Why can a DC motor run on a DC supply, but a transformer will be damaged if connected to the same DC supply?
1 Answer
The reason lies in their fundamentally different operating principles: Back EMF in a DC motor versus Mutual Induction in a transformer.
1. The DC Motor: Operation via Back EMF
A DC motor works on the principle that a current-carrying conductor in a magnetic field experiences a force. When a DC voltage is applied to the motor's armature windings, current flows, and the armature begins to rotate.
The crucial concept here is Back EMF (Electromotive Force). As the armature conductors rotate and cut through the magnetic field, a voltage is induced in them, according to Faraday's Law of Induction. This induced voltage opposes the applied supply voltage and is called "Back EMF."
The effective voltage across the armature is (Supply Voltage - Back EMF). This difference drives the current, which is limited by the armature's resistance:
Armature Current (Ia) = (V - Eb) / Ra
- At startup: The motor is stationary, so the Back EMF (Eb) is zero. The current is very high (
Ia = V / Ra), which is why DC motors need starters. - During operation: As the motor speeds up, the Back EMF increases, reducing the armature current to a safe, stable level required to handle the mechanical load. The Back EMF is the motor's self-regulating mechanism; it prevents excessive current from flowing and destroying the windings during normal operation.
2. The Transformer: Operation via Mutual Induction
A transformer operates on the principle of mutual induction, which requires a changing magnetic field to induce a voltage in the secondary coil.
With an AC Supply: An alternating current (AC) in the primary winding creates a continuously changing and reversing magnetic flux in the iron core. This changing flux links with the secondary winding, inducing an AC voltage in it. The primary winding also has a high inductive reactance which opposes the flow of AC current, keeping it at a safe, low level (the no-load current).
With a DC Supply: When a DC voltage is applied to the primary winding, the current is constant. This creates a strong but steady, unchanging magnetic field.
Since the magnetic field is not changing, there is no mutual induction. No voltage will be induced in the secondary winding. The transformer completely fails to perform its function.
More dangerously, a transformer's primary winding is just a coil of wire with very low DC resistance. In an AC circuit, the current is limited by high inductive reactance. In a DC circuit, frequency is zero, so inductive reactance is also zero. The only thing limiting the current is the winding's tiny resistance. According to Ohm's Law (I = V/R), this results in an extremely large current flowing through the primary winding. This massive current causes rapid overheating (P = IĀ²R), which will quickly burn out the winding's insulation and destroy the transformer, likely causing a short circuit and a fire hazard.
In summary: A DC motor safely regulates its own current using the speed-dependent Back EMF it generates. A transformer relies on a changing magnetic field from an AC supply and has no internal mechanism to limit a DC current, leading to its rapid destruction.