I. Basic Principles of SelectionCurrently, there is no unified standard for motor protection products in the market, and there are various specifications and models.
To meet the diverse needs of users, manufacturers have developed many series of products, which brings inconvenience to users in the selection process.
When selecting, users should fully consider the actual needs of motor protection, rationally choose protection functions and modes to achieve effective protection, improve equipment reliability, reduce unplanned downtime, and minimize accidents.
II. Basic Methods of Selection
(A) Conditions Related to Selection:There are several conditions and factors related to protection, which provide references for users when making selections.Motor Aspects: It is essential to understand the model specifications, motor functional characteristics, protection types, rated voltage, rated current, rated power, power frequency, insulation class, etc.
These factors provide users with a basis for correctly using, maintaining, and selecting protection devices.Environmental Conditions: These mainly refer to normal temperature, high temperature, severe cold, corrosion, vibration, sand and wind, altitude, electromagnetic pollution, etc.Motor Application: This mainly refers to the specific requirements of mechanical equipment, such as fans, water pumps, air compressors, lathes, oil field pumps, and their different load characteristics.Control System Aspects: Control modes include manual, automatic, on-site control, remote control, standalone operation, centralized control for production lines, etc. Startup methods include direct, reduced voltage, star-delta, frequency-sensitive variable resistor, variable frequency drive, soft start, etc.
Other Aspects: This refers to the level of casualness or rigor in on-site production monitoring and management by users, and the severity of the impact of non-standard shutdowns on production.There are many other factors related to the selection of protection devices, such as installation position, power supply conditions, coordination with the distribution system, etc. Consideration should also be given to whether it is for the protection configuration of newly purchased motors, upgrading motor protection, or improving protection for accident-prone motors, as well as the difficulty of changing the motor protection mode and its impact on production.
The selection and adjustment of protection devices should be comprehensively considered based on the actual working conditions at the site.
(B) Common Types of Motor ProtectorsThermal Relay: Used for small-capacity AC motors operating in good conditions and not subjected to frequent start-ups. However, due to low accuracy and unreliable performance, it is not recommended for use.
Electronic Type: It detects the three-phase current value, and the setting current value is adjusted using potentiometer knobs or DIP switches. The circuit generally adopts an analog design and operates with inverse time or definite time characteristics. Protection functions include overload, phase loss, and locked rotor protection. Fault types are indicated by lights, and operating data is displayed on digital screens.
Intelligent Type: It detects the three-phase current value and uses a microcontroller to achieve comprehensive intelligent motor protection, integrating protection, measurement, communication, and display into one device. The setting current is digitized, and users can adjust various parameters on-site based on their actual usage requirements and protection conditions.
Display is available on digital screens or large LCD displays, and it supports multiple communication protocols such as ModBUS, ProfiBUS, etc.
This type is relatively expensive and is suitable for important applications. Intelligent protection is now widely used for high-voltage motor protection.
Thermal Protection Type: This type embeds a thermal element in the motor to provide protection based on the motor’s temperature. It offers good protection, but when used for larger capacity motors, it needs to be used in conjunction with current monitoring types to prevent a sharp rise in temperature when the motor is locked, which can cause damage to the motor windings due to the lag of the temperature sensing element.Magnetic Field
Temperature Detection Type: This type embeds magnetic field detection coils and temperature sensors in the motor to provide protection based on changes in the internal rotating magnetic field and temperature of the motor. Its main functions include overload, locked rotor, phase loss, overheat protection, and wear monitoring. The protection is comprehensive, but the drawback is the need to install magnetic field detection coils and temperature sensors inside the motor.
(C) Selection of Protector Types Under Motor Operating ConditionsFor single-machine independent operation motors with low operational requirements, simple control, and easy monitoring and management, and where shutdown does not significantly affect production, ordinary protectors can be used. These protectors have a simple structure, are easy to install, wire, and operate on-site, and offer a high cost-performance ratio.For high operational requirements where safety and continuity are critical, a higher level of automation and the need for specialized control, monitoring, and management, as well as the requirement for network monitoring and control in MCC systems, a mid-to-high grade protector with comprehensive functions should be used.For explosion-proof motors, due to bearing wear causing eccentricity and potential explosion risk due to frictional heat at the explosion-proof tolerance, wear status monitoring should be selected.
For large-capacity high-voltage submersible pumps, which are difficult to inspect and maintain, wear status monitoring should also be chosen to prevent major economic losses caused by pump barrel accidents.For protectors used in explosion-proof areas, suitable explosion-proof protectors should be selected according to the specific requirements of the application site, to prevent safety accidents.These are some of the common principles and methods for selecting motor protection devices.
Post time: Dec-07-2023