Self-locking and interlocking, as the most basic secondary line protection methods of motors, are something that every electrical person should understand.
Before looking at the principles of self-locking and interlocking, let’s briefly take a look at the function and working principle of a contactor? In electrical engineering, contactors are devices that can quickly cut off AC and DC main circuits and can frequently connect large current control circuits (up to 800A), so they are often used in motors as control objects and can also be used to control factory equipment. , electric heaters, working machines and various power units and other electrical loads, the contactor can not only connect and cut off the circuit, but also has low voltage release protection. The contactor has a large control capacity and is suitable for frequent operations and long-distance control. It is one of the important components in the automatic control system.
The working principle of the contactor is: when the contactor coil is energized, the coil current will generate a magnetic field. The generated magnetic field will cause the static iron core to generate electromagnetic attraction to attract the moving iron core, and drive the AC contactor to move. The normally closed contact is open and the normally open contact is closed, and the two are linked. When the coil is powered off, the electromagnetic attraction disappears, the armature is released under the action of the release spring, causing the contacts to recover, the normally open contacts to open, and the normally closed contacts to close. The working principle of a DC contactor is somewhat similar to that of a temperature switch.
1. Self-locking
The contactor is self-locking, also called self-protection. After the motor starts, release the start button, and the contactor keeps the coil energized through its own normally open auxiliary contact, which is called self-locking. Self-protection is to connect a power cord taken from the stop button to one end of the open point on the contactor, and the other end of the open point to the contactor coil. In this way, after starting, the open point of the contactor changes to the closed point, and the coil can be taken from the stop button. The power supply is taken out, thus forming a self-protection circuit.
The main function of the lock is to ensure that the starting coil continues to be energized by controlling the self-locking of the normally open auxiliary contact, thereby ensuring that the main contact of the contactor remains closed and the load continues to operate. To give a simple example, if you want the motor to rotate for a long time, you don't need to keep pressing the start button. If you don't press the stop button and the motor is not overloaded, you only need to click the start button and the motor will keep rotating. This is the function of the self-locking control circuit to enable the motor to run continuously. In addition, the self-locking control circuit also has under-voltage and loss-of-voltage (or zero-voltage) protection functions.
As shown in the figure below, when the start button SB2 is pressed, the contactor KM1 is electrically closed, causing the motor to run. At the same time, the normally open contact of the contactor is also closed. Even if SB2 is released, the motor It will continue to run, and when the stop button SB1 is pressed, the motor stops.
2. Interlocking
Interlocking The effect of two contactors preventing each other from receiving power at the same time through their own normally closed auxiliary contacts is called interlocking.
1. Electrical interlocking
Interlocking is generally used in the forward and reverse circuits of motors. It is set up to ensure the safe operation of electrical equipment. To give a simple example, when When the motor is rotating forward, misoperation will cause the contactors controlling forward and reverse rotation to be closed at the same time, which will cause a short circuit. If an interlock circuit is added, when the motor is rotating forward, it will be pressed. The button that controls reverse rotation will not allow the motor to reverse if the forward contactor is not released in place.
The picture below is a control circuit for forward and reverse rotation of a motor. KM1 controls forward rotation and KM2 controls reverse rotation. When SB2 is pressed, coil KM1 is powered on and at the same time, the normally closed contact of KM1 acts. , at this time, contactor KM2, which controls the reverse rotation of the motor by pressing button SB3, will not close. The same is true when KM2 is electrically attracted.
When the stop button SB1 is pressed or the thermal relay FR is overloaded, the motor stops rotating. (We also see self-locking in the interlocking circuit)
2. Mechanical interlocking
Electrical interlocking is when one contactor opens and the other contactor is disconnected The coil power supply achieves the purpose of interlocking. Generally, electrical interlocking uses contactor auxiliary contacts to achieve interlocking purposes. When one contactor is turned on, the coil power supply circuit of the other contactor is cut off.
Mechanical interlocking is achieved through mechanical components. For example, if two contactors cannot be closed at the same time, a mechanical lever can be used so that when one contactor is closed, the other contactor is mechanically stuck. Close. Electrical interlocking is relatively easy to implement, flexible and simple, and the two interlocking devices can be installed in different locations.
Mechanical interlocking is more reliable and there is no need to worry about misoperation, but it is more complicated and sometimes even impossible to achieve. Usually two interlocking devices are installed in close proximity.
The following will take the Delixi Electric FR6 mechanical interlocking device as an example to explain in detail the functional principle of mechanical interlocking. A reversible contactor group usually consists of two contactors. It has the characteristics of interval switching. In order to ensure its normal and safe operation, it requires a mechanical interlocking function between the two contactors. In the event of accidental or incorrect operation, When other situations occur, it can be ensured that the two contactors will not be closed at the same time, but only after one of the contactors is disconnected, the other contactor can be closed. This can effectively prevent short circuit accidents in the distribution lines.
The mechanical interlocking structure of the existing reversible contactor group is generally used in a modular manner. It has many parts, is larger in size, takes up more installation space, and has a complex structure, making assembly inconvenient.
For example, in some installations, the interlocking components must be installed first and then the top cover. When an integrated top cover is used, the customer cannot use any one of the contactors alone, even if it is equipped with With an independent top cover, when customers need to use the contactor alone, they still need to use tools to pry open the top cover, remove the interlocking components, and then replace the top cover before use. The process is extremely complicated and can easily reduce the reliability and reliability of the contactor. service life. In addition, its operation will affect the suction reaction force characteristics of the contactor body, thereby affecting the reliability of the reversible contactor group.
The FR6 mechanical interlocking device simplifies the structure. The reversible contactor device includes a first contactor and a second contactor. The first contactor and the second contactor are arranged side by side. It is characterized by 2 contacts. The shell of the device is respectively provided with through holes, and the two through holes penetrate each other to form a channel. A locking rod that can move along the axial direction of the channel is provided in the channel.
The movable contact in the first contactor unit supports a first lockhole on the side wall close to the channel side, and the movable contact in the second contactor unit supports the side wall close to the channel. The side wall is also provided with a second keyhole, and the two ends of the interlocking rod can form locking fits with the two keyholes in turn. In this way, the mechanical interlocking element can slide freely in the channel of the housing, so that the mechanical interlocking element supports the cooperation of the side wall, the lock hole and the locking rod through the movable contact.
When the movable contact support of one contactor unit drives the contacts to connect, the movable contact support of the other contactor unit is locked and cannot drive the contacts on it to connect. Then remain disconnected. In this way, the interlocking of the reversible contactors is achieved through mechanical interlocking elements. When customers need to use two contactor units separately, they only need to separate them and remove the interlocking components and locking rods. There is no need to use any tools or disassemble parts. The structure is extremely simple and very convenient to use.