Knowledge of time relay in servo control

A time relay is a time-delay control relay. After receiving an action signal, it does not immediately make the contact move, but delays for a period of time before making the contact move. Time relays are mainly used in various automatic control systems and motor start control circuits.

1. Appearance and symbols

Time relays are divided into two types: power-on delay type and power-off delay type, and their symbols are shown in the figure below. For the power-on delay type time relay, when the coil is energized, the power-on delay type contact will act after the delay time (normally closed contact opens, normally open contact closes), and after the coil is de-energized, the contact will immediately return to normal; For the power-off delay type time relay, when the coil is energized, the power-off delay type contact will act immediately (normally closed contact opens, normally open contact closes), and after the coil is de-energized, the contact will need to return to normal after the delay time.

2. Types and characteristics

There are many types of time relays, mainly air-damped, electromagnetic, electric and electronic.

① The air-damped time relay is also called the airbag time relay. It delays based on the resistance generated by air compression. It has a simple structure, low price, and a large delay range (0.4 to 180s), but its delay accuracy is low.

Structure of air damped time relay

② The electromagnetic time relay has a short delay time (0.3~1.6s), but its structure is relatively simple and it is usually used in power-off delay occasions and DC circuits.

③ The principle of the electric time relay is similar to that of a clock, in that the internal motor drives the reduction gear to rotate to obtain the delay. This relay has high delay accuracy and a wide delay range (0.4 to 72 hours), but its structure is relatively complex and the price is very expensive.

④ Electronic time relay is also called electronic time relay, which uses delay circuit to delay. This kind of relay has high precision and small size.

A commonly used power-on delay type electronic time relay

3. Selection

When selecting a time relay, you can generally follow the following rules:

① Decide whether the time relay should be a power-on delay type or a power-off delay type based on the needs of the controlled circuit.

② Select the voltage of the time relay attraction winding according to the voltage of the controlled circuit.

③ If the delay requirement is high, you can choose a transistor time relay or an electric time relay; if the delay requirement is not high, you can choose an air damped time relay.

4. Detection

The detection of time relay mainly includes contact normal state detection, coil detection and coil power-on detection.

① Normal detection of contacts. Normal detection of contacts refers to detecting the resistance of contacts when the control coil is not energized. The normally open contact is disconnected and the resistance is infinite. The normally closed contact is closed and the resistance is close to 0Ω. The normal detection of the normally open contact of the time relay is shown in the figure below.

② Detection of control coil. The detection of time relay control coil is shown in the figure below.

③ Power on the control coil to detect the contacts. Apply the rated voltage to the control coil of the time relay, and then detect whether the contact state changes according to the type of time relay. For example, for a power-on delay time relay, after the power is on for a delay time, check whether its delayed normally open contact is closed (resistance is close to 0Ω) and whether its delayed normally closed contact is disconnected (resistance is infinite).