1. Overview of motor thermal protection
The commonly used thermal protection components of small and medium-sized three-phase asynchronous motors can be roughly divided into PTC thermistors, PT100 temperature sensors and bimetallic temperature relay. There is also a moisture-proof heating belt for internal heating of motors in humid environments. These temperature protection components are buried in the inside of the motor corresponding position, such as motor winding or motor bearing cover before and after, when the motor due to the overvoltage, under voltage, over current, lack of phase or overload or bearing failure cause motor winding temperature is too high, the signal is passed on to the motor temperature control instrument, immediately shut off the power of the motor and the protection of motor.
2. PTC thermistor
PTC thermistor is a positive temperature coefficient semiconductor resistance, more than the specified temperature (Curie temperature), its resistance value with the increase in temperature shows a step increase, up to 10-1000 times. When used, with electronic circuit detection, and then make the relay action.
PTC thermistor has small volume, small heat capacity and fast reaction speed. When the motor is manufactured, the PTC thermistor is embedded in the windings of the motor. Three series thermistors are selected for the three-phase electric machine. They are embedded in the three-phase windings of the motor, U, V and W in turn. After binding, they are dipped together with the windings to connect the lead wire to the junction box.
3. Pt100 temperature sensor
Pt100 is a platinum thermal resistance sensor with a resistance value of 100 ohms at 0 degrees. The Pt100 temperature sensor is an analog signal, which can measure the temperature of the corresponding part of the motor through the corresponding relation between its resistance value and temperature. The high precision measurement can be realized by converting the resistance signal through a special integrated circuit.
When measuring the temperature of the winding, it is embedded in the winding end winding to bind and fix, dip and dry together with the winding, and lead to the junction box.
When measuring the bearing temperature, make a hole (through) in the bearing room of the motor and tie the corresponding wire buckle, and then install it firmly, so that the end face of the sensor touches the outer ring of the bearing, and the connection leads to the junction box.
4. Bimetallic temperature relay
The bimetallic temperature relay can be directly embedded in the motor winding. When the motor is overloaded, the winding temperature rises to close to the limit value, and the bimetal sheet with a pair of contacts is heated and bent, which makes the contacts break and the circuit is cut off to realize the temperature protection of the motor.
5. Moistureproof add tropical
In a humid environment, when the motor stops working, the moisture-proof heating belt starts to work, so as to avoid the motor winding condensation due to moisture, to ensure that the motor can still operate normally in a humid environment. The heating lead is drawn together with the motor lead. When the motor is powered on, the heating belt will be cut off. When the motor is cut off, the heating belt is electrified and heated.
