Traditional Chinese Motor overheat protector: Circuit Design, Parts List & Workshop Prototype Guide

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Motor overheat protectorIn production practice, motor windings are overheated or even burned due to overloading or phase failure. This phenomenon not only affects normal production, but also brings certain economic losses to enterprises. Therefore, various motor overheating protectors have emerged, each with its own merits. For this reason, a motor overheating protection circuit is introduced to the reader.1． How the circuit worksThe circuit of the motor overheat protector is shown in Figure 49-1. It is composed of a temperature monitoring circuit and a control circuit. Figure 49-1In the figure, diodes D1~D4 form a bridge rectifier circuit, which is stabilized by DW andCapacitor C1 filters and provides 12V DC power to the temperature monitoring circuit. Thermistors Rt1 and Rt2 with positive temperature coefficients and resistors R1 and R2 respectively form two voltage dividers to provide corresponding forward voltages to the OR gate circuit composed of diodes D1 and D2. The unijunction transistor BG and its surrounding components form a relaxation oscillator, and the voltage output when it is turned on is used to control the unidirectional thyristor. Thermistors Rt1 and Rt2 are respectively embedded in any two windings of the motor to monitor whether the motor is overheated.When the motor is overloaded or runs out of phase, the current in the winding will inevitably increase, resulting in overheating. At this time, the thermistor buried in the winding will increase as the temperature increases, and the diode D1 or D2 in the OR gate circuit will be directed to charge the capacitor C2. When the charging voltage on C2 rises to the peak voltage of the unijunction transistor, the BG transistor is turned on, generating a voltage drop on the resistor R4, outputting a trigger pulse, triggering the thyristor SCR to be turned on, the relay J being electrically closed, the normally closed contact J1 being disconnected, cutting off the power supply of the AC contactor KM, then the closed contact KM1 of the AC contactor will act, cutting off the power supply to the motor, and realizing the motor overheating protection.The three-phase power supply of the circuit shown in the figure is Y (star) connected. If one phase is disconnected, the other two phase windings must be overheated, so only two thermistors need to be buried in any two phase windings. If the motor windings are connected in △ (triangle), a thermistor should be buried in each phase winding, and a voltage divider and a diode should be added accordingly to form a three-OR gate circuit.The one-way silicon controlled thyristor SCR in the picture acts as a switch.2. Component selectionJ is a 12V DC relay, and TRX?F high-sensitivity relay is selected.Rt1 and Rt2 are positive temperature coefficient thermistors, and their characteristics are that they increase with temperature.As the resistance value increases, RZK? type switching thermistor can be used. The resistance value depends on the application.It depends on the ambient temperature used, usually between 300 and 1000Ω.Resistors R1 and R2 can use 220kΩ potentiometers or trimmer resistors to adjust the temperature.Easy to control time.R3 is 510Ω, R4 is 200Ω, and R5 …

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