A “varistor” is a resistance device with non-linear volt-ampere characteristics. It is mainly used for voltage clamping when the circuit is subjected to overvoltage and absorbs excess current to protect sensitive devices. The varistor is a voltage-limiting protection device. Using the non-linear characteristics of the varistor, when an overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby realizing the protection of the subsequent circuit.
Today we mainly demostrate the application and role of the varistor in micromotors.
The application of varistor in the micromotor is mainly to absorb the reverse electromotive force of the motor at the moment of commutation. Prevent commutator pole in the moment of commutator short circuit sparks, commutator and brush surface by high-temperature burns, affect the service life of the motor, and generate electromagnetic waves, interfere with the use of other electronic products.
1.The principle of varistor: varistor, hence the name suggests, is understood to be sensitive to voltage and changes. With voltage increasing at the top of it, its resistance value can be from M Ω (Megaohm) level change to the M Ω milliohm level. When the voltage is low, the varistor works in the leakage current area, showing a large resistance and a small leakage current. When the voltage increases into the nonlinear region and the current changes in a considerable range, the voltage changes little and presents a good voltage limiting characteristic.
When the voltage rises again, the varistor enters the saturation zone presenting a small linear resistance. Due to the large current, the varistor will overheat and burn or even burst over time. The varistor is usually in the current leakage zone and enters the nonlinear zone to release the surge current when subjected to the surge impact. Generally, it cannot enter the saturation zone.
2.Selection of varistor in the motor: The selection of varistor is generally the nominal voltage E1, which is the voltage value between the two ends when the varistor flows through 1mA current.
1). The so-called varistor voltage is the breakdown voltage or threshold voltage, which refers to the voltage value under the specified current. In most cases, the voltage value measured when a 1mA DC is passed into the varistor is used. The varistor voltage range of the product can range from 1V to 9000V. It can be selected correctly according to specific needs. Generally, 1mA=1~1.5 times the peak value of the rated voltage. The selection of the voltage value of the ZnO varistor is very vital, and it is related to the protection effect and service life. If the rated power supply voltage of a motor is 12V, the voltage value of the varistor is V1mA=(1~1.5)×Vp=(1~1.5)×12V=”12~18v”, and the breakdown voltage of the varistor can be Choose between (2.0～2.2)*Vp, that is, between 24V～26.4V.
2). The so-called current capacity, that is, the peak value of the maximum pulse current, is the maximum pulse current when the change in the varistor voltage does not exceed ± 10% for the specified inrush current waveform and the specified number of inrush currents when the ambient temperature is 25℃ value. To prolong the service life of the device, the amplitude of the surge current absorbed by the ZnO varistor should be less than the maximum flow rate of the product given in the manual. However, starting from the protection effect, it is better to select a larger flow rate. In many cases, the actual flow rate is difficult to calculate accurately. Make the appropriate choice according to the actual use of the motor.