Introduction to the method of using thermal relay correctly

Relay This is a common household electrical appliance. Thermal relays are widely used in motor overload protection due to their simple structure and easy installation and use. According to the size of the motor starting current and the length of time, the correct configuration of the connection method of the thermal relay in the circuit is the key to the rational selection of the thermal relay setting current.

At present, when the thermal relay is used for a motor with light load starting, the starting current is generally about six times the rated current, and the starting time will not exceed 5 seconds. At this time, the setting current of the thermal relay can be adjusted to be equal to the rated current of the motor. When the motor is overloaded by 20%, the thermal relay connects the main and auxiliary contacts in the circuit directly in series, and quickly cuts off the power supply within the specified time to protect the motor from overload. The thermal relay can avoid six times the starting current within 5s without causing misoperation.

When the motor works at rated current, no thermal relay protection is required. Even if it is operated at an overload of 20%, the design is also allowed to last for a period of time and will not be burned immediately. However, if the motor is blocked or the dragged machinery fails, the motor will be overloaded and burned; and the thermal relay can quickly cut off the circuit during this time to ensure the normal operation of the motor.

In special occasions where the thermal relay is started under heavy load (or the starting time is long and the impact load is dragged), such as the blower and induced draft fan of the gasifier, since the starting time of these devices exceeds 5s, the setting current of the thermal relay should be adjusted to the motor 1.1 ~ 1.15 times of rated current. Sometimes in order not to cause the thermal relay to malfunction during starting, the setting multiple of the thermal relay can only be increased. This makes the overload capacity of the motor exceed 20% of the design time, and the protection of the motor is not obvious. In order to ensure the reliability of the action in heavy loads, the connection of the thermal relay in the circuit should adopt the circuit shown in Figure 1. Due to the action of TA, KA and KT, this circuit can avoid the impact of starting current even when the starting time is long, and the setting value of the thermal relay can be adjusted to be equal to the rated current value without causing errors. action.

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