The AC induction motor is often referred to as the workhorse of the industry because it offers users simple, rugged construction, easy maintenance, and cost-effective pricing. These factors have promoted standardization and development of a manufacturing infrastructure that has led to a vast installed base of motors; more than 90% of all motors used in worldwide industry are AC induction motors.

In spite of this popularity, the AC induction motor has two basic limitations. The standard motor is neither a true constant-speed machine, nor is it inherently capable of providing variable-speed operation. Both limitations require consideration, as the quality and accuracy requirements of motor/drive applications continue to increase.

This article will explore the reason for slip and discuss ways to minimize it. In addition, it will detail the best methods now available for controlling motor speed with power electronics, including technology to minimize the negative effects of slip.

Motor slip is necessary for torque generation. An AC induction motor consists of two basic assemblies: the stator and rotor. The stator structure is composed of steel laminations shaped to form poles. Copper wire coils are wound around these poles. These primary windings are connected to a voltage source to produce a rotating magnetic field. Three-phase motors with windings spaced 120 electrical degrees apart are standard for industrial, commercial, and residential use.

The rotor is another assembly made of laminations over a steel shaft core. Radial slots around the laminations' periphery house rotor bars, which are cast-aluminium or copper conductors shorted at the ends and positioned parallel to the shaft. Arrangement of the rotor bars looks like a squirrel cage, hence the well-known term, “squirrel cage induction motor” (Photo above). The term “induction motor” comes from the alternating current (AC) that's “induced” into the rotor via the rotating magnetic flux produced in the stator.

Motor torque is developed from the interaction of currents that flow in the rotor bars and the stator's rotating magnetic field. In actual operation, rotor speed always lags the magnetic field's speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque. This speed difference is called slip speed. Slip also increases with load and is necessary for producing torque.

Slip depends on motor parameters. According to the formal definition, the slip (S) of an induction motor can be found with the following equation:

s = [(n2 - n)/ns] x 100%

where ns is synchronous speed and n is actual speed.

For small values of motor slip, the slip is proportional to the rotor resistance, stator voltage frequency, and load torque. It's inversely proportional to the second power of supply voltage. The traditional way to control the speed of a wound rotor induction motor is to increase the slip by adding resistance in the rotor circuit. The slip of low-horsepower motors is higher than those of high-horsepower motors because of higher rotor winding resistance in smaller motors.

As seen in the Table above, smaller motors and lower-speed motors typically have higher relative slip. However, high-slip large motors and low-slip small motors are also available.

You can see that full-load slip varies from less than 1% in high-hp motors to more than 5% in fractional-hp motors. These variations may cause load-sharing problems when motors of different sizes are mechanically connected. At low load, the sharing is normally not a problem, but at full load, the motor with lower slip takes a higher share of the load than the motor with higher slip.

As shown in Fig. 1 at right, the rotor speed decreases in proportion to the load torque. This means that the rotor slip increases in the same proportion.

Relatively high rotor impedance is required for good across-the-line, or full voltage, starting performance. In other words, high torque is required against low current. Low rotor impedance is also necessary for low full-load speed slip and high operating efficiency. The curves in Fig. 2 show how higher rotor impedance in motor B reduces the starting current and increases the starting torque, but it causes a higher slip than in standard motor A.

Methods to reduce slip. Synchronous, reluctance, or permanent-magnet (PM) motors can solve the problem of slip because there's no measurable slip in these three types of motors. Synchronous motors are used for very high- and low-power applications, but to a lesser extent in the medium-horsepower range, where many typical industrial applications fall. Reluctance motors are also used, but their output/weight ratio isn't very good, so they're less competitive than the squirrel cage induction motor. PM motors, which are used with electronic adjustable speed drives (ASDs) offer accurate speed control without slip, high efficiency with low rotor losses, and the flexibility to choose a very low base speed, eliminating the need for gear boxes. However, PM motors are still limited to certain special applications, mainly because of high cost and the lack of standardization.

Selecting an oversized AC induction motor is another way to reduce slip. Larger motors typically have a lower slip value to begin with, and slip gets smaller with a partial, rather than full, motor load. The disadvantage with oversizing the motor is that with a larger motor comes higher energy consumption, which increases investment and operation costs.

Adjustable speed AC drive is often the best solution. The inherent limitations of the AC induction motor can be solved with ASDs. The most common AC drives today are based on pulse-width modulation (PWM). The constant AC line voltage with 60 cycles per second from the supply network is rectified, filtered, and then converted to a variable voltage and variable frequency. When this output from the frequency converter is connected to an AC motor, it's possible to adjust the motor speed.

When using an AC drive for adjusting the motor speed, there are many applications where motor slip is no longer a problem. The speed of the motor isn't the primary control parameter. Rather, it could be the liquid level, air pressure, gas temperature, or some other controlling parameter.

High static speed accuracy and/or dynamic speed accuracy are still required in many drive applications, such as printing machines, extruders, paper machines, cranes, and elevators. There are also many machines and conveyors where speed control — between sections driven by separate motors — has to be synchronized. Instead of oversizing the motors to eliminate the speed error caused by slip, it may be better to use sectional drive line-ups with separate inverters for each individual motor. The inverters are connected to a DC-voltage bus bar supplied by a common rectifier. This is a very energy-efficient solution, because the driving sections of the machinery can use the braking energy from decelerating sections (regeneration).

Slip compensation can even be added to AC drives to further reduce the effect of motor slip. A load torque signal is added to the speed controller to increase the output frequency in proportion to the load. Slip compensation can't be 100% of the slip because of rotor temperature variations that may cause overcompensation and unstable control. But the compensation can achieve accuracies up to 80%, meaning slip can be reduced from 2.4% to about 0.5%.

Peltola is emeritus director of marketing for drives and motors, ABB Oy, Helsinki, Finland.