The most comprehensive knowledge of electric motors!

Motor knowledge, the difference between various types of motors

1, DC, AC motor difference

DC motors use direct current as the power source, while AC motors use alternating current as the power source. In terms of structure, the principle of DC motors is relatively simple, but the structure is complex and not easy to maintain. While the principle of AC motor is complex but the structure is relatively simple, and it is easier to maintain than DC motor.

In terms of price, DC motors with the same power are higher than AC motors. Including speed control devices, also DC is higher than the price of AC, of course, the structure and maintenance also has a big difference.

And in terms of performance, because the DC motor speed stability, speed control precision, is the AC motor can not reach, so in the speed of the strict requirements had to use DC motor instead of AC motor. AC motor speed control is relatively complex, but it is widely used due to the use of AC power in chemical plants.

2, synchronous, asynchronous two types of motor difference

If the rotor rotates at the same speed as the stator, then it is called synchronous motor. If it is not the same, it is called asynchronous motor.

3、Difference between ordinary and inverter motors

First of all, it is clear that ordinary motor can not be used as inverter motor. Ordinary motor is designed for constant frequency and constant voltage, so it can't be fully adapted to the requirements of inverter speed control, so it can't be used as inverter motor. The impact of inverter on motor is mainly in motor efficiency and temperature rise, inverter can produce different degrees of harmonic voltage and current in operation, so that the motor runs under non-sinusoidal voltage and current, the high harmonics inside will cause the motor stator copper consumption, rotor copper consumption, iron consumption and additional loss increase. The most significant one is the rotor copper consumption, these losses will make the motor extra hot, reduce the efficiency, reduce the output power, and increase the temperature rise of the common motor by 10%-20% in general.

The frequency converter carrier frequency is from several kilohertz to more than ten kilohertz, which makes the motor stator winding to bear a very high voltage rise rate, which is equivalent to applying a steep impact voltage to the motor, so that the inter-turn insulation of the motor is subjected to a more serious test. When ordinary motor adopts inverter power supply, it will make the vibration and noise caused by electromagnetic, mechanical and ventilation factors become more complicated. And the various harmonics contained in the inverter power supply interfere with each other with the inherent space harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces, thus increasing the noise. Due to the wide working frequency range of the motor and the wide range of speed change, it is difficult to avoid the frequency of various electromagnetic force waves from the inherent vibration frequency of each structural part of the motor. When the power supply frequency is low, the loss caused by the high harmonics in the power supply is larger; secondly, when the speed of the variable motor decreases, the cooling air volume decreases proportionally to the third power of the speed, resulting in the heat of the motor not being dissipated, the temperature rise increases sharply, and it is difficult to realize constant torque output.

How to distinguish between ordinary motors and inverter motors? The difference in structure between ordinary motor and inverter motor?

Higher insulation level requirement

Generally, the insulation level of inverter motor is F or higher, to strengthen the insulation to ground and the strength of the wire-turn insulation, especially to consider the ability of insulation to withstand shock voltage.

Higher requirements for vibration and noise of inverter motor

Inverter motor should fully consider the rigidity of motor components and the whole, and try to improve its inherent frequency to avoid resonance with each force wave.

03.Different cooling methods of inverter motor

Inverter motor generally adopts forced ventilation cooling, that is, the main motor cooling fan is driven by an independent motor.

04.Different requirements of protection measures

Bearing insulation measures should be adopted for inverter motors with capacity over 160KW. The main reason is that it is easy to produce magnetic circuit asymmetry, which will also produce shaft current. When the currents produced by other high frequency components are combined together, the shaft current will be greatly increased, which will lead to bearing damage, so insulation measures should be taken in general. For constant power inverter motor, when the speed exceeds 3000/min, special grease with high temperature resistance should be used to compensate the temperature rise of the bearing.

05.Different heat dissipation system

Inverter motor cooling fan adopts independent power supply to ensure continuous cooling capacity.

Motor knowledge, motor selection

The basic elements needed for motor selection are:

Type of load to be driven, rated power, rated voltage, rated speed, other conditions.

Type of load

- DC motor

- Asynchronous motor

- Synchronous motor

The load is smooth, no special requirements for starting and braking of continuous operation of production machinery, it is appropriate to give priority to ordinary squirrel cage asynchronous motor, which is widely used in machinery, pumps, fans, etc. Starting and braking more frequently, requiring a larger starting and braking torque production machinery, such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, etc, should use wire-wound asynchronous motors. If there is no requirement for speed regulation, but the speed needs to be constant or the power factor needs to be improved, synchronous motor should be used, such as medium and large capacity water pump, air compressor, hoist, mill, etc. If the speed range is above 1:3, and the production machinery needs continuous stable and smooth speed regulation, it is appropriate to use other excitation DC motor or squirrel cage asynchronous motor or synchronous motor with frequency regulation, such as large precision machine tool, gantry planer, rolling mill, hoisting machine, etc. Production machinery requiring large starting torque and soft mechanical characteristics, use series-excited or compound-excited DC motors, such as trams, motor cars, heavy cranes, etc. Generally speaking, the motor can be roughly determined by providing the type of load to be driven, the rated power, rated voltage and rated speed of the motor.

However, these basic parameters are not enough if the load requirements are to be optimally met. Also need to provide the parameters include: frequency, operating system, overload requirements, insulation level, protection level, rotational inertia, load resistance torque curve, installation mode, ambient temperature, altitude, outdoor requirements, etc. (according to the specific situation to provide)

Motor knowledge, the steps of motor selection

When the motor is running or malfunctioning, four methods can be used to prevent and troubleshoot in time by seeing, hearing, smelling and touching to ensure the safe operation of the motor.

First, observe whether there is any abnormality during the operation of the motor, which is mainly manifested in the following cases.

When the stator winding is short-circuited, smoke may be seen in the motor.

When the motor is seriously overloaded or running out of phase, the speed will become slower and there is a heavy "humming" sound.

When the motor repair network runs normally, but stops suddenly, you will see sparks at the loose wiring; fuse is blown or a part is stuck, etc.

If the motor vibrates violently, it may be that the transmission device is stuck or the motor is poorly fixed or the foot bolt is loose, etc.

If there are discoloration, burn marks and smoke stains at the contact points and connections in the motor, it means that there may be local overheating, poor contact at the conductor connections or burned winding, etc.

When the motor is running normally, it should make a uniform and light "humming" sound, no noise and special sound.

If the noise is too big, including electromagnetic noise, bearing noise, ventilation noise, mechanical friction sound, etc, it may be a precursor of failure or failure phenomenon.

For electromagnetic noise, if the motor makes a high and low and heavy sound, the reasons may be the following.

(1) Uneven air gap between stator and rotor, at this time the sound is high and low and the time between high and low sound is unchanged, which is caused by the bearing wear and tear and thus the stator and rotor are not centered.

(2) three-phase current imbalance. If the sound is dull, it means that the motor is seriously overloaded or running out of phase.

(3) Loose iron core. In the operation of the motor, the core fixing bolt is loosened due to vibration, which causes the core silicon steel piece to be loose and makes noise.

For bearing noise, the motor should be monitored frequently during operation. The method of monitoring is: hold one end of the screwdriver against the bearing installation part, and the other end is close to the ear, then you can hear the bearing running sound. If the bearing is running normally, the sound will be continuous and small "rustling" sound, and there will be no change of high and low and metal friction sound.

If the following sounds occur, it is not normal phenomenon.

(1) bearing operation "squeaking" sound, this is the sound of metal friction, generally due to bearing oil shortage, should be disassembled bearing to fill the appropriate amount of grease.

(2) If there is a "chirp" sound, which is the sound of the ball rotation, generally caused by dry grease or lack of oil, can be filled with the appropriate amount of grease.

(3) If there is a "click" sound or "creak" 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 motor is not used for a long time and the grease is dry.

If the transmission mechanism and the driven mechanism make continuous and non-suddenly high and low sound, it can be divided into the following cases.

(1) Periodic "snap" sound, caused by the belt joint is not smooth.

(2) Periodic "thud" sound, caused by the coupling or pulley and shaft loosening and key or keyway wear.

(3) Uneven collision sound is caused by the fan cover colliding with the wind blade.

Third, the smell of the motor can also determine and prevent the failure. Open the junction box and sniff with nose to see if there is burnt smell. If a special paint smell is found, it means the internal temperature of the motor is too high; if a very heavy paste smell or burnt smell is found, it may be that the insulation layer repair network is broken or the winding has been burnt. If there is no smell, also need to use megohmmeter to measure the insulation resistance value between its winding and shell is lower than 0.5 meg, have to carry out drying treatment. The resistance value is zero, which means it has been damaged.

Fourth, the temperature of some parts of the motor can also determine the cause of failure. In order to ensure the safety, the back of the hand should touch the motor shell and bearing area. If abnormal temperature is found, the reasons may be as follows:

Poor ventilation. Such as fan off, blocked ventilation channel, etc.

Overload. The current is too large and the stator winding is overheated.

Stator winding inter-turn short circuit or three-phase current imbalance.

Frequent starting or braking.

If the temperature around the bearing is too high, it may be due to bearing damage or lack of oil.

Motor bearing temperature regulations, the reasons for abnormalities and processing procedures provide that the maximum temperature of rolling bearings does not exceed 95 ℃, the maximum temperature of sliding bearings does not exceed 80 ℃. And the temperature rise should not exceed 55℃ (temperature rise is the bearing temperature minus the ambient temperature at the time of testing). Causes of high bearing temperature rise and treatment:

(1) Cause: Shaft is bent and the centerline is not allowed. Treatment: Re-find the center.

(2) Cause: Loose foundation screws. Treatment: Tighten the foundation screws.

(3) Reason: The lubricant is not clean. Treatment: Replace the lubricating oil.

(4) Cause: The lubricant has been used for too long and has not been replaced. Treatment: Wash the bearing and replace the lubricant.

(5) Reason: The ball or roller in the bearing is damaged. Treatment: Replace the bearing with a new one.

Solution:

Open the module cover and replace the damaged insurance, charging resistor and other components in the module.

Replace the damaged optical communication subboard or protection diode.

Connect the optical fiber normally according to the label, and replace the optical fiber if it is damaged.

Replace the module power supply board.