Motor overheat protection and common sense of temperature measuring components

Motor overheat protection and common sense of temperature measuring components

1. Types and applications of motor overheat protection

The overheat protection and temperature measuring components commonly used in small and medium-sized three-phase asynchronous motors can be roughly divided into two categories: one is fixed-value temperature sensing components, such as thermistors and thermal switches; the other is thermocouples and platinum Thermal resistance. There is also a moisture-resistant heating tape to protect motors operating in wet environments. These temperature sensing elements are embedded in corresponding positions inside the motor. For example: embedded in the motor winding or the front and rear bearings of the motor, it can directly reflect the actual temperature of the motor winding and bearings. Whether it is due to overvoltage, undervoltage, overcurrent, phase loss, or overload, stalled and other faults, it is manifested by the temperature rise of the motor winding. Such signals are transmitted to the motor protector or temperature controller, which can immediately cut off the power supply of the motor to protect the motor.

2. Application of thermal switch

The local over-temperature protection of the motor usually uses the MK1 type thermal switch. When manufacturing the motor, it is embedded in the coil of the motor end winding, bound and fixed, dipped in paint and dried together with the coil, and its lead wire is drawn out with the motor lead wire, and connected to the designated terminal in the motor junction box. When the temperature rise of the operating coil of the motor reaches the operating temperature of the thermal switch, the thermal switch can cut off the power supply of the controller and disconnect the main circuit of the motor, thereby protecting the motor. Motors with different insulation levels can choose thermal switches with different temperatures.

3. Application of thermistor

PTC thermistors are usually used for motor protection. PTC thermistor has small volume, small heat capacity, and fast response speed. It is not only suitable for undervoltage, overvoltage and overload protection of motors, but also for phase loss starting protection of motors. However, it must be equipped with special motor protection when used. device (such as GRB). When manufacturing the motor, the PTC thermistor is embedded inside the motor coil. Select three thermistors in series for a three-phase motor, embed them in the three-phase coils of the motor A, B, and C in turn, press and bind them, dip the coils together with the paint, and connect the lead wires to the junction box. When the temperature of the motor coil is close to the insulation grade temperature, the resistance of the PTC thermistor increases, and this signal is sent to the GRB motor overheat protector, and the protector disconnects the motor circuit according to this signal, thereby protecting the motor.

If the motor is not equipped with a PTC thermistor, you can open the motor end cover, glue the PTC thermistor on the coil with AB glue, and lead the lead wires into the junction box, and then use it with a special protector for thermistor. The control temperature should drop by 20°C (because it is not embedded inside the coil).