The national standard stipulates that three-phase asynchronous motors are not allowed to start directly under the following two circumstances:

① The rated capacity is greater than 7.5kW;

② When the motor is powered by a special transformer, The capacity is greater than 20% of the transformer capacity;

Motors that are not suitable for direct start can be started by the following methods:

① Use a motor starter with appropriate power;

② Use a frequency converter with appropriate power;

③ Use a star-delta conversion circuit;

Using a star-delta conversion circuit to start is a relatively simple and cost-saving way. The cooperation of the three contactors causes the three-phase stator windings to form a star connection (Y) in the early stages of starting the motor. After the motor has been running for a period of time, the three-phase windings are converted to a delta connection (△).

In star connection, the phase voltage (220V) is loaded on the stator winding. In delta connection, the line voltage (380V) is loaded on the stator winding. It can be seen that star-delta start is a reduced voltage start. .

Star-delta step-down starting is suitable for motors whose stator windings are delta-connected during normal operation (the national standard stipulates that motors above 4kW should adopt delta connection). The circuit includes the main circuit and the control circuit. The control loop can be completed by buttons, contactors, and time relays. In this case, PLC does not need to be involved.

In addition, PLC can also be used to realize timing/interlocking and other control loop functions. In this case, the wiring of the control loop will be relatively simple, and a time relay can also be saved. Regardless of whether a PLC is used or not, the wiring of the main circuit remains unchanged.

In today’s article, I will share with you a function block for motor star-delta start written in SCL language.

The motor main circuit circuit diagram of this routine is as shown below:

Among them:

Q1: circuit breaker, with normally open contact;

KM1: main contact KM2: Star contactor, with normally open contacts;

KM2: Star contactor, with normally open contacts;

KM3: Triangular contactor, with normally open contacts;

FR1: Thermal relay, with normally closed contact;

Create a new function block FB5021_motorStartUp-star-delta in the Portal environment, and declare variables as shown in the following two pictures:

①Input/output variables:

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②Static variables:

The code is as follows:

I have tested this code, as shown below ：