Advanced Motor Technology – Induction Motors and Synchronous Motors

1. Induction Motor

Induction Motor is a widely used AC motor, and its working principle is based on the law of electromagnetic induction . In an induction motor, after the stator winding is connected to the power supply, a rotating magnetic field is generated due to the AC current. This magnetic field changes continuously in space and rotates at a constant speed (synchronous speed).

The rotor part is a closed conductor structure, usually a squirrel cage or winding structure. When the rotating magnetic field generated by the stator cuts the rotor conductor, according to Faraday’s law of electromagnetic induction, an induced current will be generated in the rotor conductor. These induced currents will form a torque under the action of the magnetic field, causing the rotor to start rotating.

However, since the current in the rotor is generated by electromagnetic induction rather than direct power supply, the rotor speed is always lower than the rotation speed of the stator magnetic field, that is, the synchronous speed, and there is a relatively fixed slip rate between the two. It is precisely because the rotor and the rotating magnetic field are in asynchronous operation that this type of motor is called an “asynchronous motor.”

Asynchronous motors do not require mechanical commutation devices, have simple structures, are durable and easy to maintain, and are suitable for a wide range of industrial and civil applications, such as pumps, fans, compressors, conveyor belts, and other applications that require continuous operation or frequent start and stop. At the same time, variable frequency speed regulation technology can also be used to accurately control the speed of asynchronous motors.

Induction motors, also known as asynchronous motors, can be divided into the following categories based on their structure and application:

1. **Single-phase induction motor**:

   – Single-phase induction motors are generally used in small devices such as household appliances, such as fans, refrigerator compressors, etc. Since single-phase power cannot produce a rotating magnetic field, these motors usually require special designs (such as starting windings or capacitors) to achieve self-starting.

2. **Three-phase induction motor**:

   – According to the different rotor structures, it can be divided into two categories:

     – Squirrel-cage rotor induction motor: The most common type, the rotor consists of cast aluminum or copper bars with embedded conductors, forming a closed circuit similar to a squirrel cage, without the need for additional slip rings and carbon brushes.

     – Wound rotor induction motor: The rotor winding uses terminal blocks connected at the end, and the rotor impedance can be changed by external resistors to adapt to different load requirements and speed control.

3. **Classification by protection level and installation method**:

   – Open type (OCP): The motor housing has good ventilation, but poor protection performance. It is suitable for dry, clean environments with low requirements for dust and water resistance.

   – Enclosed type (TEFC, TENV, TURBO, etc.): Depending on the degree of encapsulation, it provides different levels of dustproof, waterproof and cooling effects, suitable for harsh environments or occasions with special protection requirements.

   – Vertical, horizontal and various installation arrangements: classified according to the motor axis and installation position.

4. **Classification by power and application**:

   – Micro and small induction motors: mainly used in precision instruments, household appliances and other fields.

   – Medium and large induction motors: widely used in power drive applications such as pumps, fans, compressors, etc. in industrial production.

   – Special purpose induction motors: such as submersible motors (underwater work), explosion-proof motors (flammable and explosive environments), high temperature motors (high temperature working conditions), etc.

5. **Classification by performance characteristics**:

   – Ordinary induction motor: commonly used for continuous operation, with good starting performance and overload capacity.

   – High-efficiency and energy-saving induction motor: adopts new materials and optimized design to improve energy efficiency ratio and meet relevant national or international energy efficiency standards.

In summary, the classification of induction motors is mainly based on their electrical characteristics, mechanical structure, protection level, installation method, and specific application environment and requirements.

2. Synchronous Motor

A synchronous motor is an AC motor whose rotor speed is strictly synchronized with the grid frequency and the number of pole pairs of the motor itself, that is, the rotor rotation speed is always equal to the synchronous speed. The main features and classifications of synchronous motors include:

1. **Working Principle**:

   – A rotating magnetic field is generated when three-phase alternating current is passed through the stator winding.

   – There are two types of rotors: in permanent magnet synchronous motors, the rotor uses permanent magnets; in induction synchronous motors, the rotor forms a current through electromagnetic induction, which in turn generates a magnetic field that interacts with the stator magnetic field.

   – In the synchronous state, the rotor magnetic field and the stator magnetic field are relatively stationary, so there is no slip phenomenon.

2. **Classification**:

   – Permanent Magnet Synchronous Motor (PMSM): It uses permanent magnets as the source of the rotor magnetic field. It has high efficiency and power density and is widely used in new energy vehicles, servo drives and other fields.

   – Non-permanent magnet synchronous motor (such as hollow cup rotor synchronous motor, squirrel cage synchronous motor, etc.): No permanent magnets are used, and the rotor magnetic field is provided by an external power supply or generated by self-inductance.

   – Synchronous generator: mainly used in power plants to convert mechanical energy into electrical energy, and can be connected to the grid to provide stable voltage and frequency support for the power system.

   – Synchronous compensator (synchronous phase condenser or synchronous capacitor): used to improve the reactive power balance of the power grid and regulate the voltage stability of the power grid.

3. **Applications**:

   – Industrial drive equipment, especially where precise speed control and high torque output are required.

   – Large synchronous generators in power plants are the main source of power supply for the power system.

   – Reactive power compensation and stability control of power systems.

4. **Advantages**:

   – Precise speed control capability, suitable for transmission systems requiring extremely high precision.

   – It can provide high-quality AC power as a synchronous generator to support the frequency stability of the power grid.

   – Compact structure and high efficiency, especially in modern design, permanent magnet synchronous motors combined with permanent magnet materials have higher performance advantages.

5. **Development Trend**:

   – The development of new materials and new processes has promoted the miniaturization and high performance of permanent magnet synchronous motors.

   – The application of AI technology makes the control of synchronous motors more intelligent, optimizing motor efficiency and life prediction management.

   – Research and development of high-voltage and large-capacity synchronous motors continues to meet the growing needs of the energy sector.

3. Differences in control algorithms between the two

The differences in control algorithms between induction motors (asynchronous motors) and synchronous motors are mainly reflected in the following aspects:

1. **Startup and operation mode**:

   – Induction motor: Usually no special control strategy is required when starting, as long as the three-phase power supply is connected, it can start automatically. During operation, the rotor magnetic field is generated by electromagnetic induction from the stator rotating magnetic field, so the speed is always lower than the synchronous speed.

   – Synchronous motor: When starting, an excitation system is required to provide an initial magnetic field, or for permanent magnet synchronous motors, a permanent magnet fixed on the rotor generates a constant magnetic field. During operation, the controller needs to ensure that the rotor magnetic field is strictly synchronized with the stator magnetic field.

2. **Speed ​​control**:

   – Induction motor: Basic speed control methods include variable frequency speed regulation (V/F control, vector control, etc.), which adjusts the motor speed by changing the frequency of the power supply. Vector control can achieve more precise speed control and simulate the control effect of DC motors.

   – Synchronous motor: Especially in permanent magnet synchronous motor, its speed control is more direct and precise. By adjusting the inverter output current to keep the motor magnetic field synchronized with the stator magnetic field, it can achieve precise speed control and high dynamic performance in a wide range.

3. **Power Factor Correction**:

   – Induction motor: Due to its natural characteristics, the power factor is low when no-load or light-loaded, and active power factor correction is generally not performed. However, the power factor can be improved through vector control in situations where the load changes greatly.

   – Synchronous motors: Especially brushless synchronous motors, whose excitation magnetic field can be independently controlled, can easily achieve real-time adjustment of the power factor to make it close to 1.

4. **Control system complexity**:

   – Induction motor: Traditional control is relatively simple, while modern vector control is complex but the technology is mature and widely used.

   – Synchronous motors: In particular, the control of permanent magnet synchronous motors is relatively complex, requiring accurate estimation of rotor position information and implementation of complex field oriented control algorithms (FOC, Field Oriented Control).

5. **Positioning accuracy**:

   – Induction motors: Due to their asynchronous nature, they have limited positioning accuracy and are generally not suitable for precision positioning tasks.

   – Synchronous motor: In particular, permanent magnet synchronous motor has high positioning accuracy and is suitable for applications with high position accuracy requirements.

In summary, induction motor control relies more on the regulation of AC power frequency, while synchronous motors require fine control of magnetic field strength and direction. Synchronous motors have advantages in precise speed control, efficient operation, and positioning accuracy due to their synchronous characteristics and magnetic field controllability, and the corresponding control algorithms are more complex.

4. The design of both

The development trends of induction motors (asynchronous motors) and synchronous motors in terms of body design mainly focus on the following aspects:

1. **High efficiency and energy saving**:

   – Induction motor: As energy efficiency standards continue to increase, the design trend of induction motors is towards higher efficiency. This includes using higher-grade insulation materials, optimizing winding structures to reduce copper and iron losses, and improving cooling systems.

   – Synchronous motors: In particular, permanent magnet synchronous motors (PMSMs) have significant advantages in the field of high efficiency and energy saving due to their inherent high efficiency characteristics. The research and development trend is to improve the performance of permanent magnets, reduce the risk of demagnetization, and optimize the rotor and stator design to further improve the overall efficiency.

2. **Intelligent control and digitalization**:

   – Both are developing towards intelligence, using advanced control algorithms and digital signal processors (DSP), microcontrollers (MCU), etc. to achieve precise speed, torque control and fault diagnosis functions. For example, vector control and direct torque control technology are widely used in synchronous motors.

3. **Miniaturization and Lightweight**:

   – In the fields of new energy vehicles, aerospace, etc., the requirements for miniaturization and lightweight of motors are getting higher and higher. Through the application of new materials (such as high-performance silicon steel sheets, composite rotors), optimized electromagnetic field distribution design, and integrated packaging technology, the motor size is made more compact and the weight is lighter.

4. **Environmental protection and sustainability**:

   – Environmental protection concepts have promoted the development of motor material selection, such as seeking more environmentally friendly permanent magnet material alternatives in permanent magnet synchronous motors and reducing the use of rare earth elements; at the same time, attention is also paid to environmental friendliness and recyclability throughout the entire life cycle.

5. **Customization and modularization**:

   – With the diversification of market demand, motor design tends to be modular and customized in order to quickly respond to the needs of different application scenarios, such as adapting to different voltage levels, power ranges and working requirements under special environmental conditions.

In short, whether it is an induction motor or a synchronous motor, future development will focus more on improving energy conversion efficiency, enhancing control performance, meeting specific application requirements, and taking into account the goals of environmental protection and sustainable development.

5. Who to choose for electric vehicles

When choosing a drive motor for an electric vehicle, two types are mainly considered: induction motor (AC asynchronous motor) and permanent magnet synchronous motor.

1. Induction motor (AC asynchronous motor):

   – Advantages: simple structure, strong and durable, low maintenance cost, suitable for high-speed operation and able to withstand large overload shocks.

   – Disadvantages: Relatively low efficiency, especially at partial load, requires large copper and iron losses, and has no inherent magnetic field control capability, requiring a frequency converter to adjust the speed.

2. Permanent Magnet Synchronous Motor (PMSM):

   – Advantages: high efficiency, high power density, small size, light weight, especially high torque output in the medium and low speed range, with good energy saving effect. Because permanent magnets are used as the rotor magnetic field source, its speed is strictly synchronized with the power frequency, and precise speed control can be achieved.

   – Disadvantages: Permanent magnet materials are expensive and may demagnetize in high temperature environments; in addition, high requirements are placed on the control system to ensure stability and safety.

The current market trend tends to use permanent magnet synchronous motors as the main drive motors for electric vehicles, especially for pure electric vehicles that pursue mileage and power performance. Permanent magnet synchronous motors have become a better choice due to their high efficiency and high power density. However, in some applications such as commercial vehicles or cost-sensitive applications, induction motors may still be selected due to their reliability and cost advantages.

6. Coreless Motor

The coreless motor, also known as the coreless DC motor or coreless permanent magnet motor, is a special motor design with the notable feature that the rotor adopts an ironless core structure. The rotor of a traditional motor usually consists of a laminated core and windings, while the rotor of a coreless motor is a “cup” shape made of pure copper or aluminum without an iron core, which is hollow inside and the winding is directly wound on the rotor wall.

The main advantages of coreless motors include:

1. Low inertia: Since there is no core mass, the coreless motor has a very low moment of inertia, which enables it to achieve very high acceleration and fast response.
2. High efficiency: The core loss is eliminated, the electromagnetic conversion efficiency is improved, especially when running at medium and low speeds, it can maintain a high power density and efficiency.
3. Excellent speed regulation performance: It can provide good speed regulation performance in a wide speed range, especially suitable for applications that require frequent speed changes.
4. Good stability: No cogging effect, low vibration, low noise and excellent dynamic performance during operation.
5. Small size and light weight: Suitable for miniaturized equipment and precision control fields.

It has a wide range of applications, such as in servo control systems in aerospace, medical equipment, robotics, precision instruments, office automation equipment, home appliances, and new energy vehicles.

The original and most common form of hollow cup motor is the DC motor, which is characterized by the use of ironless rotor design. However, as technology develops, there are AC motor designs that use similar “hollow cup” structural principles.

For example, there is a special type of AC motor called “ironless induction motor”. Although its rotor structure is not a typical hollow cup shape, it also has the characteristics of low iron loss and low rotational inertia. This motor may adopt a similar ironless or less iron core design concept in some applications with high response requirements or where the core loss needs to be reduced.

Strictly speaking, the expression “coreless AC motor” is not common, but if the definition is relaxed to include AC motors based on the concept of ironless or extremely small iron core rotors, such motor designs do exist and may be used in certain high-end applications.

At present, there is no widely used or mass-produced “coreless synchronous motor” product on the market. Usually, the coreless motor we are discussing mainly refers to a special design of DC motor, which is characterized by the ironless rotor structure.

The working principle of synchronous motors (including permanent magnet synchronous motors and electrically excited synchronous motors) is strictly synchronized with the frequency of the AC power supply, and most of them have an iron core structure. However, in theory, a synchronous motor design based on a hollow cup structure can be envisioned, that is, by applying permanent magnets or other methods to generate a constant magnetic field to an ironless or extremely small iron core rotor to achieve synchronous operation. However, in practical applications, due to the high requirements of synchronous motors on structural strength, electromagnetic performance and stability, traditional iron core designs are more common.

If in the future research or product development can produce a true “hollow cup synchronous motor”, it will be an innovative technological breakthrough and may be used in specific fields such as aerospace, precision control, etc.

7. Linear Motor

A linear motor is an electric motor that directly converts electrical energy into linear motion, without the need for rotary motion and mechanical transmission devices (such as gears, belts, screws, etc.) to indirectly generate linear motion. Its working principle is similar to that of a rotary motor, but the structure is to expand the rotary motor radially so that the magnetic field generated by the stator interacts with the mover (equivalent to the role of the rotor in a linear motor), thereby achieving linear motion along the magnetic track.

This picture comes from the Internet

The basic structure of a linear motor usually includes:

1. **Primary part (stator)**: The stationary part, containing the coil windings, which generates an electromagnetic field when current passes through it.

2. **Secondary part (motor or slider)**: The part that moves along the stator and can carry the load. Some designs contain magnetic materials or permanent magnets, which interact with the magnetic field generated by the stator to generate thrust.

Advantages of linear motors:

– High acceleration and high speed: Without the limitations of traditional mechanical transmission mechanisms, linear motors can provide very high acceleration and operating speeds.

– Precise control: It can achieve precise position and speed control, suitable for applications with high precision requirements.

– No wear: Due to the contactless drive, mechanical wear is reduced, and the reliability and life of the system are improved.

– Low maintenance cost: Since there are fewer transmission parts, maintenance requirements are reduced.

Linear motors have a wide range of applications, such as:

– Magnetic levitation system for high-speed trains (Maglev)

– Industrial automation production line

– Semiconductor manufacturing equipment

– CNC machine tools

– Linear elevators and conveyor systems

– Cylinder drives in the printing and paper industry

– Medical equipment, such as MRI scanners, etc.

Furthermore, in the electric vehicle industry, linear motors are also being explored as a possible option for propulsion systems, i.e. linear drive electric vehicles.

While linear motors offer significant advantages in many areas, they also present some potential disadvantages and challenges:

1. **High initial cost**: The R&D, design and manufacturing costs of linear motors and their supporting systems are usually higher than those of traditional rotary motors and transmission mechanisms. Especially for applications requiring long travel, the cost of magnetic tracks and movers will increase significantly.

2. **Heat dissipation problem**: Due to the structural characteristics of linear motors, their heat dissipation conditions may not be as ideal as those of rotary motors. Especially when running at high power or high speed, heat management is a key issue and an effective cooling system is required to ensure motor performance and life.

3. **Complex magnetic circuit design**: Designing an efficient and stable magnetic field distribution is a technical challenge for linear motors , especially in long-stroke applications, where ensuring magnetic field uniformity and stability is even more difficult.

4. **Air gap problem**: The thrust of a linear motor is closely related to the size of the air gap between its stator and mover. A larger air gap may lead to a decrease in thrust and lower efficiency, while a smaller air gap may cause safety problems (such as collisions) and increase maintenance difficulties.

5. **Protection and packaging**: The moving parts of linear motors are exposed, unlike rotary motors, which can be effectively prevented from entering by sealing structures such as bearings. Therefore, special protective measures may be required to adapt to harsh environments.

6. **Control Complexity**: The dynamic characteristics of linear motors are often complex and require advanced control systems to achieve precise position, velocity and acceleration control.

7. **Electromagnetic interference**: The electromagnetic field generated by the high-voltage and high-current coil may interfere with surrounding electronic equipment, and corresponding shielding measures need to be taken.

Despite these challenges, with the advancement of technology and the growth of application demand, linear motors still show strong competitiveness in many fields and are becoming more widely used in the process of continuously overcoming the above-mentioned shortcomings.

The difficulties faced by linear motors in control algorithms mainly include the following aspects:

1. **High-precision position control**:
   – Linear motors have extremely high requirements for position control accuracy due to their direct drive characteristics. In order to achieve nanometer or even sub-micron position control accuracy, advanced servo control systems and precise feedback mechanisms (such as grating scales, magnetic encoders, etc.) and complex control algorithms such as model predictive control , sliding mode control or adaptive control are required.

2. **Nonlinear dynamic characteristics**:
   – The dynamic behavior of linear motors is more complex than that of rotary motors, with strong electromagnetic force nonlinearity, thrust fluctuations caused by air gap changes, parameter changes caused by thermal effects, etc. These need to be compensated by the controller, which may require the use of nonlinear control theory, such as fuzzy logic control, neural network control or inverse system method.

3. **Balance between fast response and stability**:
   – In order to meet the acceleration requirements during high-speed motion, linear motors need to have fast response capabilities. However, fast response often introduces greater shock and vibration, so the control algorithm must take into account both the speed and stability of the system, and sometimes a feedforward control strategy is required to predict and offset the impact of load changes.

4. **Anti-interference ability**:
   – External disturbance factors such as temperature changes, load fluctuations, unstable power supply voltage, etc. have a significant impact on the performance of linear motors. The control algorithm should have good robustness and adaptability to adjust control parameters in real time to ensure operational stability and accuracy.

5. **Saturation effect processing**:
   – When the current is too large, the winding of the linear motor will enter the magnetic saturation state, resulting in a non-linear relationship between thrust and current. Designing an effective control algorithm to handle this saturation effect is another technical difficulty.

6. **Trajectory tracking and smooth transition**:
   – In applications that require precise following of a predetermined trajectory, such as precision machining, semiconductor manufacturing, etc., how to ensure that the motor moves smoothly and accurately along the predetermined trajectory requires the controller to effectively handle complex trajectory planning and dynamic optimization problems.

In short, the design of linear motor control algorithm is a comprehensive challenge that combines multiple disciplines such as electrical engineering, automatic control, and mechanical dynamics. It requires continuous theoretical research and technical iteration to overcome the above difficulties.

There is a motor design that can achieve both rotation and linear motion, which is called a “rotary-linear motor” (also called a rotary linear motor or RLA/RLL motor). This motor design combines the characteristics of a rotary motor and a linear motor, and is able to convert the power output form through an internal mechanism without changing its physical structure.

The working principle of rotary-linear motors is usually that in an axial magnetic circuit, a part of the motor can both rotate and produce linear motion along the axis. Specifically, this type of motor may use a specially designed rotor and stator structure in its internal structure, and generate rotational torque in certain working modes by switching the magnetic field distribution or using the principle of electromagnetic coupling, and convert it into linear thrust in other modes.

However, this type of motor is not widely used in the market at present, and is more often used in specific high-end equipment or products in the laboratory research stage. In actual applications, if both rotation and linear motion functions are required, an independent rotation motor and linear motor combination is usually used, or a transmission mechanism such as gears and belts is used to convert rotational motion into linear motion.

8. Direct drive or transmission

In motor control systems, direct drive and belt drive each have their own advantages and disadvantages. The specific choice depends on factors such as application scenarios, performance requirements, cost considerations, and maintenance convenience.

1. Direct drive motor system:

   – Direct drive motors are directly connected to the load, such as the inner drum of a washing machine or the joints of an industrial robot, eliminating intermediate transmission links such as belts and gears, thereby achieving higher transmission efficiency and precise control.

   – advantage:

     – Higher energy transfer efficiency: no transmission loss, theoretically can achieve 1:1 power transmission.

     – Precision control: Direct drive motors usually use brushless DC motors or other high-precision motors, which can achieve precise speed and position control, and are particularly suitable for occasions that require precise motion control.

     – Low noise and vibration: Reducing mechanical transmission parts reduces operating noise and vibration and improves overall stability.

   – shortcoming:

     – Higher cost: Direct drive motors and supporting control systems are relatively complex and costly.

     – Difficulty of maintenance: Once the motor fails, maintenance may involve disassembling the entire machine, which is relatively complicated.

2. Transmission motor system:

   – The transmission motor transmits the power of the motor to the load through mechanisms such as belts, chains, gears, etc.

   – advantage:

     – Moderate cost: Compared with direct drive systems, traditional transmission systems are less expensive, and there are many mature options for motors and transmission components.

     – Easy maintenance: Some transmission components can be replaced and repaired independently, which reduces maintenance costs to a certain extent.

     – Flexibility: The transmission ratio can be adjusted to suit different speed and torque requirements.

   – shortcoming:

     – Energy loss: There is a certain amount of transmission loss, resulting in the overall efficiency being slightly lower than that of a direct drive system.

     – Limited control accuracy: Due to the addition of a transmission link, there may be certain challenges in the application of high-precision control.

     – Loud noise and vibration: Transmission components may increase noise and vibration.

In summary, in the selection of motor control systems, if there are high requirements for control accuracy, energy efficiency, quietness and space occupancy, and the budget is sufficient, direct drive technology may be a better choice. In the case of cost sensitivity, easy maintenance and certain flexibility in transmission configuration, the transmission motor system may be more suitable.

In electric vehicles (EVs), the main drive motor and the wheel hub motor are two different drive schemes:

1. Main drive motor (central drive):

   – In this configuration, the electric motor is usually installed in the center of the vehicle, and the power is transmitted to the wheels through the transmission system (such as speed reducer, differential, etc.). The main drive motor can be arranged centrally, which is convenient for efficient heat dissipation management and is easier to modify using existing vehicle platforms.

   – Advantages: The centralized design is mature and reliable, and easy to maintain; it is compatible with traditional fuel vehicle platforms; it can adapt to efficiency optimization under different speed ranges through the speed change mechanism.

   – Disadvantages: Requires additional transmission components, which increases weight, cost and potential failure points; occupies relatively large space.

2. Hub motor (distributed drive):

   – Each wheel or at least part of the wheels is equipped with an independent electric motor, which is directly integrated into the wheel or very close to the wheel, eliminating the need for complex mechanical transmission devices.

   – Advantages: Simplified vehicle structure, no need for traditional drive shafts, differentials and other components, improving space utilization; enabling more flexible power distribution, each wheel can independently control torque output, which is beneficial to improving handling and traction control; theoretically, higher transmission efficiency can be achieved.

   – Disadvantages: Increased unsprung mass may affect the dynamic response and comfort of the suspension system; the design and maintenance of the hub motor is more complex, including technical challenges in thermal management and sealing; higher reliability requirements, because once a single hub motor fails, it will affect the operation of the entire wheel; electric braking capabilities are limited, and additional braking solutions may be required.

This picture comes from the Internet

In general, mainstream electric vehicles currently use more main drive motor solutions because of their mature technology, controllable costs and easy maintenance. Although the hub motor has its innovative advantages, it has not yet been widely popularized due to its technical difficulty and application challenges. However, with the advancement of technology, the hub motor may be more widely used in certain types of electric vehicles in the future.