Circuit diagram of motor inching control (1)
Inching control is to press the button to start the motor, release the button motor power loss until stop.
The control circuit schematic diagram is as follows:
Start up:Press start button sb → contactor km coil is powered on → km main contact is closed → motor M starts running.
stop it:Release button sb → contactor km coil loses power → km main contact is disconnected → motor M loses power and stops running.
Circuit diagram of motor inching control (2)
The so-called inching control refers to:Press the button, the motor will be powered on to run; release the button, the motor will lose power and stop running. This control method is often used in the control of the electric hoist’s lifting motor and the motor control of the lathe carriage moving rapidly. The control circuit of inching and unidirectional rotation is the simplest control circuit with button contactor to control motor operation. The wiring diagram is shown in the figure below.
It can be seen from the figure that the inching forward rotation control circuit is composed of transfer switch QS, fuse Fu, start button sb, contactor km and motor M. Among them, the transfer switch QS is used as the power isolating switch, the fuse Fu is used for short-circuit protection, the button sb controls the power on and power loss of the coil of the contactor km, and the main contact of the contactor km controls the start and stop of the motor M. the working principle of the line is as follows:
When motor M needs inching, switch on the transfer switch QS first, and the motor M has not been connected to the power supply. When the start button sb is pressed, the coil of contactor km is powered on to make the armature close. At the same time, the three pairs of main contacts of contactor km are driven to close, and the motor M is connected with the power to start operation. When the motor needs to stop running, just release the start button sb to make the coil of contactor km lose power, and the armature will reset under the action of reset spring, which will drive three pairs of main contacts of contactor km to resume disconnection, and motor M will stop running after power loss.
The above diagram shows the wiring diagram of the point dynamic forward rotation control in the figure above. It looks intuitive and easy for beginners to learn and understand, but it is very troublesome to draw. Especially for some complex control circuits, because of the large number of electrical appliances used, the form of wiring diagram drawing makes people feel complicated and difficult to understand, which is not practical.
Therefore, the control circuit usually does not draw the wiring diagram, but uses the national unified electrical equipment graphic symbols and text symbols to draw the control circuit schematic diagram. The schematic diagram of inching forward rotation control circuit is shown in the right figure.
It is drawn according to the physical wiring circuit. In the diagram, symbols represent electrical components and lines represent connecting wires. It is used to express the working principle of control circuit, so it is called schematic diagram. Schematic diagram has been widely used in design department and production site.
In the analysis of various control circuit schematic diagram, in order to be simple and clear, it is usually used to indicate the working principle of the circuit with a small number of words. For example, the working principle of inching forward turning control circuit can be described as follows:
First close the power switch QS, start: press the start button sb → contactor km coil is powered on → km main contact is closed → motor M starts running.
stop it: release the start button sb → contactor km coil loses power → km main contact is disconnected → motor M loses power and stops running. When it is out of use, disconnect the power switch QS.
When the motor is required to start continuously, it is only necessary to connect a stop button in series on the control circuit in the figure above, and connect a normally open auxiliary contact of a contactor at both ends of the start button. As shown in the figure on the right. Working principle of the circuit: first close the power switch QS:
Start up:Press start button SB1 → km coil is energized → km dynamic closing auxiliary contact is closed (self-locking), KM main contact is closed → motor M is started and running continuously.
When the SBI is released, it returns to the off position. Since the SBI is connected in parallel with a contact of the contactor, the coil is energized and the contact continues to be connected. This use of the contactor itself to close the contact to keep the coil of the contactor energized is called self-locking. The auxiliary contact in parallel with the connector for self-locking is called self-locking contact.
stop it:Press stop button SB2 → km coil loses power → km self-locking contact is disconnected, KM main contact is disconnected → motor M stops running.
When SB2 is released and its normally closed contact is closed again, since the self-locking contact of the contactor KM has been disconnected when the control circuit is cut off, the SBI is also disconnected, so the contactor km cannot be powered on and the motor M will not rotate.
The contactor self-locking control circuit can not only make the motor run continuously, but also has an important feature, that is, it has the protection function of under voltage and voltage loss (or zero voltage).
Under voltage protection:“Under voltage” refers to that the line voltage is lower than the rated voltage of the motor. “Under voltage protection” is a kind of protection that when the line voltage drops to a certain value, the motor can automatically break away from the power supply voltage and stop running, so as to avoid the motor running under the under voltage.
Loss of voltage (or zero voltage) protection:Voltage loss protection means that when the motor is in normal operation, it can cut off the motor power automatically when it is suddenly cut off due to some external reasons.