figure out what the correct equations should be )
Hint:
For the input power you have to change one of the terms to something else.
For power out you have to decide by another variable.

Also, for electrical circuits, if you measure the volts and the amperage going into the motor, what do you get? energy in or power in?

So... would it be

Power In = PE/T = mgh/t ?

and

Power Out = torque x RPM ?

???

(The original equation is what my teacher gave me) haha

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need to know asap

256bits
Gold Member
Power In = PE/T = mgh/t

Does a falling mass run your motor?
You might mean Power Out.

What about volts and amps input into you motor for Power In?

Power Out = torque x RPM
That is generally correct.
You just have to be sure your units for torque and RPM are a matchup so that they multiply correctly to give watts or horsepower for the Power Out. You might need a conversion factor.

By the way, if this is homework, which it appears to be, there is a Homework Section which is great for exercise questions.

no this is for an experimental report.
anyway thank you very much for the help