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, a DC motor uses direct current (DC) as its power source, while an AC motor uses alternating current (AC) as its power source.

In terms of structure, the principle of a DC motor is relatively straightforward, but its structure is complex and challenging to maintain. On the other hand, the principle of an AC motor is complex, but its structure is relatively simple and easier to maintain compared to a DC motor.

In terms of price, DC motors with the same power are typically more expensive than AC motors. Additionally, the cost of a DC motor is higher if you include a speed regulating device to control its speed.

In terms of performance, the speed of a DC motor is stable and the speed control is accurate, which cannot be achieved by an AC motor. However, DC motors are only used as a replacement for AC motors under strict speed requirements.

Although the speed regulation of an AC motor is more complex, it is widely used due to the widespread use of AC power in chemical plants.

2. Differences between synchronous and asynchronous motors

Differences between synchronous and asynchronous motors

A synchronous motor is a type of motor where the rotation speed of the rotor is the same as that of the stator. On the other hand, an asynchronous motor is a type of motor where the rotation speed of the rotor is not the same as that of the stator.

3. Differences between ordinary and variable frequency motors

It is evident that regular motors cannot be utilized as variable frequency motors. This is because regular motors are designed to operate at constant frequency and constant voltage, which does not fully meet the requirements of frequency regulation for speed control. Hence, it cannot be used as a frequency conversion motor.

The impact of the frequency converter on the motor primarily affects its efficiency and temperature rise. The frequency converter generates various degrees of harmonic voltage and current during operation, causing the motor to operate under non-sinusoidal voltage and current. This leads to an increase in stator and rotor copper consumption, iron consumption, and additional losses in the motor.

Of all the impacts, the most significant one is the rotor copper consumption, which causes the motor to generate more heat and reduces its efficiency and output power. As a result, the temperature rise of regular motors generally increases by 10% to 20%.

Ordinary motor              Variable frequency motor with independent cooling fan

The frequency range of the frequency converter ranges from several kilohertz to over ten kilohertz, which results in a high voltage rise rate in the stator winding of the motor. This is equivalent to applying a steep impulse voltage to the motor, putting the turn-to-turn insulation of the motor to a severe test.

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

The harmonics present in the variable frequency power supply interact with the inherent space harmonics of the electromagnetic part of the motor, resulting in various electromagnetic excitation forces and increased noise.

The wide working frequency range and large speed variation range of the motor make it difficult to avoid the natural vibration frequency of each structural part, resulting in a frequency of various electromagnetic force waves.

At low power frequency, the loss caused by higher harmonics in the power supply is substantial. Additionally, as the speed of the variable motor decreases, the cooling air volume decreases proportionally to the cube of the rotating speed, leading to a sharp increase in motor temperature and difficulty in achieving 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

Typically, variable frequency motors have an insulation rating of F or higher. To enhance the insulation strength, it is important to improve the ground insulation and wire turn insulation, particularly its capacity to resist impulse voltage.

2. Variable frequency motors require higher vibration and noise

For variable frequency motors, it is important to fully consider the rigidity of both the motor components and the entire motor. Efforts should be made to improve the natural frequency of the motor to avoid resonance with any force waves.

3. Variable frequency motor has different cooling modes

The variable frequency motor typically uses forced ventilation for cooling, which means that the cooling fan of the main motor is powered by a separate motor.

4. Different requirements for protection measures

For variable frequency motors with a capacity exceeding 160 kW, measures to insulate the bearings should be implemented.

This is due to the likelihood of magnetic circuit asymmetry and shaft current generation. When high-frequency currents generated by other components combine, it can significantly increase the shaft current, leading to damage to the bearings. To prevent this, insulation measures are generally necessary.

For constant power variable frequency motor

When the rotation speed surpasses 3000 revolutions per minute, it is important to use a special grease with high-temperature resistance to counteract the rise in temperature of the bearing.

5. Different cooling systems

The cooling fan of the variable frequency motor is powered by a separate power source to guarantee its continuous cooling ability.

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 that has a stable load and no specific requirements for starting and braking, an ordinary squirrel cage asynchronous motor is the preferred choice. This type of motor is widely used in machinery, water pumps, fans, and other applications.

basic contents required for motor selection

Production machinery that frequently starts and stops and requires high starting and braking torque, such as bridge cranes, mine hoists, air compressors, and irreversible rolling mills, should use wound asynchronous motors.

Synchronous motors should be used for applications that require constant speed or improved power factor, and have no speed regulation requirement, such as medium and large-capacity water pumps, air compressors, hoists, and mills.

The speed regulation range should be greater than one to three.

For production machinery that requires continuous, stable, and smooth speed regulation, separately excited DC motors, squirrel cage asynchronous motors, or synchronous motors with variable frequency speed regulation should be used, such as large precision machine tools, gantry planers, rolling mills, and hoists.

Production machinery with high torque and soft mechanical characteristics should start with series or compound excitation DC motors, such as trams, electric locomotives, and heavy cranes.

It should be noted that these basic parameters may not be sufficient 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 malfunction, four methods can be utilized to prevent and rectify the issue in a timely manner, thus ensuring the safe operation of the motor.

1. Look

Observe for any abnormalities during the motor operation, which are primarily indicated by the following scenarios:

1). If the stator winding experiences a short circuit, the motor may produce smoke.

2). If the motor operates under severe overload or phase loss, the speed will decrease and a loud “buzzing” sound will be heard.

3). If the motor’s maintenance network operates normally but suddenly stops, sparks may be observed at loose parts of the wiring. This could be due to a blown fuse or a stuck component.

4). If the motor vibrates excessively, it could be due to a stuck transmission device, poor fixation of the motor, or a loose foot bolt.

5). Discoloration, burn marks, and smoke marks at the internal contacts and connections of the motor may indicate local overheating, poor contact at conductor connections, or winding burnout.

2. Listen

The motor should emit a uniform and light “buzzing” sound during normal operation, without any additional noises or special sounds. If the noise level is too high, including electromagnetic, bearing, ventilation, mechanical friction, etc., it may indicate a potential problem or malfunction.

(1) For electromagnetic noise, if the motor produces a loud and heavy sound, the possible causes are:

  • Uneven air gap between the stator and rotor, resulting in high and low sounds with a consistent interval. This can be caused by bearing wear leading to non-concentricity of the stator and rotor.
  • Unbalanced three-phase current, which can be due to incorrect grounding, short circuit, or poor contact of the three-phase winding. If the sound is dull, it may indicate that the motor is significantly overloaded or out of phase operation.
  • Loose iron core, caused by vibration loosening the fixing bolts of the iron core, resulting in noise from the loosening of the silicon steel sheet.

(2) The sound of the bearings should be monitored regularly during motor operation. This can be done by pressing one end of a screwdriver against the bearing installation and holding the other end close to the ear to listen to the running sound.

If the bearing is operating normally, it should produce a continuous and small “rustling” sound, without any changes from high to low or metal friction sounds.

  • A “squeaking” sound indicates metal friction, typically caused by a lack of oil in the bearing. The bearing should be disassembled and refilled with an appropriate amount of grease.
  • A “pumping” sound is the result of ball rotation, usually caused by dry grease or a lack of oil. More grease can be added as needed.
  • A “clicking” or “creaking” sound is due to irregular movement of the ball in the bearing, caused by ball damage or drying of the lubricating grease after a long period of inactivity.

(3) If the transmission mechanism and driven mechanism produce a continuous, rather than uncertain, sound, it may be caused by the following:

  • A periodic “snap” sound is caused by an unsmooth belt joint.
  • A periodic “thump” sound is caused by looseness between the coupling or pulley and the shaft, or wear of the key or keyway.

3. Smell

Faults in a motor can be detected and prevented by using the sense of smell.

To check for faults, open the junction box and smell it for any burnt or unusual odors.

If there is a smell of paint, it could indicate that the internal temperature of the motor is too high.

If a strong, pungent smell or a burnt smell is present, it may indicate that the insulation or winding has been damaged.

Even if there is no noticeable odor, it is still important to measure the insulation resistance between the winding and the shell using a megger.

If the insulation resistance is lower than 0.5 trillion ohms, the motor should be dried. A resistance value of zero indicates that the motor has been damaged.

4. Touch

Touching the temperature of various parts of the motor can also help diagnose faults.

For safety reasons, it is best to use the back of the hand to touch the motor shell and parts near the bearing when checking the temperature.

If an abnormal temperature is detected, it could be due to several reasons such as:

  • Poor ventilation, such as if the fan has fallen off or the ventilation duct is blocked.
  • Overheating of the stator winding due to excessive current.
  • Stator winding inter-turn short circuit fault or three-phase current imbalance.
  • Frequent starting or braking.

If the temperature around the bearing is excessively high, it could be caused by bearing damage or a lack of lubrication oil.

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

According to regulations, the maximum temperature of rolling bearings must not exceed 95℃ and the maximum temperature of sliding bearings must not exceed 80℃, with a temperature rise not exceeding 55℃ (calculated as the difference between the bearing temperature and ambient temperature during testing).

Potential causes and solutions for excessive temperature rise in bearings include:

  • Bent shaft or inaccurate centerline – realign the center.
  • Loose foundation screws – tighten the screws.
  • Dirty lubricating oil – replace the oil.
  • Old lubricating oil – clean the bearing and replace the oil.
  • Damaged ball or roller in the bearing – replace the bearing.

For the solution section, the following revisions should be made:

  • Replace the damaged components (fuse, charging resistor, etc.) within the module by opening the cover plate.
  • Replace any damaged luminous daughter boards or protective diodes.
  • Ensure that the optical fiber is properly connected as indicated, and replace if damaged.
  • Replace the module power board.

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