What Is an Induction Motor?

Induction motor ^[1] , also known as "asynchronous motor", is a device that puts the rotor in a rotating magnetic field and obtains a rotating torque under the action of the rotating magnetic field, so the rotor rotates. The rotor is a rotatable conductor, usually in the shape of a squirrel cage. Invented by electrical engineer Nikola Tesla in 1887. The entry introduces the concept, inventor, working principle, basic structure, working method, braking method, asynchronous characteristics, specifications and fault inspection of the induction motor.

Induction motor is also called "asynchronous motor", that is, the rotor is placed in a rotating magnetic field.

Nikola Tesla (July 10, 1856 ~ January 7, 1943),

Through the relative movement of the rotating magnetic field generated by the stator (its rotational speed is synchronous rotational speed n ₁ ) and the rotor winding, the rotor winding cuts the magnetic induction line to generate an induced electromotive force, so that an induced current is generated in the rotor winding. The induced current in the rotor winding interacts with the magnetic field to generate electromagnetic torque and rotate the rotor. As the rotor speed gradually approaches the synchronous speed, the induced current gradually decreases, and the electromagnetic torque generated correspondingly decreases. When the asynchronous motor works in the motor state, the rotor speed is smaller than the synchronous speed. In order to describe the difference between the rotor speed n and the synchronous speed n ₁ ,

Basic structure of single-phase asynchronous motor

A single-phase asynchronous motor is a motor that requires only a single-phase AC power supply ^[2]

1 Starting method of asynchronous motor ^[4]

Electric braking method of three-phase induction motor ^[6]

1. Generally speaking, small asynchronous motors refer to induction-type asynchronous motors. This motor uses auxiliary coils and capacitors not only during startup, but also during operation. Although the starting torque is not very large, it has a simple structure, high reliability and high efficiency ^[7]

project	matter
Insulation resistance	Insulation resistance between room temperature metal resistance and motor housing exceeds 100M
Insulation pressure	Voltage between normal temperature metal winding and motor case
The temperature rises	There is no problem when inputting at 1500V, 50 / 60Hz for 1min.
Insulation class	Type E (120 ° C)
Overheating protection device	Open (120 ± 5 ); return (77 ± 5 )
Operating temperature	-10 ~ 50 (UL CE standard motor use temperature is -10 ~ 40 )

(1) Inspection and repair of reasons why the motor does not turn after power is turned on

Induction engine

The motor does not turn after it is powered on. Generally there are the following reasons ^[8] :

1) The wiring of control equipment is wrong;

2) The setting value of the overcurrent relay adjustment is too small;

3) The power is not turned on, such as the fuse is blown, the switch is faulty or the contact is poor, and the lead wire is open;

4) The connection between the power supply and the motor is faulty;

5) The motor winding is faulty, such as short-to-phase, grounding, wrong wiring, open circuit, etc .;

6) Wrong-type rotor asynchronous motor start misoperation or start resistance is too small;

7) The motor bearing is faulty and stuck;

8) The rubbing of the stator and rotor cores (sweeping) is equivalent to an excessive load;

9) The motor is overloaded or the mechanical rotation part is stuck.

2) Motor overheating inspection and repair

It is found that the overheating of the motor in normal operation is generally due to the following reasons:

1) The power supply voltage suddenly rises and does not match the rated voltage of the motor nameplate, or the three-phase power supply voltage is seriously unbalanced;

2) The load dragged by the motor varies greatly, and the motor is temporarily overloaded;

3) Due to bearing failure or gap wear exceeding the limit, the shaft is bent, the core is overheated and deformed, and the rotor is axially oscillated.

4) Environmental dust enters the inside of the motor and adheres to the insulation surface and blocks the cooling air ducts and cooling ducts, which makes the motor poorly ventilated, the cooling effect is greatly reduced, and the motor overheats;

5) The motor cooling device fails, the air temperature adjustment device is faulty, causing the motor to overheat;

6) Single-phase operation of three-phase motors;

7) The winding is faulty, such as short circuit, open circuit, grounding, wrong connection, etc .;

8) The air gap is uneven.

After rewinding, the motor overheats due to:

1) Wiring error;

2) Too many or too few coil turns;

3) The coil wire is too thin, and the coil pitch is too small or too large;

4) The assembly quality of the motor is not good, the iron cores are not aligned, the axial gap of the stator and rotor cores causes axial magnetic tension, and the air gap assembly and adjustment are uneven. As the insulation level of the motor continues to increase and the allowable temperature rise limit is also increased, a higher temperature rise of the motor casing may be normal. However, if the temperature rise of the housing and the bearing of the department is tested with an alcohol thermometer, and compared with the temperature rise allowed by the insulation level of the motor, it is confirmed that the motor is overheated, then the following steps can be performed to check ^[9] .

1) First check whether the voltage of the three-phase power supply is balanced, and whether the degree of voltage fluctuation is greater than the guaranteed value of the manufacturing plant (± 10 [%]). Due to voltage imbalance, three-phase unbalanced current is generated, which causes increased motor loss and motor heating, so it must be corrected in time. Power frequency variation also affects (± 5 [%]) motor heating, but the actual change is not large, so it is generally not considered in the analysis.

2) Check whether the motor is single-phase running, whether the contacts of the three-phase contactor are in good contact, whether the fuse of the switch is blown, and whether the wiring (single-phase) is disconnected. Take action after troubleshooting.

3) Check whether the three-phase current exceeds the rated value. If it exceeds the rated value, check the cause. When dealing with this kind of failure, we must find out the cause of the chamber: the shaft is bent; the bearing is faulty. A slight iron core bore does not affect the normal operation of the motor. When the bore is severe, you can use a turning tool to lightly cut the surface of the rotor (the general turning diameter is about 0.2mm is appropriate).

4) The dust covering the insulation affects the heat dissipation of the motor, the filter screen is blocked, the ventilation duct and the ventilation pipe are blocked, etc., will cause the motor to overheat. Therefore, dust cleaning measures can be taken to eliminate dust, and the motor must be disassembled for cleaning treatment if necessary.

5) If there is a fault in the winding, check the winding for short circuit and grounding test. According to experience, if the motor winding has a short-to-turn short circuit, the motor will vibrate, and if it is rotating for a long time, it will not be long and it will smoke. However, the inter-turn short circuit causes the motor to heat up, and the chance of continuing for a long period of time is rare.

After the rewinding and overhaul, the temperature rise of the motor exceeds the limit, which may be caused by poor insulation processing technology, incorrect coil data, wiring errors, and assembly quality. At this time, the motor should be disassembled and checked against the original records, and the correctness of the winding data should be verified.