Question

Provide a thorough explanation on what causes the rotating field within the DC (asynchronous) machine, and how this field impacts motor performance.

Answer 1

DEFINITION :

DC Asynchronous Motor:

A DC motor is an electrical machine that converts electrical energy into mechanical energy.

The working of DC motor is based on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a mechanical force.

The direction of the mechanical force is given by Fleming’s Left-hand Rule and its magnitude is given by F = BIL Newton.

Where F represents force, B magnetic field, I current and L is for length of the conductor. As the force is a vector quantity it must have a direction too. For finding direction of this force we apply Fleming’s left hand rule. If we know the magnitude as well as direction then we can get useful work from a DC Motor.

Performance of DC

Motor Electromotive Force (EMF)
If the shaft of a DC motor is turned by an external force, the motor will act like a generator and produce an Electromotive force (EMF). During normal operation, the spinning of the motor produces a voltage, known as the counter-EMF (CEMF) or back EMF, because it opposes the applied voltage on the motor.

The back EMF is the reason that the motor when free-running does not appear to have the same low electrical resistance as the wire contained in its winding. This is the same EMF that is produced when the motor is used as a generator (for example when an electrical load, such as a light bulb, is placed across the terminals of the motor and the motor shaft is driven with an external torque). Therefore, the total voltage drop across a motor consists of the CEMF voltage drop, and the parasitic voltage drop resulting from the internal resistance of the armature's windings.

Conclusion

As an unloaded DC motor spins, it generates a backwards-flowing electromotive force that resists the current being applied to the motor. The current through the motor drops as the rotational speed increases, and a free-spinning motor has very little current. It is only when a load is applied to the motor that slows the rotor that the current draw through the motor increases