November 2008

Magnetism gives life to motion, motors

By Dave Polka

The squirrel-cage induction motor is probably the most widely used motor in industry today.

Traditional applications for AC induction motors include fans and pumps. The AC induction motor has been widely accepted in many demanding industrial applications, compared with the DC motor, because less maintenance is required.

It is quite common to find AC motors in applications such as compressors, machine tools, conveyors, mixers, crushers, ski lifts, and extruders.

With its efficient operation and energy savings characteristics, the AC induction motor will increase in prominence throughout the next several decades.

All AC motors can be classified into single-phase and polyphase motors (poly meaning many-phase, or three-phase). Because polyphase motors are the most commonly used in industrial applications, we will take a closer look at these units.

Keep in mind there are also single-phase AC motors in use for applications such as small appliances, residential fans, furnaces, and many other low-horsepower applications.

For industrial applications, however, mainly three-phase induction motors are the choice. The main advantage of using three-phase motors is efficiency. Three-phase motors are much simpler in construction than other types and require less maintenance.

A more powerful motor can build into a smaller frame compared with a single-phase motor. The three-phase motor will operate at a higher efficiency compared with the single-phase motor.

There are several types of polyphase motors: induction, wound rotor, and synchronous. The most common type of motor in this group is the squirrel cage induction motor. We will use this motor type as the basis for understanding the general AC motor principles.

AC induction motor

The main parts in an AC induction motor are the rotor (the rotating element) and the stator (stationary element that generates the magnetic flux).

The rotor construction looks like a squirrel cage, hence the traditional name: squirrel cage induction motor. The squirrel cage motor is the simplest to manufacture and the easiest to maintain.

As to operation, the three-phase current produces a rotating magnetic field in the stator. This rotating magnetic field causes a magnetic field to be set up in the rotor also. The attraction and repulsion between these two magnetic fields causes the rotor to turn.

The squirrel cage motor is a constant-speed motor with either a normal or a high starting torque. These characteristics fulfill the requirements of the majority of industrial applications.

Each magnetic pole pair is wound in such a way that allows the stator magnetic field to "rotate." The stator of the motor consists of groups of coils wound on a core, which a frame encloses.

A simple two-pole stator has three coils in each pole group. (A two-pole motor would have two poles × three phases = six physical poles.) Each coil in a pole group connects to one phase of the three-phase power source.

One characteristic of three-phase power is the phase current reaches maximum value at different time intervals.

The magnetic fields one of these coils, phase "A" coils for example will be at almost maximum value. At the same instant, the currents of phase "B" are at zero and phase "C" currents are slightly more than "A."

At a later instant in time, the current in the "B" coils is close to maximum with consequent maximizing of the magnetic field of the "B" coils. At this same instant, the field of the "C" phase is slightly less than maximum. The "A" coil fields are at zero value.

This same process repeats as the magnetic field of each of the phases reaches maximum, all at different times (different degrees of magnetic field rotation). The maximum field thus sequentially repeats at "A," "C," and "B" continuously around the stator and essentially defines a rotating magnetic field.

The wound coils of the stator are such that they are diametrically opposite coils. This means they carry the same phase current but connect so their magnetic fields are of opposite polarity. Thus, the motor turns and powers equipment functions.

magnetism

magnetism2

 

ABOUT THE AUTHOR

Dave Polka (dave.polka@us.abb.com) has a BS in industrial education. He is the Training Center Manager for ABB Drives & Power Electronics in Wisconsin. His book is Motors & Drives: A practical technology guide, ISA Press, 2003.