I. Differences of various engines
1 . Differences between DC and AC motors
DC Motor Schematic Diagram
AC Motor Schematic Diagram
As the name suggests, a DC motor uses direct current (DC) as a power source, while an AC motor uses alternating current (AC) as a power source.
In terms of structure, the principle of a DC motor is relatively simple, but its structure is complex and difficult to maintain. On the other hand, the working 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 precise, 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 speed regulation of an AC motor is more complex, it is widely used due to the widespread use of AC power in chemical plants.
two . Differences between synchronous and asynchronous motors
A synchronous motor is a type of motor where the rotational speed of the rotor is equal to that of the stator. On the other hand, an asynchronous motor is a type of motor where the rotational speed of the rotor is not the same as that of the stator.
3 . Differences between common and variable frequency motors
It is clear that normal motors cannot be used as variable frequency motors. This is because normal motors are designed to operate at constant frequency and voltage, which does not fully meet the frequency regulation requirements for speed control. Therefore, it cannot be used as a frequency conversion motor.
The impact of the frequency converter on the motor mainly affects its efficiency and temperature rise. The frequency converter generates varying 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 is the consumption of copper in the rotor, which causes the engine to generate more heat and reduces its efficiency and power output. As a result, the temperature rise of normal engines generally increases by 10% to 20%.
The frequency range of the frequency converter varies from several kilohertz to more than ten kilohertz, which results in a high rate of voltage rise in the motor stator winding. This is equivalent to applying a sharp impulse voltage to the motor, subjecting the motor's turn-to-turn insulation to a severe test.
When a motor is powered by a frequency converter, 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 spatial 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 several waves of electromagnetic force.
At low power frequencies, the loss caused by higher harmonics in the power supply is substantial. Additionally, as variable engine speed decreases, the volume of cooling air decreases in proportion to the cube of rotational speed, leading to a sharp increase in engine temperature and difficulty in achieving constant torque output.
So how to distinguish between ordinary motor and variable frequency motor?
II. Structural differences between ordinary motor and variable frequency motor
1 . Higher insulation class requirements
Variable frequency motors typically have an insulation rating of F or higher. To increase insulation resistance, it is important to improve insulation from earth and insulation of wire turns, particularly their ability to resist impulse voltage.
two . Variable frequency motors require greater vibration and noise
For variable frequency motors, it is important to fully consider the stiffness of 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 main motor's cooling fan is powered by a separate motor.
4 . Different requirements for protective measures
For variable frequency motors with a capacity greater than 160 kW, measures must be implemented to isolate the bearings.
This is due to the probability of asymmetry of the magnetic circuit and generation of current in the shaft. When high frequency currents generated by other components combine, it can significantly increase shaft current, causing damage to bearings. To avoid this, isolation measures are generally necessary.
For constant power variable frequency motor
When the rotation speed exceeds 3,000 revolutions per minute, it is important to use a special grease with high temperature resistance to counteract the increase in bearing temperature.
5 . Different refrigeration systems
The variable frequency motor cooling fan is powered by a separate power source to ensure its continuous cooling ability.
III. Engine Selection
The basic contents required for engine selection:
Load type, rated power, rated voltage, rated speed and other driven conditions.
1. Types of driven loads
This must be addressed from the characteristics of the motors, which can be simply categorized into direct current (DC) motors and alternating current (AC) motors, with AC motors divided into synchronous and asynchronous motors.
(1) DC Motors
The advantage of DC motors is the convenience of speed regulation through voltage adjustments, combined with the ability to provide considerable torque. They are suitable for loads that require frequent speed adjustments, such as rolling mills in steel mills and elevators in mines.
However, with the advancement of frequency conversion technology, AC motors can also adjust speed by changing frequency. Although the cost of a variable frequency motor is not much higher than that of normal motors, the price of the inverter constitutes a significant part of the total cost of the equipment. Thus, another advantage of DC motors is their cost-benefit ratio.
A disadvantage of DC motors is their complex structure, which inevitably leads to increased failure rates. DC motors, compared to AC motors, not only have more complex windings (excitation, commutation, compensation and armature windings), but also include additional components such as slip rings, brushes and commutators.
These requirements not only require high manufacturing precision, but also result in higher maintenance costs in the long term.
Therefore, DC motors are in an awkward position in industrial applications, gradually losing popularity but still useful during the transition phase. If the user has sufficient resources, it is recommended to choose an AC motor with an inverter, given the various benefits that inverters offer.
(2) Asynchronous Motors
The advantages of asynchronous motors lie in their simple structure, stable performance, easy maintenance and low cost. They also have the simplest manufacturing process. As an old shop technician once said, the labor hours spent assembling a DC motor could complete approximately two synchronous motors or four asynchronous motors of similar power. This says a lot about the widespread use of asynchronous motors in industry.
Asynchronous motors are divided into squirrel cage and wound rotor types, differentiated by their rotors. The rotor of a squirrel cage motor is made of metal bars, either copper or aluminum.
Aluminum is cheaper and, as China is rich in bauxite, it is widely used where requirements are not high.
However, the mechanical and electrical properties of copper are superior to those of aluminum, and most rotors I have encountered are made of copper. Squirrel cage motors, once the problem of broken bars has been resolved, demonstrate significantly greater reliability than those with wound rotors.
A disadvantage, however, is that the torque generated by a metal rotor cutting magnetic lines in a rotating stator field is relatively small and the starting current is large, making it difficult to handle loads that require high starting torque.
Although increasing the length of the engine core can generate more torque, the effect is quite limited. Wound rotor motors, on the other hand, energize the rotor winding through slip rings at start-up, creating a rotor magnetic field. The resulting relative motion with the stator's rotating field produces a higher torque.
During starting, the starting current is reduced by the use of water resistors, the resistance of which is controlled by a mature electronic control device that changes its value during the starting process. This is suitable for loads such as rolling mills and elevators.
However, because wound rotor asynchronous motors add components such as slip rings and water resistors, the overall equipment cost is somewhat higher. Compared to DC motors, they have a narrower speed adjustment range and relatively smaller torque, so their value is lower.
However, because asynchronous motors establish a rotating magnetic field by energizing the stator winding, which is an inductive component that does no work, they draw reactive power from the grid, creating a significant impact.
For example, when a large inductive appliance is connected to the grid, the grid voltage drops and the brightness of electric lights decreases abruptly.
Therefore, power companies may restrict the use of asynchronous motors, which many factories must take into consideration. Some large electricity consumers, such as steel and aluminum factories, choose to establish their own power plants, forming independent networks, to alleviate these usage restrictions.
Therefore, in order for an asynchronous motor to meet the needs of high power loads, it must be equipped with a reactive power compensation device. In contrast, synchronous motors can supply reactive power to the grid via excitation devices. The greater the power, the more evident are the advantages of synchronous motors, thus creating a stage for their use.
(3) Synchronous Motors
In addition to compensating reactive power in an overexcited state, the advantages of synchronous motors also include:
1) The speed of the synchronous motor strictly follows n=60f/p, allowing precise speed control.
2) They offer high operational stability; In the event of a sudden drop in grid voltage, the excitation system would normally boost the excitation to ensure stable operation, while the torque of an induction motor (proportional to the square of the voltage) would decrease significantly.
3) The overload capacity is greater than that of a comparable induction motor.
4) They have high operational efficiency, especially in the case of low-speed synchronous motors.
Synchronous motors cannot start directly; they require induction or frequency conversion startup. Induction starting refers to the process in which a starting winding similar to the squirrel cage winding of an induction motor is installed on the rotor of the synchronous motor.
A supplementary resistor with a resistance value about ten times that of the excitation winding is connected in series in the excitation circuit to form a closed circuit, allowing the synchronous motor stator to be connected directly to the grid.
The motor then starts like an induction motor and when the speed reaches subsynchronous speed (95%), the additional resistor is disconnected. The start of frequency conversion is not elaborated here. Consequently, one of the disadvantages of synchronous motors is the need for additional starting equipment.
A synchronous motor operates with excitation current. Without excitation, the motor is asynchronous. The excitation is a direct current system applied to the rotor, with rotational speed and polarity consistent with the stator.
If there are problems with the excitation, the motor will lose synchronization, will not be able to adjust and will activate a protection mechanism, causing the motor to trip due to “excitation failure”. Therefore, another disadvantage of synchronous motors is the need for an additional excitation device.
Previously this was provided directly by a DC motor, but is now mainly provided by thyristor rectification. As the saying goes, the more complex the structure and the more equipment, the more potential points of failure, therefore, the higher failure rate.
Based on the performance characteristics of synchronous motors, their applications are mainly found in hoists, grinding machines, fans, compressors, rolling mills, water pumps and other loads.
In summary, the principle for selecting an engine is that as long as the engine performance meets the requirements of production machinery, priority should be given to engines with simpler structures, lower prices, reliable operation and convenient maintenance.
In this respect, AC motors are superior to DC motors, AC induction motors are superior to AC synchronous motors, and squirrel cage induction motors are superior to wound rotor induction motors.
For production machines running continuously with constant loads and without special requirements for starting or braking, it is preferable to use a standard squirrel cage induction motor, which is widely used in machines, water pumps, fans and more.
Production machines that require frequent starts and stops and require high starting and braking torque, such as overhead cranes, mine hoists, air compressors, and irreversible rolling mills, must use a wound rotor induction motor.
In cases where there is no need for speed adjustment, constant speed or improvement in power factor is required, synchronous motors must be used. They are suitable for medium and large capacity water pumps, air compressors, hoists, grinding machines and more.
For production machines that require a speed adjustment range greater than 1:3 and that require smooth and stable speed regulation, it is recommended to use separately excited DC motors or squirrel cage induction motors or synchronous motors with control of frequency speed. They are suitable for high precision machine tools, gantry planers, rolling mills, winches and more.
Production machines that require high starting torque and have smooth mechanical properties should use series or compound excited DC motors. They are ideal for electric vehicles, electric locomotives, heavy cranes and more.
2. Rated power
The rated power of an electric motor refers to its output power, also known as shaft power or capacity, which is a characteristic parameter of the motor. When people ask about engine size, they are usually referring to the power rating and not the physical dimensions.
Power rating is the most important metric when quantifying engine load capacity and is a necessary parameter when selecting an engine.
(Where Pn refers to the rated power, Un refers to the rated voltage, In refers to the rated current, cosθ is the power factor and η is the efficiency)
The principle of choosing the correct engine capacity should be based on the premise that the engine can meet the load requirements of production machinery and decide the engine power in the most economical and reasonable way.
If the power is chosen too high, it will result in increased investment and equipment waste, and the motor will often run underload, resulting in low efficiency and power factor. On the other hand, if the power is chosen too low, the engine will run overloaded, causing premature damage.
There are three main factors that determine the power of an engine:
1) Heating and increasing engine temperature, which is the most important factor in determining engine power;
2) Short-term overload capacity of the motor;
3) For squirrel cage asynchronous motors, the starting capacity must also be considered.
Firstly, the specific production machinery, based on heating, temperature rise and load requirements, calculates and selects the load power. The motor then preliminarily selects the rated power based on the load power, duty cycle and overload requirements.
After the motor's nominal power has been pre-selected, it must undergo checks for heating, overload capacity and, when necessary, starting capacity. If any of these checks fail, the engine must be reselected and rechecked until all parameters pass.
Therefore, duty cycle is also a necessary requirement to be provided. If there is no requirement, it will be processed according to the most common S1 work cycle; motors with overload requirements also need to provide the overload multiples and corresponding operating time; Squirrel cage asynchronous motors that drive high inertia loads such as fans also need to provide the load moment of inertia and starting torque curve for starting capability checks.
All nominal power selections mentioned above are carried out under the premise of a standard ambient temperature of 40°C. If the ambient temperature where the engine operates changes, the rated power of the engine must be revised.
Based on theoretical calculations and practice, at different ambient temperatures, the engine power can increase or decrease approximately according to the table below.
Therefore, in areas with harsh climates, ambient temperature must also be provided. For example, in India, the room temperature needs to be checked at 50°C.
Furthermore, high altitude can also affect engine power; the higher the altitude, the greater the increase in engine temperature and the lower the power output. Engines used at high altitudes also need to consider the effects of corona discharge.
As for the current engine power range on the market, I provide the following data from my company's performance table for reference:
- DC Motor: ZD9350 (grinder) 9350kW
- Asynchronous Motor: Squirrel Cage Type YGF1120-4 (Blast Furnace Fan) 28000kW
- YRKK1000-6 type wound rotor (raw material grinder) 7400kW
- Synchronous Motor: TWS36000-4 (Blast Furnace Fan) 36000kW (One test unit reached 40000kW)
3. Rated voltage
The rated voltage of a motor refers to the line voltage under its rated operating conditions. The choice of the motor's nominal voltage depends on the supply voltage of the installation's power system and the motor capacity.
The selection of voltage rating for AC motors depends mainly on the supply voltage level at the location of use. The common low voltage network is 380 V, so the nominal voltage is normally 380 V (Y or Δ connection), 220/380 V (Δ/Y connection) or 380/660 V (Δ/Y connection).
When the power of low voltage motors reaches a certain level (such as 300KW/380V), it becomes difficult or very expensive to increase the current due to the limitation of wire capacity.
In such cases, it is necessary to obtain high power by increasing the voltage. The normal supply voltage for the high voltage network is 6,000 V or 10,000 V, but in foreign countries there are also voltage levels of 3,300 V, 6,600 V and 11,000 V.
High voltage motors have the advantages of high power and strong shock resistance. However, they also have the disadvantage of high inertia, making starting and braking difficult.
The rated voltage of a DC motor also needs to match the source voltage. It is usually 110V, 220V or 440V. The commonly used voltage level is 220V, but for high-power motors, it can be increased to 600–1000V.
When the AC power supply is 380V and a three-phase bridge controllable silicon rectification circuit is used for power supply, the rated voltage of the DC motor should be set at 440V. If powered by a three-phase half-wave controllable silicon rectification source, the nominal voltage of the DC motor should be 220V.
4. Rated speed
The rated speed of an electric motor refers to its speed under designated operating conditions.
Both the electric motor and the machinery it drives have their own nominal speeds. When choosing the speed of an electric motor, it must be considered that too low a speed is not desirable. This is because the lower the rated speed of an electric motor, the more stages it has, resulting in a larger size and higher cost.
At the same time, the speed of the electric motor should not be too high, as this would complicate the transmission mechanism and make maintenance difficult.
Furthermore, at a fixed power, the engine torque is inversely proportional to the speed.
For those with low starting and braking requirements, a comprehensive comparison can be made from the perspectives of initial investment, space occupation and maintenance costs, considering several different rated speeds, and then the final rated speed can be determined.
For those who start, brake and reverse frequently, but the transition time has little impact on productivity, the speed ratio and rated speed of the electric motor are chosen mainly to minimize losses in the transition process, as well as considering the initial investment. For example, elevator motors, which require frequent reversals and have high torque, have low speed. This results in a large and expensive engine.
When the engine speed is high, the critical speed of the engine must also be considered. Each engine rotor vibrates during operation and the rotor amplitude increases with speed.
At a certain speed, the amplitude reaches its maximum (also known as resonance), and beyond this speed, the amplitude gradually decreases with increasing speed and stabilizes within a certain range. This speed, where the rotor amplitude is at its maximum, is known as the critical rotor speed.
This speed is equal to the natural frequency of the rotor. As the speed continues to increase and approaches twice the natural frequency, the amplitude will increase again. The speed equal to twice the natural frequency is called the second order critical speed. This continues with the third order, fourth order, and so on.
If the rotor operates at critical speed, severe vibrations will occur and shaft bending will increase markedly, which over time can result in severe bending deformation or even shaft breakage. The first-order critical speed of an engine is generally above 1,500 rpm, so the impact of critical speed is generally not considered for conventional low-speed engines.
On the other hand, for high-speed 2-pole motors, with a nominal speed close to 3000 rpm, the impact of this effect must be considered, and the motor must not be operated at critical speed for a prolonged period.
Generally speaking, the type of driven load, rated power, rated voltage and rated speed of the motor can approximately determine the motor.
However, if you want to optimally meet load requirements, these basic parameters are far from sufficient.
Additional parameters required include frequency, duty cycle, overload requirements, insulation class, protection class, rotational inertia, load torque curve, installation method, ambient temperature, altitude, external requirements, etc., provided accordingly with specific circumstances.
4. Engine Maintenance
In the event of engine operation or malfunction, four methods can be used to prevent and correct the problem in a timely manner, thus ensuring the safe operation of the engine.
1 . To look
Observe any abnormalities during engine operation, which are mainly indicated by the following scenarios:
1). If the stator winding is short-circuited, the engine may produce smoke.
two). 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 engine maintenance network operates normally but stops suddenly, sparks may be observed in loose parts of the wiring. This could be due to a blown fuse or a stuck component.
4). If the engine vibrates excessively, it may be due to a stuck transmission device, poor engine mounting, or a loose foot screw.
5). Discoloration, burn marks, and smoke marks on internal contacts and motor connections may indicate local overheating, poor contact in conductor connections, or burnout of windings.
two . To hear
The engine should emit a uniform, light “hum” sound during normal operation, without any additional noises or special sounds. If the noise level is very high, including electromagnetic, bearing, ventilation, mechanical friction, etc., this may indicate a possible problem or malfunction.
(1) For electromagnetic noise, if the motor produces a loud and heavy sound, the possible causes are:
- Uneven air gap between stator and rotor, resulting in high and low sounds with a consistent range. This may be caused by bearing wear, leading to non-concentricity of the stator and rotor.
- Unbalanced three-phase current, which can be caused by incorrect grounding, short circuit or poor contact of the three-phase winding. If the sound is muffled, it may indicate that the motor is significantly overloaded or out of phase.
- Loose iron core, caused by vibration loosening the iron core fixing screws, resulting in silicon steel sheet loosening noise.
(2) Bearing sound should be monitored regularly during engine operation. This can be done by pressing one end of a screwdriver against the bearing installation and holding the other end close to your ear to listen for the operating sound.
If the bearing is operating normally, it should produce a small, continuous “rustling” sound without any changes from high to low or metallic rubbing sounds.
- A “squealing” sound indicates metal friction, normally caused by a lack of oil in the bearing. The bearing must be disassembled and refilled with an appropriate amount of grease.
- A “pumping” sound is the result of the ball rotating, usually caused by dry grease or lack of oil. More grease can be added as needed.
- A “clicking” or “creaking” sound is due to uneven movement of the ball in the bearing, caused by damage to the ball or drying out of the lubricating grease after a long period of inactivity.
(3) If the transmission mechanism and the driven mechanism produce a continuous sound rather than an uncertain sound, it may be caused by the following:
- A periodic “popping” sound is caused by an uneven belt joint.
- A periodic “thump” sound is caused by play between the coupling or pulley and the shaft, or key wear or keyway.
3 . Smell
Faults in an engine can be detected and avoided using the sense of smell.
To check for faults, open the junction box and smell it for burning or unusual odors.
If there is a paint smell, it may indicate that the internal temperature of the engine is too high.
If there is a strong, pungent smell or a burning smell, this 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 housing using a megger.
If the insulation resistance is less than 0.5 trillion ohms, the motor must be dried. A zero resistance value indicates that the motor has been damaged.
4 . Touch
Measuring the temperature of various engine parts can also help diagnose faults.
For safety reasons, it is best to use the back of your hand to touch the motor housing 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, as if the fan has fallen out or the ventilation duct is blocked.
- Overheating of the stator winding due to excessive current.
- Short-circuit fault between turns of the stator winding or three-phase current imbalance.
- Frequent starting or braking.
If the temperature around the bearing is excessively high, it may be caused by bearing damage or lack of lubricating oil.
Motor bearing temperature regulations and causes of abnormalities and solutions
According to regulations, the maximum temperature of bearings should not exceed 95°C and the maximum temperature of sliding bearings should not exceed 80°C, with a temperature rise of not more than 55°C (calculated as the difference between the temperature of the bearing and the ambient temperature during the test).
Possible causes and solutions for excessive temperature rise in bearings include:
- Bent shaft or inaccurate center line – realign center.
- Loose foundation screws – tighten screws.
- Dirty lubricating oil – replace the oil.
- Old lubricating oil – clean the bearing and replace the oil.
- Damaged ball or roller in bearing – replace bearing.
For the solutions section, the following revisions must be made:
- Replace damaged components (fuse, load resistor, etc.) inside the module by opening the cover.
- Replace any damaged luminous daughter boards or protection diodes.
- Make sure the optical fiber is connected correctly as indicated and replace if damaged.
- Replace the module power board.
