A three-phase motor is an electric motor that operates using three-phase alternating current (AC) power. Designed with three sets of windings, each powered by one of the three phases of the AC supply.
Definition
Three-phase motors represent a very important power equipment in modern industries and commerce. Motors consume about 60% of global industrial electricity, and over 75% of the load is carried by three-phase motors.
The primary advantage of a three-phase motor is that the rotating magnetic field is smooth and unbroken. Consider a three-phase motor with 15,000 watts output and an energy loss rate less than 5%, while a single-phase motor could waste up to 10% energy. It is estimated that a three-phase motor can save around $1,800 dollars every year for over 10,000 hours of operation at 0.12 dollars per kWh.
In 1888, the great scientist Tesla came up with the proposal of a three-phase motor. He developed a three-phase electrical-powered induction motor. More than 3 billion three-phase motors exist around the world today, with 1.5-horsepower motors in small-scale processing factories up to 10,000-horsepower super-large ones at hydroelectric stations, all with principal dependence on that technology.
The long-term operating cost of three-phase motors is much lower. For instance, a 5-kilowatt three-phase motor usually costs about 500 USD, while a single-phase one can be as cheap as 350 USD. However, three-phase motors generally do not require major component replacements within 10 years, while the average maintenance cycle for single-phase motors is 5 years.
Equipped with three-phase motors to drive effective production, at a rate of more than 500 cars per hour, the Volkswagen factory in Germany has managed to reduce energy consumption by 15% over the last five years, saving 3 million euros annually.
The average full load efficiency for three-phase motors can be in the range of 90%-95%, while for single-phase motors, it is only in the range of 75%-80%. Assuming that if a motor runs 24 hours a day, running 300 days a year, the energy utilization of a three-phase motor will be more than 20% higher than for a single-phase motor.
The three-phase motors in the baggage conveyor systems of airports have over 100 units. Running without failure for 5,000 hours continuously, these motors operate stably in temperature environments from -20°C to 50°C. In shipyards and mining equipment, the power of three-phase motors reaches between 500 kilowatts and 5,000 kilowatts, driving several thousand tons of equipment.
Other challenges that face three-phase motors include higher initial investment costs and higher starting currents. However, these issues are being progressively solved with the increasing usage of VFD technology. A three-phase motor system equipped with a VFD can save over 30% in electricity costs within 5 years, with the equipment payback period typically under 3 years.
How It Works
Three-phase AC is the primary technological foundation of three-phase motors. Opposite to single-phase electricity, the three-phase current is formed of three sine waves 120 degrees apart. 92% is an average energetic efficiency of a three-phase motor. Most single-phase motors can’t have more than 75-85% energy efficiency.
For example, a 10-kilowatt three-phase motor has a true rated output of 9.5 kilowatts with losses of under 5%. Losses for single-phase motors under heavy loads can be as high as 10%-15%. A factory using 50 three-phase motors that operate 16 hours per day can save over 50,000 USD per year in electrical costs alone.
The three-phase power creates a rotating magnetic field inside the motor automatically; thus, the rotor will rotate directly without the need for additional mechanisms for starting. Comparatively, single-phase motors with start capacitors have a failure rate of 12% after they run for 5,000 hours, while three-phase motors have only a 2% failure rate.
In the case of a three-phase motor, its rotor is propelled by a rotating magnetic field with the induction principle. In this induction, Faraday’s Law of Electromagnetic Induction applies, given as E = -dΦ/dt, with Φ being the rate of change of magnetic flux. A three-phase power supply operating at 50Hz will therefore have a synchronous speed of 3,000 revolutions per minute.
For example, in the low-speed state, the fluctuation rate of three-phase motor torque does not exceed 3%, while that of single-phase motors can be over 15%. In automobile manufacturing, three-phase motors play a very important role, as the robots in body spray painting rely on constant torque to ensure that the painting error does not exceed 0.1 millimeters.
Generally speaking, the power factor of three-phase motors falls within a range of 0.85 to 0.95 and for single-phase motors it falls within the range of 0.7 to 0.8. A steel factory upgraded its motor system, improved its power factor from 0.8 to 0.92 and was able to save 200,000 USD a year in grid penalty fees.
The 50-kilowatt crane motor of Boeing aircraft production develops the rated speed in 5 seconds while lifting a 10-ton wing with a stable speed of 1,000 revolutions per minute.
The global market size of the three-phase motor reached USD 26 billion in 2022 and will increase to USD 40 billion by 2030. There will be one three-phase motor per wind turbine for every 20 years of useful life, plus only 2% of all investments will relate to maintenance, contributing a lot toward lowering the long-run operation costs in renewable projects.
Thermal relays, circuit breakers, and overload protection devices that automatically cut off the power supply in case of overheating of the motor, overload, or short circuit are also installed for three-phase motors. Due to an overload failure, a mining company lost 1 million USD, then after the company reduced its downtime by 30% after deploying smart protection devices.
Types
Three-phase motors are classified into Induction Motors, Synchronous Motors, and Brushless DC Motors. They account for over 80% of the global motor market share. The global usage of induction motors has exceeded 500 million units, with 70% being three-phase motors.
They hold about 90% of the global industrial application market. Induction motors generally lie in the range of 200-5,000 USD. A 7.5-kilowatt three-phase induction motor costs around 800 USD, while for synchronous motors, one may have to pay 1,000 USD or even more. Induction motors also incur lower maintenance costs since their only routine maintenance is generally carried out once a year, which amounts to around 150 USD.
About 120,000 synchronous motors are applied annually in high-load industries such as wind power, shipbuilding, and cement plants. They have a higher initial investment cost, with a 20-kilowatt synchronous motor priced at around 5,000 USD, but their average lifespan can reach 25 years, far exceeding the 15-year lifespan of induction motors.
The BLDC has an efficiency of more than 95%, whereas for induction motors, it’s only approximately 85% on average. The power of each Tesla motor system is about 250 kW, and energy utilization in high-speed driving modes exceeds 92%; in conventional internal combustion engine vehicles, thermal efficiency amounts to a mere 35%-40%.
There are squirrel cage rotors and wound rotors. Squirrel cage motors possess simple structure, low price, and easy maintenance, accounting for more than 60% in the industrial motor market. A squirrel cage rotor motor usually costs about 1,500 USD, within the range of a 15-kilowatt power source, while wound rotor motors will probably cost 20%-30% more than the former one. Up to now, over 70% of the wound rotor motors are used in mining industries, whose starting torque could achieve 150%-200% of rated torque.
Every year, over 200,000 explosion-proof three-phase motors are used for industries like petroleum, chemicals, and coal mining. They can bear the temperature higher than 50°C and guarantee stability in explosive gas. Explosion-proof motors are 1.5 times more expensive than the normal ones. However, explosion-proof motors are below 3% failure rate, while normal motors can fail up to 8%-10%.
The permanent magnet synchronous motors of industrial robot joint drive systems possess high power density and rapid responses. They are capable of making precise positioning within less than 10 ms. The induction motor used is squirrel cage type; therefore, these induction motors used at large water pump stations or wind farms can keep long hours of operation in continuous operation without deterioration. A squirrel cage rotor motor with 500 kW has an average annual maintenance cost around 8 000 USD whereas its annual downtime does not usually exceed 10 hours.
Key Components
The core components of three-phase motors can be divided into the Stator, Rotor, Bearings, Windings, and Cooling System. About 60% of the motor manufacturing cost comes from the materials and processing of these key components.
A high-quality stator costs about 4.5 USD per kilogram, and the weight of a stator typically ranges from 20 kilograms to 500 kilograms. A 10-kilowatt three-phase motor’s stator weighs about 35 kilograms, costing around 160 USD. A 100-kilowatt industrial motor’s stator weighs over 300 kilograms. Using high-grade silicon steel sheets for the stator can increase the motor’s energy efficiency by 3%-5%.
The rotor is divided into squirrel cage rotors and wound rotors. Over 85% of induction motors use squirrel cage rotors, which have a simple structure, low manufacturing cost, and high reliability. A 15-kilowatt motor’s squirrel cage rotor costs about 200 USD, while the cost of a wound rotor is 25%-30% higher. Squirrel cage rotors are made of aluminum or copper bars, with copper rotors being 8%-12% more efficient than aluminum rotors, but their cost is higher. Copper rotor motors can extend the motor’s lifespan by 3-5 years, which is particularly important for industrial equipment that needs to run continuously for 24 hours.
Windings are typically made from copper wire, but in cost-sensitive applications, aluminum wire windings are also used. Copper wire windings typically cost about 8-10 USD per kilogram, while aluminum wire costs about 2.5 USD per kilogram. However, copper wire has 60% higher conductivity than aluminum wire. The failure rate of copper wire winding motors is about once every 5,000 hours, while the failure rate of aluminum wire winding motors is once every 3,000 hours.
The average lifespan of bearings is 20,000 hours, but in high-load or high-temperature environments, the bearing lifespan can be reduced by 30%-50%. Regular bearings cost about 20-50 USD, while high-performance bearings can exceed 200 USD. In high-end applications like aerospace and wind power generation, ceramic bearings are commonly used, as they can withstand higher temperatures and pressures with a lower friction coefficient. Motors using ceramic bearings typically have a lifespan extended by 40% or more, with a 35% reduction in failure rates.
Cooling methods include natural cooling, air cooling, and water cooling. For small motors below 5 kilowatts, natural cooling is typically used, while industrial motors above 10 kilowatts mostly adopt air cooling or water cooling designs. The cost of an air cooling system typically ranges from 50-200 USD, while water cooling systems can cost over 1,000 USD. Lowering the motor’s operating temperature by 20-30°C can extend the motor’s lifespan and maintenance cycle.
Siemens designed a three-phase motor for a large hydroelectric power station that uses water cooling and copper rotor designs. This made the motor’s energy efficiency reach 97%, saving 500,000 USD annually. Additionally, the motor’s maintenance cycle was extended from every 2 years to every 5 years.
Efficiency and Losses
More than 60% of the global industrial power consumption comes from motor drive systems, with over 70% of these motors being three-phase motors.
The average efficiency of three-phase motors typically ranges from 85% to 96%. A standard 10-kilowatt induction motor operates at an efficiency of about 92% under full load, consuming 10.87 kWh of electricity per hour, with 0.87 kWh converted to heat loss. If this motor runs for 16 hours per day for 300 days per year, its total energy loss will amount to 4,176 kWh, resulting in an annual loss of 501.12 USD in electricity costs. Using a higher-efficiency IE3 or IE4 motor could reduce losses by 20% to 30%.
Motor losses include copper losses, iron losses, mechanical losses, and stray losses. Copper losses account for 40% to 60% of total losses. For a 50-kilowatt motor under full load, the copper loss is around 2 kW, which means that 4% of the energy consumed is wasted as heat in the windings. Increasing the diameter of copper wire by 10% reduces resistance by about 19%, lowering copper losses by 5%-8%, though the cost of copper wire would rise by 15%-20%.
Iron losses account for 20% to 30% of total losses. To reduce iron losses, motor manufacturers typically use silicon steel sheets for the core. High-grade silicon steel costs about 2,500 USD per ton, more than 30% more expensive than regular steel, but can reduce iron losses by 15%-20%. Siemens’ high-efficiency motors use non-oriented silicon steel, reducing iron losses by 17%, and achieving 96.5% efficiency in their IE4 motors.
Mechanical losses primarily come from bearing friction and fan cooling. Ordinary bearings have friction losses that account for 5%-10% of total losses, while high-performance ceramic bearings can reduce friction losses by 30%-40%. A motor equipped with high-performance bearings running at 8,000 rpm has a bearing loss of only 0.3 kW, while ordinary bearings can have losses of 0.5 kW. In air cooling systems, using high-efficiency cooling fans can reduce cooling losses by 15% to 25%, and optimizing the cooling system can improve motor efficiency by 2%-3%.
For a large manufacturing enterprise with a total equipment power of 1,000 kW running 20 hours a day and 7,000 hours per year, increasing the average motor efficiency from 90% to 94% can save nearly 300,000 kWh annually, equivalent to a savings of 36,000 USD. The initial investment cost for replacing motors with high-efficiency models is about 150,000 USD, which can be recouped in 4 years, while the motor’s average lifespan is 15 years, meaning the company can continue to benefit from energy savings for the next 11 years.
Wind turbines use permanent magnet synchronous motors as their core drive equipment, which typically operate with 97% efficiency. A 1.5-megawatt wind turbine at a wind speed of 12 m/s can generate 1,500 kWh per hour. If the motor efficiency drops by 1%, it will lose about 131,400 kWh of energy per year, equivalent to a loss of 13,140 USD at the current rate of 0.1 USD/kWh.
Over 50% of industrial motors operate under low-load conditions, which results in a significant decrease in efficiency. For instance, a 30-kilowatt motor running at 50% load may see its efficiency drop from 93% to 87%, resulting in an additional loss of 1.8 kWh per hour. Using a motor system with a variable frequency drive (VFD) can save 20% to 50% in energy consumption, with a typical payback period of 1.5 to 2 years.
Purchase Cost
The price of three-phase motors ranges from 200 USD to 50,000 USD. A 7.5-kilowatt standard industrial motor costs about 800 USD, while a 500-kilowatt high-efficiency motor may cost over 30,000 USD.
The copper windings and silicon steel sheets of the motor account for 50%-60% of production costs. The price of copper fluctuates between 8,500 USD and 10,000 USD per ton, while silicon steel costs about 2,500 USD per ton. A 10-kilowatt motor requires about 15 kilograms of copper wire, costing around 130 USD in materials. If companies opt for aluminum wire windings instead of copper, the cost can be reduced by 30%-40%, but motor efficiency will drop by about 10%.
Motors from well-known brands typically cost 20%-30% more than those from local or non-branded manufacturers. A 15-kilowatt ABB motor costs about 1,200 USD, while a local brand motor might cost between 800 USD and 900 USD. Branded motors typically have an average failure rate of once every 10,000 hours, while non-branded motors can fail as frequently as once every 5,000 hours.
Variable frequency drive (VFD) motors are typically 30%-50% more expensive than traditional motors but can achieve 20%-40% energy savings by adjusting motor speed. A 30-kilowatt VFD motor costs between 2,500 USD and 3,000 USD, while a standard motor costs about 1,800 USD. Motors with VFD control can recover their initial investment costs within 2-3 years through electricity savings. A food processing plant saved 45,000 kWh of electricity annually by replacing traditional motors with VFD motors, equivalent to a savings of 5,400 USD in electricity costs.
IE4 motors are 15%-20% more efficient than IE1 motors, but their price is typically 25%-40% higher. A 75-kilowatt IE1 motor costs around 3,500 USD, while the same IE4 motor may cost between 4,800 USD and 5,200 USD. Under 8,000 hours of continuous operation, an IE4 motor can save about 3,000 USD in electricity costs, which means the extra purchase cost is recovered in 2 years.
In North America and Europe, the average price of a 10-kilowatt three-phase motor is between 1,000 USD and 1,200 USD, while in Asia, the price typically ranges from 600 USD to 800 USD. The manufacturing cost of motors in China is typically 20%-30% lower than in Europe.
A 100-kilowatt industrial motor typically weighs between 500 kilograms and 1,000 kilograms, and the transportation cost can range from 100 USD to 1,000 USD. The installation cost for high-power motors may account for 10%-15% of the total purchase cost. A mining company that purchased 5 motors of 500 kW spent 150,000 USD on equipment but had an installation cost of 25,000 USD.
Bulk purchasing typically provides a 10%-15% discount, and long-term contracts with suppliers may offer discounts of up to 20%. An automobile manufacturer purchasing over 1,000 motors annually negotiated a 3-year long-term contract, lowering the cost of each motor by 18%, saving over 200,000 USD in procurement costs.
Common Applications
More than 70% of industrial equipment relies on three-phase motors in driving core machinery.
A large automobile manufacturing plant, which produces about 500,000 automobiles every year, has each robot and every conveyor belt driven by three-phase motors along the assembly line. The ratings of these motors range from 2 kW to 100 kW. They deploy more than 5,000 three-phase motors and can save more than 2 million USD every year in energy consumption. Generally, three-phase motors service life is over 10 years, and the maintenance cost of the company is very low, usually occupying 3%-5% of the total investment in equipment.
In the electrical industry, notably in wind and hydropower stations, three-phase motors are the central parts in power generation units. A 1.5 MW wind turbine typically uses a high-efficiency synchronous three-phase motor with an efficiency of up to 98%. Up to 2024, more than 700,000 wind turbines were installed worldwide, generating more than 1,500 terawatt-hours of electricity each year. For a wind turbine, the mean time between failures is more than 20,000 hours on average.
The main three-phase motors used in the transportation sector are subways, light rail, electric buses, and electric vehicles. For example, electric vehicle core drive system from Tesla uses a permanent magnet synchronous three-phase motor, while the Model 3 motor showed an efficiency of 97% in city drive and maintained 93% or higher on highways. For the subway system, a typical six-car train has 12 three-phase induction motors rated at 100 kW to 300 kW each, which moves more than 100,000 passengers during rush hours every day.
Agriculture and food processing industries are other major users of three-phase motors for driving irrigation pumps, crushers, and food processing equipment. In large farms, a 15-kilowatt irrigation pump pumps more than 500 cubic meters of water per hour. For this kind of operation, it requires a very efficient and reliable three-phase motor that would run for long, continuous hours. The irrigation systems using three-phase motors save 15%-20% in energy consumption compared to the traditional single-phase motors, saving thousands of dollars in annual electricity costs. In food processing, a typical 30-kilowatt mixer at 2,000 RPM is usually driven by three-phase motors.
In those industries, huge cranes, mixers and elevators were driven by the three-phase motors. For example, when constructing the Shanghai Tower, 632 meters tall, more than 200 three-phase motors were used for driving the elevator system. The elevators have a maximum load capacity of 2,000 kg and a speed of 10 meters per second. The motors are usually equipped with VFD systems that automatically adjust the motor speed and save up to 30% in energy costs.
The application areas of three-phase motors include main drives for drilling rigs, hoists, and compressors in the mining and petrochemical industries. A three-phase motor for a drilling platform, for instance, has ratings that range from 500 kW to 2,000 kW and maintains output steadily for operation over 24 hours continuously. Over 30% of oil and gas extraction equipment around the world relies on three-phase motors, and such systems normally have a maintenance cycle every 5 years.
For systems where heating, ventilation, and air conditioning are needed, large numbers of three-phase motors are used in HVAC systems in large commercial buildings and industrial plants. In general, such systems come equipped with fans, compressors, and pumps in sizes between 5 kW and 500 kW. The average annual consumption of a medium-sized shopping mall using such an HVAC system is around 1 million kWh, while savings of more than 15% in energy consumption are achieved for the same system using high-efficiency three-phase motors. An optimized HVAC system can save more than 100,000 USD of annual expenditure on electricity.
