With this system, the motor starts at reduced voltage because resistors are inserted in series with the windings. When the speed stabilises, the resistors are eliminated and the motor is connected directly to the mains.
This process is usually controlled by a timer.
This starting method does not alter the connection of the motor windings so the ends of each winding do not need outputs on a terminal board.
The resistance value is calculated according to the maximum current peak on starting or the minimum starting torque required for the resistance torque of the machine to drive. The starting current and torque values are generally:
– SC = 4.5 RC
– ST = 0.75 RT
During the acceleration stage with the resistors, the voltage applied to the motor terminals is not constant but equals the mains voltage minus the voltage drop in the starting resistance.
The voltage drop is proportional to the current absorbed by the motor. As the current weakens with the acceleration of the motor, the same happens to the voltage drop in the resistance. The voltage applied to the motor terminals is therefore at its lowest on starting and then gradually increases.
As the torque is proportional to the square of the voltage at the motor terminals, it increases faster than in star-delta starting where the voltage remains constant throughout the star connection.
This starting system is therefore suited to machines with a resistive torque that increases with the speed, such as fans and centrifugal pumps.
It has the drawback of a rather high current peak on starting. This could be lowered by increasing the resistance value but that would cause the voltage to drop further at the motor terminals and thus a steep drop in the starting torque.
On the other hand, resistance is eliminated at the end of starting without any break in power supply to the motor, so there are no transient phenomena.