随州Advantages and disadvantages of three-phase asynchronous motors

1. Advantages of three-phase asynchronous motors

The rotor speed of three-phase asynchronous motor is lower than the speed of rotating magnetic field, and the rotor winding generates induced electromotive force and current due to the relative movement between the rotor winding and the magnetic field, and interacts with the magnetic field to generate electromagnetic torque to realize energy conversion. Compared with single-phase asynchronous motors, three-phase asynchronous motors have better operating performance and can save various materials. According to the different rotor structures, three-phase asynchronous motors can be divided into cage type and winding type. The asynchronous motor with cage rotor has a simple structure, reliable operation, light weight, and low price, which has been widely used. Its main drawback is the difficulty in speed regulation. The rotor and stator of a wound three-phase asynchronous motor are also equipped with three-phase windings and connected to an external rheostat through slip rings, brushes. Adjusting the resistance of the rheostat can improve the starting performance of the motor and adjust the speed of the motor.

2. Disadvantages of Asynchronous Motors

2.1 Cage induction motors have the following three main drawbacks.

(1) The starting torque is not large, making it difficult to meet the needs of starting with load. The majority of solutions to this problem in current society are to increase the power capacity (i.e., increase capacity) of the electric motor to increase its starting torque, which leads to serious "big horse pulling small car" problems. This not only increases the investment in purchasing equipment, but also wastes a large amount of electricity due to low load operation in long-term applications, which is very uneconomical. The second method is to purchase a hydraulic coupling, which starts the motor with no load and then drives the load through the hydraulic coupling. This method also requires an increase in investment in purchasing equipment, and because the efficiency of the hydraulic coupling is lower than 97%, at least 3% of the electrical energy is wasted. Therefore, the efficiency of the entire drive device is very low, and electricity is also wasted. Moreover, after adding the hydraulic coupling, the operational reliability of the unit greatly decreases, significantly increasing maintenance difficulties, and therefore is not a good method.

(2) Large torque is not large and is used to drive loads that often experience short-term overload, such as crushers used in mines, which often stop and burn out the motor. So that it can only operate under light load conditions, which not only reduces production but also wastes electrical energy.

(3) The high starting current increases the capacity of the required power supply transformer, thereby increasing a significant investment. Another approach is to use step-down starting to reduce the starting current, while also increasing the investment in purchasing step-down devices and further worsening the already poor starting characteristics.

2.2 Wound type induction motor

When the winding induction motor operates normally, the three-phase winding is short-circuited through the collector ring. When starting, in order to reduce the starting current, a starting resistor can be connected in series to the rotor, and an appropriate resistor can be connected in series to the rotor. This not only reduces the starting current, but also increases the starting torque due to the increase in the rotor power factor and active component of the rotor current. This type of motor can also be adjusted for speed by changing the external resistance. Although the starting and operating characteristics of wound type motors are both excellent, they still have the following drawbacks:

(1) Due to the presence of a collector ring and brush on the rotor, not only does it increase manufacturing costs, but it also reduces the reliability of starting and running. The sliding contact between the collector ring and brush is the main reason for the failure of this type of motor. Especially sparks can be generated between the collector ring and the brush, which limits the application range of traditional wound motor in places with explosion-proof requirements such as mines, underground mines, petroleum, and Huagong, and even in places with high dust and ash concentrations.

(2) In order to improve reliability, most traditional wound type motors do not use brushes, so there is a waste of electrical energy during operation: friction loss between the collector ring and the brush, electrical loss on the contact resistance, and electrical loss on the three cables between the brush and the control cabinet short circuit switch. If the distance between the electric machine and the control cabinet is very long, this loss will be very serious. Moreover, due to the generation of carbon powder, sparks, and noise from the collector ring and brushes, long-term pollution of the surrounding environment can harm the health of management personnel and surrounding residents.

(3) The starting torque of traditional wound motor is slightly higher than that of cage motor, but it often cannot meet the needs of full load starting, and even needs to increase capacity to form a "big horse pulling small car".

The fundamental reason for the serious shortcomings of the traditional induction motor mentioned above is the difficult to reconcile contradiction between "starting", "running", and "reliability". Therefore, it is inevitable to consider one and lose the other, and cannot be both superior.

Starting method of three-phase asynchronous motor

The three-phase AC asynchronous motor is directly started. Although the control circuit structure is simple and easy to use and maintain, the starting current is large (about 4-7 times the normal working current). If the power supply capacity is not many times larger than the motor capacity, the starting current may significantly affect the normal operation of other electrical equipment in the same power grid. Therefore, for squirrel cage asynchronous motors, methods such as stator series resistance (reactance) step-down starting, stator series autotransformer step-down starting, and star delta step-down starting can be adopted; For wound asynchronous motors, rotor series resistance starting or rotor series frequency sensitive varistor starting can also be used to limit the starting current.

Direct Start

Definition: Direct start refers to using a knife switch or contactor to directly connect the stator winding of the motor to the AC power supply, and the motor is directly started at rated voltage.

Advantages: When the transformer capacity allows, the squirrel cage asynchronous motor should be started directly at full voltage as much as possible. The control circuit is simple, which can improve the reliability of the control circuit and reduce the maintenance workload of the electrical equipment.

Disadvantages: The starting current for direct starting can generally reach 4-7 times the rated current. Excessive starting current can reduce the lifespan of the motor, significantly reduce the secondary voltage of the transformer, reduce the starting torque of the motor itself, and even prevent the motor from starting. Excessive current can also cause fluctuations in the power supply voltage, affecting the normal operation of other equipment in the same power supply network.

Zhejiang Jinsu Motor Co., Ltd. is an enterprise that manufactures various types of DC, AC, and three-phase asynchronous motors. The company covers a total area of over 30 acres and currently employs more than 150 employees; Having a modern garden style factory building and introducing excellent and efficient production and testing equipment from both domestic and international sources, we aim to create a sustainable manufacturing enterprise that provides customers with excellent products and services. As a modern enterprise that integrates scientific research, production, sales, and service, we are committed to building a sustainable business.