# Power Input for Motor and Back EMF

Hello everyone, I'm a bit confused about the power drawn by a motor. Since back emf reduces current when the motor is rotating at high speed, is it true that back emf reduces the power required at high speed? THX :D

## Answers and Replies

NascentOxygen
Staff Emeritus
Science Advisor
Hello everyone, I'm a bit confused about the power drawn by a motor. Since back emf reduces current when the motor is rotating at high speed, is it true that back emf reduces the power required at high speed? THX :D
Hi psycho. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

Yes. At the higher speed back emf is greater, so current reduces. Of course, you'll only see such high range speeds when the motor is unloaded, or very lightly loaded, so it isn't being required to produce much mechanical power (compared to its full capabilitiy) under those conditions, anyway.

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Hello nascent, how about the power drawn by the motor? Is it Also minimum at highspeed and maximum when stall?

NascentOxygen
Staff Emeritus
Science Advisor
Hello nascent, how about the power drawn by the motor? Is it Also minimum at highspeed and maximum when stall?
If you operate your DC motor from a fixed voltage, then the motor draws maximum electrical power when stalled. Note, that when stalled the motor is delivering zero mechanical power to its shaft, ω=0.

meBigGuy
Gold Member
Google dc motor efficiency curves and look at torque, current, rpm and power output (efficiency) curves.

When a motor is unloaded it spins at maximum rpm and draws little current and outputs no power. As you load it down it spins slower, draws more current and outputs more power. If the motor were 100% efficient, the Voltage X Current drawn would equal the output power. As the motor slows down due to load, the back emf reduces so the current (and output power) increases.