A three‑phase cage motor at standstill is like a transformer with a shorted secondary. The rotor is not turning, so nothing opposes the supply but the windings' impedance. The motor draws its locked-rotor current: often six to eight times its full load current. A starter exists to bring that current down. Every way of doing it has a cost in torque.
The two rules
At standstill:
- The current follows the voltage at the motor terminals. Half the voltage, half the current.
- The torque follows the square of that voltage. Half the voltage, a quarter of the torque.
So a starter that cuts the voltage cuts the torque faster than the current.
The methods
| Method | Line current | Starting torque |
|---|---|---|
| Direct on line | The locked-rotor current | The locked-rotor torque |
| Star-delta | One third | One third |
| Autotransformer at tap k | k squared of it | k squared of it |
| Primary resistance leaving k | k of it | k squared of it |
| Soft starter, current limit | The limit | The limit over the locked-rotor current, squared |
| Variable speed drive | About its current limit | About its current limit, as a share of full load torque |
- Star-delta. In star each winding sees the line voltage over the square root of 3. Its current falls by that factor, and the line current falls by it again, because the windings are in star. The result is a third of the current and a third of the torque.
- Autotransformer. The motor sees the tap voltage, so its current and torque fall as before. The transformer then steps the current down again on the line side. The line current and the torque both fall with the tap squared: a 65 % tap gives about 42 % of each.
- Soft starter. It holds the current at a limit by lowering the voltage. The torque is still the voltage squared, so a soft starter at 3.5 times full load current on a motor of 6 times gives about a third of the locked-rotor torque.
- Variable speed drive. It starts the motor at low frequency with the voltage scaled to keep the flux. The motor runs on the steep part of its curve, so it gives full load torque at about full load current. It is the only method that cuts the current without cutting the torque.
Will it start the load?
A method starts the load where its starting torque is above the torque the load needs to break away. A fan or centrifugal pump needs little at standstill, so every method starts it. A loaded conveyor, a crusher or a positive displacement pump may need full load torque or more. Star-delta's third is then not enough.
The dip
The starting current flows through the supply's impedance and pulls the voltage down. A rough estimate is the starting current over the fault current at the motor: 290 A from a 4 kA supply is about a 7 % dip.
Compare the methods for a motor with the motor starting tool. A worked conveyor is in Starting a loaded conveyor.