Start procedure

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.

02. Start

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 I_DOL = 5 × I_n (I_n = motor rated current)

The motor voltage during the start U_s is 61% of the rated voltage U_n (U_s/U_n = 0,61).

Therefore the motor current I_Mot = U_s/U_n × I_DOL × F
(F is a factor depending on the motor)

I_Mot = 0,61 × 5 × 0,91 = 2,8 × I_n

Calculation of the mains current I_S:

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.

U_n × I_S = U_s/U_n × I_Mot => I_S = U_s/U_n × I_Mot / U_n
I_S = 0,61 × 2,8 = 1,7 × I_n
For the other starting methods the following is valid: the mains current I_S is equal to the motor current Imot.

I_S = I_Mot = 2,8 × I_n

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.