Medium Voltage Application Guide

13531.A
SOFT STARTERS
Slip ring motor control
The principle of a slip-ring motor is that external rotor resistance provides the necessary motor torque during
acceleration to full speed. Once the motor is close to full speed, the external rotor resistance is shorted out and the
motor operates as a standard three phase induction motor.
Old slip-ring motor systems typically consist of a liquid resistance tank with an electrode, or else a series of cast-iron
or wire wound resistor banks with a changeover switch. These systems require mechanical intervention for motor
starting, can become mechanically unreliable, and require regular maintenance.
AuCom medium voltage soft starters include a unique function specifically for slip-ring motor control. This function
is not suitable for applications where a slip-ring motor is being used for speed control or to develop excess start
torque (ie more than 100% motor full load torque to break away).
Some rotor resistance is required in order to start the motor. This rotor resistance (R1) is shorted when the motor
is close to full speed, using rotor resistance contactor K3. The contactor must be AC2 rated for the nameplate
rotor current.
AuCom medium voltage soft starters use a "Dual Ramp" start function. This provides a voltage ramp with constant
current control while the rotor resistance is in the circuit. This is followed by a smooth transition when shorting out
the rotor resistance. A second voltage ramp with constant current control is provided for acceleration to full
running speed.
Typical slip-ring motor starting system using a soft starter for control
R1
M1
M
3
T1
T2
SST
L1
L2
K1
1
T3
L3
K3
K2
Mains supply R1 Rotor resistance (single stage)
K1 Main contactor SST Soft starter
K2 Bypass contactor M1 Slip ring (wound rotor) motor
K3 Rotor resistance contactor
Operating sequence
NOTE
The rotor resistance must be engineered to provide the necessary acceleration torque during motor
starting. This example assumes a soft starter which offers a dedicated slip-ring motor control function
(eg AuCom MVS or MVX).
Starting control sequence
1. The soft starter SST is given a start command and main contactor K1 closes.
2. The soft starter performs a series of prestart checks, then ramps up to full voltage using Ramp 1
3. Once the rotor has reached a constant speed, the voltage on the output of the soft starter SST is backed off and
rotor resistance contactor K3 closes, shorting out rotor resistance R1.
4. The output of the soft starter SST is ramped-up to full voltage using Ramp 2, accelerating the motor to full
speed.
5. Bypass contactor K2 closes and the starting sequence is complete.
Page 66 Medium Voltage Application Guide 710-12280-00A