At present, bimetallic mechanical thermal relay is widely used in motor protection. However, the protection function of mechanical thermal relay to motor is very limited. According to incomplete market statistics, mechanical thermal relays can provide protection less than 30 times out of every 100 times that a motor may burn out. The main reason is that there are three defects in the bimetallic mechanical thermal relay: first, the setting error is large and the protection value cannot be set accurately; Second, the bimetallic sheet is prone to thermal fatigue, which will cause parameter changes after several times of overheating; Third, there is no lack of phase protection.
Motor protector is produced for these defects. However, over the past 30 years, the motor protector has not been able to completely replace the mechanical thermal relay market position. Why is that? There are many reasons, but the bottom line is value for money. Cheap, the performance is not very good; The performance is good, but the price is too expensive.
Low voltage motor, the general cause of damage, is basically caused by over-current.There are many reasons for the over-current, mainly overload, lack of phase, over-voltage, under-voltage, three phase imbalance and so on. Therefore, having a high precision sine wave current measurement method is the key to the development of motor protector. At present, most of the low-end and cheap motor protectors in the market are realized by the analog electronic circuit, which not only has low precision, but also has serious temperature drift, which cannot realize the high-precision current measurement. For high-end and expensive motor protectors, high-precision CT is used for current sampling and powerful microprocessor is used for digital processing. It can be seen that there is a big difference between the two.
Post time: Jan-13-2020