Current Draw with Different motor voltages

[mull982]

Hello

Can someone please explain to why when using a higher voltage for a motor you draw less current. I was trying to think about this, and using ohms law V=IR I cannot see this. Since the current is equal to V/R it would appear from this that if the voltage increased, then the current would increase assuming that the resistance was fixed. Is that my problem, is the resistance not fixed? Sould I be using some other equation instead of V=IR to model this relationship? Are there inductive properties to take into account?

I would appreciate any explanations or point me to any references where I could find information on this subject. A scientific explanation would be great if anyone knows.

Thanks

Mull982

 

[barbeer]

Maybe try using the wattage of the motor if available since it does not change.

 

[CAPS]

Barbeer is correct motor power would be a constant. A dual volatage motor will require a wiring reconfiguration to run on different voltages so the resistance is not the same. The lower voltage has the two halves of the winding in parallel and at the higher voltage has them in series. The resistance at the lower voltage is 1/4 that of the higher voltage.

 

[winnie]

The resistance of a motor is not fixed, so trying to use E=IR to figure out the current flow to the motor is a lost cause.

A resistor is explicitly not a constant power load. As the applied voltage increases, the current flowing through the resistor will increase, and the power delivered will scale proportionately to the square of the applied voltage.

In the normal range of operating voltage for a motor, the efficiency is approximately constant, and so the electrical power input will be approximately constant. If you have a load with constant power input, then the current flowing to that load will be inversely proportional to the applied voltage. Note that if you work Ohm's law in this situation, you will find that as the supply voltage goes up, the apparent resistance of the motor will also go up.

This is just a rough approximation; depending upon the specific mechanical load, the design of the motor, and the magnitude of the change in voltage, the current flowing through a motor may increase or decrease when the applied voltage changes.

-Jon