Synchronous motors are motors where the rotors are synchronized with the stator magnetic field and are usually used in applications involving high powers. Their rated speed depends on the power supply frequency. At 50 Hz, the speed of a four-pole motor is 1500 rpm.
Definition
In a 50 Hz AC system, the rated speed for the four-pole synchronous motor is 1500 revolutions per minute, whereas the six-pole rated speed is 1000 revolutions per minute.
Such synchronous motors have an efficiency above 90%, whereas high-performance models can reach 98%. A synchronous motor of 500 kW is able to consume as little as 510 kWh, which would allow it to last under full load for about one hour, compared to the average efficiency provided by an asynchronous motor, amounting to just 85%.
Some very large motors can achieve an ideal power factor of up to 1.0, which improves the power utilization efficiency of the grid. A large factory using synchronous motors may reduce about 100,000 RMB per year in annual maintenance costs for reactive power compensation equipment.
Synchronous motors for cement production normally fall within the 1000 to 6000 kW power range. In the Three Gorges Project, one unit can reach up to 700 MW capacity.
For a 300 kW synchronous motor, the saving is about 400,000 RMB procurable cost but saves around 60,000 RMB through electricity cost; payback 5 to 7 years and a life-span of 20 to 30 years. Actually, some last more than 50 years, such as equipment with long lifespan running.
The cost of the starter system for a 2000 kW rated power synchronous motor may expend as much as 10% to 15% of the overall price of the equipment.
The speed deviation of synchronous motors applied in high-end equipment of some types is controlled within 0.01%.
A 2 MW rated wind turbine can produce more than 5 million kWh of clean energy per year by dispelling around 4000 tons of CO2 emission.
Construction
The diameter of the stator of a synchronous motor rated at 1000 kW is generally between 1.5 to 2 meters, and the overall weight of the windings exceeds 500 kg.
Hydropower station synchronous generators operate between 100 and 500 revolutions per minute. High-speed motors may reach as high as 3000 revolutions per minute. The diameter of the rotor in a salient pole synchronous generator can be up to 8 meters, and its total weight is over 200 tons.
For an industrial synchronous motor rated at 500 kW, the excitation current is between 40 and 60 amperes, while the output voltage ranges between 200 and 400 volts. The bearing rated load capacity of the industrial synchronous motor is 25 tons with more than 100,000 hours lifespan.
A synchronous motor with a rated power of 2000 kW requires about 200 liters of cooling water per hour to keep the operating temperature below 75°C.
The conductivity of synchronous motors can reach over 99%. The total length of copper wire in the stator windings of a 1500 kW synchronous motor exceeds 2000 meters, with a total weight of about 300 kg.
The shell thickness of the large synchronous motor can be as high as 30 mm and can stand an outer pressure up to 10 MPa. Its protection level can even be up to IP55.
At work, such parameters like voltage, temperature, vibration, and speed can be monitored in a 500 kW synchronous motor control system with a data sampling frequency as high as 100 times per second.
Neodymium-iron-boron magnets possess a magnetic energy product that exceeds 400 kJ/m³. Permanent magnetic materials applied to the rotor of wind power synchronous motors possess a price exceeding 300,000 RMB per ton.
Working Principle
If the rotor of a synchronous motor can run in synchronism with the above rotating magnetic field generated by the 50 Hz system, then that synchronous motor, too, shall run at its speed of 3000 revolutions per minute.
Typical excitation current for 500 kW motor is around 50 A.
For example, a large 2000 kW synchronous motor would typically employ the starting motor that accounts for about 5% to 10% of the power in the motor; therefore between 100 kW and 200 kW.
For overload applications, synchronous motors tend to vary by less than 0.01%. Synchronous motor scanners are used in MRI machines; this typically introduces an error less than 0.1 mm. In synchronous motor use in one plant, the requirement for reactive power is reduced to 30%.
For a synchronous motor, rated torque is 1000 Nm and stator magnetic field strength is 1.5 Tesla, the magnetic flux of the rotor should be maintained around 0.8 Weber.
The amplitude of mechanical vibration of a synchronous motor is usually less than 1 mm in operation. In aerospace applications, the operating noise of synchronous motors is less than 60 dB.
Efficiency may be improved by 2%-3% by using low-loss silicon steel for the stator. High-end synchronous motors have operating efficiencies of up to 99%.
For a 100 kW synchronous motor, the rotor machining accuracy has to be up to the micron level.
Advantages
Efficiency is normally higher than 90% under full load, and for high-power models it can be as high as 98%. This, for example, means that a 500 kW synchronous motor will need 510 kWh of energy per hour. Its asynchronous counterpart with an efficiency of 85% will require 588 kWh. If such an asynchronous motor operated for 8000 hours per year, the use of a synchronous motor would save more than 620,000 kWh of electricity, or even nearly 500 tons of CO2.
The speed variation of devices that use synchronous motors is generally less than 0.01%. A weaving machine with a synchronous motor reduces the defect rate of the fabric to less than 1%, whereas the device using a regular motor can have up to 5% defect rate.
The power factor has increased by about 2 million kWh of reactive power loss per annum, from 0.85 up to 0.98 in the whole plant.
The asynchronous motor systems consume around 10 million kWh a year. Once the system is replaced with synchronous motors, the annual consumption would be decreased to 8.5 million kWh, saving direct electricity costs by 1.5 million RMB.
Synchronous motors have standard lifespans of 20 to 30 years, however, high-grade equipment can stay for more than 50 years if well serviced. A typical synchronous motor on a steel manufacturing site has survived for over 35 years while operating for as long as 300,000 hours.
Gadgets served with synchronous motors can have noise ratings of less than 60 dB, which means they are generally quieter than any similar asynchronous engine with a value of 75 dB.
A high-voltage synchronous motor with a rated power of 5000 kW can be operated continuously for 2 hours at loads up to 110%.
A synchronous generator with a rated power of 2 MW at a wind farm has an average time between failures of over 5000 hours, which extends operating time by about 20% compared to asynchronous generators.
Synchronous motors have an annual average failure rate of only 1%, while the failure rate of asynchronous motors is 3%-5%, and the maintenance intervals can be extended to 3-5 years.
Common Applications
A unit capacity of over 100 MW is very common in the design of large synchronous generators. The Three Gorges Project is one example: one single generator unit with a capacity of 700 MW and efficiency of 98.5% can last on average for over 30 years.
Of course, the power rating of large synchronous motors for rolling mills should be more than 10 MW. A 12,000 kW synchronous motor installed at a steel mill had an annual failure rate of less than 2%, which saves about 300,000 RMB per year in maintenance cost.
This motor rated at 5000 kW in chemical plants can carry a volume of liquid chemicals of about 1500 cubic meters/hour. Approximately, 8% of energy consumption is saved on the synchronous motor, which accounts for around 1200 tons yearly in terms of reducing CO2 emission.
A 2500 kW synchronous motor in a cement plant can process over 200 tons of raw materials per hour, saving maintenance costs of up to 500,000 RMB annually.
A subway train usually has 8 synchronous motors. A total of up to 5000 kW of power. Compared to DC motors, which are traditional by nature, synchronous motors extend the maintenance interval by 30%, thus reducing energy consumption by 15% and saving close to 20% in maintenance costs on a yearly basis.
High-precision motors are used in flight simulators and inertial navigation systems. The speed deviation can be regulated within 0.01%. One model of an inertial navigation system uses a synchronous motor that stably operates in the temperature range from -40°C to 85°C.
A 3 MW rated wind turbine will yield around 9 million kWh electricity in a year. This could supply around 3000 average homes. By the year 2030, this would be surpassed by more than 70%.
A synchronous motor-controlled MRI machine ensures a scanning error in the range of 0.1 mm.
The positioning accuracy up to 0.005 mm is possible when the CNC is driven by synchronous motors.
