Three-phase AC flows into the stator of the induction motor, forming a rotating magnetic field. This then interacts with the rotor conductor to induce current and create torque. Speed is measured in terms of the number of pole pairs. Working under a 50 Hz power supply condition, for example, the synchronous speed for a two-pole motor would be 3000 rpm. Operating under such conditions, slip of a rotor is generally in the area of 2 to 5%, when the efficiency of the motor can be over 90%.
Generating a Rotating Magnetic Field
The principle behind generating a rotating magnetic field is that three-phase alternating current generates a dynamic magnetic field within the stator. In three-phase alternating current, there is a phase difference of 120 degrees, and the peak of every phase current is staggered in time. This precise time interval can be achieved through current cycle distribution of 20 milliseconds.
When a two-pole induction motor operates under 50 Hz power supply conditions, it runs at 3000 rpm. Increasing the number of pole pairs to 4 will drop it to synchronous 1500 rpm. This flexibility has so far driven the penetration rate of induction motors to more than 85% globally, making them mainstream in the industrial motor market.
The general efficiency of induction motors can be between 85% to 97%. A 100-kilowatt-rated power motor consumes 800 kilowatt-hours of electricity every day for 8 hours of operation. Raising the efficiency from 85% to 95% could save 29,200 kilowatt-hours of electricity every year. Calculated at an average electricity price of 0.7 yuan per kilowatt-hour, this could save more than 20,000 yuan for the company.
Inside the induction motor, the stator material generally adopts high magnetic conductivity silicon steel sheets, whose magnetic permeability reaches between 4000 and 6000, while the ordinary steel only has about 1000. The resistance of high-quality copper wire is very small, with a low of 0.017 ohms per square millimeter, thereby reducing heat generated during long time running and increasing the service life by 20% to 30%.
For the aspect of maintenance alone, an appropriately operational induction motor necessitates nearly 2% to 3% per year towards its total equipment cost while, in the case of a DC motor, it demands over more than double due to having a commutator and carbon brushes.
A drop in voltage by 10% would likely lead to a drop in output power of the induction motor from 15% to 20%, with further slipping. Presently, an induction motor is normally designed to include automatic voltage regulators and intelligent control systems. It is supposed to cut the loss in power that may arise because of voltage fluctuation below 5%.
In 1892, Nikola Tesla, the inventor of the induction motor, experimentally demonstrated the advantages of three-phase current in generating dynamic magnetic fields and designed the first practical induction motor based on this. By the early 20th century, more than 30% of factories in the United States had replaced traditional steam power with induction motors.
It involves changes in the frequency of the input power supply using inverters. The inverted technology has been employed in a Japanese factory, transforming the induction motors from a fixed speed to variable speed. As a result, the company has reduced electricity consumption by 15%, about 500,000 yen in electricity bills per year after transformation.
This would raise the output power of a wind turbine by 3% to 5%. Assuming that one wind turbine produces 10 million kWh of electricity every year, this small increment could supply 300,000 to 500,000 kWh to the grid, enough to meet the annual needs of about 300 households.
Cutting Rotor Conductors
The idea behind cutting rotor conductors is the principle of electromagnetic induction. The rotor conductor is usually a cage-like structure in the form of aluminum or copper. In an induction motor rated at an average power of 50 kW, the strength of the induced current on the rotor conductor is about 100 amperes. This huge current provides a strong magnetic force on the surface of the rotor, which acts together with the direction of the rotating magnetic field and eventually drives the rotor to rotate.
The magnitude of the induced current produced by cutting a conductor depends upon the percentage slip. The slip is the difference between the rotor speed and the synchronous speed of the rotating magnetic field. It is usually expressed as a percentage. If the synchronous speed is 1500 rpm and the actual rotor speed is 1450 rpm, the slip is 3.3%. The greater the slip, the greater the induced current. Engineering design keeps the slip below 5%.
The resistivity of copper is as low as 0.017 micro-ohm meters, while that of aluminum is 0.028 micro-ohm meters. For conductors of the same size, copper generates more than 30% less heat than aluminum. The market price of copper per kilogram is about three times that of aluminum. In the case of lower energy consumption and longer service life, copper could significantly offset operating costs in the entire life cycle.
These losses can be decreased by 15% to 20% for which the eddy current and hysteresis losses optimization of form and size of the conductor and silicon steel sheets of 0.5 mm thickness only are used. Improvement in this increases the overall efficiency of the motor and makes it above 95% of energy conversion at full load.
Mechanical Vibrations and Noise Generation: Mechanical vibrations and noise generation take place while cutting the conductor of the rotor. These vibrations are primarily excited by the air gap anisotropy between the rotor and the stator. The air gap design size normally lies between 0.5 and 2 mm. The greater the size of the air gap, the lesser the torque; the smaller the size, the greater the mechanical friction and difficulty in manufacturing. The error in air gap can be controlled within 0.1 millimeters with some high-quality induction motors that have CNC machining and precision detection technologies; its noise level usually does not exceed 70 decibels.
A 100,000 watt induction motor produces a maximum heat of 10,000 kcal per hour with high load conditions. The forced air cooling and water cooling are usually adopted for modern induction motors in order to ensure the temperature increase lower than 80 ℃.
The operating speed of a wind turbine rotor is generally in the range of 1000~3000 rpm, and centrifugal force can cause over 10 MPa pressure on the conductor. If the tensile strength of the conductor material is not good enough, it will be easily damaged mechanically. The tensile strengths of aluminum and copper are 90 MPa and 220 MPa, respectively; thus copper rotors have been widely used in high-speed motors.
The cutting of the rotor conductor is not just a question of the depth of cut but also that of magnetic field and current direction. By adjusting the shape and position of the rotor slots, engineers are able to get an optimal distribution of induced current. Application of trapezoidal or wavy slot design increases starting torque for certain high-performance industrial motors by over 10%.
Induction Current Generation
The essence of induced current is to transfer the energy of the rotating magnetic field to the rotor through the principle of electromagnetic induction. In the case of a 50-kilowatt-rated induction motor, for example, the magnitude of the electromotive force that is induced in the rotor will fall within the range of tens of volts to hundreds of volts, depending on the value of slip and resistivity of the rotor material.
When the slip is zero, the rotor rotates synchronously with the revolving magnetic field, there would be no electromotive force generated hence, no current; therefore there is no output torque produced. With 5% slip, the generated current reaches its rated value. Hence, motor efficiency and power output reach their optimal values. In general, the rotor surface current density in induction motors with a rated slip of 3% supplied from a 50 Hz supply can be as large as 10 amperes per square millimeter.
According to Lenz’s rule, the direction of the induced current is chosen in such a way as to always oppose the direction of change in the external magnetic field producing it. Typical torque output of an industrial four-pole motor has a range from 100 up to 500 Nm. This work optimizes induction current with increased starting torque up to more than 20% at motor performance.
In the case of a 75 kW induction motor, eddy current loss and hysteresis loss take around 30% to 40% of total energy loss. To reduce this type of loss, low resistivity copper is used by engineers as a rotor conductor and fill up the rotor slots by high permeability silicon steel material; hence, 10% to 15% energy losses can be reduced.
At 50 and 60 Hz power supplies, respectively, the amplitude difference of the rotor induced current for induction motors with the same power is more than 10%. The power supply frequency can be as high as 400 Hz in the traction motor of a high-speed train. This high-frequency operation mode will greatly increase the amplitude of the induced current and further put higher demands on both the material and heat dissipation capacity of the conductor.
In operation, the temperature rise of the rotor surface is generally no more than 80 degrees Celsius when the induction motor of a standard medium size is working under full load. If the distribution of induced current is bad, the temperature of a local hot point may be higher than 100 degrees Celsius and thus accelerate material aging or even lead to equipment failure. Using optimized slot design, the engineering design adopts a manufacturing process with high precision to maintain fluctuation within a 10-degree Celsius rise in temperature.
For example, if the low-frequency motor, the frequency of its induced current can be only several hertz, which will make a large fluctuation in the rotor flux density and an unstable output torque. Variable frequency speed regulation system can dynamically adjust the power supply frequency and increase the operating efficiency by 5% to 8%.
The electromagnetic noise generated by an induced current in large industrial induction motors can rise to 70 decibels and affects the normal operation of equipment around it. In general, a shielding layer or special coatings are added by motor manufacturers between the stator and the rotor. This can reduce electromagnetic noise by more than 15%, thus improving it.
The rotor is stationary when the induction motor of traditional structure starts, and the slip is 100%. Therefore, the maximum value of the induced current is reached. It causes the starting current to be 6-8 times the rated current. In modern induction motors, soft-starter or inverter technologies are used for limiting the starting current within the rated current of 2 times so as to reduce the influence on starting equipment and the power grid.
Generating Rotational Force
Rotational force generation is based on the interaction between the rotor and stator magnetic fields. When induced current is generated in the rotor conductor, a magnetic field is formed which interacts with the rotating magnetic field generated by the stator to generate torque. A 50 kW-rated induction motor has a rated output torque of about 318 Nm.
The magnitude of rotational force is thus proportional to slip. Rotor speed at a synchronous speed of 1500 rpm is 1450 rpm and therefore the slip is 3.3% whereas torque and efficiency are maximum under the slip conditions less than 5% and, consequently, more than 90% efficiency of the motor at full load is achievable.
Standard industrial induction motors use an air gap flux density of between 1.2 and 1.8 Tesla. The higher the flux density, the better the torque output. The stator and rotor in induction motors normally use high magnetic permeability silicon steel sheet. Magnetic permeability for a silicon steel sheet is three to five times higher compared to ordinary steel, thereby allowing a torque increase of 15% to 20%.
An induction motor designed properly can have fluctuations of no more than ±2%. Therefore, the induction motors find very special application to high-speed drives with high precision and stability in operation, like textile machinery or machine tools.
Usually, a four-pole motor contains 24 to 48 rotor slots. A different slot design largely affects torque output and running smoothness. The stepped or wavy slot design on rotors could more than 10% improve torque output with reduced electromagnetic noise.
It normally lies in the range from 0.5 to 2 mm. If the air gap is too big, the magnetic field strength will be weakened and the torque will decrease; if the air gap is too small, there may be an increase in mechanical losses and heating problems. For modern induction motors, the error allowed in the air gap can be controlled within 0.1 mm, and such precision in manufacturing greatly enhances the performance and reliability of the motor.
At full load, the induction motors develop the maximum torque output efficiency, which can be as high as more than 95%, while at light loads the efficiency falls to 60% to 70%. A 100 kW rated motor has an efficiency of about 75% at 50% load.
At 75 kW induction motor, the copper and iron losses are each about 40% of the total losses. Losses can be more than 15% lower with reduced resistivity conductors like copper and high permeability stator cores.
As, for example, for every 100 kW output of the induction motor develops maximum of 10,000 kcal/hr. of heat and according to latest methods of cooling, rise in temperature may be restricted below 80° C which adds over 20% to a motor’s life time.
Rotor Rotation
The principle of rotor rotation is based on the interaction between a rotating magnetic field created by the stator and the current induced in the rotor. The average speed of the rotor for induction motors of 100 kW rating is nearly equal to the synchronous speed, such as 1500 rpm, but for real applications this is usually between 1440 to 1490 rpm.
The slip is normally small at light loads, less than 2% and increases to 3% to 5% at full load. When the power frequency is 50 Hz, the synchronous speed of a four-pole induction motor is 1500 rpm and the actual rotor speed at full load slip is 1440 rpm for 4%.
Most induction motors use the cage rotor structure. The conductors of the standard rotor are mostly made of copper or aluminum. The diameter of each conductor is usually between 8 and 15 mm. Two end rings connect the conductors to form a closed loop. The damage rate of copper rotors in high-speed operation is more than 20% lower than that of aluminum rotors, but the cost is correspondingly higher.
The vibration frequency of industrial induction motors is usually concentrated in the range of 25 to 100 Hz, and the amplitude can reach 0.1 mm to 0.5 mm. Some high-end motors have greatly improved operating stability by using more precise dynamic balancing technology and high-strength bearings to reduce the amplitude to less than 0.05 mm.
The air gap is usually between 0.5 and 2 mm, and even an error of 0.1 mm can give unstable rotation or reduced efficiency. For an induction motor rated at 200 kW, increasing the air gap by 0.2 mm could drop the torque output by more than 10% while increasing the current fluctuations and heat losses.
At full load, the rotor current and magnetic field strength will increase significantly. For example, a 50 kW induction motor has an output torque of 318 Nm at full load but only about 150 Nm at light load.
In the induction motor running at 100 kW, its rotor can generate 8000 to 10,000 kcal of heat per hour. Most of the modern induction motors adopt a forced air cooling or water cooling system that controls the temperature rise of the rotor surface to less than 80 degrees Celsius.
The rotor acceleration time of a 75 kW induction motor is usually between 2 and 4 seconds according to the load inertia and the design of the motor. Later, engineers developed vector control technology that can shorten the time to 1.5 seconds with improved starting torque and control accuracy.
As the power frequency is reduced, the synchronous speed of the rotating magnetic field is lowered, and the rotor speed follows the reduction. A two-pole induction motor has a synchronous speed of 3600 rpm at 60 Hz and 3000 rpm at 50 Hz.
Speed Adjustment
It normally occurs through the variation of power frequency, slip, and the number of motor pole pairs. A 50 kW induction motor operating at a power frequency of 50 Hz has a synchronous speed of 1500 rpm. Increasing the frequency to 60 Hz increases the synchronous speed to 1800 rpm.
The essence of variable frequency speed regulation technology is to change the input frequency of the motor by using a frequency converter while keeping the ratio of voltage to frequency constant. One induction motor for industrial use has an input voltage of 400 volts and a frequency of 50 Hz. When the frequency is lowered to 25 Hz, the voltage should be reduced to 200 volts accordingly. This control method can extend the range of speed adjustment to 10% – 100% of the rated speed.
Under light load conditions, the slip of the induction motor is usually less than 2%, and the speed is close to the synchronous speed; at full load, the slip may increase to about 5%, thus reducing the speed. An induction motor with a synchronous speed of 1500 rpm has an actual speed of 1425 rpm when the slip is 5%.
Synchronous speed is inversely proportional to the number of pole pairs. Increasing the number of pole pairs reduces the synchronous speed. Thus, for a given power supply frequency of 50 Hz, a two-pole motor has a synchronous speed of 3000 rpm, a four-pole motor has 1500 rpm, and a six-pole motor has it down to 1000 rpm.
In variable frequency speed regulation, the efficiency is usually maintained above 90% within the rated speed range, but when the speed is reduced to less than 50% of the rated value, the efficiency may drop to 70% to 80%. A 100-kilowatt induction motor may increase its energy consumption by 15% to 20% when the speed is reduced to half of the rated value.
The temperature rise in the stator of a 75-kW induction motor can be in excess of 100°C at low speeds. Most modern motors, however, are provided with separate cooling systems, such as forced ventilation or even liquid cooling, that limit the temperature rise to less than 80°C.
Engineers introduced vector control technology. Compared to the traditional scalar control, vector control can dynamically adjust the input current and voltage by real-time monitoring of the flux and torque changes of the motor rotor to control the speed fluctuation within ±1%.
The price of the inverter usually accounts for 30% to 50% of the total cost of the induction motor. The inverter cost of a 100-kW motor is about RMB 20,000 to 50,000. The payback period of the variable frequency speed regulation system is usually within 2 to 3 years.
Maintaining Stable Operation
In it, stable operation is ensured through optimization of electromagnetic performance, precision in mechanical design, and proper management of the heat dissipation system. For industrial-grade induction motors, their normal operating life usually goes up to 15 to 20 years, while less than 1% of induction motors fail every year in good conditions.
The typical air gap for a normal industrial motor ranges from 0.5 to 2 mm. If the error in the air gap is more than 0.1 mm, it will create an uneven magnetic field that will consequently provide torque fluctuation with reduced efficiency. Keeping the air gap error within 0.05 mm improves efficiency by 3% to 5%.
A 75 kW induction motor may lose from 15% to 20% output power when the voltage drops by 10%. The modern induction motor is normally fitted with voltage regulators and a frequency control system that can reduce the impact of voltage fluctuations to less than 5%.
The vibration frequency of industrial motors ranges from 25 to 100 Hz, and failure to control the same in time might cause bearing wear, rotor imbalance, and other troubles. By use of high precision bearings and dynamic balancing technology, the amplitude of induction motors could be reduced less than 0.02 mm.
While a 100 kW induction motor operates, it produces 10,000 kcal/h of heat. Today, the motors are usually cooled by forced air or water and their temperature rise is possible to be controlled within 80 degrees Celsius.
The most efficient and stable torque output is usually in the range of 90%-95% under full load; for the light load operation, the slip is small, hence efficiency can be as low as 70%-80%. By properly optimizing the design of the motor and load matching, more than 10% improvement in operating efficiency can be expected by the engineers.
It causes electromagnetic interference with induction motors, particularly under high frequency conditions, to the control systems and monitoring equipment. There are induction motors that are high-end with a low-noise cable design and shielding technology, which have been introduced to reduce electromagnetic interference by over 15%.
Motor bearings are very crucial during operation, and their friction and wear should be reduced by lubrication frequently. In this regard, properly lubricated induction motors can increase the life of the bearing by more than 30%, whereas defects in lubrication can increase the temperature of the bearing up to 120 degrees Celsius, which enhances the possibility of failure considerably.
