Ac induction motors are the workhorses of industry. They are rugged, inexpensive, and easy to maintain. Two things designers should know about ac induction motors are that they are not inherently capable of variable-speed operation, nor are they true constant-speed machines.

The first limitation is addressed with power electronics. The second, due to slip (the difference between actual and synchronous motor speed), requires further examination.

Q&A

Q: How are ac motors constructed?

A: An ac induction motor consists of two basic assemblies — stator and rotor. The stator is composed of steel laminations shaped to form poles. Copper wire coils wind around the poles and connect to a voltage source that produces a rotating magnetic field.

The rotor is made of laminations over a steel shaft core; radial slots around the perimeter house rotor bars (cast-aluminum or copper conductors shorted at the ends and positioned parallel to the shaft). Arrangement of the rotor bars resembles a small cage, from which the term “squirrel-cage” induction motor derives.

Q: Why is motor slip necessary?

A: Slip is necessary for generating load-moving torque. Motor torque develops from current in the rotor bars interacting with the stator's rotating magnetic field. In actual operation, rotor speed always lags stator speed, thereby allowing the rotor bars to cut magnetic lines of force and produce useful torque. This speed difference is known as slip speed. The greater the load to be moved, the greater the slip.

Q: On what does slip depend?

A: Slip depends on synchronous and actual speed and is expressed by the equation

For small values, slip is proportional to rotor resistance, stator voltage frequency, and load torque — and is inversely proportional to the square of the supply voltage. Increasing slip is a way to control a wound rotor's speed, which involves adding resistance in the rotor circuit. The slip of low-horsepower motors is greater than those of high-horsepower, due to higher rotor winding resistance. Typically, small and low-speed motors have higher relative slip.

Q: Are there any good solutions?

A: An adjustable-speed ac drive is the best solution. Implementing sectional drive line-ups with separate inverters for each motor eliminates speed errors caused by slip. Then, these inverters can be connected to a dc voltage bus bar (supplied by a common rectifier) for an energy-efficient solution that powers sections of machinery with regenerated braking energy from decelerating sections.

Slip compensation can also be added to ac drives to reduce motor slip. Adding a load-torque signal to the speed controller increases output frequency proportionally to the load. Though compensation cannot account for 100% of slip (since rotor temperature variations can cause over-compensation and unstable control), it can achieve accuracies up to 80%, reducing slip from 2.4% to about 0.5%.

This month's handy tips provided by Mauri Peltola. For more information, call Ken Graber at 262-780-3873 or visit ABB Drives and Power Electronics' website at www.abb.com.

Squirrel cage ac induction motor

Three-phase motors with windings spaced 120° apart are standard for industrial, commercial, and residential use. The name induction motor comes from alternating current induced into the rotor via the rotating magnetic flux.

Rotating magnetic field

Rotor voltage induction occurs when the rotor's speed is less (more during braking) than the flux rotation's speed. This speed difference is slip.

Torque-speed curve

The speed curve of an induction motor shows that slip is the difference in rotor speed relative to the synchronous speed. Specifically, CD = AD - BD = AB.