Based on years of experience in selling motor protectors, the following methods for selecting motor protectors are summarized for users' reference.
I. Basic Principles for Selecting Motor Protectors
Currently, there is no unified standard for motor protectors on the market, and almost every manufacturer has its own models and specifications. To meet the needs of different customers, manufacturers have developed many products for different purposes, resulting in a wide variety that makes selection difficult for users. When selecting a protector, users should fully consider their actual motor protection needs and rationally choose the protection functions and methods to achieve good protection results, improve equipment reliability, reduce unplanned shutdowns, and minimize accident losses.
II. Basic Methods for Selecting Motor Protectors
(a) Conditions related to model selection:
The selection of motor protection involves the appropriate matching relationship between the motor and the protector. The following are some conditions and factors related to protection, which can be used as a reference for users when selecting a protection device.
1. Regarding the electric motor: First, you need to understand the model specifications, motor functional characteristics, protection type, rated voltage, rated current, rated power, power supply frequency, insulation class, etc. This information provides a basic reference for users on how to correctly select and use motor protectors.
2. Environmental conditions: mainly refers to normal temperature, high temperature, extreme cold, corrosiveness, vibration, wind and sand, altitude, electromagnetic pollution, etc.
3. Application of electric motors: such as fans, water pumps, air compressors, lathes, oilfield pumping units, etc., with different load mechanical characteristics. For example, a water pump motor protector should have an underload function. When the water pump runs out of water, the protector needs to issue a protection command.
4. Control system: Control modes include manual, automatic, local control, remote control, stand-alone operation, and centralized production line control. Starting methods include direct start, step-down start, star-delta start, frequency-sensitive resistor start, frequency converter start, and soft start.
5. Other aspects: Whether the user's on-site production monitoring and management is more casual or rigorous, and the severity of the impact of abnormal shutdowns on production, etc.
There are many other factors related to the selection of motor protectors, such as installation location, power supply conditions, and compatibility with the power distribution system. It is also necessary to consider whether it is a newly purchased motor protector, an upgrade of the motor protection, or an improvement of the protection for a faulty motor. The difficulty of changing the motor protection method and the degree of impact on production must also be considered. The selection and adjustment of motor protectors should be comprehensively considered based on the actual working conditions on site.
(II) Common Types of Motor Protectors
1. Thermal relay: For ordinary small-capacity AC motors, under good working conditions and without frequent starts or other harsh conditions, due to poor accuracy and lack of phase loss protection, reliability cannot be guaranteed, so its use is not recommended.
2. Motor Protector: This device detects three-phase current values, and the current setting is operated using a potentiometer knob or DIP switch. The circuit is generally analog and uses inverse time-limit operation. Protection functions include overload, phase loss, three-phase imbalance, and other fault protection. Fault types are indicated by indicator lights. The UL-E2 series is an example of this type of motor protector.
3. Integrated Intelligent Type: This type detects three-phase current values and uses a microcontroller to achieve intelligent integrated protection of the motor, combining protection, measurement, and display functions. The setting current is digitally set via operation panel buttons. Users can adjust various parameters on-site according to their actual usage requirements and protection needs. A digital tube display is used as the display window, or a large-screen LCD display can be used; these are more expensive and are used in more critical applications. Currently, intelligent types are commonly used for high-voltage motor protection.
4. Thermal protection type: A thermal element is embedded in the motor to protect it based on the motor temperature. The protection is generally good and has a temperature lag. 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 sensing element when the motor stalls.
5. Magnetic field temperature detection type: A magnetic field detection coil and temperature probe are embedded in the motor. It provides protection based on changes in the rotating magnetic field and temperature inside the motor. Its main functions include overload, locked rotor, phase loss, overheat protection, and wear monitoring. The protection functions are comprehensive. The disadvantage is that the magnetic field detection coil and temperature sensor need to be installed inside the motor, making it difficult to use in complex field environments. At present, it is only suitable for laboratory use.
(III) Selection of Motor Protector Type under Motor Operating Conditions
1. For single-unit independent motors with low working requirements, simple operation and control, relatively casual monitoring and management, and minimal impact on production from shutdown, ordinary type protectors can be selected. Ordinary type protectors have simple structure, are easy and convenient to install, wire, replace, and operate on site, and have high cost performance.
2. For MCC systems with high requirements for working conditions, critical safety and continuity, high degree of automation, and the need for dedicated personnel to control, monitor, and manage, and requiring network monitoring, mid-to-high-end motor protectors with more comprehensive functions should be selected.
3. For explosion-proof motors, bearing wear can cause eccentricity, potentially leading to high temperatures at the explosion-proof gap due to friction, posing an explosion hazard. Wear condition monitoring should be selected in such cases. For large-capacity high-pressure submersible pumps, due to the difficulty of inspection and maintenance, wear condition monitoring should also be selected to prevent rotor rubbing accidents and significant economic losses.
4. For motor protectors used in locations with explosion-proof requirements, the appropriate explosion-proof protector should be selected according to the specific requirements of the application site to avoid safety accidents.
