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1. Basic principles of selection

At present, low voltageMotor protectorThere is no unified standard for products, and the models and specifications are varied. In order to meet the different needs of users, manufacturers have derived many series of products. The wide variety of products brings a lot of inconvenience to users in selecting models. Users should select models based on the actual needs of motor protection to achieve the expected use value and effect.

2. Basic method of selection

(I) Conditions related to selection:

The selection of motor protection involves the reasonable matching of the motor and the protector. The following provides several conditions and factors related to protection to provide users with a reference for how to select a protector.

1. Motor: First, you need to understand the model specifications, motor functional characteristics, protection type, rated voltage, rated current, rated power, power supply frequency, insulation, etc. [1]. These contents can basically provide users with a reference for how to correctly use, maintain and select protectors.

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

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

4. Control system: The control modes include manual, automatic, local or centralized remote control, independent operation of a single unit, or production line chain automation program. The starting methods include direct, step-down, star-delta, frequency-sensitive rheostat, inverter, soft start and other starting methods.

5. Other aspects: whether the user is casual or rigorous in the on-site production monitoring and management, the severity of the impact of abnormal shutdown on production, etc.

There are many other factors related to the selection of protectors, such as installation location, connection lines and coordination of system electrical appliances, but the main factor is the impact of changes in motor working conditions and control on production. According to the above-mentioned actual working conditions and actual changes, the protector should be reasonably selected and adjusted.

(II) Common types of low voltage motor protectors

1. Ordinary type: The structure is relatively simple. The set current value is set by a potentiometer knob or a code switch. The circuit is generally analog. The action protection curve adopts inverse time or definite time working characteristics. The main function is to highlight the fault protection such as overload, phase loss (three-phase imbalance) and stall. The fault type is displayed by an indicator light, and the running power is displayed by a digital tube.

2. Digital monitoring type: The internal circuit of 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 controlled 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. Since the information displayed by the digital tube is not rich enough, this type of protector is medium and high-end.

3. Intelligent monitoring Chinese display type: Compared with the above digital monitoring type, it has more complete functional characteristics and humanized configuration. This type of product directly displays various parameters, states, and information on the Chinese LCD on the control panel unit, making the interface more intuitive and eye-catching. It can support multiple communication protocols, such as ModBUS, ProfiBUS, etc., meet many startup methods, view various information management records, etc. This type 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 the ordinary type and digital monitoring type, except that the wiring method of the internal circuit output control of the protector is different, mainly highlighting special control, such as fans, water pumps, lathes, oil pumps, etc.

(III) Selection of protector type under motor working conditions

1. For single-machine independently operated motors that do not require high working conditions, have simple operation and control, and are relatively casual in monitoring and management, and whose shutdown has little impact on production, an ordinary type protector can be selected. This is because the ordinary type protector has a simple structure, and is easy and convenient to install, wire, replace, and operate on site, and has the characteristics of high cost performance.

2. For MCC systems that have very high requirements on working conditions, safety and continuity are very critical, and the degree of automation is high, and they require dedicated control, monitoring, and management, and they require network monitoring, mid-to-high-end or high-end protectors should be used.

3. For motors with special load characteristics and frequent alternation between light and heavy loads, special protectors should be selected to achieve the dual purposes of protection control and display monitoring.

(IV) Selection of wiring method for main current of protector

The main current wiring methods are divided into:

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

2. Terminal type (can also use the secondary circuit of the peripheral current transformer)

3. Direct plug-in

The one-time through-core wiring is convenient and safe, and avoids the heating of contact resistance caused by poor contact of the terminal. If the rated current of the motor is above 5A, the one-time through-core wiring can generally be selected.

The direct plug-in wiring method is convenient for wiring, especially in cases where the space is small and suitable for installation location, the plug-in protector can be directly connected to the contactor output main contacts.

(V) Selection of the protector setting current range

In order to adapt to the selection of motors of different powers, protectors basically have a certain current adjustment range. When selecting a protector, try to choose a value in the middle of the set current range based on the rated current value of the motor.

(VI) Working power supply selection

The working power supply is mainly used to supply the internal circuit of the protector, except for the type that does not require working power supply. The working power supply levels are 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, and the user only needs to select it according to the voltage level of the motor control circuit.