Introduction ot AC Motors

[Magneto]

AC Motors

The most common and simple industrial motor is the three phase AC induction motor, sometimes known as the "squirrel cage" motor. Substantial information can be found about any motor by checking its (nameplate).

Advantages
Simple Design
Low Cost
Reliable Operation
Easily Found Replacements
Variety of Mounting Styles
Many Different Environmental Enclosures
Simple Design

The simple design of the AC motor -- simply a series of three windings in the exterior (stator) section with a simple rotating section (rotor). The changing field caused by the 50 or 60 Hertz AC line voltage causes the rotor to rotate around the axis of the motor.

The speed of the AC motor depends only on three variables:
The fixed number of winding sets (known as poles) built into the motor, which determines the motor's base speed.
The frequency of the AC line voltage. Variable speed drives change this frequency to change the speed of the motor.
The amount of torque loading on the motor, which causes slip.
Low Cost

The AC motor has the advantage of being the lowest cost motor for applications requiring more than about 1/2 hp (325 watts) of power. This is due to the simple design of the motor. For this reason, AC motors are overwhelmingly preferred for fixed speed applications in industrial applications and for commercial and domestic applications where AC line power can be easily attached. Over 90% of all motors are AC induction motors. They are found in air conditioners, washers, dryers, industrial machinery, fans, blowers, vacuum cleaners, and many, many other applications.
Reliable Operation

The simple design of the AC motor results in extremely reliable, low maintenance operation. Unlike the DC motor, there are no brushes to replace. If run in the appropriate environment for its enclosure, the AC motor can expect to need new bearings after several years of operation. If the application is well designed, an AC motor may not need new bearings for more than a decade.
Easily Found Replacements

The wide use of the AC motor has resulted in easily found replacements. Many manufacturers adhere to either European (metric) or American (NEMA) standards. (For Replacement Motors)
Variety of Mounting Styles

AC Motors are available in many different mounting styles such as:
Foot Mount
C-Face
Large Flange
Vertical
Specialty
Many Different Environmental Enclosures

Because of the wide range of environments in which people want to use motors, the AC motor has been adapted by providing a wide range of enclosures:
ODP - Open Drip Proof
TEFC - Totally Enclosed Fan Cooled
TEAO - Totally Enclosed Air Over
TEBC - Totally Enclosed Blower Cooled
TENV - Totally Enclosed Non-Ventilated
TEWC - Totally Enclosed Water Cooled

Disadvantages


Expensive speed control
Inability to operate at low speeds
Poor positioning control
Expensive speed control

Speed control is expensive. The electronics required to handle an AC inverter drive are considerably more expensive than those required to handle a DC motor. However, if performance requirements can be met -- meaning that the required speed range is over 1/3rd of base speed -- AC inverters and AC motors are usually more cost-effective than DC motors and DC drives for applications larger than about 10 horsepower, because of cost savings in the AC motor.
Inability to operate at low speeds

Standard AC motors should not be operated at speeds less than about 1/3rd of base speed. This is due to thermal considerations. A DC motor should be considered for these applications.
Poor positioning control

Positioning control is expensive and crude. Even a vector drive is very crude when controlling a standard AC motor. Servo motors are more appropriate for these applications.