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Classification of AC motors
**Classification of AC Motors**
According to statistics, the classification of AC motors can be done in two main ways: by function and by type. These classifications help in understanding the different roles and characteristics of AC motors in various applications.
**By Function**
AC motors are generally categorized into generators, motors, and synchronous machines based on their function. Due to the reversible nature of motor operation, the same device can function as both a generator and an electric motor depending on the operating conditions. However, some motors are primarily designed to work as generators, while others are mainly used as motors.
This distinction is not always clear-cut, but it helps in identifying the primary purpose of a particular motor design.
**By Type**
AC motors are divided into two major categories: synchronous motors and asynchronous motors (also known as induction motors). In a synchronous motor, the rotor rotates at the same speed as the rotating magnetic field, which is referred to as the synchronous speed. This speed (Ns) is determined by the frequency of the power supply (f) and the number of pole pairs (P) in the motor:
$$ N_s = \frac{60f}{P} $$
In China, where the standard power frequency is 50 Hz, a three-phase motor with one pair of poles will have a synchronous speed of 3000 rpm, while a two-pole motor will operate at 1500 rpm, and so on. In contrast, the rotor speed of an asynchronous motor is always slightly less than or greater than the speed of the rotating magnetic field, and this difference is called "slip." The slip is typically within 10%.
**Frequently Asked Questions**
**1. Why does the motor generate shaft current?**
Shaft current refers to the current that flows through the motor shaft, bearing, and base circuit. It can be caused by several factors, including magnetic field asymmetry, harmonics in the power supply, poor manufacturing or installation leading to uneven air gaps, or improper stator core construction.
The effects of shaft current include damage to the bearing surface, increased friction, heat, and ultimately, bearing failure. To prevent this, measures such as eliminating harmonics and insulating the bearing housing or end cover are recommended.
**2. Why can’t general motors be used in highland areas?**
High altitude affects motor performance in three key areas: temperature rise, corona discharge (especially in high-voltage motors), and commutation in DC motors. At higher altitudes, the air is thinner, which reduces cooling efficiency, increases temperature rise, and may cause insulation issues. Anti-corona measures and proper carbon brush selection are essential for reliable operation in such environments.
**3. Why should the motor not run at light load?**
Running a motor at light load leads to low power factor and inefficiency, resulting in wasted energy and higher operational costs. Common causes include incorrect load, phase loss, blocked airflow, prolonged low-speed operation, and excessive power harmonics.
**4. Why can't I start the motor in a cold environment at will?**
Low temperatures can cause insulation cracking, freezing of bearing grease, and soldering issues at wire joints. Therefore, it’s important to store the motor properly and inspect its windings and bearings before operation in cold conditions.
**5. What are the reasons for motor three-phase current imbalance?**
Three-phase current imbalance can result from unbalanced voltage, poor internal connections, winding short circuits, or wiring errors. These issues can lead to overheating and reduced motor efficiency.
**6. Why can't a 60Hz motor be connected to a 50Hz power supply?**
Connecting a 60Hz motor to a 50Hz supply increases the magnetic flux, which can cause excessive excitation current, higher copper losses, and potential overheating of the windings. This may eventually damage the motor.
**7. What are the reasons for motor phase loss?**
Phase loss can occur due to poor switch contact, transformer or line disconnection, blown fuses, loose terminal screws, poor internal wiring, or damaged windings.
**8. What are the causes of abnormal vibration and sound in the motor?**
Abnormal vibrations and noises can be caused by mechanical issues like poor lubrication, worn bearings, loose fasteners, or debris inside the motor. Electromagnetic problems such as overload, current imbalance, or winding faults can also contribute.
**9. What are the causes of motor bearing overheating?**
Overheating of motor bearings can result from tight fits, misalignment, improper bearing selection, poor lubrication, or axial currents. Poor installation, over-tightened pulleys, or old and degraded grease can also play a role.
**10. What are the reasons for low insulation resistance in the motor?**
Low insulation resistance may be caused by moisture, dust accumulation, aging insulation, or damaged leads and terminal blocks.
