Motors are everywhere in the field of equipment.
Motor types, soft start methods, selection steps, damage causes and treatment methods, the difference between good and bad motors...Each of these questions is an important reflection of the motor happiness index. Let's take a look at them together.
The difference between various types of motors
1. The difference between DC and AC motors
Schematic diagram of DC motor structure
Schematic diagram of AC motor structure
As the name implies, DC motors use direct current as a power source. AC motors use alternating current as a power source.
In terms of structure, the principle of DC motors is relatively simple, but the structure is complex and not easy to maintain. The principle of AC motors is complex but the structure is relatively simple, and it is easier to maintain than DC motors.
DC Motor
In terms of price, DC motors with the same power are higher than AC motors, including the speed control device, which is also more expensive than AC speed control devices. Of course, there are also great differences in structure and maintenance.
In terms of performance, because DC motors have stable speed and precise speed control, which are unmatched by AC motors, DC motors have to be used instead of AC motors under strict speed requirements.
AC motor speed regulation is relatively complex, but it is widely used because chemical plants use AC power.
2. The difference between synchronous and asynchronous motors
If the rotation speed of the rotor is the same as that of the stator, it is called a synchronous motor. If it is not the same, it is called an asynchronous motor.
Single-phase induction motor
Working principle of stepper motor
3. Difference between ordinary and variable frequency motors
First of all, ordinary motors cannot be used as variable frequency motors.
Ordinary motors are designed according to constant frequency and constant voltage, and it is impossible to fully adapt to the speed regulation requirements of frequency converters, so they cannot be used as variable frequency motors.
The impact of frequency converter on motor is mainly on motor efficiency and temperature rise
Frequency converter can generate harmonic voltage and current of different degrees during operation, making the motor run under non-sinusoidal voltage and current. The high-order harmonics will cause the increase of stator copper loss, rotor copper loss, iron loss and additional loss of the motor, the most significant of which is the rotor copper loss. These losses will cause additional heat generation in the motor, reduce efficiency and output power, and the temperature rise of ordinary motor will generally increase by 10% - 20%.
Ordinary motors
Inverter motors with independent cooling fans
Insulation strength of motors
The carrier frequency of the inverter ranges from several thousand to more than ten kilohertz, which makes the stator winding of the motor bear a very high voltage rise rate, which is equivalent to applying a very steep impact voltage to the motor, making the inter-turn insulation of the motor bear a more serious test.
Harmonic electromagnetic noise and vibration
When ordinary motors are powered by inverters, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors become more complicated. The harmonics contained in the variable frequency power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces, thereby increasing the noise. Due to the wide operating frequency range and large speed variation range of the motor, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequencies of the motor's structural parts.
Cooling problems at low speeds
When the power frequency is low, the losses caused by the high-order harmonics in the power supply are large; secondly, when the motor speed is reduced, the cooling air volume decreases in proportion to the cube of the speed, causing the heat of the motor to be unable to dissipate, the temperature rise increases sharply, and it is difficult to achieve constant torque output.
Three-phase stator
Small electric motor
How to distinguish between ordinary motors and variable frequency motors?
The structural differences between ordinary motors and variable frequency motors
(1) Higher insulation level requirements
Generally, the insulation level of variable frequency motors is F or higher, and the insulation strength of the ground and the wire turns is strengthened, and the ability of the insulation to withstand impulse voltage should be considered in particular.
(2) Higher vibration and noise requirements for variable frequency motors
The variable frequency motor should fully consider the rigidity of the motor components and the whole, and try to increase its natural frequency to avoid resonance with each force wave.
(3) Different cooling methods for variable frequency motors
Variable frequency motors generally use forced ventilation cooling, that is, the main motor cooling fan is driven by an independent motor.
(4) Different protection measures requirements
For variable frequency motors with a capacity of more than 160KW, bearing insulation measures should be adopted. The main problem is that it is easy to produce magnetic circuit asymmetry and shaft current. When the currents generated by other high-frequency components are combined, the shaft current will increase greatly, causing bearing damage, so insulation measures are generally required. For constant power variable frequency motors, when the speed exceeds 3000/min, special high-temperature resistant grease should be used to compensate for the temperature increase of the bearing.
(5) Different cooling systems
The cooling fan of the variable frequency motor is powered by an independent power supply to ensure continuous heat dissipation.
Motor cross-section
Steps for selecting a motor
The basic contents required for selecting a motor include: the type of load driven, rated power, rated voltage, rated speed, and other conditions.
Load type
DC motor
Asynchronous motor
Synchronous motor
For continuous-running production machinery with a stable load and no special requirements for starting and braking, ordinary squirrel-cage asynchronous motors should be used first. They are widely used in machinery, water pumps, fans, etc.
For production machinery that starts and brakes frequently and requires large starting and braking torques, such as bridge cranes, mine hoists, air compressors, irreversible steel rolling mills, etc., wound-rotor asynchronous motors should be used.
For situations where there is no speed regulation requirement, constant speed is required or the power factor needs to be improved, synchronous motors should be used, such as medium and large capacity water pumps, air compressors, hoists, grinders, etc.
For production machinery that requires a speed regulation range of more than 1:3 and requires continuous, stable and smooth speed regulation, separately excited DC motors or squirrel cage asynchronous motors or synchronous motors with variable frequency speed regulation are suitable, such as large precision machine tools, gantry planers, steel rolling mills, hoists, etc.
For production machinery that requires large starting torque and soft mechanical characteristics, series-excited or compound-excited DC motors should be used, such as trams, electric locomotives, heavy cranes, etc.
Generally speaking, the motor can be roughly determined by providing the driven load type, rated power, rated voltage and rated speed of the motor. However, if you want to optimize the load requirements, these basic parameters are far from enough. Other parameters that need to be provided include: frequency, working system, overload requirements, insulation level, protection level, moment of inertia, load resistance torque curve, installation method, ambient temperature, altitude, outdoor requirements, etc., which should be provided according to specific circumstances.
Summary of experience in handling motor faults
When the motor is running or fails, you can prevent and eliminate the fault in time by looking, listening, smelling and touching to ensure the safe operation of the motor.
1. Look
Observe whether there are any abnormalities during the operation of the motor, which are mainly manifested in the following situations.
1. When the stator winding is short-circuited, you may see smoke coming out of the motor.
2. When the motor is seriously overloaded or running in phase loss, the speed will slow down and there will be a heavy "buzzing" sound.
3. When the motor is running normally, but suddenly stops, you will see sparks coming out of the loose connection, the fuse is blown, or a part is stuck.
4. If the motor vibrates violently, it may be that the transmission device is stuck or the motor is not fixed well, the base bolts are loose, etc.
5. If there are discoloration, burn marks and smoke marks on the contact points and connections inside the motor, it means that there may be local overheating, poor contact at the conductor connection, or burning of the winding.
2. Listening
When the motor is running normally, it should emit a uniform and lighter "buzzing" sound, without noise or special sounds. If the noise is too loud, 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 emits a high-low and heavy sound, the reasons may be as follows:
(1) The air gap between the stator and the rotor is uneven. At this time, the sound is high-low and low-low, and the interval between high and low sounds remains unchanged. This is caused by bearing wear, which makes the stator and rotor non-concentric.
(2) Unbalanced three-phase current. This is caused by incorrect grounding, short circuit or poor contact of the three-phase winding. If the sound is very dull, it means that the motor is seriously overloaded or running in a phase loss.
(3) Loose core. During the operation of the motor, the vibration causes the core fixing bolts to loosen, causing the core silicon steel sheet to loosen and make noise.
2. For bearing noise, you should monitor it frequently during the operation of the motor. The monitoring method is: put one end of the screwdriver against the bearing installation part and the other end close to your ear, and you can hear the sound of the bearing running. If the bearing is running normally, the sound is a continuous and fine "rustling" sound, without any fluctuations or metal friction sounds.
If the following sounds occur, it is an abnormal phenomenon:
(1) When the bearing is running, there is a "squeaking" sound, which is a metal friction sound. It is generally caused by the lack of oil in the bearing. The bearing should be disassembled and an appropriate amount of grease should be added.
(2) If there is a "chirping" sound, this is the sound made by the ball when it rotates. It is generally caused by the drying up of the grease or lack of oil. An appropriate amount of grease can be added.
(3) If there is a "clicking" or "squeaking" sound, it is the sound produced by the irregular movement of the ball in the bearing. This is caused by the damage of the ball in the bearing or the drying up of the grease due to the long-term non-use of the motor.
3. If the transmission mechanism and the driven mechanism make a continuous sound instead of a fluctuating sound, it can be handled according to the following situations.
(1) Periodic "snap" sound is caused by uneven belt joint.
(2) Periodic "thump" sound is caused by loose coupling or pulley and shaft and wear of key or keyway.
(3) Uneven collision sound is caused by blade collision with fan cover.
3. Smell
Faults can also be judged and prevented by smelling the motor. Open the junction box and smell it. See if there is a burnt smell. If a special paint smell is found, it means that the internal temperature of the motor is too high; if a strong burnt smell or burnt smell is found, it may be that the insulation maintenance net is broken or the winding has been burned. If there is no smell, it is necessary to use a megohmmeter to measure the insulation resistance between the winding and the shell. If it is less than 0.5 megohms, it needs to be dried. If the resistance is zero, it means it is damaged.
Fourth, touch
Touching the temperature of some parts of the motor can also determine the cause of the fault. To ensure safety, use the back of your hand to touch the motor housing and the parts around the bearing. If the temperature is abnormal, the reasons may be as follows.
1. Poor ventilation. Such as fan falling off, ventilation duct blockage, etc.
2. Overload. The current is too large and the stator winding overheats.
3. The stator winding turns are short-circuited or the three-phase current is unbalanced.
4. Frequent starting or braking.
5.If the temperature around the bearing is too high, it may be caused by bearing damage or lack of oil.
Motor bearing temperature regulations, causes and treatment of abnormalities
The regulations stipulate that the maximum temperature of rolling bearings shall not exceed 95°C, and the maximum temperature of sliding bearings shall not exceed 80°C. And the temperature rise shall not exceed 55°C (the temperature rise is the bearing temperature minus the ambient temperature during the test);
For details, see HG25103-91 Causes and treatment of excessive bearing temperature rise:
(1)Cause: The shaft is bent and the center line is not accurate.
Treatment: Re-center.
(2)Cause: The foundation screws are loose.
Treatment: Tighten the foundation screws.
(3)Cause: The lubricating oil is not clean.
Treatment: Replace the lubricating oil.
(4) Reason: The lubricating oil has been used for too long and has not been replaced.
Handling: Clean the bearing and replace the lubricating oil.
(5) Reason: The ball or roller in the bearing is damaged.
Handling: Replace with a new bearing.
You know the difference between good and bad motors
Source: Electrical Engineering Technology Service Platform
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