At present, most of the motors used in coal mine system equipment use thermal relays as the overload protection components of the motors. They are widely used in production due to their small size, simple structure, and low cost. However, due to the low sensitivity, large error, and poor stability of the thermal relay, the protection is unreliable. Affected by the environment, temperature, and its own materials, it has been unable to meet the needs of modern coal mine production in terms of protecting motors.
1 Causes of damage to coal mine equipment motors and the purpose of improvement
During the maintenance process in the maintenance workshop, we found that most of the damaged motors were caused by overcurrent, blocked rotor, overload, open phase, and under voltage. If early warning and timely treatment can be achieved, the damage to the equipment motor can be greatly avoided, its service life can be prolonged, maintenance costs can be reduced, and the operating coefficient can be improved.
At present, there is no unified standard for motor protection products on the market, and there are various models and specifications. In order to meet the different needs of users, manufacturers derive a lot of series of products, with a wide variety and complex functions, which bring a lot of inconvenience to the use and selection. Therefore, we have to choose the protection mode and method according to our actual situation and considering the actual requirements.
2 Field application of intelligent motor protector
2.1 Under voltage, overvoltage, and phase loss protection
2.1.1 Under voltage
When an abnormal situation occurs in the power supply system and the voltage drops to a certain level, the motor will become fatigued, stalled, and reach several times the overcurrent. Therefore, access to under voltage protection is very necessary.
External cause: caused by lightning strikes.
Operating overvoltage: The overvoltage induced by the circuit inductance at both ends of the device caused by switching operations such as opening and closing.
When the phase voltage of the power supply system is higher than our preset protection voltage value, the protector will act, disconnect the normally closed contact, power off the KM, the main contact will be disconnected, and the motor will stop. Thereby protecting the motor.
2.1.3 Lack of phase
In the coal mine system, a considerable part of the equipment uses three-phase ungrounded power supply. When a certain phase is cut off in the power supply system, the equipment motor current will suddenly increase and the input voltage will be abnormal. When the voltage value is abnormally higher than the set value, the protector will act. Cut off the KM contactor.
2.2 Overcurrent protection
Overcurrent protection is the main point in motor protection, and it is also harmful to the motor. Mainly include overload, short circuit, locked-rotor, three-phase unbalance, etc. We use current transformers as detection and input components.
If the actual power of the motor exceeds the rated power of the motor under certain circumstances, this phenomenon is called motor overload. The working current of an overloaded motor is greater than its rated current, usually within 1.5 times the normal range.
2.2.2 Short circuit
The short circuit of a three-phase motor is more complicated. There are single-phase grounding short-circuits and phase-to-phase short-circuits; when a single-phase grounding short-circuit occurs, any phase is directly short-circuited with the PE wire. Generally, the motor will not be burned, but it will be charged in the motor shell. Cause an electric shock. The other is a short-circuit between phases. There are two-phase short-circuits and three-phase short-circuits. The three-phase short-circuit current will be much larger.
2.2.3 Stalled Rotor
When the load suddenly increases, the internal problems of the motor itself (bearing damage, bore sweeping), transmission mechanism, transmission, etc., any of the problems in the above links are the cause of the stall. Locked-rotor will cause the motor current to continue to increase, up to 7 times the rated current, and the motor will be burned out for a longer time. When the current value reaches the protector setting value, the protector will act.
The motor protector is usually selected, and the overload capacity of the motor should be fully considered to avoid damage. Improve reliability and production continuity. Strive to be economically reasonable. In the selection of functions, factors such as the value of the motor itself, the type of load, the use environment, the importance of the main equipment, and whether the stopping of the motor will affect production and the degree of impact should be comprehensively considered.
Through the above transformation, the stability of the power supply and power consumption system will definitely be improved, and the automation level of the coal mine electrical control system will be improved. At the same time, the maintenance cost will be greatly reduced, the operation efficiency of the motor will be improved, and the economic benefit will be increased. Coal production has a propelling effect.