In small household appliances such as soymilk machines, mixers, eggbeaters, etc., in order to avoid the "difficult" large block caused by motor blockage and burned motor, design engineers usually design temperature or overcurrent protection measures, commonly used There are: 1 over-current protection, recoverable temperature protection (also called bimetal thermostat), and now also use PTC for stall protection.
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When the motor is blocked, the current flowing through the armature will be several times the normal operating current of the motor, usually at least 4 times to 5 times or higher. Such a large current flows through the armature of the motor, causing the motor. The overall temperature rises sharply, resulting in a significant drop in electrical insulation performance, which eventually causes the insulation layer of the enameled wire of the rotor or stator winding to melt, causing dangerous phenomena such as short-circuiting of the motor.
Since the motor will start to work to overcome the frictional moment, the instantaneous operating current will be relatively large. After the start-up, the operating current will be a constant value without stalling, because the primary fuse is rated at work. When the current is 2 times, it will act quickly, and it will appear to be blown, showing an open circuit, which will cause the motor to fail to start. Therefore, when using the primary use as the overcurrent protection of the motor in the early stage, in order to overcome this shortcoming of the primary insurance, it is usually The rated working current of the selected primary fuse will be much larger than the normal working current of the motor. This avoids the problem of false protection at the moment of motor starting, but if the motor is blocked or overheated, it will result in untimely protection. Protection effect.
If the bimetal thermostat is used as the motor's blocking or overheat protection, the bimetal thermostat will not operate until the inrush current of the motor starting moment is not converted into the heat of the whole machine, although it can effectively avoid the selection. The disadvantage of the one-time fuse, but the installation position of the bimetal thermostat is very particular. It is necessary to stick the thermostat to the stator or field winding of the motor. Otherwise, if the motor is blocked, the armature temperature is higher than the setting. Fixed value, but because the heat conduction is not timely, or the amount of heat conduction to the temperature controller is not enough, it is not enough to cause the temperature controller to operate, which will result in a great protection effect.
As a stall protection for small appliances, the bimetal thermostat has a problem. If the thermostat is activated, the power supply of the motor is cut off, and the motor will stop running at this moment, so that it no longer generates heat, along with the overall heat of the motor. Dissipate, the thermostat will automatically resume conduction, the motor will resume rotation again. If the user is just cleaning the blade at this moment, then automatically recovering the rotating blade will cause the user to use the danger.
If PTC is used as the motor's stall protection, the above problems will be effectively avoided. Firstly, PTC is used as a recoverable fuse for polymer materials. The internal part relies on Joule's law Q=Iwu 2IRJT action, which shows the heat generated inside and flowing through it. The square of the current is proportional to the current. When the current increases a little, the internal heat will increase sharply in the square of the current; the large current at the moment of motor start-up, because the accumulated heat is not enough to cause the PTC to act, there will be no false protection; once the motor A stall occurs. Since the current blocked by the lock is several times the normal operating current, the PTC can start protection as long as the current flowing through it is twice the normal operating current. After protection, it is necessary to manually interrupt the power supply, so that the PTC heat can be restored to the factory low resistance state. If the current is not cut off, the PTC will always have a small residual current flowing through the PTC to maintain the high resistance value, even at this moment. The user is constantly cleaning the blade of the mixer and ensuring that the motor does not rotate, thus protecting the user's personal safety.
Since the PTC is not polarized, as long as it is connected in series in the load circuit, it is not limited by space, and the installation position is extremely flexible and convenient, so it has received more and more customers' approval.
The “Wanhe†brand self-resetting overcurrent protector (WH) series products, which are professionally produced by Wanrui Company, have been widely used in the following international and domestic products: home appliances, communication products, audio-visual equipment, electric toys, electric Tools, various power supplies, chargers, various lamps, computers and peripherals, automotive electronics, hairdressing products, medical equipment, security products, industrial automation control equipment, etc., have become mainstream suppliers in the industry.
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