The function of the motor protector is to provide comprehensive protection for the motor. When the motor is overloaded, phase-deficient, blocked, short-circuited, over-voltage, under-voltage, leakage, three-phase unbalance, overheating, bearing wear, and eccentricity of the rotor, alarm is given. Or protected device.
First, the basic principle of motor protector selection
At present, there are no uniform standards for motor protection products on the market, and the model specifications are varied. In order to meet the different needs of users, manufacturers have derived a large number of series of products, which brings a lot of inconvenience to the selection of users. Users should fully consider the actual needs of motor protection when selecting the model, and reasonably choose the protection function and protection method. Achieve good protection results, improve equipment operation reliability, reduce unplanned parking, and reduce accident losses.
Second, the basic method of motor protector selection
(1) Conditions related to the selection:
The selection of the motor protector has a reasonable relationship between the motor and the protector. The following provides several conditions and factors related to protection, which provide a reference for the user to select.
1. Motor: First understand the model specifications, motor function characteristics, protection type, rated voltage, rated current, rated power, power frequency, insulation level, etc. These contents basically provide a reference for users to correctly use and maintain and select the protector.
2, environmental conditions: mainly refers to normal temperature, high temperature, high cold, corrosion, vibration, sand, altitude, electromagnetic pollution.
3, motor use: mainly refers to the characteristics of the drag mechanical equipment, such as fans, pumps, air compressors, lathes, oil field pumping units and other different mechanical characteristics.
4. Control system: The control mode includes manual, automatic, local control, remote control, stand-alone operation, centralized control of the production line, etc. The starting mode includes direct, buck, star triangle, frequency sensitive varistor, inverter, soft start and other starting methods.
5. Other aspects: The user's on-site production monitoring management is relatively random or rigorous, and the severity of the impact of abnormal shutdown on production.
There are many factors related to the selection of the motor protector, such as the installation position, power supply, and the coordination with the power distribution system. It is also considered whether the new motor protection configuration, or the motor protection upgrade, or the accident The improvement of motor protection, etc.; also consider the difficulty of changing the protection mode of the motor and the degree of influence on production; the selection and adjustment of the protector should be considered comprehensively according to the actual working conditions on site.
(2) Common types of motor protectors
1. Thermal relay: ordinary small-capacity AC motor, good working conditions, no frequent working conditions, etc. Due to poor precision, reliability cannot be guaranteed and is not recommended.
2. Electronic type: Detect three-phase current value. The setting current value is operated by potentiometer knob or dial switch. The circuit is generally analog, with anti-time or time-limited operating characteristics. The protection function includes fault protection such as overload, phase loss, and stalling. The fault type is displayed by indicator light, and the running power is displayed by digital tube.
3. Intelligent type: The three-phase current value is detected, and the protector uses the single-chip microcomputer to realize the intelligent comprehensive protection of the motor, integrating protection, measurement, communication and display. The setting current is digitally set and operated by the operation panel button. The user can modify the various parameters on the spot according to the actual use requirements and protection conditions. The digital tube can be used as the display window or the large screen liquid crystal display can be used. Support a variety of communication protocols, such as ModBUS, ProfiBUS, etc., the price is relatively high, for more important occasions, the current high-voltage motor protection is intelligent.
4. Thermal protection type: The thermal element is embedded in the motor and protected according to the temperature of the motor. The protection effect is good, but when the motor capacity is large, it needs to be used together with the current monitoring type to avoid the temperature rise sharply when the motor is blocked. The hysteresis of the temperature measuring element causes damage to the motor windings.
5. Magnetic field temperature detection type: The magnetic field detection coil and temperature probe are embedded in the motor, and the protection is based on the change of the internal rotating magnetic field of the motor and the change of temperature. The main functions include overload, stall, phase loss, overheat protection and wear monitoring. The protection function is perfect, and the disadvantage is that a magnetic field detecting coil and a temperature sensor are installed inside the motor.
(3) Selection of motor protector type under motor working conditions
1. For the single-machine independent operation motor with low requirements on working conditions, simple operation control, relatively simple monitoring and management, and no impact on production, the ordinary type protector can be used. Because the structure of the ordinary protector is simple, the wiring is installed on site. It is simple, convenient, and cost-effective.
2, for the working conditions are very high, safety and continuity are critical, and the degree of automation is high, and requires special personnel to control, monitor, manage, MCC system that needs network monitoring, should choose medium-to-high-end, full-featured Protector.
3. For explosion-proof motors, eccentricity due to bearing wear may cause high temperature friction at the explosion-proof gap, which may cause explosion hazard. The wear status monitoring function should be selected. For large-capacity high-pressure submersible pumps, due to the difficulty in inspection and maintenance, the wear status monitoring function should also be selected to avoid major economic losses caused by broom accidents.
4. Applicable to protectors with explosion-proof requirements, according to the specific requirements of the application site, select the corresponding explosion-proof protector to avoid safety accidents.
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Label: Motor protector analysis
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