Protection work of permanent magnet motor

The protection of permanent magnet motors is the protection of the motor, that is, when the motor has overload, phase loss, blocked rotor, short circuit, overvoltage, undervoltage, leakage, three-phase imbalance, overheating, bearing wear, stator and rotor eccentricity, etc. , to give an alarm or protection; the device that provides protection for the motor is a motor protector, including thermal relays, electronic protectors and intelligent protectors. Currently, large and important motors generally use intelligent protection devices. So what are the protection tasks for permanent magnet motors? Please see the detailed introduction below!

The protection work of permanent magnet motors includes: short circuit protection, undervoltage protection, voltage loss protection, field weakening protection, overload protection, etc., which are as follows:

(1) Short circuit protection

When short-circuit faults occur due to damage to the insulation of permanent magnet motor windings and wires, damage to control appliances and circuits, misoperation of wires, etc., we can use measures to protect the appliances and quickly cut off the power supply for short-circuit protection. Commonly used short-circuit protection appliances include fuses and automatic air circuit breakers.

(2) Undervoltage protection

When the grid voltage decreases, the permanent magnet motor operates under undervoltage. Since the motor load has not changed, the motor torque decreases under undervoltage, and the stator winding current increases accordingly, which affects the normal operation of the motor or even damages the motor. At this time, we can protect the electrical appliance and cut off the power supply for undervoltage protection. Electrical appliances that implement undervoltage protection include contactors and electromagnetic voltage relays. Fuses and thermal relays cannot be protected against undervoltage because when the motor is running under undervoltage, its stator winding does not increase enough to cause the fuse and thermal relay to operate, so these two electrical appliances cannot be protected against undervoltage.

(3) Pressure loss protection

When the production machinery is working, a sudden power outage occurs in the power grid for some reason. When the power supply is restored, the protective appliance must ensure that the production machinery can be operated after it is restarted to avoid personal and equipment accidents. This protection is voltage loss (zero pressure) protection. Electrical appliances that realize voltage loss (zero voltage) protection include contactors and intermediate relays.

(4) Weak magnetic protection

Protective appliances are used to ensure that the DC motor operates under a strong magnetic field, so that the magnetic field does not weaken or disappear, and the motor speed increases rapidly, or even overruns. This kind of protection is weak magnetic protection. In the DC motor excitation circuit, a field weakening relay (i.e., an undercurrent relay) is inserted in series to realize field weakening protection. The working principle of the undercurrent relay is: during the starting and running process of the DC motor, when the excitation current value reaches the action value of the undercurrent relay, the relay will close, causing the normally open contact connected in series in the control circuit to close. The motor is allowed to start or maintain normal operation; but when the excitation current decreases a lot or disappears, the under-current relay is released, its normally open contact is disconnected, the control circuit is cut off, the contactor coil loses power, and the motor stops spinning.

(5) Overload protection

If the motor is overloaded for a long time and the temperature rise of its winding exceeds the allowable value, the insulation material of the winding will become brittle, and the motor life of the permanent magnet motor will be reduced. In severe cases, the motor may even be damaged. The greater the overload current, the shorter the time to reach the allowable temperature rise. A commonly used overload protection component is a thermal relay. The thermal relay can meet such requirements: when the motor is running at the rated current, the temperature rise of the motor is the rated temperature rise, and the thermal relay will not act. When the overload current is small, the thermal relay will take a long time to act. Overload When the current is larger, the thermal relay will operate in a shorter time.

Due to thermal inertia, the thermal relay will not act instantaneously due to the influence of short-term overload surge current or short-circuit current of the motor. Therefore, while using the thermal relay for overload protection, short-circuit protection must also be provided. The rated current of the fuse used for short-circuit protection should not exceed 4 times the rated current of the heating element of the thermal relay.

When the working environment temperature of the motor and the working environment temperature of the thermal element are different, the reliability of the protection will be affected. Nowadays, better thermal relays generally use thermistors as temperature measurement components. By stringing the thermistors in the motor windings, the temperature rise of the motor windings can be measured more accurately.

The ideal motor protector is not one with many functions, nor is it so-called advanced, but should meet the actual needs of the site, achieve the unity of economy and reliability, and have a high performance-price ratio. Reasonably select the type and function of the protector according to the actual situation on site. At the same time, the installation, adjustment, and use of the protector should be simple and convenient. More importantly, a high-quality protector should be selected.