When comparing between an electric motor, and a generator. I find them being quite similar. I can use and motor as a generator and vice versa.

What's confusing is the principle, the Lorentz force applied in the case of a motor, it causes the torque & acceleration of the armature. Simultaneously, causes the motional back-emf on the motor when it reaches high speeds.

What about the case of a generator? Certainly there is emf due to the Lorentz force, and when there is a load connected to a generator, current flows inside that magnetic field,is there a Lorentz force due to that generated current flow that opposes the applied mechanical input that induced the voltage of the generator?

  • \$\begingroup\$ Of course. That's how the mechanical power is extracted. \$\endgroup\$
    – user16324
    Nov 30, 2015 at 13:03
  • \$\begingroup\$ Back EMF is always present, if BEMF is higher than applied voltage then its in a generator mode, otherwise its in motor mode. Both principles (motor and generator) are present all the time, the dominant one determines the mode of operation. \$\endgroup\$ Nov 30, 2015 at 14:18
  • \$\begingroup\$ @BrianDrummond I don't understand, so there is a Lorentz force opposite to the direction of the mechanical input in every single generator? That's why when connected to a load, it's difficult to generate current? \$\endgroup\$
    – AxtII
    Nov 30, 2015 at 14:28

1 Answer 1


Energy is preserved all the time, when it works like a motor it ouputs torque to mech. load, when acts like a generator makes a negative torque that has to be feed from mechanical drive. When there is no electrical load at generator output then there is no negative torque, the mech. drive (ex. diesel motor) is unloaded. As soon there is a load present at generator output, then the engine is loaded - it has to supply enough torque to spin the generator.


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