According to statistics, the classification of AC motors can be done in the following two ways:
By functionAC motors are generally classified into alternators, AC motors, and synchronous cameras according to their functions. Due to the reversibility of the working state of the motor, the same motor can be used as both a generator and an electric motor.
It is not very clear that the motor is divided into a generator and a motor. However, some motors mainly operate as generators, and some motors mainly operate as motors.
By varietyAC motors are divided into two categories: synchronous motors and asynchronous motors. The rotational speed ns of the synchronous motor rotor is the same as the rotational speed of the rotating magnetic field, and is called synchronous rotational speed. There is a strict relationship between ns and the frequency of the connected alternating current (f) and the number of magnetic pole pairs (P) of the motor.
Ns=60f/P
In China, the power frequency is 50 Hz, so the synchronous speed of the one-pole motor in the three-phase AC motor is 3000 rpm, and the synchronous speed of the two-pole motor in the three-phase AC motor is 1500 rpm, and so on. The speed of the asynchronous motor rotor is always lower or higher than the rotational speed of its rotating magnetic field, and the name of the asynchronous is thus derived. The difference between the rotor speed of an asynchronous motor and the rotational magnetic field speed (called the slip) is usually within 10%.
Frequently Asked Questions 1. Why does the motor generate shaft current?The shaft of the motor---bearing---the current in the base circuit is called the shaft current.
Reasons for shaft current generation:
(1) Asymmetry of the magnetic field;
(2) There are harmonics in the supply current;
(3) The manufacturing and installation are not good, and the air gap is not uniform due to the eccentricity of the rotor;
(4) There is a gap between the two semicircles of the detachable stator core;
(5) The number of tiles with fan-shaped stator cores is not properly selected.
harm:
The surface of the motor bearing or the ball is eroded, and the shaped micro-holes are formed, which deteriorates the running performance of the bearing, increases the friction loss and heat, and ultimately causes
The bearing burned.
prevention:
(1) Eliminate the pulsating flux and power supply harmonics (such as adding an AC reactor on the output side of the inverter);
(2) When designing the motor, the bearing housing and the base of the sliding bearing are insulated, and the outer ring and the end cover of the rolling bearing are insulated.
2. Why can't general motors be used in highland areas?Altitude has an adverse effect on motor temperature rise, motor corona (high voltage motor) and DC motor commutation. The following three aspects should be noted:
(1) The altitude is high, the larger the motor temperature rises, the smaller the output power. But when the temperature rises with altitude, it is enough to compensate for the effect of altitude on temperature rise.
When the rated output power of the motor can be changed;
(2) Anti-corona measures should be taken when the high-voltage motor is used in the plateau;
(3) Altitude is unfavorable for DC motor commutation. Pay attention to the choice of carbon brush material.
3. Why should the motor not run at light load?When the motor is running at light load, it will cause:
(1) The motor power factor is low;
(2) The motor is inefficient.
It will cause equipment waste and uneconomical operation.
(1) The load is too large;
(2) lack of phase;
(3) blocked air ducts;
(4) The low speed running time is too long;
(5) The power supply harmonics are too large.
4. Why can't I start the motor in a cold environment at will?The motor is too long in a low temperature environment and will:
(1) Motor insulation cracking;
(2) Bearing grease freezes;
(3) Wire joint solder powder.
Therefore, the motor should be stored in a cold environment and the windings and bearings should be inspected before operation.
5. What are the reasons for the motor three-phase current imbalance?(1) Three-phase voltage imbalance;
(2) The welding of a certain phase branch inside the motor is poor or the contact is not good;
(3) Short circuit between motor windings or short circuit to ground and phase;
(4) Wiring error.
6. Why can't a 60Hz motor be connected to a 50Hz power supply?When designing the motor, the silicon steel sheet is generally operated in the saturation region of the magnetization curve. When the power supply voltage is constant, reducing the frequency will increase the magnetic flux, and the excitation current
The increase in flow leads to an increase in motor current and an increase in copper consumption, which ultimately leads to an increase in the temperature rise of the motor. In severe cases, the motor may be burnt due to overheating of the coil.
7. What are the reasons for the motor phase loss?Power supply:
(1) Poor contact in the switch;
(2) Transformer or line disconnection;
(3) The fuse is blown.
Motor:
(1) The motor junction box screws are loosely connected;
(2) Poor internal wiring;
(3) The motor winding is broken.
8. What are the causes of abnormal vibration and sound of the motor?Mechanical aspects:
(1) poor bearing lubrication and bearing wear;
(2) The bearing chamber is too large.
(3) The fastening screw is loose;
(4) There are debris in the motor.
Electromagnetic aspects:
(1) Motor overload operation;
(2) Three-phase current imbalance;
(3) lack of phase;
(4) Short circuit failure of the stator and rotor windings;
(5) The welded part of the cage rotor is broken by welding.
9. What are the causes of motor bearing overheating?The motor itself:
(1) The inner and outer rings of the bearing are too tight;
(2) There are problems with the shape and position tolerance of the parts, such as the coaxiality of the parts such as the base, end cover and shaft;
(3) improper selection of bearings;
(4) The bearing is poorly lubricated or the bearing is not cleaned, and there is debris in the grease;
(5) Axis current.
Use aspect:
(1) Improper installation of the unit, such as the shaft coaxiality of the motor shaft and the driven device is not satisfactory;
(2) The pulley is pulled too tightly;
(3) The bearing is not well maintained, the grease is insufficient or exceeds the service life, and the hair is deteriorated.
11. What are the reasons for the low insulation resistance of the motor?
(1) The winding is damp or has water intrusion;
(2) accumulation of dust or oil on the windings;
(3) Insulation aging;
(4) The insulation of the motor lead or terminal block is broken.
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