Design polymer resettable fuse overcurrent protection circuit
En los pequeños electrodomésticos como leche de soja, batidora y batidora de huevos, para evitar que el motor bloquee y queme el motor. Los ingenieros de diseño generalmente han diseñado medidas de protección contra sobrecorriente o temperatura. Los más utilizados son: protección de sobrecorriente de una sola vez, protección de temperatura reiniciable (también llamado termostato bimetálico). Hoy en día, el polímero PTC también se utiliza para la protección de rotor bloqueado.
Cuando el motor está bloqueado, la corriente que fluye a través del inducido será varias veces la corriente de trabajo normal del motor, generalmente al menos 4 a 5 veces o más. Una corriente tan grande que fluye a través del inducido del motor hará que la temperatura general del motor aumente bruscamente, dando como resultado una disminución significativa en el rendimiento del aislamiento eléctrico. Finalmente, la capa de aislamiento del alambre esmaltado del devanado del rotor o del estator se derrite, provocando fenómenos peligrosos como cortocircuitos y quemaduras del motor.
Since the motor is starting, in order to overcome the friction torque, the instantaneous working current will be relatively large. After the start is completed, the operating current will be a constant value if there is no stall. When the current fuse is twice its rated operating current, it will act quickly, and it will blow out and present an open circuit, causing the motor to fail to start. When using current fuse as the overcurrent protection of the motor, in order to overcome the shortcomings of the one-time fuse, the rated working current of the one-time fuse usually selected will be much larger than the normal working current of the motor. Although this avoids the problem of false protection at the moment of motor startup, if the motor is blocked or overheated, the protection will not be timely and the protection effect will be affected.
If the bimetal thermostat is used as the motor's stall or overheat protection, the bimetal thermostat will not operate until the inrush current at the moment of motor start is not converted into the heat of the motor. Although it can effectively avoid the disadvantages of using disposable fuses, the installation location of the bimetal thermostat is very particular. The thermostat needs to be close to the stator or field winding of the motor. Otherwise, if the motor is blocked, although the actual temperature of the armature is higher than the allowable value, the heat conduction is not timely enough to cause the thermostat to operate, which will greatly reduce the protection effect.
As the stall protection of small household appliances, the bimetal thermostat has another problem. If the thermostat is activated and the power supply of the motor is cut off, the motor will stop running at this moment, so it no longer generates heat. As the overall heat of the motor is dissipated, the thermostat will automatically resume conduction, and the motor will resume rotation again. If the user happens to be cleaning the blade at the moment, the blade that automatically recovers and rotates at a high speed will cause the user to be injured!
If the PTC fuse is used as the motor's blocking protection, the above problems will be effectively avoided. First of all, as a recoverable fuse made of polymer materials, PTC is internally operated by Joule's law Q=I^2*R*T. It can be seen that the heat generated inside is proportional to the square of the current flowing. When the current increases a little, the internal heat will increase sharply in the square of the current; The high current impact at the moment the motor starts, the accumulated heat is not enough to cause the PTC action, so no false protection will occur; Once the motor is locked, the current due to the locked rotor is several times the normal working current. The PTC can start the protection as long as the current flowing through is twice the normal working current, and the overall temperature of the motor is not over-temperature at this moment. Avoid the impact of high temperature on the motor!
After the PTC starts the protection, manual intervention is required to cut off the power, so that the PTC can be restored to the factory low resistance state after the heat is dissipated. If the current is not cut off, there will always be a small residual current flowing through the PTC to maintain the high resistance of the PTC. Even if the user is cleaning the blade of the mixer with power at the moment, it is ensured that the motor will not rotate, thereby protecting the personal safety of the user. This is the biggest highlight of PTC used for over-current protection of such small home appliances!
Since PTC has no polarity, it only needs to be connected in series in the load circuit without being restricted by space. The installation location is extremely flexible and convenient, so it has been recognized by more and more customers.
The resettable overcurrent protector products have been widely used in the following products both at home and abroad: Home appliance, Communication Products, Audio-visual equipment, Electric toys, electrical tools, power supply, Chargers, various lamps, computer peripherals, automotive electrical appliances, beauty salon products, medical equipment, security products, industrial automation control and other fields.