Difference between Various Motors and How to Select a Motor?

Ⅰ. The difference between various types of motors

1. The difference between DC and AC motors

Schematic Diagram of DC Motor Structure

Schematic Diagram of the Structure of the AC Motor

As the name implies,

The power source for DC motors is direct current, while the power source for AC motors is alternating current.

The DC motor's principle is relatively simple structurally, but the structure is complex and inconvenient to maintain.

The AC motor has a complicated theory but a basic structure that makes it easier to maintain than a DC motor.

A DC motor with the same power is more expensive than an AC motor.

The price of DC is more than that of AC because it includes the speed control mechanism that regulates the speed. Of course, the construction and upkeep varied significantly.

In terms of performance, the DC motor must be replaced by the AC motor under rigorous speed restrictions due to the DC motor's steady speed and accurate speed control, which cannot be reached by the AC motor.

Although AC motor speed regulation is complicated, it is commonly employed in chemical plants that use AC power.

2. The difference between synchronous and asynchronous motors

The rotor rotates at the same speed as the stator, so it is called a synchronous motor. If not, it is called an asynchronous motor.

3. The difference between ordinary and variable frequency motors

To begin with, regular motors clearly cannot be employed as variable frequency motors.

Ordinary motors are built for constant frequency and constant voltage, and because they can't fully meet the requirements of frequency converter speed regulation, they can't be utilized as frequency converter motors.

Influence of inverter on motor

Mainly in the efficiency and temperature rise of the motor

During operation, the inverter can generate various degrees of harmonic voltage and current, allowing the motor to function on non-sinusoidal voltage and current.

The copper loss of the rotor is the most severe of these. The motor will generate more heat as a result of these losses, which will diminish its efficiency and output power. Ordinary motors have a temperature rise of 10% to 20%.

The frequency converter's carrier frequency ranges from a few kilohertz to over ten kilohertz, requiring the motor stator winding to withstand a high voltage rise rate, which is equivalent to applying a steep impulse voltage to the motor, causing the motor's inter-turn insulation to withstand more serious problems. test. The vibration and noise induced by electromagnetic, mechanical, ventilation, and other elements become more complicated when an ordinary motor is powered by a frequency converter.

The variable frequency power supply's harmonics interact with the electromagnetic part of the motor's intrinsic space harmonics to produce varied electromagnetic excitation forces, increasing noise.

The frequencies of various electromagnetic force waves find it difficult to avoid the natural vibration frequency of each structural part of the motor due to the motor's vast operating frequency range and wide range of rotational speed fluctuation.

When the frequency of the power supply is low, the loss caused by high-order harmonics in the power supply is large; second, as the speed of the motor decreases, the cooling air volume decreases in direct proportion to the cube of the speed, preventing the heat from the motor from being dissipated and causing the temperature to rise rapidly. It is challenging to maintain a steady torque output as the size of the engine grows.

How to distinguish between ordinary motors and variable frequency motors?

The difference between ordinary motor and variable frequency motor structure

01. Higher insulation level requirements

The frequency converter motor's insulation grade should be F or above, and the ground insulation and insulation strength of the turns should be improved, particularly the insulation's capacity to withstand impulsive voltage.

02. The vibration and noise requirements of variable frequency motors are higher

To minimize resonance with each force wave, the frequency converter motor should thoroughly consider the rigidity of the motor components and the whole, and try to raise its natural frequency.

03. The cooling method of the variable frequency motor is different

In most frequency conversion motors, forced ventilation cooling is used, which means that the main motor cooling fan is operated by a separate motor.

04. Different requirements for protection measures

Variable-frequency motors with a capacity of more than 160 KW should have bearing insulation. The fundamental reason is that asymmetrical magnetic circuits are simple to make and create shaft currents. When the currents generated by other high-frequency components combine, the shaft current increases dramatically, causing bearing damage, hence insulation is usually used. When the speed of a constant power variable frequency motor surpasses 3000/min, special grease with a high-temperature resistance should be used to compensate for the bearing's temperature rise.

05. Different cooling systems

To maintain continuous cooling capacity, the variable frequency motor cooling fan is powered by an independent power source.

Ⅱ. Motor selection

The basic contents required for motor selection are:

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

load type

DC

Asynchronous motor

Synchronous motor

Ordinary squirrel-cage asynchronous motors, which are commonly used in machinery, water pumps, fans, and other applications, should be favored for production machinery with a stable load and no specific requirements for starting and braking.

Wound asynchronous motors should be used in manufacturing machinery that requires a lot of starting and braking force, such as bridge cranes, mine hoists, air compressors, irreversible rolling mills, and so on.

Synchronous motors should be utilized in applications where there is no need for speed regulation, the rotational speed must be constant, or the power factor must be enhanced, such as medium and large capacity water pumps, air compressors, hoists, mills, and so on.

Separately excited DC motors, squirrel-cage asynchronous motors or synchronous motors with variable frequency speed regulation are recommended for production machinery that requires a speed regulation range of 1:3 or more and requires continuous, stable, and smooth speed regulation, such as large precision machine tools, gantry planers, Rolling mills, hoists, and so on. Trams, electric locomotives, hefty cranes, and other production gear that require a large starting torque and soft mechanical qualities use series or compound excitation DC motors.

In general, the motor can be approximated by stating the type of load driven, the motor's rated power, rated voltage, and rated speed.

However, if the load requirements are to be satisfied optimally, these basic characteristics are insufficient.

Frequency, working system, overload requirements, insulation class, protection class, a moment of inertia, load resistance torque curve, installation technique, ambient temperature, altitude, outdoor requirements, and so on are all parameters that must be specified. (given under specified circumstances)

Ⅲ. Steps for motor selection

When the motor is running or malfunctioning,

You can prevent and eliminate faults in time by looking, listening, smelling, and touching four methods.

To ensure the safe operation of the motor.

1. look

Examine the motor's operation for any abnormalities, which are most commonly seen in the following scenarios.

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

2. When the motor is severely overloaded or experiencing phase loss, the speed slows and a strong "humming" sound is heard.

3. The motor maintenance network is operating normally, but when it abruptly stops, sparks can be observed at the loose connection; a fuse has blown or a component has become stuck.

4. If the motor vibrates excessively, the transmission device may be stuck, the motor may not be correctly fixed, the foot bolts may be loose, and so on.

5. If the contact points and connections in the motor have discoloration, burn marks, or smoke traces, there may be local overheating, poor contact at the conductor connection, or winding burning.

2. Listen

During normal functioning, the motor should make a uniform and mild "humming" sound, with no murmurs or unusual noises.

If the noise is excessive, it could be a sign of failure or a failure phenomenon, such as electromagnetic noise, bearing noise, ventilation noise, mechanical friction noise, and so on.

1. If the motor creates a loud, high-pitched sound due to electromagnetic noise, the following could be the cause:

(1) There is a discrepancy in the air gap between the stator and the rotor. The sound swings between high and low at this period, and the gap between high and low sounds remains constant. This is due to bearing wear, which causes the stator and rotor to be out of alignment.

(2) There is an imbalanced three-phase current. This is caused by improper three-phase winding grounding, short circuits, or poor contact.

(3) The iron core is strewn everywhere. The iron core fastening bolts loosen during motor operation due to vibration, causing the iron core silicon steel sheet to loosen and create noise.

2. During the running of the motor, the bearing noise should be monitored often.

The way for monitoring is to place one end of the screwdriver against the bearing installation component and the other end close to the ear, and listen for the sound of the bearing running. If the bearing is operating normally, a constant and small "rustling" sound will be heard, with no sudden high and low fluctuations or metal friction sound.

It is abnormal if the following sounds appear:

(1) When the bearing is running, it makes a "squeak" sound. This is the sound of metal rubbing on metal, which is usually produced by a lack of oil in the bearing. The bearing should be dismantled and greased according to the manufacturer's instructions.

(2) If the ball makes a "chirp" sound while it rotates, it is most likely due to dry grease or a shortage of oil, and a suitable amount of grease can be applied.

(3) A "click" or "crunch" sound is created by irregular movement of the balls in the bearing, which is caused by bearing damage or grease drying if the motor is not used for a long time.

3. If the transmission and driven mechanisms create a continuous sound rather of high and low tones, the following situations can be addressed.

(1) The unequal belt junction produces a periodic "crack" sound.

(2) A loose coupling or pulley and shaft, as well as key or keyway wear, generate a periodic "dong dong" sound.

(3) The uneven collision sound is created by the impact of the fan blades against the fan cover.

3. Smell

The smell of the motor can also be used to diagnose and prevent faults.

Sniff the junction box once it's been opened.

Examine the area for a burning odor. If you smell a unique paint smell, it implies the motor's internal temperature is too high; if you notice a heavy sticky scent or a burnt smell, it means the insulation layer maintenance net has broken down or the winding has burned.

If there is no odor, a megohmmeter must be used to test the insulating resistance between the winding and the shell, which must be less than 0.5 megabytes, and the winding must be dried. It has been damaged since the resistance value is zero.

4. Touch

The temperature of various sections of the motor can also be used to determine the source of the problem.

Touch the motor housing and parts around the bearing with the back of your hand to confirm safety.

If an anomalous temperature is discovered, the following causes may apply:

1. Inadequate ventilation. The fan, for example, may break off, or the ventilation duct may become obstructed.

2. Excessive use. As a result, the stator windings are overheated and the current is high.

3. The three-phase current is imbalanced or the stator winding is short-circuited between spins.

4. Excessive braking or beginning.

5. A high temperature around the bearing could be due to bearing damage or a shortage of lubrication.

Ⅳ. Motor bearing temperature regulations, abnormal causes, and treatment

The maximum temperature of rolling bearings must not exceed 95 degrees Celsius, and the maximum temperature of sliding bearings must not exceed 80 degrees Celsius, according to the standards. And the temperature does not reach above 55 degrees Celsius (the temperature rise is the bearing temperature minus the ambient temperature during the test).

Causes and treatment of bearing temperature rise too high:

(1) Reason: the shaft is bent and the centerline is not allowed.

  Treatment: Find the center again.

(2) Reason: The foundation screw is loose.

  Remedy: Tighten the foundation screws.

(3) Reason: 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.

  Treatment: Clean the bearing and replace the lubricating oil.

(5) Reason: The balls or rollers in the bearing are damaged.

  Treatment: Replace the bearing with a new one.

Solution:

1. Remove the damaged fuses, charging resistors, and other components from the module by opening the cover.

2. Replace the light pass daughterboard or protection diode that has been broken.

3. The optical fiber is connected in a standard manner, as shown by the mark. Replace the optical fiber if it is damaged.

4. Replace the module power board.