I have heard that the motor can work as generator when the supply is stopped because back EMF flows from stator coils & charges the battery. Now, even when the motor is running & we supply the voltage to stator, the rotor magnetic field is still cutting the stator coils & generating a back EMF which will oppose the voltage currently being applied to the stator. So what is the role of this back EMF generated & how it affects the motor performance & stator current & voltage ? If conceptually wrong, correct me.

  • $\begingroup$ Related and possibly a duplicate: Back emf and power? $\endgroup$ – John Rennie Dec 30 '17 at 11:39
  • $\begingroup$ I checked it & it's a lil bit complex & confusing.....but still thanx ! $\endgroup$ – Saket Joshi Dec 31 '17 at 7:00

When working as a motor, a electrical machine "uses" its back EMF to regulate the current through the machine. When there is no mechanical load, the angular speed rises until the back EMF perfectly balances the supply voltage, so no current flows and no work is done by the electrical supply on the electrical machine.

When a load is imposed on the machine, the rotation speed slows and the back EMF no longer balances the supply. Current thus flows through the machine, and it is this current which feels the Lorentz force and does work. The higher the load, the lower the back EMF and the greater the current, so that the rate of electrical working on the motor by the supply equals the rate of mechanical work output. When a DC motor is stopped and the supply is switched on, the current and torque can be enormous; a DC motor's torque is typically maximal at zero angular speed. This is an ideal torque speed characteristic for getting heavy loads moving, which is why the DC motor is heavily used in industry and transportation.

  • $\begingroup$ So this means the back EMF of rotor directly opposes the voltage applied to stator winding & the resultant voltage produces required forces, right ? $\endgroup$ – Saket Joshi Dec 31 '17 at 7:06
  • $\begingroup$ As you said, when supply is switched on, current is enormous. So will the V=IR rule decide how much current flows through the coil of stator ? Here, V being applied battery or grid voltage & R being resistance of copper winding......Is it right ? $\endgroup$ – Saket Joshi Dec 31 '17 at 7:12

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