About the knowledge of motors, the most selected and comprehensive! (II)
Basic knowledge of electric motors - Selection steps for electric motors - The basic content required for motor selection includes: the type of load driven, rated power, rated voltage, rated speed, and other conditions. Load type: DC motor, asynchronous motor, synchronous motor. For continuous operation production machinery with stable load and no special requirements for starting and braking, ordinary squirrel cage asynchronous motor should be preferred, which is widely used in machinery, water pumps, fans, etc. Production machinery that requires high starting and braking torque due to frequent starting and braking, such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, etc., should adopt wound asynchronous mode
Steps for selecting a motor
The basic content required for motor selection includes: the type of load driven, rated power, rated voltage, rated speed, and other conditions.
For production machinery with stable load and continuous operation without special requirements for starting and braking, it is advisable to prioritize the use of ordinary squirrel cage asynchronous motors, which are widely used in machinery, water pumps, fans, etc.
Production machinery that requires high starting and braking torque due to frequent starting and braking, such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, etc., should use wound asynchronous motors.
In situations where there is no requirement for speed regulation and constant speed or improvement of power factor is required, synchronous motors should be used, such as medium and large capacity water pumps, air compressors, elevators, mills, etc.
For production machinery that requires a speed regulation range of 1:3 or above and requires continuous, stable, and smooth speed regulation, it is recommended to use separately excited DC motors or squirrel cage asynchronous motors or synchronous motors with variable frequency speed regulation, such as large precision machine tools, gantry planers, rolling mills, elevators, etc.
For production machinery with large starting torque and soft mechanical characteristics, series excited or compound excited DC motors shall be used, such as tramcars, electric locomotive, heavy cranes, etc.
Generally speaking, providing the type of load driven, rated power, rated voltage, and rated speed of the motor can roughly determine the motor. But if we want to optimize to meet the load requirements, these basic parameters are far from enough. The parameters that need to be provided include: frequency, working system, overload requirements, insulation level, protection level, moment of inertia, load resistance moment curve, installation method, environmental temperature, altitude, outdoor requirements, etc., according to the specific situation.
Basic knowledge of electric motors
Summary of experience in handling motor faults
When the motor is running or malfunctioning, there are four methods to prevent and eliminate faults in a timely manner, namely, looking, listening, smelling, and touching, to ensure the safe operation of the motor.
Observe if there are any abnormalities during the operation of the motor, which are mainly manifested in the following situations.
When the stator winding is short circuited, smoke may be seen from the motor.
When the motor is severely overloaded or runs out of phase, the speed will slow down and there will be a heavy "buzzing" sound.
3. The motor maintenance network operates normally, but when it suddenly stops, sparks may be seen at the loose connection; The phenomenon of a fuse blowing or a component getting stuck.
4. If the motor vibrates violently, it may be due to jamming of the transmission device, poor fixation of the motor, loose foundation bolts, etc.
5. If there is discoloration, burning marks, and smoke stains at the internal contacts and connections of the motor, it indicates that there may be local overheating, poor contact at the conductor connections, or burnt windings.
The motor should emit a uniform and light "buzzing" sound during normal operation, without any noise or special sounds. If too much noise is emitted, including electromagnetic noise, bearing noise, ventilation noise, mechanical friction noise, etc., it may be a precursor or phenomenon of a malfunction.
1. For electromagnetic noise, if the motor emits a loud and heavy sound, there may be several reasons:
(1) The air gap between the stator and rotor is uneven, causing the sound to fluctuate from high to low and the interval between high and low frequencies to remain unchanged. This is due to bearing wear, which causes the stator and rotor to be not concentric.
(2) The three-phase current is unbalanced. This is due to incorrect grounding, short circuit, or poor contact of the three-phase winding. If the sound is very dull, it indicates that the motor is severely overloaded or running out of phase.
(3) Loose iron core. The vibration of the motor during operation causes the fixing bolts of the iron core to loosen, causing the silicon steel sheet of the iron core to loosen and emit noise.
2. For bearing noise, it should be monitored frequently during motor operation. The monitoring method is to press one end of the screwdriver against the mounting area of the bearing, and the other end is close to the ear to hear the sound of the bearing running. If the bearing operates normally, its sound will be a continuous and small "rustling" sound, without any fluctuations in height or metal friction sound.
If the following types of sounds occur, it is considered abnormal:
(1) There is a "squeaking" sound when the bearing is running, which is a metal friction sound, usually caused by a lack of oil in the bearing. The bearing should be disassembled and added with an appropriate amount of lubricating grease.
(2) If there is a "chirp" sound, it is the sound made when the ball rotates, usually caused by the drying of lubricating grease or lack of oil. An appropriate amount of grease can be added.
(3) If there is a "clicking" or "creaking" sound, it is the sound generated by the irregular movement of the ball in the bearing, which is caused by the damage of the ball in the bearing or the long-term use of the motor, and the drying of the lubricating grease.
If the transmission mechanism and the driven mechanism emit continuous rather than fluctuating sounds, they can be handled in the following ways.
(1) Periodic "popping" sounds are caused by uneven belt joints.
(2) Periodic "thumping" sound is caused by loose coupling or pulley between shafts, as well as worn keys or keyways.
(3) The uneven collision sound is caused by the wind blades colliding with the fan cover.
By smelling the odor of the motor, faults can also be identified and prevented. Open the junction box and sniff with your nose. Check if there is a burnt smell. If a special paint smell is found, it indicates that the internal temperature of the motor is too high; If a strong paste or burnt odor is found, it may be due to the breakdown of the insulation repair mesh or the burning of the winding. If there is no smell, it is necessary to use a megohmmeter to measure the insulation resistance between the winding and the shell, which is less than 0.5 megawatts. Drying treatment must be carried out. A resistance value of zero indicates damage.
Touching the temperature of some parts of the motor can also determine the cause of the malfunction. To ensure safety, the back of the hand should be used to touch the surrounding parts of the motor casing and bearings when touching. If temperature abnormalities are found, there may be several reasons.
1. Poor ventilation. Such as fan detachment, blocked ventilation ducts, etc.
2. Overload. Causing excessive current and overheating of the stator winding.
3. Short circuit between stator windings or three-phase current imbalance.
4. Frequent starting or braking.
If the temperature around the bearing is too high, it may be caused by bearing damage or lack of oil.
Temperature regulations for motor bearings, causes of abnormalities, and solutions
The regulations stipulate that the maximum temperature of rolling bearings shall not exceed 95 ℃, and the maximum temperature of sliding bearings shall not exceed 80 ℃. And the temperature rise shall not exceed 55 ℃ (the temperature rise is the bearing temperature minus the ambient temperature during testing);
Please refer to HG25103-91 for the reasons and solutions for excessive temperature rise of bearings:
(1) Reason: The shaft is bent and the centerline is not accurate.
handle; Find the center again.
(2) Reason: The foundation screws are loose.
Handling: Tighten the foundation screws.
(3) Reason: The lubricating oil is not clean.
Treatment: Replace the lubricating oil.
(4) Reason: The lubricating oil has been used for a long time and has not been replaced.
Treatment: Clean the bearings and replace the lubricating oil.
(5) Reason: The ball or roller in the bearing is damaged.
Handling: Replace with a new bearing.
Basic knowledge of electric motors
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