Starting Induction Motors
Induction Motors can usually be started by simply connecting them to the supply line voltage. This can, however, cause power system problems. Start currents (or "in-rush" currents) are high, often about 6 times rated current. Starting a motor directly from the supply line can cause the supply voltage to dip, or "sag", due to the higher than normal voltage drop across supply line and transformer impedances. In severe cases, this voltage sag can cause supply instability or the failure of sensitive power electronics connected to the system.
Wound Rotor Motors
Start behaviour of wound rotor machines can be controlled by adding resistance to the rotor circuit, as shown below:
In the circuit model above, the actual resistance RE has been referred to the rotor as R′E. It can be seen from the circuit model that the increased impedance will reduce the start current. In addition, the extra rotor resistance will increase the motor torque during starting, reducing the start time and overall impact of the in-rush current.
Cage Machines
Cage motors cannot have additional resistance added to the rotor circuit. A number of options are available for cage motor starts:
- Full Voltage, or direct-on-line start
- Wye-Delta Start
- Adding additional circuitry to the stator, including
- Autotransformer
- Additional resistance
- Electronic soft-start or variable speed start