First, the basic principles of selection

At present, there is no same standard for low-voltage motor protection products on the market, and there are various models and specifications. In order to meet the different needs of users, manufacturers have derived a lot of series of products, which brings a lot of inconvenience to the selection of the majority of users. Users should select the motors according to the actual needs of the protection, in order to achieve the expected use value and effect.

Second, the basic method of selection

(I) Conditions related to type selection:

The selection of motor protection exists the motor and the protector two how to balance the relationship, the following provides several conditions and factors related to protection, for the user how to choose the protector to provide reference.

1, motor: to first understand the model specifications [1], motor functional characteristics, protection type, rated voltage, rated current, rated power, power frequency, flange, etc. These contents can basically give the user how to use and maintain correctly and select the protector to provide a reference basis.

2, environmental conditions: mainly refers to room temperature, high temperature, high cold, corrosion degree, vibration, sand, altitude, electromagnetic pollution, etc.

3, motor use: mainly refers to the requirements of driving mechanical equipment characteristics, such as fans, water pumps, air compressor, lathe, oil field pumping unit and other different load mechanical characteristics.

4. Control system: the control mode is manual, automatic, local or centralized remote control, single unit independent operation, or production line chain automation program and other work operation. Start mode has direct, step-down, star triangle, frequency sensitive rheostat, frequency converter, soft start and other start mode.

5, other aspects: the user to the site production monitoring management is more casual sex or rigor, non-normal shutdown to affect the severity of production.

There are many related factors with the selection of protector, such as installation position, connection line and system with electrical appliances, but mainly the variation of motor working conditions and control of the change of production impact, according to the actual working conditions requirements and actual changes, the reasonable selection and adjustment of the protector.

(two) the common types of low voltage motor protector

1, ordinary type: the structure is relatively simple, setting the current value is set by the potentiometer knob or code switch control, the circuit generally adopts analog, action protection curve using inverse time limit or fixed time limit working characteristics. The main function is to highlight fault protection such as overload, lack of phase (three-phase unbalance), blocking rotation, etc. The fault type is displayed by indicator light, and the running power is displayed by digital tube.

2, digital monitoring type: the use of single chip microcomputer in the internal circuit of the protector, to achieve intelligent comprehensive protection of the motor, set protection, measurement, communication, display as one. The setting current adopts the digital setting, through the operation of the panel button to operate the vertical, the user can according to their actual use requirements and the protection situation in the scene to correct the setting of various parameters, using the digital tube as the display window, because the digital tube display information is not rich enough, this kind of protector boundary is medium and high grade.

3, intelligent monitoring of Chinese phenotype: compared with the above digital monitoring type, it has more perfect functional characteristics and humanized configuration. This kind of product for all kinds of parameters, status, information directly on the control surface panel unit Chinese display liquid crystal display, make the interface more intuitive and eye-catching, can support a variety of communication protocols, such as MODBUS, PROFIBUS, to meet many ways to start, view a variety of information management records and so on. This kind of protector belongs to the high grade type in the current market.

4, special type: structural characteristics, basic protection function, circuit control mode is roughly the same as the ordinary type and digital monitoring type, but the internal circuit output control wiring method is different, to highlight the special control, such as fan, water pump, lathe, pumping unit, etc..

(3) Selection of protector type under the working condition of the motor

1, low requirements for the working conditions, simple operation control, monitoring, governance, more casual, downtime for production of little influence on single independent motor operation, can choose ordinary protector, because ordinary protector has simple structure, in the field wiring installation, replacement, manipulation is simple, convenient, high cost-effective, etc.

2, for the working conditions are very high requirements, security and continuity is very critical, and the degree of automation is high, and the need for special person control, monitoring, management, network monitoring of the MCC system, should be selected in the high-grade or high-grade protector.

3. For those motors with special load characteristics and alternating light and heavy loads, special protectors should be selected for the dual purposes of protection and control and display and monitoring.

(4) the connection mode of the main current of the protector

Main current wiring mode is divided into:

1. One-pass core type (secondary circuit of peripheral current transformer can also be used)

2, terminal type (can also use secondary circuit of peripheral current transformer)

3. Direct insertion type

One - time through the core connection is convenient and safe, avoiding the contact resistance heating caused by the bad contact of the terminal. When the rated current value of the motor is above 5A, a core-piercing connection can be selected in general.

Direct plug wiring is convenient for wiring, especially for those with small space and suitable for installation, the plug protector can be directly connected with the main contact output of the contactor.

(5) selection of the setting current range of the protector

In order to adapt to the selection of different power motors, protectors are basically provided with a certain range of current regulation, in the selection of protectors, according to the rated current value of the motor as far as possible to select the value of the middle range of the current setting.

(6) selection of working power supply

The working power supply is mainly for the internal circuit of the protector to work, except for the working power supply type, the working power supply level is generally divided into: AC380V, 220V, 110V, 36V. There is no special requirement for the selection of working power supply, because it is an independent power supply unit, the user only needs to choose according to the voltage level of the motor control loop.