Classification of AC motors
According to statistics, AC motors can be classified in the following two ways:
By function
AC motors are generally classified into several categories according to their function, including AC generators, AC motors, and synchronous condensers. Due to the reversibility of the motor's operating state, the same motor can function as both a generator and a motor.
It's not entirely accurate to divide motors into generators and electric motors; some motors primarily function as generators, while others primarily function as electric motors.
By variety
AC motors are broadly classified into two categories: synchronous motors and asynchronous motors. In a synchronous motor, the rotor speed ns is the same as the rotational speed of the rotating magnetic field; this speed is called the synchronous speed. ns has a strict relationship with the frequency (f) of the AC current and the number of pole pairs (P) of the motor.
ns=60f/P
In China, the power frequency is 50 Hz. Therefore, the synchronous speed of a single-pole motor in a three-phase AC motor is 3000 rpm, and the synchronous speed of a two-pole motor in a three-phase AC motor is 1500 rpm, and so on. The rotor speed of an asynchronous motor is always lower or higher than the speed of its rotating magnetic field, hence the name "asynchronous." The difference between the rotor speed and the rotating magnetic field speed of an asynchronous motor (called slip) is usually within 10%.
Frequently Asked Questions
1. Why does a motor generate shaft current?
The current in the motor's shaft-bearing housing-base circuit is called shaft current.
The cause of shaft current:
(1) Magnetic field asymmetry;
(2) There are harmonics in the power supply current;
(3) Poor manufacturing and installation, resulting in uneven air gap due to rotor eccentricity;
(4) There is a gap between the two semicircles of the detachable stator core;
(5) The number of stator core pieces stacked in a fan shape is not appropriate.
harm:
This erosion of the motor bearing surface or balls creates pinhole-like micropores, deteriorating bearing performance, increasing frictional loss and heat generation, and ultimately causing...
The bearing burned out.
prevention:
(1) Eliminate pulsating magnetic flux and power supply harmonics (e.g., by adding an AC reactor to the output side of the frequency converter).
(2) When designing the motor, the bearing housing and base of the sliding bearing are insulated, and the outer ring and end cover of the rolling bearing are insulated.
2. Why can't regular electric motors be used in high-altitude areas?
Altitude negatively impacts motor temperature rise, motor corona (in high-voltage motors), and commutation in DC motors. The following three aspects should be considered:
(1) The higher the altitude, the greater the temperature rise of the motor and the smaller the output power. However, the decrease in air temperature with increasing altitude is sufficient to compensate for the effect of altitude on temperature rise.
At this time, the rated output power of the motor can remain unchanged;
(2) Anti-corona measures should be taken when high-voltage motors are used at high altitudes;
(3) Altitude is detrimental to the commutation of DC motors, so attention should be paid to the selection of carbon brush materials.
3. Why is it not advisable for motors to operate under light loads?
When a motor is running under light load, it will cause:
(1) The motor has a low power factor;
(2) The motor has low efficiency.
This will result in equipment waste and uneconomical operation.
(1) Overload;
(2) Phase loss;
(3) Air duct blockage;
(4) The low-speed operation time is too long;
(5) Excessive power supply harmonics.
4. Why can't you start a motor in a cold environment?
If a motor is kept in a low-temperature environment for an extended period of time, it will:
(1) Cracking of motor insulation;
(2) The bearing grease has frozen;
(3) The solder of the wire joint is powdered.
Therefore, motors should be heated and stored in cold environments, and the windings and bearings should be inspected before operation.
5. What are the causes of unbalanced three-phase current in a motor?
(1) Three-phase voltage imbalance;
(2) Poor welding or poor contact in a certain phase branch inside the motor;
(3) Short circuit between turns of the motor winding or short circuit to ground or between phases;
(4) Wiring error.
6. Why can't a 60Hz motor be connected to a 50Hz power supply?
In motor design, silicon steel sheets are typically designed to operate in the saturation region of the magnetization curve. When the power supply voltage is constant, reducing the frequency increases the magnetic flux and excitation current.
Increased current leads to increased motor current and copper loss, which in turn leads to increased motor temperature rise. In severe cases, the motor may even burn out due to overheating of the coil.
7. What are the causes of phase loss in a motor?
Regarding the power supply:
(1) Poor switch contact;
(2) Transformer or line disconnection;
(3) Circuit breaker.
Regarding the motor:
(1) Loose screws in the motor junction box, resulting in poor contact;
(2) Poor internal wiring soldering;
(3) The motor winding is broken.
8. What are the causes of abnormal vibration and noise in a motor?
Mechanical aspects:
(1) Poor bearing lubrication, bearing wear;
(2) The bearing chamber is too large.
(3) The fastening screws are loose;
(4) There are foreign objects inside the motor.
Electromagnetic aspects:
(1) The motor is operating under overload;
(2) Three-phase current imbalance;
(3) Phase loss;
(4) A short circuit fault occurred in the stator and rotor windings;
(5) The welded part of the cage rotor is broken, causing the bar to break.
9. What are the causes of motor bearing overheating?
The motor itself:
(1) The inner and outer rings of the bearing are too tightly fitted;
(2) There are problems with the form and position tolerances of the parts, such as poor coaxiality of parts such as the base, end cover, and shaft;
(3) Improper bearing selection;
(4) Poor bearing lubrication or unclean bearing, with impurities in the grease;
(5) Axis current.
In terms of usage:
(1) Improper installation of the unit, such as the coaxiality of the motor shaft and the shaft of the driven device not meeting the requirements;
(2) The pulley is pulled too tight;
(3) The bearing is not well maintained, the grease is insufficient or has exceeded its service life, and it has become dry and deteriorated.
11. What are the reasons for low insulation resistance in motors?
(1) The winding is damp or water has entered;
(2) Dust or oil accumulates on the windings;
(3) Insulation aging;
(4) Damage to the insulation of the motor leads or terminal block.
