Tips for selecting motor protectors

1. Basic principles of selection

At present, there is no unified standard for motor protection products on the market, and the models and specifications are varied. In order to meet the different needs of users, manufacturers have derived many series of products with various types, which brings many inconveniences to users in selecting models; users should fully consider the actual needs of motor protection when selecting models, and reasonably choose protection functions and protection methods to achieve good protection effects, improve equipment operation reliability, reduce unplanned shutdowns, and reduce accident losses.

2. The method of selection

1. Conditions related to selection

1) Motor parameters: First, you need to understand the motor's specifications, functional characteristics, protection type, rated voltage, rated current, rated power, power frequency, insulation level, etc. These contents can basically provide users with a reference for correctly selecting a protector.

2) Environmental conditions: mainly refers to normal temperature, high temperature, extreme cold, corrosion, vibration, wind and sand, altitude, electromagnetic pollution, etc.

3) Motor purpose: mainly refers to the requirements of driving mechanical equipment, such as fans, water pumps, air compressors, lathes, oil field pumping units and other different load mechanical characteristics.

4) Control mode: The control modes include manual, automatic, local control, remote control, stand-alone operation, centralized control of production line, etc. The starting modes include direct, step-down, star-angle, frequency-sensitive rheostat, inverter, soft start, etc.

5) Other aspects:

Installation location, power supply conditions, distribution system conditions, etc.; also consider whether to configure protection for newly purchased motors, upgrade motor protection, or improve accident motor protection, etc.; also consider the difficulty of changing the motor protection method and the degree of impact on production; the selection and adjustment of the protector must be comprehensively considered according to the actual working conditions on site.

2. Common types of motor protectors

1) Thermal relay: For ordinary small-capacity AC motors, the working conditions are good and there is no severe working conditions such as frequent starting. Due to its poor accuracy and unguaranteed reliability, it is not recommended.

2) Electronic type: detect three-phase current value, set current value with potentiometer or DIP switch, the circuit is generally analog, with inverse time or definite time working characteristics. Protection functions include overload, phase loss, stall, etc., fault type is displayed with indicator light, and running power is displayed with digital tube.

3) Intelligent type: Detect three-phase current value, the protector uses a single-chip microcomputer to realize intelligent comprehensive protection of the motor, integrating protection, measurement, communication and display. The set current is set digitally and operated through the buttons on the operation panel. Users can modify and set various parameters on site according to the specific situation of the motor; use digital tubes as display windows, or use large-screen LCD displays, and can support multiple communication protocols, such as ModBUS, ProfiBUS, etc. The price is relatively high and is used in more important occasions; currently, high-voltage motor protection uses intelligent protection devices.

4) Thermal protection type: A thermal element is embedded in the motor to provide protection based on the temperature of the motor windings, with good protection effects. However, when the motor capacity is large, it needs to be used in conjunction with the current monitoring type to prevent the motor windings from being damaged due to the lag of the temperature measuring element when the temperature rises sharply when the motor is stalled.

5) Magnetic field temperature detection type: Magnetic field detection coils and temperature measuring elements are embedded in the motor to provide protection based on changes in the rotating magnetic field and temperature inside the motor. The main functions include overload, stall, phase loss, overheating protection and wear monitoring. The protection function is complete, but the disadvantage is that magnetic field detection coils and temperature sensors need to be installed inside the motor.

3. Choice of protector type

1) For stand-alone independently operated motors that do not require high working conditions, have simple operation and control, and have little impact on production when shut down, ordinary protectors can be selected. Ordinary protectors have the characteristics of simple structure, convenient installation and wiring on site, easy replacement, simple operation, and high cost performance.

2) For motors working under harsh working conditions and with high reliability requirements, especially those involved in automated production lines, mid- to high-end intelligent protectors with more comprehensive functions should be selected.

3) For explosion-proof motors, eccentricity caused by bearing wear may cause high temperature at the explosion-proof gap and explosion hazard. Wear status monitoring function should be selected. For special bearings such as large-capacity high-pressure submersible pumps, the temperature of the bearings should be measured to avoid major economic losses caused by bore sweeping accidents.

4) For protectors used in places with explosion-proof requirements, the corresponding explosion-proof protectors should be selected according to the specific requirements of the application site to avoid safety accidents.