Principle Diagram of Three-phase Asynchronous Motor Forward and Reverse Rotation Control
Main electrical components: 3 button switches, 2 contactors and 1 Thermal overload. It is better to add 3 fuses to protect 3 fire wires.
In order to achieve positive and reverse control, the motor can adjust any two relative phases of its power supply (we call commutation). Usually, the V phase is unchanged, and the U phase and W phase are relatively adjusted. In order to ensure that the two contactors can reliably change the phase sequence of the motor when they move, the upper connection of the contactor should be kept consistent, and the lower connection of the contactor should be phased. Because the two phases are aligned in phase sequence, it is necessary to ensure that the two KM coils can not get electricity at the same time, otherwise serious inter-phase short circuit fault will occur, so interlocking must be adopted.
1 Electrical Principle Analysis
In order to achieve positive and reverse control, the motor can adjust any two relative phases of its power supply (we call commutation). Usually, the V phase is unchanged, and the U phase and W phase are relatively adjusted. In order to ensure that the two contactors can reliably change the phase sequence of the motor when they move, the upper connection of the contactor should be kept consistent, and the lower connection of the contactor should be phased. Because the two phases are aligned in phase sequence, it is necessary to ensure that the two KM coils can not get electricity at the same time, otherwise serious inter-phase short circuit fault will occur, so interlocking must be adopted. For the sake of safety, the double interlocking positive and reverse control circuit of button interlocking (mechanical) and contactor interlocking (electrical) is often used (as shown in the figure below); the button interlocking is used, even if the positive and negative buttons are pressed simultaneously, the two contactors used for phase modulation can not get electricity at the same time, and the short circuit between phases is avoided mechanically. In addition, due to the application of contactor interlocking, as long as one of the contactors is electrified, its long closed contacts will not be closed, so in the application of mechanical and electrical double interlocking, the power supply system of the motor can not be short-circuited between phases, effectively protecting the motor, but also avoiding the phase-to-phase short-circuiting caused by accidents, burning the contactor.
2 Electrical Principle Explanation
In the figure, two contactors are used in the main circuit, i. e. forward and reverse contactors KM1 and reverse contactors KM2. When three pairs of main contacts of contactor KM1 are connected, the phase sequence of three-phase power supply is connected to motor according to U-V-W. When three pairs of main contacts of contactor KM1 are disconnected and three pairs of main contacts of contactor KM2 are connected, the phase sequence of three-phase power supply is connected to the motor according to W-V-U, and the motor rotates in the opposite direction. The circuit requires that contactor KM1 and contactor KM2 can not be connected at the same time, otherwise their main contacts will be closed at the same time, resulting in short circuit of U and W two-phase power supply. For this reason, a pair of auxiliary normal closed contacts are connected in series with each other in the branches of KM1 and KM2 coils to ensure that contactors KM1 and KM2 will not be connected to power at the same time. The two pairs of auxiliary normal closed contacts, KM1 and KM2, play a role in the circuit called interlocking or interlocking. These two forward start-up processes are called interlocking or interlocking contacts for auxiliary normal closed contacts.
Forward startup process
Press the start button SB2, the contactor KM1 coil is electrified, and the auxiliary open contacts of KM1 parallel to SB2 are closed to ensure that the KML coil is continuously electrified, the main contacts of KM1 in series in the motor circuit are continuously closed, and the motor is continuously running forward.
Stopping process
Press the stop button SB1, the contactor Kml coil is cut off, and the auxiliary contact of KM1 parallel to SB2 is disconnected to ensure that the Kml coil is continuously losing power, the main contact of Kml in series in the motor circuit is continuously disconnected, the stator power supply of the motor is cut off, and the motor is stopped.
Reverse startup process
Press the start button SB3, the contactor KM2 coil is electrified, and the auxiliary open contacts of KM2 parallel to SB3 are closed to ensure that the KM2 coil is continuously electrified, the main contacts of KM2 in series in the motor circuit are continuously closed, and the motor is continuously running backward.
Forward and Reverse Control of Contactor Interlocking in 3-Phase Asynchronous Motor
Forward and reverse control of three-phase asynchronous motor
Two electromagnetic contactors, KM1 and KM2, used for forward and reverse respectively, are used to switch the voltage phase of the motor. At this time, if the electromagnetic contactor KM1 is used, the power supply and the motor pass through the main contact of the contactor KM1, so that the L1 phase and the U phase, the L2 phase and the V phase, the L3 phase and the W phase correspond to each other, so the motor rotates forward. If the contactor KM2 operates, the power supply and the motor are connected through the main contact of KM2 so that the L1 phase and W phase, L2 phase and V phase, L3 phase and U phase are connected respectively. Because the L1 phase and L3 phase are exchanged, the motor rotates backwards.
Safety measures for forward and reverse control of three-phase asynchronous motor
In the forward and reverse control operation of the motor, what happens if the forward and reverse electromagnetic contactors are mistakenly operated at the same time to form a closed circuit? The voltage between the L1 and L3 phases of the three-phase power supply forms a complete short-circuit state by reversing the main contacts of the electromagnetic contactor, so there will be a large short-circuit current flowing through and burning the circuit. Therefore, in order to prevent short circuit accidents of two-phase power supply, the main contacts of contactors KM1 and KM2 must not be closed at the same time.
As shown in the figure above, in order to ensure that one contactor can't operate electronically when the other contactor can't operate electronically to avoid short circuit between power sources, the normal closed auxiliary contacts of the reverse contactor KM2 are connected in series in the forward control circuit, while the normal closed of the forward contactor KM1 is connected in the reverse control circuit. Auxiliary contacts. When the contactor KM1 is operated electronically, the normally closed contacts of KM1 Series in the inversion control circuit are broken, and the inversion control circuit is cut off, which ensures that the main contacts of KM1 can not be closed when the main contacts of KM1 are closed. Similarly, when the contactor KM2 is electrically operated, the normally closed contacts of KM2 are broken and cut.