In this method, a voltage-dropping resistance is placed in series with the motor during starting. The impedance seen by the power system then is that of the resistance plus that of the motor.
START: R OPEN, S CLOSED
RUN: R CLOSED, S OPEN
CLOSED TRANSITION (R CLOSES BEFORE S OPENS)
Series Resistance Reduced Voltage Starting
- Motor terminal voltage is reduced from line voltage.
- Motor current equals line current.
- Starting torque is reduced by the square of the terminal voltage.
- Usually on low voltage (less than 600 v).
- Where current reduction requirements are low, or where load torque during acceleration is minimal.
- Not often used with large motors because of the high heat loss in the resistors.
- May be used for full acceleration or for system voltage recovery.