Starting of cage type three-phase asynchronous motor

There are two methods for starting squirrel cage asynchronous motors, namely direct starting (full voltage starting) and reduced pressure starting.

The simplest starting method for squirrel cage asynchronous motors is direct starting, also known as full voltage starting. The wiring is shown in the following figure.

The commonly used starting devices for direct starting include three-phase knife switches, iron shell switches, and electromagnetic switches. When starting, the rated voltage is directly applied to the stator winding through the switch to start the motor. The advantage of this starting method is that the equipment is simple and starts quickly; The disadvantage is that the starting current is high.

When the power capacity (i.e. transformer capacity) is large enough and the motor capacity is small, this method can be used without causing the power supply voltage to decrease due to the starting of a certain motor

There are significant fluctuations. In this regard, each local power department has its own regulations. In general, whether the capacity of the power supply can allow the motor to start directly at the rated voltage can be determined by the following empirical formula:

Ist/IN<3/4+Transformer capacity (kVA)/[4 x Motor power (KW)]

If the calculation result cannot satisfy the above equation, depressurization starting should be used. In general, motors above 10KW should not be started directly and should be started with reduced pressure.

Reduced voltage starting refers to the starting method of an electric motor, where the voltage applied to the stator winding is reduced during start-up, and the rated voltage is applied at the end of start-up.

Although depressurization starting can reduce the starting current of the motor, the torque of the motor is proportional to the square of the voltage, so the torque of the motor decreases more during depressurization starting. Therefore, this method is only suitable for starting the motor under no-load or light load. There are several methods for starting under reduced pressure.

This starting method involves connecting appropriate resistors in series in the stator winding circuit, as shown in Figure a. First, close switch S1. At this point, the starting current generates a voltage drop across resistor R, and the voltage between the stator windings is lower than the power supply voltage. After the motor speed increases, close switch S2 and short-circuit resistor R. Apply all the line voltage to the stator winding, and the motor will work normally. Starting with a series resistor for voltage reduction consumes a large amount of electrical energy on the resistor, so it cannot be used in situations where it is frequently started. But its advantages are simple equipment, easy operation, and low price. Replacing resistors with reactors for starting does not have the aforementioned drawbacks, but the equipment cost is relatively high.

  2) The autotransformer voltage reduction starting method uses an autotransformer to reduce the voltage applied to the stator winding of the motor. The principle circuit diagram is shown in Figure b. When starting, first close S1 and then throw S2 to the starting position. At this time, the autotransformer reduces the power supply voltage and applies it to the motor, limiting the starting current. After the motor speed increases, throw S2 to the running position, and the motor will operate normally at the rated voltage. At this point, the autotransformer has been disconnected from the power grid.

If the voltage ratio of the autotransformer is K and the primary voltage is U1, then the secondary voltage is U2=U1/K, and the secondary current decreases proportionally. Also, because the current relationship between the primary and secondary sides of the transformer is I1=I2/K, it can be seen that the primary current is smaller than the current flowing directly through the stator winding of the motor. At this time, the starting current supplied by the power supply to the motor is 1/K2 times that of direct starting. Therefore, using an autotransformer for voltage reduction starting is very effective in limiting the starting current supplied by the power supply to the motor. Due to a decrease in voltage by 1/K times, the torque of the motor has also decreased by 1/K2 times.

There are 2-3 sets of taps on the secondary side of the autotransformer, and its voltage can be divided into 80%, 65% or 80%, 60%, 40% of the primary voltage U1

three

(3) The star delta (Y - △) depressurization starting method is only applicable to motors that operate in a delta connection. When starting, first connect the winding into a star shape, and then change it into a triangle shape after the motor speed increases. The principle is shown in the following figure. When starting, place the Y - △ conversion switch S2 in the starting position, and connect the ends U2, V2, and W2 of the motor stator three-phase winding to a common point. The three-phase power sources L1, L2, and L3 supply power to the ends U1, V1, and W1 of the motor stator three-phase winding through switch S1, and the motor starts in a star connection. The voltage applied to each phase stator winding is 1/√ 3 times the power line voltage, so the starting current is relatively small. When the start of the motor is about to end, turn S2 to the running position. The three-phase stator windings of the motor are connected in a triangular shape. At this time, the voltage applied to each phase winding of the motor is the line voltage U1, and the motor runs normally.

When starting with Y - △ pressure reduction, the starting current is 1/3 of the starting current when directly using △ connection, so it is very effective for reducing the starting current. However, the starting torque is only 1/3 of that when directly starting with △ connection, which means the starting torque is greatly reduced. Therefore, it can only be used on equipment with light load or no-load starting. The biggest advantage of this method is that the required equipment is simple and the price is low, thus it has been widely used.

4) The Yanbian triangle starting method is similar to the star triangle method in that it uses a mixture of star and triangle connections. The stator winding is partially connected in a star shape and partially connected in a triangle shape, which looks like the three sides of the triangle have been extended, hence it is called the "Yanbian triangle". This method can compensate for the insufficient starting torque during Y - △ starting.

The above-mentioned pressure reducing starting methods all reduce the starting current and starting torque at the same time, so cage motors are usually started under no-load or light load, and then run with mechanical load after starting.