Count several commonly used protection circuits (I)

Motor overheat protection circuit:

Continuously operating electromechanical equipment such as automatic lathes, electric heating ovens, and ball mills used in production, as well as other unattended equipment, need to take corresponding security measures because of accidents caused by motor overheating or malfunction of the thermostat. The PTC thermistor overheat protection circuit can easily and effectively prevent the above accidents.

Taking the motor overheat protection as an example, a control circuit composed of a PTC thermistor and a Schmitt circuit. RT1, RT2, and RT3 are three step-type PTC thermistors with the same characteristics, which are embedded in the windings of the stator of the motor. Under normal conditions, PTC thermistors are at normal temperature and their total resistance is less than 1KΩ. At this time, V1 is turned off, V2 is turned on, and the relay K is electrically connected to the normally open contact, and the motor is operated by the commercial power supply.

When the motor is partially overheated due to a fault, as long as one PTC thermistor is heated above the preset temperature, its resistance value will exceed 10KΩ. Then V1 is turned on, V2 is turned off, VD2 displays a red alarm, K is de-energized, and the motor stops running for protection purposes.

The choice of PTC thermistor depends on the insulation rating of the motor. The Curie temperature of the PTC thermistor is usually selected in a range of about 40 ° C lower than the limit temperature corresponding to the motor insulation level. For example, for a B1 insulated motor, the limit temperature is 130 ° C, and a PTC thermistor with a Curie temperature of 90 ° C should be selected.