Electrical machinery is everywhere in the field of equipment.
It's not a lonely device. Reliable pump also needs reliable motor, which directly affects the normal operation of the equipment.
Motor type, soft start mode, selection steps, damage reason treatment method, what's the difference between good and bad motors..... All these problems are important reflection of motor happiness index. Let's take a look at them.
電機基礎(chǔ)知識
Differences between different types of motors
1. Difference between DC and AC motors
DC motor structure diagram
Structure diagram of AC motor
As the name implies, DC motors use DC as the power supply. The AC motor uses AC as the power supply.
In terms of structure, the principle of DC motor is relatively simple, but the structure is complex and not easy to maintain. The principle of AC motor is complex but its structure is relatively simple, and it is easier to maintain than DC motor.
In terms of price, the DC motor with the same power is higher than the AC motor, including the speed regulating device which controls the speed, and the DC speed regulating device is higher than the AC speed regulating device, of course, the structure and maintenance are also very different.
In terms of performance, because the speed of DC motor is stable and the speed control is precise, which can not be achieved by AC motor, DC motor has to be used instead of AC motor under the strict requirements of speed.
The speed regulation of AC motor is relatively complex, but it is widely used because of the use of AC power in chemical plants.
2. Difference between synchronous and asynchronous motors
If the rotating speed of the rotor is the same as that of the stator, it is called synchronous motor, if not, it is called asynchronous motor.
3. Difference between ordinary and variable frequency motors
First of all, ordinary motor can not be used as frequency conversion motor.
Ordinary motor is designed according to constant frequency and constant voltage, which can not fully meet the requirements of frequency converter speed regulation, so it can not be used as frequency converter motor.
1、 The effect of frequency converter on motor mainly lies in motor efficiency and temperature rise
The frequency converter can produce different levels of harmonic voltage and current in operation, so that the motor can operate under non sinusoidal voltage and current. The high-order harmonic in it will cause the increase of stator copper consumption, rotor copper consumption, iron consumption and additional loss of the motor. The most significant one is rotor copper consumption, which will make the motor generate extra heat, reduce efficiency, reduce output power and common motor temperature Generally, the increase is 10% - 20%.
General motor frequency conversion motor with independent cooling fan
2、 On the insulation strength of motor
The carrier frequency of the frequency converter is from several thousand to ten kilohertz, which makes the stator winding of the motor bear a high voltage rise rate, which is equivalent to applying a steep impulse voltage to the motor, which makes the turn to turn insulation of the motor bear a more serious test.
3、 Harmonic electromagnetic noise and vibration
The vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will be more complex when the common motor is powered by frequency converter. The harmonics contained in the variable frequency power supply interfere with the inherent space harmonics of the electromagnetic part of the motor, forming various electromagnetic exciting forces, thus increasing the noise. Because of the wide range of working frequency and the wide range of rotating speed, it is very difficult to avoid the natural vibration frequency of various structural parts of the motor.
4、 Cooling problem at low speed
When the power frequency is low, the loss caused by the high-order harmonics in the power supply is large; secondly, when the speed of the variable motor is reduced, the cooling air volume is directly proportional to the third power of the speed, so that the heat of the motor can not be emitted, and the temperature rise increases sharply, so it is difficult to achieve the constant torque output.
How to distinguish ordinary motor and variable frequency motor?
The difference of structure between common motor and variable frequency motor
1. Higher insulation level requirements
Generally, the insulation grade of variable-frequency motor is f or higher. Strengthen the insulation strength to ground and wire turn, especially consider the ability of insulation to withstand impulse voltage.
2. Higher requirements for vibration and noise of variable frequency motor
The rigidity of the motor components and the whole body should be fully considered for the variable frequency motor, and its natural frequency should be increased as much as possible to avoid resonance with each force wave.
3. Different cooling modes of variable frequency motor
The frequency conversion motor is usually cooled by forced ventilation, that is, the main motor cooling fan is driven by an independent motor.
4. Different requirements for protection measures
Bearing insulation measures shall be adopted for variable frequency motors with capacity over 160kW. It is easy to produce asymmetric magnetic circuit and shaft current. When the current generated by other high-frequency components is combined, the shaft current will increase greatly, resulting in bearing damage. Therefore, insulation measures should be taken generally. For constant power variable frequency motor, when the speed is more than 3000 / min, special grease with high temperature resistance shall be used to compensate the temperature rise of bearing.
5. Different cooling systems
The frequency conversion motor cooling fan adopts independent power supply to ensure continuous cooling capacity.
Basic knowledge of motor
Motor selection steps
The basic contents of motor selection include: type of load, rated power, rated voltage, rated speed and other conditions.
Load type
·DC motor
·Asynchronous motor
·Synchronous motor
For the production machinery with stable load and continuous operation without special requirements for starting and braking, the ordinary squirrel cage asynchronous motor should be preferred, which is widely used in machinery, water pump, fan, etc.
The starting and braking are frequent, and the production machinery with large starting and braking torque, such as bridge crane, mine hoist, air compressor, irreversible rolling mill, etc., shall adopt wound asynchronous motor.
Where there is no requirement for speed regulation, constant speed or improvement of power factor is required, synchronous motors shall be used, such as medium and large capacity water pumps, air compressors, hoists, mills, etc.
For the production machinery whose speed range is required to be more than 1:3 and which needs continuous, stable and smooth speed regulation, it is advisable to adopt the separately excited DC motor or the squirrel cage asynchronous motor or synchronous motor with frequency conversion speed regulation, such as large-scale precision machine tool, gantry planer, rolling mill, elevator, etc.
It is required to start production machinery with large turning distance and soft mechanical characteristics, using series or compound excitation DC motor, such as tramcar, electric locomotive, heavy crane, etc.
Generally speaking, the motor can be roughly determined by providing the type of driving load, rated power, rated voltage and rated speed of the motor. But these basic parameters are far from enough if we want to meet the load requirements optimally. Parameters to be provided also include: frequency, working system, overload requirements, insulation level, protection level, moment of inertia, load resistance moment curve, installation mode, ambient temperature, altitude, outdoor requirements, etc., which shall be provided according to specific conditions.
Motor selection steps
When the motor runs or breaks down, it can be prevented and eliminated in time by four methods of seeing, listening, smelling and touching to ensure the safe operation of the motor.
1、 Look
Observe whether there is any abnormality during the operation of the motor, which mainly shows the following situations.
1. When the stator winding is short circuited, smoke may be seen from the motor.
2. When the motor is seriously overloaded or running without phase, the speed will slow down and there will be a heavy "buzzing" sound.
3. When the maintenance network of the motor operates normally, but stops suddenly, sparks will appear at the loose part of the wiring; the fuse is blown or a part is stuck. 4. If the motor vibrates violently, it may be that the transmission device is stuck, the motor is not fixed properly, the foot bolt is loose, etc.
5. If there are discoloration, burn mark and smoke mark at the internal contact and connection of the motor, it indicates that there may be local overheating, poor contact at the conductor connection or burning of the winding, etc.
2、 Listen
During normal operation of the motor, a uniform and light "buzzing" sound shall be emitted without noise and special sound. If the noise is too large, including electromagnetic noise, bearing noise, ventilation noise, mechanical friction noise, etc., it may be a precursor or fault phenomenon.
1. For electromagnetic noise, if the motor makes a high, low and heavy sound, the causes may be as follows:
(1) The air gap between the stator and the rotor is uneven. At this time, the sound fluctuates from high to low and the interval between the high and low frequencies remains the same. This is caused by the bearing wear which makes the stator and the rotor not concentric.
(2) Three phase current imbalance. This is the reason of three-phase winding's wrong grounding, short circuit or poor contact. If the sound is very dull, it means that the motor is seriously overloaded or running without phase.
(3) The core is loose. During the operation of the motor, the fixed bolts of the iron core are loosened due to the vibration, resulting in the loosening of the silicon steel sheet of the iron core and the noise.
2. For bearing noise, it shall be monitored frequently during motor operation. Monitoring method: put one end of the screwdriver against the installation position of the bearing and the other end close to the ear, then you can hear the running sound of the bearing. If the bearing operates normally, its sound is continuous and small "sanding" sound, and there will be no change from high to low and metal friction sound.
The following sounds are abnormal:
(1) When the bearing is running, there is a "squeak" noise, which is caused by metal friction. Generally, it is caused by the lack of oil in the bearing. The bearing should be disassembled and filled with a proper amount of grease.
(2) If there is a "pump" sound, it is the sound produced when the ball rotates. Generally, it is caused by the dry or lack of oil of lubricating grease. It can be filled with appropriate amount of grease.
(3) If there is a "click" or "creak" sound, it is caused 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 grease.
3. If the transmission mechanism and the driven mechanism make continuous rather than high or low sounds, they can be handled in the following situations.
(1) The periodic "crack" sound is caused by the unsmooth belt joint.
(2) The periodic "thumping" sound is caused by the looseness between the coupling or pulley and the shaft and the wear of the key or keyway.
(3) The uneven impact sound is caused by the impact of the blades on the fan cover.
3、 Smell
It can also judge and prevent faults by smelling the motor. Open the junction box and sniff. Check whether there is burnt smell. If there is a special smell of paint, it means that the internal temperature of the motor is too high. If there is a heavy smell of paste or burnt smell, it may be that the insulating layer maintenance network is broken down or the winding is burnt. If there is no smell, it is necessary to measure the insulation resistance between the winding and the shell with a megohmmeter, which is lower than 0.5 m, and then dry it. The resistance value is zero, indicating that it is damaged.
4、 Touch
Touching the temperature of some parts of the motor can also determine the cause of the fault. In order to ensure safety, the back of the hand should be used to touch the motor shell and the surrounding part of the bearing when touching with the hand. If the temperature is abnormal, there may be several reasons.
1. Poor ventilation. Such as fan falling off, ventilation channel blocking, etc.
2. Overload. The current is too high and the stator winding is overheated.
3. Stator winding turns short circuit or three-phase current imbalance.
4. Start or brake frequently.
5. If the temperature around the bearing is too high, it may be caused by bearing damage or oil shortage.
The temperature regulation of motor bearing, the cause of the abnormity and its treatment
According to the regulations, the maximum temperature of rolling bearing shall not exceed 95 ℃, and the maximum temperature of sliding bearing shall not exceed 80 ℃. And the temperature rise shall not exceed 55 ℃ (the temperature rise is the bearing temperature minus the ambient temperature at the time of test);
For details, see hg25103-91 bearing temperature rise causes and treatment:
(1) Cause: the shaft is bent and the center line is not correct.
Handle; center again.
(2) Cause: loose foundation screws.
Treatment: tighten the foundation screws.
(3) Cause: the lubricating oil is not clean.
Treatment: change the lubricating oil.
(4) Cause: the lubricating oil has been used for too long and has not been replaced.
Treatment: clean the bearing and change the lubricating oil.
(5) Cause: the ball or roller in the bearing is damaged.
Treatment: replace the new bearing.
resolvent:
Open the cover plate of the module and replace the damaged fuse, charging resistor and other components in the module.
Replace the damaged fairway or protective diode.
The optical fiber shall be connected normally according to the mark, and the optical fiber shall be replaced if it is damaged.
Replace the module power board.
