Brief analysis of the basic knowledge, technical parameters and principles of relays

1. Functions, characteristics and classification of relays

1. The function of control relay

A control relay is an automatic electrical appliance that is suitable for remotely connecting and disconnecting AC and DC small-capacity control circuits, and is used for control, protection and signal conversion in electric traction systems. The input quantity of a control relay is usually electrical quantities such as current and voltage, or non-electrical quantities such as temperature, pressure, and speed. The output quantity is the electrical signal sent when the contact is in action or the parameter change of the output circuit.

2. Characteristics of control relay

The characteristic of a control relay is that the output quantity will change abruptly only when the change in the input quantity reaches a certain level, that is, the contacts of the relay will open and close, and the control circuit will be operated through the opening and closing action of the contacts.

3. Classification of control relays

Control relays can be divided into: voltage relays, current relays, time relays, thermal relays, speed relays, temperature relays and pressure relays according to the different signals they reflect.

2. Structure of general relay

Most control relays use electromagnetic structures. An important feature of electromagnetic relays is that different coils or damping coils can be used in the same electromagnetic system, which can be made into voltage relays, current relays and time relays. This type of relay is universal in structure, so it is also called a universal relay. Electromagnetic relays are composed of an electromagnetic system and a contact system. Control relays generally do not use arc extinguishing devices, and the contact structure is relatively simple.

1. Electromagnetic system

The electromagnetic system reflects the input quantity, including the iron core, armature, coil and reaction spring.

1) The DC magnetic system is made of a whole piece of steel and has a U-shaped snap-on structure. The armature is made into a plate shape and rotates around the edges. When the coil is not energized, the armature opens by the action of the reaction spring.

2) The AC magnetic system uses silicon steel sheets stacked into a double E structure and added with a short-circuit ring.

2. Contact system

The contact system reflects the output quantity. Most relay contacts adopt a bridge double-break structure with a small rated current, generally 5~10A. Since the relay contacts are used in the control circuit, the power of the control circuit is generally not large, so the rated current and conversion capacity of the relay contacts are not required to be high.

3. The principle of general relay

When voltage is applied to both ends of the relay coil, current flows through the coil to generate electromagnetic effect. Under the attraction of electromagnetic force, the armature overcomes the reaction force of the spring and is attracted to the iron core, driving the movable contact of the armature to move, so that the normally open contact is attracted and the normally closed contact is disconnected; when the coil is powered off, the electromagnetic attraction disappears, and the armature drives the contact to return to the original position under the action of the reaction spring, so that the normally closed contact is attracted and the normally open contact is disconnected. Thus, the purpose of conducting and cutting off the control circuit is achieved.

4. Commonly used control relays

1. Voltage relay

The coil of a voltage relay is called a voltage coil, which is used to receive the input voltage signal. When the voltage reaches the action value, the voltage relay is activated, and the voltage coil is connected in parallel with the signal voltage in the circuit. In order to generate a certain magnetic flux potential in the relay magnetic circuit without affecting the normal operation of other circuits, the current flowing through the voltage coil is required to be as small as possible, so the voltage coil has a larger resistance and more turns, and the wire used is thinner. Voltage relays can be divided into overvoltage relays and undervoltage relays according to the size of the pull-in voltage.

1) Overvoltage relay

The armature does not attract when the coil is at rated voltage. When the coil voltage is higher than the rated voltage, the armature moves to attract. Then, when the circuit voltage drops to the relay release voltage, the armature returns to the released state. The voltage release value of the overvoltage relay is lower than the action value, and the adjustment range of the attraction voltage is 1.05~1.2U N.

2) Undervoltage relay

When the coil voltage is lower than the rated voltage, the armature will be attracted, and when the coil voltage is very low, the armature will be released. Generally, the DC undervoltage relay has a pull-in voltage U 0 = 0.3 ~ 0.5U N , and a release voltage Ux = 0.07 ~ 0.2U~N~; the AC voltage relay has a pull-in voltage U~0~ = 0.6 ~ 0.85U~N~, and a release voltage Ux = 0.1 ~ 0.35U~N~.

2. Current relay

The coil of a current relay is called a current coil, which is used to receive the input current signal. When the current reaches the action value, the current relay is activated, and the current coil is connected in series with the signal current in the circuit. The current flowing through the current coil is large, and the voltage drop generated by the current coil is required to be as small as possible, so the number of turns of the current coil is small, generally a few turns to dozens of turns, the wire is thicker, and the resistance is very small. Current relays can be divided into overcurrent relays and undercurrent relays according to the size of the pull-in current.

1) Overcurrent relay

When the load current flows through the relay coil during normal operation, the armature will not be attracted even if it is the rated load current. When the load current is a certain value greater than the rated current, the armature is attracted to drive the contact to operate. Overcurrent relays are often used for overcurrent protection of circuits. The AC overcurrent relay attracts current I 0 =1.1~3.5I N, and the DC overcurrent relay attracts current I 0 =0.75~3I N.

2) Undercurrent relay

During normal operation, the load current flowing through the electromagnetic coil is greater than the pull-in current of the relay, and the armature is in the pull-in state. When the load current is reduced to the relay release current, the armature is released and the contacts are moved. In a DC circuit, if the DC motor excitation circuit is disconnected, the DC motor will run away, which is a serious consequence. Therefore, an undercurrent relay must be used for protection, but there is no undercurrent relay in the AC circuit.

3. Intermediate relay

1) The intermediate relay is actually an electromagnetic voltage relay, which is generally used to control various electromagnetic coils. The difference is that it has more contacts, a compact structure, and a sensitive response. The intermediate relay is mainly used to expand the number of contacts and contact capacity.

2) The electromagnetic system of the intermediate relay adopts a solenoid electromagnet. When the coil is energized, the moving iron core is attracted to the conical stopper and drives the crossbeam, so that the moving contact brackets on both sides move upward, so that the contacts are switched. After the coil is de-energized, the moving iron core and the moving contact bracket return to their original positions under the action of the reaction spring.

5. Basic knowledge of time relay

A relay that can only execute actions after a delay after receiving an external signal is called a time relay. There are several major types of time relays, including air type, electric type, transistor type, and DC electromagnetic type. The delay mode includes power-on delay type and power-off delay type.

1. Symbolic representation of time relays

2. Electromagnetic damping time relay

The commonly used electromagnetic damping time relay is the JT18 series time relay, which is suitable for DC circuits.

1) Structure

By putting a copper or aluminum sleeve on the iron core column of a DC electromagnetic voltage relay, it becomes an electromagnetic damping time relay.

2) Principle

According to the law of electromagnetic induction, when the coil is powered on, an induced electromotive force and an induced current will be generated in the copper or aluminum sleeve, and an induced magnetic flux will be generated. Under the action of the induced magnetic flux, the magnetic flux will slowly increase, thereby extending the time to reach the attraction magnetic flux value, the armature will delay attraction, and the contacts will also delay action. When the coil is disconnected from the power supply, the magnetic flux will slowly decrease due to the action of the copper or aluminum sleeve, thereby extending the time to reach the release magnetic flux value, the armature will delay opening, and the contacts will delay action.

When the armature is in the open position, the air gap is large, the magnetic resistance is large, the magnetic flux is small, and the effect of the damping sleeve is not obvious, so the delayed action of the contact is not obvious. When the armature is in the closed position, the magnetic flux is large, and the effect of the damping sleeve is obvious, so the release delay obtained by de-energizing the coil is obvious, which can reach 0.3~0.5s. In the electric traction automatic control system, the coil de-energizing delay type is often used. The length of the delay can be adjusted by changing the air gap size after the armature is attracted with a non-magnetic gasket, or by changing the tightness of the release spring.

3) Advantages and Disadvantages