I don't understand how current reduces speed in an electric motor,while voltage increases it. Wouldn't a stronger magnetic field mean there is a stronger "force" thus should be a higher acceleration, therefore a higher speed of rotation?

  • \$\begingroup\$ Field current or armature current? \$\endgroup\$ Jun 3 '14 at 8:36

Assuming you are referring to the motor slowing when you increase the field current (in a motor with a separate field winding, rather than a permanent magnet motor:

Remember what controls the speed of a lightly loaded motor : the speed increases until the back-EMF approximately equals the driving voltage. The back-EMF is generated by the motor acting as a generator. If the motor was perfectly efficient and completely unloaded, the motor would run fast enough so the back EMF equals the driving voltage; with any further increase in speed it would exceed the driving voltage and the motor would return power to the supply.

In practice the motor doesn't quite reach this ideal speed : the difference in voltage drives current through the motor's resistance, and this supplies the torque to overcome friction and the load.

Now consider what happens if you increase the FIELD current. As you expect, that increases the magnetic field. But remember the motor is acting as a generator, to generate back-EMF. Increasing the field will increase the voltage generated at any given speed : therefore the back-EMF equals the driving voltage at a lower speed. So, the motor slows down.

EDIT re : armature current. From the above explanation it should be clear that increasing armature current (by loading the motor harder) drops more voltage across the armature resistance : V=IR. Subtract this voltage from the driving voltage and you have the new, lower, back EMF : corresponding to a lower speed.

  • \$\begingroup\$ You can not believe how enlightened I feel right now... Its as if something has been blocked in a my mind for a while, now it's free to think and understand and move on! Thank you! \$\endgroup\$
    – Pupil
    Jun 3 '14 at 8:50
  • \$\begingroup\$ What if we increases the armature's current alone? I assume the same would happen? \$\endgroup\$
    – Pupil
    Jun 3 '14 at 8:52
  • \$\begingroup\$ Brian, if back-emf never was never induced increasing current would create a stronger magnetic field, thus a stronger forces, thus the speed would indeed increase? That's why if we increase more power to the electric motor... if it did not heat up it would increase it speed due to the additional current. So in reality the whole concept revolves around the BACK-EMF the current is the cause of the strong force, and that strong force is responsible for higher accelerations. \$\endgroup\$
    – Pupil
    Jun 3 '14 at 13:34
  • \$\begingroup\$ Essentially yes. You can stop back-emf being generated, simply by stalling the motor. Enormous current (maybe 10x the rated current) and enormous torque. But very low efficiency and output power because it's stopped... The current WILL heat it up so don't stall it for long! This happens momentarily when starting; a decent sized motor will trip breakers unless they are "motor rated" to take the abuse. A second revelation here : the IR loss is simply wasted power. So the key to efficiency is to minimise it : run a motor as fast as possible (maximum practical V) and as lightly loaded (minimum I) \$\endgroup\$ Jun 3 '14 at 15:00
  • \$\begingroup\$ ... as lightly loaded (minimum I) as practical. Of course bearings, brushes in old-fashioned brush motors and so on impose mechanical drag, so the peak in efficiency occurs close to maximum speed, minimum load - maybe 80-90% of its unloaded speed. \$\endgroup\$ Jun 3 '14 at 15:09

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