Motor Basics: Differences, Selection, Maintenance

Differences of various motors

Differences of various motors

1. Differences between DC and AC motors

Schematic diagram of DC motor

Schematic diagram of DC motor

Schematic diagram of AC motor

Schematic diagram of AC motor

Just as its name implies, DC motor uses DC as the power supply, while AC motor uses AC as the power supply.

In terms of structure

The principle of the DC motor is relatively simple, but the structure is complex and inconvenient for maintenance.

The principle of the AC motor is complex, but the structure is relatively simple, and it is easier to maintain than the DC motor.

In terms of price

DC motors with the same power are higher than AC motors. Including the speed regulating device to control the speed, the price of DC is higher than that of AC. of course, there are great differences in structure and maintenance.

In terms of performance

The speed of DC motor is stable and the speed control is accurate, which can not be achieved by AC motor.

DC motor has to be used to replace AC motor under the strict requirements of speed.

AC motor speed regulation is relatively complex, but it is widely used because of the use of AC power supply in chemical plants.

2. Differences between synchronous and asynchronous motors

Differences between synchronous and asynchronous motors

If the rotation 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. Differences between ordinary and variable frequency motors

First of all, it is clear that ordinary motors cannot be used as variable frequency motors.

The 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 a frequency conversion motor.

The influence of the frequency converter on the motor mainly lies in the efficiency and temperature rise of the motor.

The frequency converter can produce different degrees of harmonic voltage and current during operation, so that the motor operates under non-sinusoidal voltage and current.

The high-order harmonic will increase the stator copper consumption, rotor copper consumption, iron consumption and additional loss of the motor.

The most significant one is the rotor copper consumption, which will make the motor generate additional heat, and reduce efficiency and output power.

The temperature rise of ordinary motors generally increases by 10% ~ 20%.

Ordinary motor              Variable frequency motor with independent cooling fan

The carrier frequency of the frequency converter ranges from several kilohertz to more than 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, so that the turn to turn insulation of the motor bears a serious test.

When an ordinary motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complex.

The harmonics contained in the variable frequency power supply interfere with the inherent space harmonics of the electromagnetic part of the motor to form various electromagnetic excitation forces, so as to increase the noise.

Due to the wide working frequency range and large speed variation range of the motor, the frequency of various electromagnetic force waves is difficult to avoid the natural vibration frequency of each structural part of the motor.

When the power frequency is low, the loss caused by higher harmonics in the power supply is large;

Secondly, when the speed of the variable motor decreases, the cooling air volume decreases in direct proportion to the third power of the rotating speed.

As a result, the heat of the motor cannot be dissipated, the temperature increases sharply, and it is difficult to achieve constant torque output.

So how to distinguish between ordinary motor and variable frequency motor?

Structural differences between ordinary motor and variable frequency motor

1. Higher insulation class requirements

Generally, the insulation grade of the variable frequency motor is F or higher.

Strengthen the ground insulation and wire turn insulation strength, especially the ability of insulation to withstand impulse voltage.

2. Variable frequency motors require higher vibration and noise

For variable frequency motors, the rigidity of motor components and the whole shall be fully considered.

Try to improve its natural frequency to avoid resonance with each force wave.

3. Variable frequency motor has different cooling modes

The variable frequency motor is generally cooled by forced ventilation, that is, the cooling fan of the main motor is driven by an independent motor.

4. Different requirements for protection measures

Bearing insulation measures shall be taken for variable frequency motors with a capacity of more than 160kW.

The main reason is that it is easy to produce magnetic circuit asymmetry and shaft current. When the currents generated by other high-frequency components act together, the shaft current will increase greatly, resulting in bearing damage, so insulation measures should be taken generally.

For constant power variable frequency motor

When the speed exceeds 3000r / min, special grease with high-temperature resistance shall be used to compensate for the temperature rise of the bearing.

5. Different cooling systems

The cooling fan of the variable frequency motor adopts an independent power supply to ensure continuous cooling capacity.

Selection of motor

The basic contents required for motor selection:

Load type, rated power, rated voltage, rated speed and other conditions driven.

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 pump, fan, etc.

basic contents required for motor selection

Production machinery with frequent starting and braking and requiring large starting and braking torque.

Such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, etc., shall adopt wound asynchronous motors.

Synchronous motors shall be used where there is no speed regulation requirement, and constant speed is required or power factor is required to be improved.

For example, medium and large-capacity water pumps, air compressors, hoists, mills, etc.

The speed regulation range shall be more than one to three.

Production machinery requiring continuous, stable and smooth speed regulation should adopt separately excited DC motor or squirrel cage asynchronous motor or synchronous motor with variable frequency speed regulation.

For example, large precision machine tools, gantry planers, rolling mills, hoists, etc.

It is required to start production machinery with large torque and soft mechanical characteristics, and use series excitation or compound excitation DC motors, such as trams, electric locomotives, heavy cranes, etc.

It is required to start production machinery with large torque and soft mechanical characteristics, and use series excitation or compound excitation DC motors, such as trams, electric locomotives, heavy cranes, etc.

However, these basic parameters are far from enough if we want to optimally meet the load requirements.

Parameters to be provided include:

frequency, working system, overload requirements, insulation grade, protection grade, the moment of inertia, load resistance moment curve, installation mode, ambient temperature, altitude, outdoor requirements, etc. (provided according to specific conditions).

Maintenance of motor

In case of motor operation or fault, four methods can be used to prevent and eliminate the fault in time, so as to ensure the safe operation of the motor.

1. Look

Observe whether there is any abnormality during the operation of the motor, which is mainly manifested in the following situations.

1). When the stator winding is short-circuited, the motor may smoke.

2). When the motor runs under severe overload or phase loss, the speed will slow down and there will be a heavy “buzzing” sound.

3). When the motor maintenance network operates normally but suddenly stops, sparks will be seen at the loose part of the wiring; The fuse is blown or a component is stuck.

4). 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.

5). If there are discoloration, burn marks and smoke marks at the internal contact and connection of the motor, it indicates that there may be local overheating, poor contact at the conductor connection or winding burning, etc.

2. Listen

The motor shall emit uniform and light “buzzing” sound without noise and special sound during normal operation.

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

(1) For electromagnetic noise, if the motor makes a loud 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 and low, and the interval between high and low sounds remains unchanged, which is caused by bearing wear, resulting in the non concentricity of the stator and the rotor.

2) The three-phase current is unbalanced. This is due to the wrong grounding, short circuit or poor contact of the three-phase winding. If the sound is very dull, it indicates that the motor is seriously overloaded or out of phase operation.

3) The iron core is loose.

During the operation of the motor, the fixing bolts of the iron core are loosened due to vibration, resulting in the loosening of the silicon steel sheet of the iron core and noise.

(2) Bearing noise shall be monitored frequently during motor operation.

The monitoring method is:

Press one end of the screwdriver against the installation part of the bearing and the other end close to the ear to hear the running sound of the bearing.

If the bearing operates normally, its sound is a continuous and small “rustle” sound, and there will be no change from high to low and metal friction sound.

1) There is a “squeak” sound when the bearing is running, which is the sound of metal friction.

It is generally caused by the lack of oil in the bearing.

The bearing should be disassembled and filled with an appropriate amount of grease.

2) If there is a “pumping” sound, it is the sound made when the ball rotates.

It is generally caused by dry grease or lack of oil. An appropriate amount of grease can be added.

3) If there is a “click” 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 drying of the lubricating grease after the motor is not used for a long time.

(3) If the transmission mechanism and the driven mechanism emit continuous rather than uncertain sound, it can be handled in the following situations.

1) The periodic “snap” sound is caused by the unsmooth belt joint.

2) The periodic “thump” 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 collision sound is caused by the blade colliding with the fan cover.

3. Smell

Faults can also be judged and prevented by smelling the smell of the motor.

Open the junction box and smell it with your nose to see if there are burnt smell.

If there is a special paint smell, it indicates that the internal temperature of the motor is too high;

If a heavy paste smell or burnt smell is found, the insulation maintenance network may be broken down or the winding has been burned.

If there is no smell, it is also necessary to measure the insulation resistance between the winding and the shell with a megger.

If it is lower than 0.5 trillion, it shall be dried. If the resistance value is zero, it indicates that it has been damaged.

4. Touch

Touching the temperature of some parts of the motor can also judge the cause of the fault.

To ensure safety, the back of the hand shall be used to touch the motor shell and the parts around the bearing when touching.

If the abnormal temperature is found, the reasons may be as follows:

  • Poor ventilation. For example, the fan falls off and the ventilation duct is blocked.
  • The current is too large and the stator winding is overheated.
  • Stator winding inter-turn short circuit fault or three-phase current imbalance.
  • Start or brake frequently.
  • If the temperature around the bearing is too high, it may be caused by bearing damage or lack of oil.

Regulations on motor bearing temperature and causes of being abnormal and solutions

According to the regulations, the maximum temperature of the rolling bearing shall not exceed 95 ℃, the maximum temperature of the 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 during the test).

Causes and treatment of excessive bearing temperature rise:

(1) Cause: the shaft is bent and the center line is inaccurate.

Processing: find the center again.

(2) Cause: the foundation screw is loose.

Treatment: tighten the foundation screws.

(3) Cause: the lubricating oil is not clean.

Treatment: replace the lubricating oil

(4) Cause: the lubricating oil has been used for too long and has not been replaced.

Treatment: clean the bearing and replace the lubricating oil.

(5) Cause: the ball or roller in the bearing is damaged.

Treatment: replace the bearing with a new one.


  • Open the module cover plate and replace the damaged fuse, charging resistor and other components in the module.
  • Replace the damaged luminous daughter board or protective diode.
  • The optical fiber shall be connected normally according to the mark. If the optical fiber is damaged, it shall be replaced.
  • Replace the module power board.

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