Introduction
In the industrial field, small and medium-sized three-phase asynchronous motors are currently the most widely used motor products.
Different induction motors have different applications, with the continuous development of electric motor manufacturing technology and research on the working principle of industrial motors.
There are also many new types of electric motors, including rare earth permanent magnet motors, switched reluctance motors, synchronous reluctance motors, etc.
A brief introduction to the various types of electric motors
Small and medium-sized three-phase asynchronous motors are currently the most widely used electrical motors products for industrial applications.
The three-phase asynchronous motor is a type of AC motor, also known as an induction motors.
It has a series of advantages such as simple structure, easy manufacturing, robustness, easy maintenance, low cost and low price.
Therefore, it is widely used in industry, agriculture, national defence, aerospace, scientific research, construction, transportation and people’s daily life.
It has a low power factor and is somewhat limited in its application.
1. Structure of three-phase asynchronous motor
Prepare a three-phase induction motor to be disassembled and look at its structure from the outside in.
The following diagram shows the outline and sectional view of three-phase induction motors.
From the outside of the electrical motors you can see the fan cover, the fan, the left and right end caps, the nameplate and the terminal box.
Open the terminal box and you can see the terminal posts and lead wires.
Remove the fan cover, fan and end caps, pull out the rotor and you can see the stator core, rotor core, shaft, bearings and other parts inside the motor.
The stator core is loaded in the housing and the rotor core is mounted on the motor shaft.
The stator core has coils (windings) and the rotor core has a cage shaped aluminium bar cast into it.
Winding linear rotor iron core with rotor windings.
As can be seen, the basic structure of the squirrel cage electrical motors consists of a fixed and immovable part, a rotor and other parts, as shown in the figure below:
The rare earth permanent magnet motor
It is a new type of permanent magnet motor that emerged in the early 1970s.
The magnetic properties of rare-earth permanent magnet materials are so excellent that they can be magnetised to create a strong permanent magnetic field without the need for additional energy.
The rare earth permanent magnet motor is not only highly efficient, but also has a simple structure, reliable operation and is small in size and light in weight.
They can achieve high performance (e.g. high efficiency, high speed, high response) that cannot be matched by traditional electrically excited motors, and can also be made into special motors that can meet specific operating requirements.
High performance rare earth permanent magnet motors are the basis for many new technologies and high technology industries.
Combined with power electronics and microelectronic control technology, it can produce a variety of mechatronic products with excellent performance, such as CNC machine tools, machining centres, flexible production lines, robots, electric vehicles, high-performance household appliances, computers and many more.
With the development of permanent magnet synchronous motors, high efficiency permanent magnet synchronous motors for high temperature, high vacuum, electric vehicles, internal inverters, high torque at low speeds and self-starting are gradually entering specific applications.
Due to the specific performance requirements of the motors for different operating conditions, the general development of permanent magnet motors is gradually showing some specific characteristics.
High power and ultra-high speed as a direction of development for permanent magnet motors
To increase the rated power of the induction motor, subject to a certain volume, it is necessary to increase the speed of the electrical ac motor considerably.
Rare earth permanent magnet motors do not require excitation windings, have a relatively simple structure, have no heat source in the magnetic field section, do not require cooling devices, have high coercivity of the material.
It can take on larger values for the air gap length, thus making it possible to increase the speed considerably.
High performance direction
Modern equipment to the motor industry put forward a variety of high-performance requirements.
Such as military equipment requirements to provide a variety of high-performance signal motor, mobile power station, automation equipment with servo systems and such motors, aerospace with high-performance, high reliability permanent magnet motor, chemical fiber equipment with high speed regulation accuracy frequency synchronous motor, CNC machine tools, machining centres, robots with high speed than rare earth permanent magnet servo motor, computer with high-precision swing motor and spindle motor, etc., to the direction of special industrial motors development.
Development in the direction of lightness
Aerospace products, electric vehicles, CNC machine tools, computers, audio-visual products, medical devices, portable optical mechatronics products, etc., all put forward strict requirements for squirrel cage induction motor with small size and light weight.
The permanent magnet synchronous motor has been developed and applied in lift technology for its small size, energy saving, good control performance, easy to make low-speed direct drive, eliminating gear reduction device, and speed regulation through frequency change.
Switched Reluctance Drive (SRD)
It is the latest generation of stepless speed control system developed after the frequency control system and brushless DC motor speed control system. It is an integrated high technology of light, electric machine and electricity integrating modern micro-electronics, digital technology, power electronics, infrared photoelectric technology and modern electromagnetic theory, design and production technology.
It has a speed control system both DC, AC two types of speed control system advantages.
Britain, the United States and other economically developed countries on the switch reluctance motor speed control system research started earlier, and has achieved significant results, product power level from a few w to hundreds of kw, widely used in household appliances, aviation, aerospace, electronics, electric machines and electric vehicles and other fields.
The convex poles of the stator and rotor of the switched reluctance motor are laminated from common silicon steel sheets, a process that minimises eddy current and hysteresis losses in the motor.
There are no windings or permanent magnets on the rotor poles, no commutators, no slip rings, etc. The stator poles are wound with concentrated windings, and the two radially opposite windings are connected in series to form one phase.
The switched reluctance motor is electrical motors that generates torque by using uneven rotor reluctance, also known as a reactive synchronous motor, whose structure and working principle are very different from those of traditional synchronous AC motors and DC motors.
It does not rely on the interaction of the magnetic fields generated by the stator and rotor winding currents to produce torque, but on the “principle of minimum reluctance” to produce torque.
This means that “the magnetic flux always closes along the path of least resistance, thus creating a magnetic pull, which in turn creates an electromagnetic torque of a magnetoresistive nature” and “the magnetic lines of force have the nature of trying to shorten the path of the flux in order to reduce the resistance and increase the permeability”.
Magneto-resistive synchronous motor
It is evolved from the same cage type asynchronous motor, whose rotor has a cage type cast aluminium winding resistance, but with a reaction slot corresponding to the number of stator poles (only the convex part of the role, no excitation winding and permanent magnets), used to generate reluctance synchronous torque.
Magneto-resistive synchronous motors are divided into single phase motor , single-phase capacitor-running, single-phase capacitor-starting and single-phase double-value capacitor types.
Application areas: reluctance synchronous motors are mainly used in industrial and agricultural production, transportation, national defence, commercial and household appliances, medical and electrical equipment, etc.
SWOT analysis of the above four types of electric motors
1) Small and medium-sized three-phase asynchronous motors
Advantages: high reliability, low cost, high technological maturity, high market share.
Disadvantages: low efficiency of ordinary motors, low technical threshold, fierce competition, insufficient technical innovation.
Opportunities: In places with high requirements for reliability of working conditions, ordinary working conditions can be met, and the promotion of high-efficiency industrial motor is the focus of energy-saving efforts.
Threats: The market for rare earth permanent magnets is expanding, especially in the market for high efficiency motors.
(2) Rare-earth permanent magnet motors
Advantage: not only high efficiency, but also simple structure, reliable operation, but also small size and light weight.
Disadvantages: Cost is 30% higher than asynchronous motors, and demagnetisation will occur at high temperatures for a long time.
Opportunities: oil fields, textile and chemical fibre industry, ceramic and glass industry and fans and pumps with long annual running time.
Market share gradually expanding.
Threats: cannot be used in places with too much power, mainly because the cost is too high and the production process is difficult.
(3) Switched reluctance motors
Advantages: simple motor structure, low cost, can be used for high speed operation. Simple and reliable power circuit, large starting torque, low starting current, suitable for frequent starting and stopping, forward and reverse conversion operation, good speed regulation performance.
Disadvantages: high noise and significant torque pulsation at low speeds. It needs to be used with a controller, and the cost of both together is high. The current power range is 8kw-400kw and is only suitable for special applications. In general purpose areas, the advantages over AC asynchronous and DC inverter speed control systems are not obvious.
Opportunities: electric vehicles, textile industry, coke industry, home appliance industry, for areas where it can be used to its advantage, i.e. areas requiring high starting torque, high speed requirements, hot and humid environments and frequent random revolutions.
Threats: The problem of high noise levels is difficult to overcome and development is limited.
(4) Synchronous reluctance motors
Advantages: Compared with permanent magnet synchronous motors, the cost of the motor is greatly reduced under the same power conditions, while the range of use of the motor is broadened and the reliability of the motor operation is improved. Due to the simple structure, the rotor does not have electromagnetic losses and can avoid the disadvantages of switching reluctance motors such as high noise and significant torque pulsation at low speeds.
Disadvantages: low power factor and low output compared to asynchronous motors of the same size.
Opportunities: Industrial and agricultural production, transport, defence, commercial and domestic appliances, medical and electrical equipment, etc.
Threats: At the research stage.
The current research is on fractional horsepower motors, which have little place in AC systems.
Comparative analysis of motor products
| Projects | Small and medium-sized three-phase asynchronous motors | Rare earth permanent magnet motors | Switched reluctance motors | Synchronous reluctance motors |
| Cost | Low | Medium | High | Medium |
| Reliability | High | Low | Low | Low |
| Technical maturity | Very mature | Fairly mature | Fair | Immature |
| Market share | Extremely large | less | Less | None |
| Efficiency | average | High | Fair | Fair |
| Copper density | High | Lower | Lower | Lower |
| Simple structure | poor | Good | Fair | Good |
| Size and quality | Large, heavy | Small, Light | Small, Light | Small, Light |
| Noise | Low | Low | High | Low |
From a comparative analysis of the various competitive products on several levels, the small and medium-sized three-phase asynchronous motor is still the most technically mature product with the largest market share.
However, in the future, we need to further optimise the process, improve the technology and make further improvements in terms of efficiency and compactness in order to maintain a high level of competitiveness.
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