Start control circuit diagram of frequency sensitive rheostat for three phase motor

The frequency sensitive rheostat is applied to start the control circuit of the wound rotor asynchronous motor as shown in the figure below. The starting process is as follows: turn on the power switch QS first and then:

Press the start button SB1, connect the contactor KM1, connect the motor to the power supply, and connect the rotor circuit into the frequency sensitive rheostat RFs in series. At this time, the value of the equivalent resistance and reactance of the frequency sensitive rheostat is very large, which limits the starting current and improves the starting torque. After the time relay KT is connected, after a period of time, its normally open contact of delay closing is closed, and the intermediate relay KA is connected. At this time, the motor has been started and accelerated for a period of time, and the equivalent resistance and reactance of the frequency sensitive rheostat are also very small. Ka’s dynamic closing contact closes the contactor km2, and its dynamic closing contact shortens the frequency sensitive rheostat, and the start-up process ends.

  

TA in the figure is a current transformer, its primary side is connected in series with the motor stator circuit, and its secondary side is connected with the thermal relay fr. In the process of starting, Ka’s normally closed electric shock shortens fr’s heating element to avoid thermal relay misoperation due to long starting process.

Starting control circuit diagram of rotor series resistance of three-phase wound motor

In order to limit the starting current and improve the starting torque, several starting resistances or frequency sensitive resistors can be connected in series in the rotor circuit. This paper will describe the starting control circuit of series resistance in the rotor circuit of three-phase wound motor, and its circuit diagram is shown in the figure below.

The working principle is: after the power switch QS is closed, more than KT3 of time relays kt1 and kt2 are connected, and their normally closed contacts of delay closing are immediately opened, so that KM1, km2 and KM3 will not be connected temporarily, so that when the motor stator winding is started with rated voltage, the starting resistors RI, R2 and R3 are connected in series in the rotor circuit to limit the starting current and improve the starting torque.

  

When starting, first press button SB1 to connect undervoltage relay KAV, its dynamic contact is closed. When the power supply voltage is seriously reduced or the circuit suddenly loses power, KAV’s dynamic contact is disconnected to protect the motor. Then press button SB2 to connect the line contactor km, and the stator winding of the electric motor is started with rated voltage. In the control circuit, the auxiliary contact of KM is opened, the time relay kt1 is powered off, its normally closed contact of delay closing is closed for a period of time, the contactor KM1 is connected, and the starting resistance R1 of motor rotor circuit series connection is cut off. At this time, the motor only operates on the artificial characteristics of starting resistances R2 and R3 in the rotor circuit, and continues to accelerate.

After the contactor KM1 is connected, its dynamic contact is open, which makes the time relay kt2 power off, its normally closed contact with delay closing is closed for a period of time, and then the contactor km2 is connected, and the starting resistance R2 in the motor rotor is cut off. The motor operates on the artificial characteristics with only resistance R3, and continues to accelerate. After the contactor km2 is connected, its normally closed contact is open, the time relay KT3 is powered off, and its normally closed contact with time delay is closed for a period of time, so that the contactor KM3 is connected, and the starting resistance R3 is removed. So far, the rotor circuit of the motor has no external resistance and operates on the natural characteristics. This concludes the startup process.

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