Working Principle of DC Motor | Back EMF & Types Explained

Theoretically, the same DC machine can function as either a dc motor or a dc generator. Therefore, the construction of a DC motor is identical to that of a DC generator.

Working Principle of a DC Motor

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

The basic operating principle is:

“Whenever a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force.”

The direction of this force is determined by Fleming's Left-Hand Rule, and the magnitude is given by:

F = BIL, where:

• B = Magnetic flux density
• I = Current through the conductor
• L = Length of the conductor in the magnetic field

Fleming's Left Hand Rule

If the thumb, forefinger, and middle finger of the left hand are held mutually perpendicular:

• First finger → Direction of magnetic field
• Second finger → Direction of current
• Thumb → Direction of force (motion)

Animation: Working of a DC Motor (Credit: Lookang)

As seen in the animation, when the armature winding is connected to a DC supply, current flows through it. This current-carrying conductor placed in a magnetic field experiences a force, causing motion.

To ensure unidirectional torque, a commutator is used. Without it, the direction of the force would reverse every half-turn of the conductor.

What is Back EMF in a DC Motor?

According to fundamental laws of nature, no energy conversion is possible until there is something to oppose the conversion. In case of generators this opposition is provided by magnetic drag, but in case of dc motors there is back emf.

According to electromagnetic induction principles, any conductor moving in a magnetic field generates an EMF. In a DC motor, this induced EMF is called back EMF (E_b).

Back EMF opposes the supply voltage and reduces the net voltage across the armature. Its direction, as per Lenz’s Law, is such that it opposes the armature current (I_a).

Diagram: Direction of Back EMF and Armature Current

The magnitude of back EMF can be calculated using the EMF equation of a DC generator, as the working principle is the same in reverse.

Significance of Back EMF

Back EMF makes the DC motor self-regulating:

• When load decreases: Motor speed increases → back EMF increases → armature current decreases → torque decreases to match the lower load.
• When load increases: Motor slows down → back EMF decreases → armature current increases → torque increases to meet new load.

This self-adjusting mechanism is crucial for stable motor operation.

Types of DC Motors

DC motors are classified based on their excitation method:

• Separately Excited: Field winding is powered by an external source.
• Self-Excited:
  • Series Wound: Field winding in series with the armature.
  • Shunt Wound: Field winding in parallel with the armature.
  • Compound Wound:
    • Long Shunt
    • Short Shunt

See detailed classification of DC machines for diagrams and comparisons.