The main components of the motor starter are: starting transformer T, By-pass switch B and star-point switch S. The mains switch is located in the MV distribution.
01. Before the start
N is open. The motor is standing still.
B open, S closed, N closed. The motor is started with reduced voltage.
Transformation provides a lower start current
Compared to other starting methods (electronic soft starter, starting reactor etc.) the mains current is lower than the motor current during the starting process. This results in a lower voltage drop respectively a higher motor torque.
Example for a calculation:
The DOL starting current of the motor is IDOL = 5 × In (In = motor rated current)
The motor voltage during the start Us is 61% of the rated voltage Un (Us/Un = 0,61).
Therefore the motor current IMot = Us/Un × IDOL × F
(F is a factor depending on the motor)
IMot = 0,61 × 5 × 0,91 = 2,8 × In
Calculation of the mains current IS:
A transformer has the same power at primary and secondary, which leads to the fact that the products of current and voltage are the same.
Un × IS = Us/Un × IMot => IS = Us/Un × IMot / Un
IS = 0,61 × 2,8 = 1,7 × In
For the other starting methods the following is valid: the mains current IS is equal to the motor current Imot.
IS = IMot = 2,8 × In
03. Switch over
N remains closed. S will be opened. Immediately after B will be closed. During the switching over (S already opened, B not yet closed) the starting transformer works as a reactor. This results in a continuous voltage supply of the motor.
B closed, S open, N closed. The motor is running with rated voltage.
N will be opened. The motor stops.