When a DC motor starts from rest, its speed is zero. At this moment, the back electromotive force (back EMF) is also zero. If the motor is connected directly to the supply mains in this condition, a very high current flows through the armature conductors. This happens because the armature resistance is very small, and the only opposition to current flow is this low resistance.
Why High Starting Current is a Problem
From the armature circuit equation:
At starting, , so:
For example, consider a 400 V, 20 kW DC motor with a total armature resistance of 0.5 Ω:
If the motor’s full-load current is 64 A, the starting current would be:
This high inrush current can cause several issues:
- Sparking and flash-overs at the commutator.
- Overheating of the armature winding due to excessive current.
- Mechanical damage from sudden high torque and acceleration.
- Voltage dips in the supply network.
Role of a Starter
To protect the motor during startup, a starter is used. It adds a high resistance in series with the armature at the moment of starting. This resistance is gradually reduced as the motor speeds up and back EMF builds. Once the motor reaches normal speed, this resistance is completely removed.
If this resistance is left in the circuit, it will:
- Cause extra energy loss, reducing motor efficiency.
- Lower the operating speed of the motor.
When a Starter is Not Necessary
For small fractional kW DC motors, a starter may not be required. This is because:
- The natural resistance and inductance of small motor windings are high enough to limit starting current.
- The inertia of a small armature is low, allowing it to reach full speed quickly and reduce the harmful effects of high current.
Key Takeaways
- A DC motor starter is essential for medium and large motors to avoid damage from high starting current.
- Small DC motors can be started directly, but only under specific conditions.
- Gradual reduction of starting resistance ensures safe acceleration and longer motor life.