Example of Forward and Reverse Rotation of Squirrel Cage Asynchronous Motor

Analysis of forward and reverse control of squirrel cage asynchronous motor

In the production process, as the production machinery that our maintenance electricians often come into contact with, it is required that the moving parts operate frequently in the forward and reverse directions.

Here are two control circuits to analyze one by one. In figure a, the control circuit of double interlocking of button and contactor is adopted. On the basis of interlocking by using the normally closed auxiliary contact of positive and negative transfer contact, composite buttons SB2 and Sb3 are added. Interlock protection. In this circuit, even if two start buttons are connected at the same time, the positive and negative transfer contacts cannot be powered on. In addition, the composite button is used. After the motor runs in the forward direction, it is not necessary to press the stop button SB1 first. You can directly press the reverse start button to make the motor run in the reverse direction.

In the actual production process, if the operation is frequent, the forward and reverse contactors may still be connected at the same time. In order to avoid simultaneous closing of forward and reverse contactors, an intermediate relay KA can be added to the control circuit to prolong the conversion process. As shown in Figure B. In other occasions, special mechanical interlocking contactors can be selected as appropriate.

Experimental report on forward and reverse control of three-phase squirrel cage asynchronous motor

In the process of motor teaching, many students asked about the forward and reverse control of single-phase motor. Next, we will discuss the forward and reverse control method of capacitive single-phase asynchronous motor.

1、 Working principle of single phase motor

Theoretically, the single-phase AC motor has only one winding. The rotating hand is of squirrel cage type. When the single-phase sinusoidal current passes through the stator winding, the motor will produce an alternating magnetic field. The strength and direction of the magnetic field change sinusoidally with time, but it is fixed in spatial orientation. Therefore, it is also called alternating pulsating magnetic field. The alternating pulsating magnetic field can be decomposed into two rotating magnetic fields with the same speed and opposite rotation directions. When the rotor is stationary, the two rotating magnetic fields produce two torques with equal size and opposite directions in the rotor, so that the synthetic torque is zero, so the motor cannot rotate. When the motor is rotated in a certain direction by external force (such as clockwise rotation), the cutting magnetic line of force between the rotor and the rotating magnetic field in the clockwise rotation direction becomes smaller, and the cutting magnetic line of force between the rotor and the rotating magnetic field in the counterclockwise rotation direction becomes larger. In this way, the balance is broken, and the total electromagnetic torque generated by the rotor will no longer rotate. To make the single-phase motor rotate automatically, a starting winding can be added to the stator. The space difference between the starting winding and the main winding is 90 degrees, and the starting winding (auxiliary winding) should be connected with a suitable capacitor in series, so that the difference seems to be 90 degrees, which is the so-called phase separation principle. In this way, when two currents with a time difference of 90 degrees pass through two windings with a space difference of 90 degrees, a (two-phase) rotating magnetic field will be generated in space. In this way, a rotating magnetic field will be generated in the stator, and its rotating magnetic field is clockwise. Under the action of this rotating magnetic field, the rotor can start automatically. After starting, when the speed rises to a certain value, the starting winding is disconnected with the help of a centrifugal switch or other automatic control device installed on the rotor. Only the main winding works during normal operation. Therefore. The starting winding can be made into short-time working mode. But there are many times when the winding is started and continuously opened. This kind of motor is called capacitive single-phase motor.

2、 Principle of forward and reverse rotation of single-phase motor

The rotation principle of asynchronous motor is to form a rotating magnetic field in the stator winding, and the direction of the rotating magnetic field determines the direction of the motor.

As long as the direction of the rotating magnetic field is changed, the rotation direction of the motor can be changed. As long as the phase sequence of three-phase motor is changed, the direction of rotating magnetic field can be changed, which also changes the forward and reverse rotation of three-phase motor. The single-phase motor divides the single-phase electricity into two-phase electricity with a difference of less than 900 through the phase separation element, capacitor or the resistance of the coil itself, in which the one on the main winding represents one phase electricity and the one on the auxiliary winding represents the other phase electricity.

Therefore, the essence of single-phase motor is "two-phase motor". To change the direction of the single-phase motor, you can change it by changing the phase sequence. However, unlike the three-phase motor, it is not to change the phase sequence of the power supply, but to change the current sequence in the main and auxiliary windings. There are two ways to change the phase sequence in the current of the main and auxiliary windings. One is to change the impedance of the main and auxiliary windings, which is realized by connecting capacitors in series at different positions of the main and auxiliary windings. The second is to change the polarity of the main or auxiliary winding to change the current phase sequence. The following takes two typical single-phase motors as examples.

1. Forward and reverse rotation mode of single-phase washing motor

The operation of single-phase washing motor requires that the parameters of the main and auxiliary windings are consistent in the midship, the forward and reverse output of the motor are the same, and the main and auxiliary windings can be working windings for each other. The single-phase motor forms a rotating magnetic field through the phase separation of the main and auxiliary windings. The difference between the main and auxiliary windings is 900 phase angle. The main winding is directly connected with I. and N, and the auxiliary winding is connected with the power supply after series capacitance. Let the current flowing through the main winding be la and the current flowing through the auxiliary winding be IR. The main winding is basically inductance, the impedance is inductive reactance, which is inductive load, and La lags behind the power supply voltage in phase. The impedance of the secondary winding will decrease and the phase angle with the power supply will also decrease due to the series capacitance. Therefore, IR is one phase angle ahead of La. As long as the appropriate capacitor is selected, IR can exceed La 90 degrees. In this way, the phase difference is generated by the two-phase current and a forward rotating magnetic field is formed to make the motor rotate forward. Similarly, if the secondary winding is directly connected with L and N at this time, the main winding is connected with the power supply after series capacitance.

Then IA is 90 ahead of IR. The phase angle forms a reverse rotating magnetic field to reverse the motor. The circuit diagram is shown in the figure below. A single pole three throw switch is used to switch the position where the capacitor is connected to the main and auxiliary windings. When the knife switch is in the middle position, it is in the stop state. During forward rotation, winding La is directly connected to LN, winding LR and capacitor are connected in series, winding LR is the starting winding, the current is advanced, and it is set as forward rotation. Then change the connection method, winding LR is directly connected to LN, winding La and capacitor are connected in series, winding La is the starting winding, and if the current is ahead, it will rotate in the opposite direction. It can also be seen from the above analysis that the main and auxiliary windings can also be the starting windings of each other.

2. Forward and reverse rotation mode of single-phase electric planer motor

The working requirements of single-phase electric planer motor are that the parameters of main and auxiliary windings are inconsistent, and the forward and reverse output of the motor are also different. The main winding is the working winding. Obviously, this type of motor cannot reverse the motor by using the above method. However, it is known from the above analysis that the secondary winding is connected in series with a capacitor, which generates a phase difference of IR exceeding la900 and forms a rotating magnetic field to make the motor rotate forward. So just let IR lag behind La 90 degrees (or in other words, let La lead ir90).

The motor can be reversed. It's easy to do this. As long as the IR flips 180 degrees (or La flips 180.), it can be done. So how to achieve IR flip 180 degrees (or La flip 180A)? As can be seen from the following wiring diagram, in fact, the main winding and the auxiliary winding are connected to the same power supply, that is, their voltage is in the same phase. If the l-connection WL and n-connection W2 of the auxiliary winding are changed to l-connection W2 and n-connection W1, the main winding remains unchanged. Maintain ln connection uiu2. Then there is a phase difference of 180 degrees between the voltage of the main winding and the voltage of the auxiliary winding, that is, the current of the auxiliary winding is also reversed by 180 degrees. At this time, IR lags behind IA by 90 degrees (or in other words, IA leads ir90 degrees). The direction of the rotating magnetic field is counterclockwise, which realizes the reversal.

In the actual circuit, the secondary winding and capacitor are connected to the power supply through forward and reverse switches. What this circuit changes is not which winding the capacitor is connected to, but the polarity of the winding. In theory, if you want to change the steering of single-phase motor, you can change the connection form of main and auxiliary windings, adjust the head and tail of main winding, or adjust the head and tail of auxiliary winding, which can change the rotation direction of single-phase motor. Because the current of the main winding is much larger than that of the auxiliary winding, it is generally used to change the wiring of the auxiliary winding to change the rotation direction of the single-phase motor. When the motor is working, the rotation of the rotating magnetic field always changes from the winding with advanced current to the winding with backward current. If the terminals of windings are reversed, the current phase of the winding will be changed to reverse, from advanced (backward) to backward (Advanced). The rotation of the rotating magnetic field will change accordingly. The circuit diagram is shown in the figure below. A double pole double throw switch is used to switch the access polarity of main and auxiliary windings. During forward rotation, the main windings UI and U2 are directly connected to LN, and the auxiliary windings are connected through switches WL and L. W2 and N are connected. The auxiliary winding is the starting winding, the current is advanced, and is set to forward rotation. Then change the connection method, the main windings UL and U2 are still directly connected to LN, and the auxiliary windings are connected through switches W2 and L. W1 and N are connected, and the auxiliary winding is still the starting winding. If the current lags behind, it will rotate in the opposite direction. From the above analysis, it can be seen that the secondary winding is always the starting winding.

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