Why is there induced current in a DC motor?

[shirozack]

TL;DR Summary

why is there induced current in a dc motor?

why is the induced force in an ac generator not given by flemming's left hand rule but his right hand rule instead?

I just saw a question saying there's an induced current in a dc motor. i always thought dc motors' electromagnetism is separate from the ac generators' electromagnetic induction? if there's really an induced current, how will the overall system work then? wouldn't the rotation be opposed?

also, why does the force from an induced current not go in the direction of flemming's left hand rule but its right instead? what's the reason?

thanks

 

[Baluncore]

I just saw a question saying there's an induced current in a dc motor.

There is an induced voltage, proportional to RPM, called the back EMF.

 

[sophiecentaur]

"why is there induced current in a dc motor?" (Best to say there is an induced EMF) When the motor is spinning, you have a coil moving through a magnetic field. At the same time, the current through the coil (from the supply) produces a force on the coil. The force is proportional to the current and the current is a result of the supply volts and the opposing induced EMF. The faster the motor runs, the greater the 'back EMF' so the less resultant volts to drive current through. The motor speed will increase to a limit, set by the mechanical load on the motor (which can be its own frictional forces). The torque is maximum when the motor starts at 0rpm.

A motor can destroy itself by over-revving if the coils move apart due to rotational forces before friction imposes a limit. Not a problem with a naff, hand-wound toy model or with a properly built one.