ALTERNATING CURRENT (AC) ELECTRIC MOTOR

An alternating current (AC) electric motor is a type of electric motor that uses alternating current (AC) directly from the grid supply to produce rotational motion. The alternating current (AC) electric motor consist of two basic parts, an  external stator supplied with alternating current to produce a magnetic field and an internal rotor connected to an output shaft producing a second magnetic field. The stator is stationary and consists of a set of coils wound around a magnetic core. While the rotor is the rotating part which can be either a squirrel cage rotor or a slip ring rotor.

There are various types of alternating current (AC) electric motors and they are; induction, synchronous and wound rotor alternating current (AC) motor.

Induction motors are the most common type of alternating current (AC) electric motor. These types of motors use electromagnetic induction to produce rotational motion.

Induction motor characteristics are: They have a simple and rugged construction, low cost. They are reliable and easy to maintain. They have a high starting torque and speed control is achieved using a variable frequency drives (VFD).

They find application as motor of choice for industrial machinery such as pumps, compressor and fans, house hold appliances such as refrigerators, air conditioners and washing machines, power tools such as drills, saws, sanders etc.

Synchronous motors are alternating current (AC) electric motors that operate at synchronous speeds which is determined by the frequency of the alternating current (AC) supply.

Synchronous motors characteristics are: They have a high efficiency especially a high loads. They have a high power factor with the accompanied reduction in energy loss and improved efficiency. They also have precise speed control using advanced control system. They have relatively low vibration in their operation.

They have widespread applications as motor of choice for power generation such as hydroelectric and thermal power plants, industrial machinery such as motors for paper mills, textile mills, cement plants etc.

Wound rotors are alternating current (AC) motors that have a rotor with windings which are connected to a power source through slip rings.

The characteristics of wound rotor motors are: They have a high starting torque. Their speed can be controlled using external resistance. They also have a high efficiency especially at high loads and low vibration in their operation.

They have found applications as the motor of choice for industrial machinery such as textile mills, paper mills, cement plants, cranes, and hoist and in applications where a high starting torque is desirable.

Alternating current (AC) motors are also classified as single phase, three phase and multiphase or polyphase motors.

Single phase alternating current motors use one phase of the alternating grid supply as input. They have less power , less efficiency especially at  a high load and they often require a starting capacitor or other starting methods.

Three phase alternating current motors use three phase from the alternating current grid supply as input. They are more powerful, efficient and have a wide range of operating speeds than single phase motors. They also have a high starting torque and do not require at external mean to start.

Polyphase or multiphase motors use more than three phase as alternating current input. These motor are used for specialized applications where high power, high reliability and increased flexibility in terms of speed control are desired.

The advantages of alternating motors in general are as follows: alternating current motors require less maintenance comparatively. They are highly efficient with efficiencies ranging from 80% to 90% and they can operate for long periods without failure or need for maintenance. They are very cost effective compared to direct current (DC) motors.

Their disadvantages are: They require complex control systems which is expensive. They are sensitive to voltage fluctuations which can affect their performance and efficiency. Alternating current (AC) motors have limited control range as their speed is determined by the frequency of the alternating current supply.

The future of alternating current (AC) motors is based on the trends and development in the following technologies: Development of new advanced material and design techniques will increase their power density as well as efficiency. The development of smart alternating current (AC) motors that can communicate with other systems in real time will enable real time monitoring and control. The increased use of alternating current motor especially for electric vehicles and renewable energy will trigger research and developments. The developments in multiphase or polyphase alternating current motors will ultimately offer motors with improved efficiency, reduced harmonics and improved reliability compared to traditional three phase alternating current motors. Also we expect in the near future a more suitable and less expensive speed control drive with a wider speed range will become the norm.

 

SOURCES:

• Basics of AC motor by Siemens.
• Electric motors and Drives: Fundamentals, types and applications by Austin Hughes.
• AC motors and Drives by Thomas W. Bartlett.
• Electrical machines, Drives and power systems by Theodore Wildi.
• Electric motor control: DC, AC, and Brushless Motors by Irving L. Kosow.