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Selection principle and method of motor protector

Basic principles of model selection


At present, there is no same standard for low-voltage motor protection products in the market, and there are various models and specifications.

 In order to meet the different use needs of users, manufacturers have derived many series of products, which bring many inconveniences to

 the selection of users. Users should choose the type according to the actual needs of motor protection in order to achieve the expected use value and effect.

July 15, 2022

Basic method of model selection


(1) Conditions related to model selection:


There is a reasonable relationship between motor and protector in the selection of motor protection. The following provides several

 conditions and factors related to protection to provide reference for users on how to select protector.


  1. In terms of motor: first understand the model and specification of [1], motor functional characteristics, protection type, rated voltage,

  2. rated current, rated power, power frequency, margin, etc. These contents can basically provide a reference basis for users on how to

  3. correctly use, maintain and select protectors.


2. Environmental conditions: mainly refer to normal temperature, high temperature, high cold, corrosion, vibration, wind and sand,

 altitude, electromagnetic pollution, etc.


3. Purpose of motor: it mainly refers to the required characteristics of driving mechanical equipment, such as fan, water pump, 

air compressor, lathe, oilfield pumping unit and other mechanical characteristics of different loads.


4. In terms of control system, the control modes include manual, automatic, local or centralized remote control, independent 

operation of single unit, or interlocking automation program of production line. The starting methods include direct, step-down, 

star delta, frequency sensitive rheostat, frequency converter, soft start and other starting methods.


5. Other aspects: it is the user's supervision and management of on-site production more casual or rigorous, and the severity 

of abnormal downtime affecting production.


There are many other factors related to the selection of protectors, such as the installation position, connecting lines and the 

coordination of system electrical appliances, but it is mainly the variation of motor working conditions and the influence of

 control changes on production. According to the above actual working conditions and actual changes, the protectors are 

reasonably selected and adjusted.

(2) Common types of low voltage motor protectors


  1. Ordinary type: the structure is relatively simple, the setting current value is controlled by potentiometer knob or code pulling

  2. switch, the circuit is generally analog, and the action protection curve adopts inverse time limit or fixed time limit working

  3. characteristics. Its main function is to highlight fault protection such as overload, phase loss (three-phase imbalance) and

  4. locked rotor. The fault type is displayed by indicator light, and the running power is displayed by digital tube.


2. Digital monitoring type: the internal circuit of the protector uses a single chip microcomputer to realize the intelligent 

comprehensive protection of the motor, integrating protection, measurement, communication and display. The setting

 current adopts digital setting, which is operated by the buttons on the control panel. Users can modify and set various

 parameters on site according to their actual use requirements and protection conditions. The digital tube is used as the 

display window. Because the information displayed by the digital tube is not rich enough, this kind of protector is of 

medium and high grade.


3. Intelligent monitoring Chinese display type: compared with the above digital monitoring type, it has more perfect

 functional characteristics and personalized configuration. This kind of products directly display various parameters,

 states and information in Chinese LCD on the control panel unit, making the interface more intuitive and eye-catching.

 It can support a variety of communication protocols, such as modbus, PROFIBUS, etc., meet many startup methods, 

view a variety of information management records, etc. This kind of protector is a high-end type in the current market.


4. Special type: the structural features, basic protection functions, circuit control mode, etc. are roughly the same as 

those of ordinary type and digital monitoring type, but the internal circuit output control wiring method of the protector

 is different, focusing on special control, such as fans, water pumps, lathes, pumping units, etc.


(3)Selection of Motor  protector type under motor working conditions


  1. For single machine independent operation motors with low requirements for working conditions, simple operation and

  2. control, random monitoring and management, and little impact on production from shutdown, ordinary protectors can

  3. be selected. Because of the simple structure of ordinary protectors, they are simple and convenient to install, replace and

  4. operate on site, and have the characteristics of high cost performance.


2. For MCC systems with high requirements for working conditions, safety and continuity, high degree of automation, 

and the need for special personnel to control, monitor and manage, and network monitoring, medium and high-grade 

or high-grade protectors should be selected.


3. For those motors with special load characteristics and frequent alternation of light and heavy loads, special protectors 

should be selected in order to achieve the dual purposes of protection control and display monitoring.

(4) Wiring mode selection of main current of  Motor protector


The main current wiring mode is divided into:


1. Primary through core type (secondary circuit of peripheral current transformer can also be used)


2. Wiring post type (secondary circuit of peripheral current transformer can also be used)


3. Direct insertion type


The one-time core threading wiring is convenient and safe, avoiding the heating of contact resistance caused by poor 

contact of terminals. When the rated current of the motor is above 5a, one-time core wiring can generally be selected.


The direct plug-in wiring method is convenient for wiring. Especially for those cases with small space and suitable installation

 position, the plug-in protector can be directly connected with the output main contact of the contactor.


(5) Selection of setting current range of  Motor protector


In order to adapt to the selection of motors with different power, the protector is basically equipped with a certain current 

regulation range. When selecting the protector, the value in the middle area of the setting current range shall be selected as

 far as possible according to the rated current value of the motor.


(6) Selection of working power supply


The working power supply is mainly used for the internal circuit of the  Motor protector, except for the type without working power 

supply. The working power supply level is generally divided into: AC380V, 220V, 110V, 36V. There are no special requirements

 for the selection of working power supply, because it is an independent power supply unit, users only need to choose according 

to the voltage level of motor control circuit.

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