In the big family of motors, both asynchronous motors and torque motors have a wide range of applications. They play important roles in different fields, but many people are not very clear about their differences. In this article, we will discuss the differences between asynchronous motors and torque motors to help readers better understand the characteristics and application scenarios of these two types of motors.
A. Principle of operation
a. Asynchronous motor
Asynchronous motor, also known as induction motor, its principle of operation is to use the stator winding in the three-phase alternating current to generate a rotating magnetic field, in the rotating magnetic field of the rotor conductor cutting the magnetic lines of force to produce induced electromotive force and induced current. As the rotor current interacts with the rotating magnetic field, an electromagnetic torque is generated, causing the rotor to rotate.
The speed of the asynchronous motor is always lower than the synchronous speed because the rotor speed needs to lag behind the rotating magnetic field speed to generate the induced current and electromagnetic torque. The synchronous speed of an asynchronous motor is related to the frequency of the power supply and the number of motor poles, which is calculated by the formula: n = 60f/p, where n is the synchronous speed, f is the frequency of the power supply, and p is the number of motor poles.
b. Torque motor
Torque motor is a special motor with soft mechanical characteristics and wide speed range. Its working principle is to adjust the output torque of the motor by changing the input voltage or current of the motor. Torque motors usually use permanent magnets or excitation windings to generate a magnetic field. When the motor is energized, the magnetic field interacts with the current in the armature windings to generate electromagnetic torque.
The output torque of a torque motor is proportional to the input current, i.e., the output torque of the motor can be precisely controlled by controlling the magnitude of the input current. At the same time, the rotational speed of the torque motor decreases automatically with the increase of the load, and it has a good blocking characteristic, which can produce a large torque under the blocking condition without burning the motor.
B. Structural Characteristics
a. Asynchronous motor
Asynchronous motor is mainly composed of two parts: stator and rotor. The stator consists of stator core, stator winding and machine base. The stator core is made of silicon steel sheets laminated together to form the magnetic circuit of the motor. The stator winding is made of enameled wire, which generates a rotating magnetic field after being energized by three-phase alternating current. The base is used to fix the stator core and support the motor as a whole.
The rotor consists of the rotor core, rotor winding and rotor shaft. The rotor core is also made of silicon steel sheets laminated together to form the magnetic circuit of the motor. The rotor winding is divided into two types: squirrel cage and wire wound. The squirrel cage rotor winding is made of copper or aluminum bars, shaped like a squirrel cage, with a simple and durable structure. The wire-wound rotor winding is made of enameled wire and is connected to the external circuit through collector rings and brushes, which can realize the speed regulation of the motor.
b. Torque motor
The structure of a torque motor is different from that of an ordinary motor. It usually has a flat structure with a larger diameter stator and rotor and a shorter axial length. This structure increases the torque output capacity of the motor and reduces the size and weight of the motor.
The stator of a torque motor usually consists of a stator core, an excitation winding, and a seat. The excitation windings are used to generate the magnetic field, and the base is used to hold the stator core and support the motor as a whole. The rotor of a torque motor usually consists of a rotor core, armature windings, and a shaft. The armature winding is the key component of a torque motor, which generates electromagnetic torque by interacting with the stator magnetic field.
C. Asynchronous motors and torque motors differ in speed regulation in the following ways:
a. Asynchronous motor:
-Frequency conversion speed regulation: mainly change the synchronous speed of AC asynchronous motor, according to the frequency conversion method there are mainly two kinds of direct frequency conversion and indirect frequency conversion. It has the advantages of large speed range, hard characteristics, high precision, small additional loss, high speed efficiency, wide range of applications, etc. It is suitable for AC cage-type asynchronous motor, and it is the main speed control method for asynchronous motor nowadays. The disadvantages are complicated control device, high cost and difficult maintenance.
- Variable-pole speed control: It is only applicable to cage-type asynchronous motors, and it changes the number of magnetic poles of the motor by changing the wiring of the stator windings, so as to change the synchronous rotational speed. This type of speed control has hard mechanical characteristics, good stability; no rotational loss, high efficiency; simple wiring, convenient control, low price. But the disadvantage is that there are levels of speed regulation, level difference is large, can not get smooth speed regulation. Can be used in conjunction with voltage regulator, electromagnetic differential clutch to obtain high efficiency and smooth speed characteristics. Applicable to production machinery that does not require stepless speed regulation, such as metal cutting machine tools, elevators, lifting equipment, fans, pumps and so on.
- String level speed regulation: only applicable to wound AC asynchronous motor, is one of the ways to change the rate of speed regulation, in the wound motor rotor circuit into the string adjustable additional potential to change the motor's rotational difference, to achieve the purpose of speed regulation. Most of the differential power is absorbed by the additional potential, and then the device that generates the additional potential is used to return the absorbed differential power to the grid or convert the energy for utilization. It is characterized by high cost-effective, can feed back the rotational loss reuse, high efficiency; device capacity and speed range is proportional to the investment in provincial, applicable to the speed range in the rated speed of 70% -90% of the production machinery; speed control device failure can be switched to full-speed operation, to avoid production stoppages. The disadvantage is that the power factor of thyristor series speed control is low, and the harmonic influence is large. Suitable for fans, pumps and rolling mill, mine hoist, extruder.
- Wire-wound motor rotor string resistance speed control: only for wire-wound AC asynchronous motor, by changing the wire-wound asynchronous motor rotor string into the value of additional resistance, to change the rate of motor rotation, and then change the motor speed. This method is simple, easy to control, but the rotational power is consumed in the form of heat in the resistance, is a graded speed regulation, mechanical characteristics are softer. In the potential load is used more, such as bridge cranes, trolley and winch control are used in this way.
- Stator voltage regulation: When changing the stator voltage of the motor, a set of different mechanical characteristic curves can be obtained, so as to obtain different speeds. Since the motor torque is proportional to the square of the voltage, changing the stator voltage will seriously affect the maximum torque, the speed range is small, and the cage motor application is not good. In order to expand the speed range, voltage regulation should be used to regulate the speed of cage motors with large rotor resistance, such as torque motor speed control, or a series of frequency-sensitive resistors on the wire-wound motor. When the speed regulation exceeds 2:1, feedback control should be used to achieve the purpose of automatic regulation. Voltage regulator line is simple and easy to realize automatic control, but the rotational power in the process of voltage regulation is consumed in the form of heat in the rotor resistance, which is less efficient. Generally applicable to production machinery below 100kw, mainly applicable to fans and pumps load or potential load.
b. Torque motor:
- Variable pole-pair number regulation method: change the pole-pair number of the cage motor by changing the way of stator winding to achieve the purpose of regulation.
- Frequency regulation: change the frequency of the motor stator power supply, thus changing its synchronous speed. Frequency regulation system is the main equipment to provide frequency inverter power supply, can be divided into AC - DC - AC frequency converter and AC - AC frequency converter two categories, at present the most used in China is the AC - DC - AC frequency converter.
- Serial regulation method: The adjustable potential is attached to the rotor ring of the winding line motor to change the differential rate of the motor to achieve the purpose of speed regulation. Most of the differential power is absorbed by the additional potentials fed in, which are then used to generate additional equipment that returns the absorbed differential power to the grid or converts it to energy for use. Depending on how the power is absorbed and utilized, series regulation can be divided into motor series regulation, mechanical series regulation and thyristor series regulation.
- Rotor series resistance regulation: winding asynchronous motor rotor series resistance, so that the motor rotation rate increases, so that the motor runs at a lower speed. The larger the series resistance, the lower the motor speed. The method of equipment is simple, easy to control, but the differential power in the form of heating consumed in the resistance, is a graded speed regulation, mechanical characteristics of soft.
- Stator voltage regulation method: when changing the stator voltage of the motor, a set of curves with different mechanical characteristics can be obtained, and then different speeds can be obtained. Due to the motor torque and voltage is proportional to the square, large torque is reduced a lot, the regulation range is small, making it difficult to apply the general cage motor. In order to expand the range of regulation, should be used with a larger rotor resistance value of the cage motor, such as torque motor speed control, or in series with a frequency-sensitive resistor on the wire-wound motor. When the speed regulation exceeds 2:1, feedback control should be used to achieve the purpose of automatic regulation. Commonly used regulation methods include series saturation reactor regulation, autotransformer regulation and thyristor regulation.
- Electromagnetic speed regulation method: It is composed of cage motor, electromagnetic torque clutch and DC excitation power supply (controller). By changing the conduction angle of the thyristor, the size of the excitation current can be changed, thus changing the output torque and speed of the clutch. This speed control method is suitable for medium and small power, the requirement of flat sliding, short-time low-speed operation of the production machinery.
- Hydraulic coupler speed regulation method: hydraulic coupler generally consists of pump wheel and turbine, placed in a sealed shell, the shell is filled with a certain amount of working fluid. When the pump wheel is driven by the prime mover to rotate, the liquid inside the pump wheel is driven by the blade to rotate, and under the action of centrifugal force, it enters the turbine along the outer ring of the pump wheel, and pushes the turbine blade to rotate, and then drives the operation of the production machine. During the working process, the turbine speed of the coupler can be changed by changing the liquid filling rate to realize stepless speed regulation. It is suitable for speed regulation of fans and pumps.
D. Performance characteristics
a. Asynchronous motor
(1) Advantages:
①Simple structure, durable and low cost.
② Reliable operation and easy maintenance.
③It is suitable for different kinds of loads and working environments.
(iv) It can be speed-regulated by changing the frequency of power supply and the number of poles.
(2) Disadvantages
① Poor speed regulation performance, narrow speed range.
② Starting current is large, the impact on the power grid is large.
(iii) Lower power factor, the need for reactive power compensation.
(iv) Lower efficiency in low speed operation.
b. Torque motor
(1) Advantages
① It has soft mechanical characteristics, the rotational speed decreases automatically with the increase of load, and it can produce large torque under the blocked state without burning the motor.
② Wide speed range, stepless speed regulation can be realized.
③High precision, can accurately control the output torque.
④ Fast response speed, good dynamic performance.
(2) Disadvantages:
① Complex structure, high cost.
② Specialized controller is needed for speed control.
(iii) Heat dissipation problem is more prominent, need to take effective heat dissipation measures.
E. Application Scenarios
a. Asynchronous motors
Asynchronous motors are widely used in industry, agriculture, transportation, household appliances and other fields. For example, in industrial production, it drives compressors, water pumps, crushers, etc., and can also be used to drive blowers, coal mills, rolling mills, winches and other equipment; in agricultural production, asynchronous motors are commonly used to drive irrigation equipment, agricultural processing equipment, etc.; in the field of transportation, asynchronous motors are commonly used to drive electric cars, electric bicycles, etc.; in the field of household appliances, single-phase asynchronous motors are commonly used to drive washing machines, fans, air conditioners, etc. In the field of household appliances, single-phase asynchronous motors are commonly used to drive washing machines, electric fans, air conditioners, etc.
b. Torque motor
Torque motors are mainly used in occasions that require precise control of torque and speed, such as textile machinery, paper-making machinery, printing machinery, packaging machinery, medical equipment, CNC machine tools, etc.
- Textile machinery: In the process of textile, the tension of yarn or fabric needs to be precisely controlled, the torque motor can provide constant torque at different speeds to ensure the stability of fabric tension in the process of printing, dyeing, weaving, winding, etc., and to prevent yarn breakage or fabric deformation, e.g., in the case of fabric transferring through several rollers, the torque motor can ensure the torque to be constant and the tension of fabric to be constant under any speed. For example, when the fabric is passed through several roller shafts, the torque motor can ensure constant torque and constant tension of the fabric at any speed.
- Wire and cable manufacturing: In the winding and unwinding process of wire and cable, the motor is required to automatically adjust the output torque with the change of the reel diameter to keep the tension of the cable constant, the soft mechanical characteristics and wide speed range of the torque motor make it very suitable for this application scenario, which can avoid the cable from being too loose or too tight, and ensure the production quality.
- Paper-making machinery: paper needs to maintain a certain tension and line speed during the production process, the torque motor can accurately control the tension of the paper, to ensure that the paper in the copying, coating, slitting and other parts of the smooth progress, and can adapt to the requirements of different paper thickness and width.
- Metal Processing: For example, metal sheet winding, uncoiling and other processing processes require motors to provide stable torque and precise speed control, torque motors can meet these requirements and improve processing accuracy and product quality.
- Printing machinery: in the printing process, the transportation, winding and driving of the printing cylinder all need precise torque and speed control, and the torque motor can ensure the register precision and quality stability of the printed products.
In summary, asynchronous motors and torque motors in the working principle, structural characteristics, performance characteristics and application scenarios, there are obvious differences. Asynchronous motor has simple structure, low cost, reliable operation, and is suitable for a variety of different loads and working environments; torque motor has soft mechanical characteristics, wide speed range, high precision, and is suitable for occasions that require precise control of torque and speed. In practical application, the appropriate motor should be selected according to the specific needs in order to give full play to its performance advantages and improve production efficiency and product quality.