The importance of motor protection
In the practical application of the motor, the motor may fail or even burn out due to improper protection.
occur. For this reason, the motor should generally implement the following electrical protection measures:
(1) Short-circuit protection: timely cut off the large short-circuit current of the motor and its lines, otherwise the large short-circuit current will burn out the motor quickly. Lines and other electrical equipment, causing heavy losses. For low-voltage motors below 500V, a fuse or an electromagnetic instantaneous release of a circuit breaker is generally used for short-circuit protection.
(2) Overload (overload) protection: For the overload current of the motor, the fuse may not be blown, so a protection device to cut off the overload current should be set up separately. When the motor is overloaded by 20%, the relay will act within 20 minutes to cut off the power supply. Motors generally use thermal relays (cooperating with contactors) or thermal trippers of circuit breakers for overload protection.
(3) Open-phase operation protection (also known as open-phase operation protection or two-phase operation protection): open-phase operation protection is also a kind of overload protection, and general thermal relays cannot reliably protect the motor from open-phase operation (with open-phase operation) except thermal relays of protective devices). Therefore, when the conditions permit, a phase-opening operation protection device should be set up separately. There are many methods and devices for motor phase failure protection, but the components that perform phase failure protection are divided into: electromagnetic phase failure protection that directly drives the action of the electromagnetic relay by using the phase failure signal; Phase protection; phase failure protection using thermal element action. Commonly used protection methods are:
① Use a thermal relay with a phase failure protection device for phase loss protection;
② Undercurrent relay phase failure protection;
③ Zero sequence
Voltage relay phase failure protection;
④ Breaking fuse voltage relay phase failure protection;
⑤ Use fast saturation current transformer protection.
(4) Voltage loss and undervoltage (low voltage) protection: In order to prevent the motor from starting and running under excessively low voltage, and the self-starting when the motor is suddenly powered off during operation and then resumes power supply, voltage loss and undervoltage are generally used. pressure protection. The electromagnetic mechanism of the AC contactor, the voltage loss release of the circuit breaker, the undervoltage release of the auto-decompression starter and the voltage relay can all play the role of voltage loss and undervoltage protection. When the power supply voltage is as low as 35% to 70% of the rated voltage, the electromagnet will be released, and the voltage loss release will act to cut off the power supply.
(5) Grounding or zero-connection protection: When the motor casing is charged, it is a protective device that prevents people from touching the casing and getting an electric shock. In order to ensure the normal operation of the motor, in addition to the normal use according to the operating procedures and the normal monitoring and maintenance during operation, regular inspections should also be carried out to do a good job in motor maintenance. In this way, some faults can be eliminated in time, the occurrence of faults can be prevented, and the safe and reliable operation of the motor can be ensured. The time interval of regular maintenance can be determined according to the type of motor and the usage environment.