With the continuous improvement of people's living standards, more and more small household electrical appliances have entered everyone's kitchen. Many small household appliances use DC motors because they have a high cost performance and provide a large starting torque. These small household appliances include, but are not limited to, soymilk machines, food processors, juicers, blenders, meat grinders, beaters, coffee machines, and the like.
Everyone knows that DC motors have no back electromotive force when they are blocked, so they will generate a large current that is several times the normal working current. When such a high current flows through the brush to the coil, it will generate a high amount of heat. When the coil temperature rises to Above the extreme temperature, the insulation of the coil is destroyed, causing the motor to burn completely.
At present, the overcurrent protection scheme of the DC motor mainly includes a disposable fuse, a bimetal, and a PPTC. The following are the characteristics of three schemes: disposable fuse, bimetal and polymer positive temperature coefficient resistor PPTC.
Disposable fuses are characterized by their small size and low price. Since the starting current of the motor is relatively large, in order to prevent the malfunction of the disposable fuse, the fuse current of the selected fuse is usually larger than the normal working current of the motor, so that when the motor is blocked, the problem of slow operation is likely to occur, resulting in protection. timely. In addition, the replacement of the fuse also increases the after-sales cost.
The bimetal is protected by deformation of the bimetal when the temperature changes. For temperature changes, the bimetal can move quickly. Since it is only temperature sensitive, the location of the installation should be very reasonable, otherwise it will not protect. After the bimetal is protected, it will restart due to the temperature drop. If the user forgets to turn off the power and cleans the blade, it may cause harm to the user. The contact of the bimetal is prone to sparking for a strong inductive load, so that problems such as contact sticking or resistance increase are likely to occur after a plurality of switches.
The positive temperature coefficient resistance of the polymer PPTC is very low under normal current conditions. When there is a stall, the current increases rapidly. Since the heating of the PPTC is proportional to the square of the current, the temperature of the PPTC also rises rapidly, when it rises to the transition temperature. At this time, the PPTC impedance will suddenly increase. This reduces the current in the motor coil. After that, there will be a continuous small current flowing through the PPTC to maintain the high resistance state. This small current is not enough to support the motor restart, so even if the user is constantly cleaning the blade of the mixer, the motor will not rotate, thus protecting The personal safety of the user. Only the customer cleans up the blade, and the power is turned off for a period of time. After the PPTC is cooled down and then powered on again, the motor will restart.
PPTC devices are insensitive to shock and inductive loads and are not sensitive to transient high currents. Therefore, it is very suitable for small appliance motor stall protection applications. The PPTC is characterized by its sensitivity to ambient temperatures. Therefore, the protection time of the same stall current is different at high temperature and low temperature. Protection time at high temperatures will be much faster than low temperatures. Many people may think that the protection time at high temperature is satisfied. The protection time will be too slow at low temperature, and the protection effect is not good. However, in fact, the motor coil is damaged at a low temperature and the stall time is much longer than that at high temperature, so even low temperature The lower PPTC device takes longer to operate and it still protects the motor from damage. TE's PPTC products have been around for more than 30 years. A wide range of products are available for motor protection, including high pressure and low pressure products.
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