An asynchronous motor, also known as an induction motor, is an AC motor that converts electromechanical energy into mechanical energy by generating electromagnetic torque through the interaction of a rotating magnetic field in the air gap and the induced current in the rotor windings. Asynchronous motors are classified into two types according to their rotor structure: squirrel-cage asynchronous motors and wound-rotor asynchronous motors. The development of asynchronous motors has been rapid; for asynchronous motors of the same size, the rated power increased from 5.5kW in 1897 to 74.6kW in 1976. An asynchronous motor is an AC motor, and its speed under load is not a constant ratio to the frequency of the connected power grid. Asynchronous motors have high operating efficiency and good working characteristics, operating at near constant speed from no-load to full-load, and can meet the transmission requirements of most industrial and agricultural production machinery.
Structural principle diagram of an asynchronous motor
An asynchronous motor is an AC motor whose speed under load is not a constant ratio to the frequency of the connected power grid. It varies with the load size. The greater the load torque, the lower the rotor speed. Asynchronous motors include induction motors, doubly-fed asynchronous motors, and AC commutator motors. Induction motors are the most widely used, and in cases where this will not cause misunderstanding or confusion, they are generally referred to as asynchronous motors .
An asynchronous motor gets its name from the fact that its rotor speed is less than the speed of the rotating magnetic field. Its basic principle is based on electromagnetic induction, and its main structure consists of two parts: a stator and a rotor.
1. What is an asynchronous motor?
The stator winding of a typical asynchronous motor is connected to the AC power grid, while the rotor winding does not require connection to any other power source. Therefore, it boasts advantages such as simple structure, ease of manufacturing, use, and maintenance, reliable operation, small size, and low cost. Asynchronous motors have high operating efficiency and good working characteristics, operating at near constant speed from no-load to full-load, meeting the transmission requirements of most industrial and agricultural machinery. Asynchronous motors are also easy to derive into various protection types to adapt to different environmental conditions. However, when an asynchronous motor is running, it must draw reactive excitation power from the power grid, which degrades the power factor of the grid. Therefore, synchronous motors are often used for driving high-power, low-speed machinery such as ball mills and compressors. Because the speed of an asynchronous motor has a certain slip relationship with the speed of its rotating magnetic field, its speed regulation performance is relatively poor (except for AC commutator motors). For transportation machinery, rolling mills, large machine tools, printing and dyeing machinery, and papermaking machinery requiring a wider and smoother speed regulation range, DC motors are more economical and convenient. However, with the development of high-power electronic devices and AC speed control systems, the speed control performance and economy of asynchronous motors suitable for wide speed ranges are now comparable to those of DC motors.
2. Basic Structure of Asynchronous Motors
The basic structure of a three-phase asynchronous motor can be divided into two main parts: the stator and the rotor.
(1) Stator of asynchronous motor
The stator consists of three parts: the frame, the stator core, and the stator windings.
The stator core is part of the electric motor's magnetic circuit and is housed in the frame. To reduce iron losses in the stator core, it is made of laminated silicon steel sheets, with insulating varnish applied to both sides of the sheets. The diagram below shows stator slots, where (a) is an open slot used in large and medium-capacity high-voltage asynchronous motors; (b) is a semi-open slot used in medium-sized asynchronous motors below 500V; and (c) is a semi-closed slot used in low-voltage small asynchronous motors.
The stator winding is made of insulated copper (or aluminum) wire and is embedded in the stator slot.
The base serves to fix and support the machine.
(2) Asynchronous motor rotor
The rotor consists of a rotor core, rotor windings, and a shaft.
The rotor core is part of the electric motor's magnetic circuit, and it is made of laminated silicon steel sheets. The core is fixed to the shaft or rotor support, and the entire rotor has a cylindrical shape.
Rotor windings are divided into two types: cage type and wound type.
A squirrel-cage winding is a self-short-circuited winding. A conductor is placed in each slot of the rotor and connected to the two ends of the iron core with end rings to form a short-circuited winding. If the rotor iron core is removed, the remaining winding can be seen to resemble a squirrel cage, as shown in Figure (a), hence the name squirrel-cage rotor. The conductor bars are made of either copper or aluminum.
If copper is used, pre-made bare copper bars need to be inserted into the slots on the rotor core, and then copper end rings are used to fit the extended ends of the copper bars, and finally welded together, as shown in Figure (b). If cast aluminum is used, the end rings and fan are cast together in one piece, as shown in Figure (c). The squirrel-cage rotor has a simple structure, is easy to manufacture, and is an economical and durable motor, so it is widely used.
A wound rotor has three-phase windings made of insulated wires embedded in its slots, typically connected in a Y configuration. The three leads of the rotor windings are connected to three slip rings and then led out using a brush assembly, as shown in the figure. This allows an external resistor to be connected in series with the rotor winding circuit to improve the motor's starting performance or regulate its speed.
Compared with squirrel-cage rotors, wound rotors have a slightly more complex structure and are slightly more expensive. Therefore, they are only used in applications that require low starting current, high starting torque, or smooth speed regulation.
3. Working principle of asynchronous motor
When a three-phase asynchronous motor is connected to a three-phase AC power supply, the three-phase stator windings generate a three-phase magnetomotive force (stator rotating magnetomotive force) through which three-phase symmetrical current flows, and a rotating magnetic field is generated.
The rotating magnetic field has a relative cutting motion with the rotor conductor. According to the principle of electromagnetic induction, the rotor conductor generates an induced electromotive force and an induced current.
According to the law of electromagnetic force, a current-carrying rotor conductor is subjected to electromagnetic force in a magnetic field, forming electromagnetic torque, which drives the rotor to rotate. When the motor shaft is loaded with mechanical load, it outputs mechanical energy.
The structure and principle of asynchronous motors are very simple, they are inexpensive to manufacture and easy to maintain, so asynchronous motors are widely used and play a crucial role in people's daily life and industrial manufacturing.
Illustrated guide to disassembling and installing a three-phase asynchronous motor
Disassembly, assembly, and repair of three-phase asynchronous motors. The disassembly, assembly, and repair of asynchronous motors is an important part of electrical maintenance training and a key skill training area commonly emphasized in technical schools. This paper presents and analyzes the skill training requirements for disassembly and assembly of three-phase squirrel-cage asynchronous motors, including the identification of the start and end points of the stator windings and quality inspection.
First, it is essential to understand the nameplate of the asynchronous motor and be familiar with its basic structure. Preparing for disassembly is a crucial step in the process of assembling and disassembling an asynchronous motor. You must be well-informed and avoid acting blindly.
Secondly, it is essential to correctly master the use of disassembly and assembly tools and instruments, such as hammers, copper rods, pullers, wrenches, megohmmeters, multimeters, and other similar tools.
Third, it is essential to master safe operating procedures. This requires training in the safe use of electrical equipment, safe operating procedures for fitters, and relevant experience in fire prevention and explosion protection to ensure the safe and successful completion of skills training and to guarantee the safety of personnel, equipment, and property.
Fourth, special attention should be paid to the requirements for disassembling and assembling the motor installed on the equipment: (1) The power supply should be cut off first, and the connection between the motor and the three-phase power line should be removed. The phase sequence of the power line should be marked and the insulation treatment should be done. (2) When disassembling the connection between the motor and the frame, pulley, and coupling, the corresponding positioning marks should be made first to ensure that the motor is safely separated from the main equipment. (3) The loosening and tightening of the end cover screws must be rotated in the order of diagonal, up, down, left and right. (4) When hoisting the rotor of a large motor, the symmetrical balance steel wire rope should be used. Wooden pads should be laid on the ground. When slowly moving the rotor out, the action should be careful. Push and guide at the same time to prevent scratching the stator winding and rotor winding. (5) The fan cover, fan blade, end cover, bearing, and rotor should be disassembled, cleaned, inspected and replaced in sequence.
When performing regular maintenance, repairs, and overhauls of a motor, the first step is to disassemble and reassemble it. Improper disassembly and reassembly methods can damage some components and cause new malfunctions. Therefore, correct disassembly and reassembly of the motor is a prerequisite for ensuring the quality of repairs. When learning motor repair, the correct disassembly and reassembly techniques should be learned first.
Disassembly of a three-phase asynchronous motor
1. Preparations before disassembly
(a) Disconnect the power supply, disconnect the motor from the power supply, and mark the corresponding power supply wires to avoid confusing the phase sequence when restoring. Insulate the ends of the power supply wires.
(b) Prepare all disassembly tools, especially special tools such as pullers and sockets.
(c) Be familiar with the structural characteristics and disassembly/reassembly techniques of the motor being disassembled.
(d) Measure and record the distance between the coupling or pulley and the shaft.
(e) Mark the phase sequence of the power cord in the junction box, the direction of the motor shaft output, and the exit direction of the lead wire on the frame.
2. Disassembly steps
The disassembly steps are briefly described with reference to Figure 1:
(a) Remove the pulley or coupling and remove the fan cover at the tail of the motor.
(b) Remove the positioning key or screws and remove the fan.
(c) Unscrew the front and rear end cover fastening screws and remove the front bearing outer cover.
(d) Place a wooden board at the front end of the shaft and use a hammer to knock the rotor and the rear end cover out of the stop.
(e) Remove the rotor.
(f) Insert the wooden block into the stator core to hold the front end cover in place, then use a hammer to strike the wooden block to remove the front end cover. Finally, disassemble the front and rear bearings and the inner cover of the bearings.
3. Disassembly methods for major components
(a) Disassembly of the pulley (or coupling): First, mark the dimensions on the shaft extension end (coupling end) of the pulley (or coupling). Then, loosen the fixing screws on the pulley or knock off the locating pin. Add kerosene to the junction of the inner hole of the pulley (or coupling) and the shaft. After a short while, allow the rusted parts to loosen. Then, use a puller to slowly pull out the pulley (or coupling), as shown in Figure 2. If it cannot be pulled out, use a blowtorch to heat the outer bushing of the pulley with a strong flame. When heating, wrap the shaft with asbestos or a damp cloth and continuously pour cold water onto the shaft to prevent it from expanding along with the outer bushing, which would affect the pulling out of the pulley.
Note: The heating temperature should not be too high, and the heating time should not be too long, to prevent deformation.
(a) Marking the position of the pulley (b) Removing the pulley with a puller
Figure 2 Disassembling the pulley
(b) Bearing removal: The following three methods can be used to remove the bearing.
When using a puller for disassembly, the puller's claws must grip the inner ring of the bearing firmly to avoid damaging the bearing, as shown in Figure 3.
To disassemble the bearing, align the copper rod with the inner ring of the bearing and strike the copper rod with a hammer, as shown in Figure 4. When using this method, be careful to strike opposite sides of the inner ring of the bearing alternately, avoiding striking only one side, and do not apply excessive force until the bearing is disassembled.
Until then.
When disassembling the bearing inside the end cover, the method shown in Figure 5 can be used. Place the end cover with the stop facing upwards and place a wooden board under the outer ring of the bearing, but do not support the bearing. Then, use a copper rod or other metal tube with a diameter slightly smaller than the outer edge of the bearing to hold the outer ring of the bearing, and tap it from top to bottom with a hammer to make the bearing come out from below.
The bearing inner ring is clamped between two thick iron plates for disassembly. The ends of the plates are supported by reliable supports, suspending the rotor in the air.
As shown in Figure 6, a thick wooden board is then placed on the upper end face of the shaft and hammered to dislodge the bearing.
Figure 6 shows the iron plate supporting the disassembly of the bearing (c) and the removal of the rotor: Before removing the rotor, thick cardboard should be placed under the air gap and the winding ends of the rotor to prevent damage to the core and windings when removing the rotor. For small motors, the rotor can be removed directly by hand. Hold the shaft with one hand and pull the rotor out a little, then support the rotor core with the other hand and gradually move it outward, as shown in Figure 7.
When disassembling larger motors, two people can work together, each holding one end of the shaft and gradually moving the rotor outwards.
Figure 7 Disassembly of the rotor of the small motor
If the shaft is too long and one end is difficult to exert force, a metal tube can be fitted onto the shaft to act as a dummy shaft, making it easier to exert force.
As shown in Figure 8.
Large motors must be lifted out using lifting equipment, as shown in Figure 9.
Disassembly of a single-phase asynchronous motor
Because single-phase asynchronous motors are simpler in structure, lighter, and smaller than three-phase asynchronous motors, anyone who can disassemble a three-phase motor can usually disassemble a single-phase motor. Only single-phase motors with a start switch are relatively more complex to disassemble; therefore, care must be taken not to damage the start switch during disassembly.
