# What is the formula for working out the volts, amps and watts of a DC Motor?

Can anyone remind of the correct way of working out the volts amps and watts on a DC motor?

I know it is something like:

$V=I \times R$

$I=\dfrac{V}{R}$

$W=\dfrac{V}{I}$

or something like that...

So, basically, to get the answer of one you multiply or divide the other two? I forget what order it goes in.

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What I suppose I really need to work out basically how I can get what size motor I have on my scooter when it has no labelling on whatsoever. Is there a way of measuring the power of the motor i.e horse power watts etc what is the formula for that. It uses(the scooter that is) 2 12v 33Ah batteries? so we need to know is it power and the watts as we have the voltage im guessing is the 2 batteries? –  Stuart Nov 11 '12 at 10:22
Note that your battery capacity does not give you much of a clue as to how much current the motor draws. All you know is that the motor is probably running on either 12V or 24V. If the 12V batteries are connected in series then the motor probably operates on 24V. Current draw of the motor will change depending upon load. You can not directly infer the motor current from the battery capacity unless you did a complicated set of experiments. So in the end you'll have to measure the motor current with an ammeter or put a small value resistor in series and measure its voltage drop. –  Michael Karas Nov 11 '12 at 13:15
Is it an electric scooter? –  damien Nov 16 '12 at 9:45

The power of a motor is the product of its speed and torque. The power output is greatest at about half way between the unloaded speed (maximum speed, no torque) and the stalled state (maximum torque, no speed).

The output power in watts is about (torque) x (rpm) / 9.57.

The efficiency of a motor η = Shaft Power Out/Electric Power In

(vector) Input Power = V·I and Loss = I²R

If you can measure real power V*I but cannot measure HP, assume 87~90% for η

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