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Electrical Connection: Slip of AC Induction Motors and How To Minimize It

This article brought to you by: ABB

Electrical article index page | Motor Maintenance Course

(By Mauri Peltola, ABB Drives)

The AC induction motor is often referred to as the workhorse of the industry. This is 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 per cent of all motors used in industry worldwide are AC induction motors.

In spite of this popularity, the AC induction motor has two basic limitations:

  1. the standard motor is not a true constant-speed machine; and
  2. is not inherently capable of providing variable-speed operation.

Both of these limitations require consideration, as the quality and accuracy requirements of motor/drive applications continue to increase.

This article explains the reason for the first limitation – slip -- and ways to minimize it. And, the best methods to control motor speed with power electronics now available are detailed, including technology to minimize the negative effects of slip.

Before the article, you might want to watch this video:

 

Motor Slip is Necessary for Torque Generation

An AC induction motor consists of two basic assemblies – 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 – 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. The name "induction motor" comes from the alternating current (AC) "induced" into the rotor via the rotating magnetic flux produced in the stator.




Motor torque is developed from the interaction of currents flowing in the rotor bars and the stators’ 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 it is necessary for producing of torque.
 

AC Induction Motor cutaway.jpg (363670 bytes) Figure 1. Squirrel cage ac induction motor opened to show the stator and rotor construction, the shaft with bearings, and the cooling fan

Click on image to enlarge.

 

Slip Depends on Motor Parameters

According to the formal definition, the slip (s) of an induction motor is:

Slip formula, where

ns = synchronous speed

n = actual speed

For small values of motor slip, the slip (s) is proportional to the rotor resistance, stator voltage frequency, and load torque -- and is in inverse proportion 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.

Smaller motors and lower-speed motors typically have higher relative slip. However, high-slip, large motors and low-slip, small motors also are available.

You can see that full-load slip varies from less than one percent (in high-HP motors) to more than five percent (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 about correct, 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 Figure 2, the rotor speed decreases in proportion to the load torque. This means that the rotor slip increases in the same proportion.
 

Figure 2 - Speed Curve of An Induction Motor.jpg (36414 bytes) The speed curve of an induction motor. The slip is the difference in rotor speed relative to that of the synchronous speed. CD = AD – BD = AB.

Click on image to enlarge.

 

Relatively high rotor impedance is required for good across-the-line (full voltage) starting performance (meaning high torque against low current), and low rotor impedance is necessary for low full-load speed slip and high operating efficiency. The curves in Figure 3 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.
 
Figure 3 - Torque-Current Speed Curves.jpg (45216 bytes) Figure 3. Torque/speed and current/speed curves for a standard motor A (full lines) and a high-torque motor B (dotted lines).)

Click on image to enlarge.

 

Methods to Reduce Slip – Motor Choice, Oversizing

The use of synchronous motors, reluctance motors or permanent-magnet motors can solve the problem of slip, because there is no measurable slip in these three types of motors. Synchronous motors are used for very high-power and very low-power applications, but to a lesser extent in the medium-horsepower range, where many typical industrial applications are. Reluctance motors also are used, but their output/weight ratio is not very good and, therefore, they are less competitive than the squirrel-cage induction motors.

A potential growth market is for permanent magnet (PM) motors -- used with electronic adjustable speed drives (ASDs). The main benefits are: accurate speed control without slip; high efficiency with low rotor losses; and the flexibility of choosing a very low base speed (eliminating the need for gear boxes). The use of PM motors is still limited to certain special applications, mainly because of high cost and the lack of standardization.

Selecting an oversized AC induction motor is a second way to reduce slip. Why? Larger motors typically have a smaller quantity of slip, and slip gets smaller with a partial (rather than full) motor load.

 

Adjustable Speed AC Drive is Often the Best Solution

The inherent limitations of the AC induction motor mentioned at the beginning of this article -- no constant speed and no speed control -- can be solved through use of adjustable speed control (ASDs). The most common AC drives today are based on pulse-width modulation (PWM). The constant AC line voltage with 60 or 50 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 is possible to adjust the motor speed.
 

Figure 4 - Control System With AC Drive.jpg (81252 bytes) Figure 4. A simple control system with an AC drive: controlling the speed of the pump controls the water level in a water tower.

Click on image to enlarge.

 

When using an AC drive for adjusting the motor speed, there are many applications where motor slip is no problem anymore. The speed of the motor is not the primary control parameter; rather, it could be the liquid level (as in Fig. 4), air pressure, gas temperature -- or something else. There are still many drive applications where high static speed accuracy and/or dynamic speed accuracy are required. Such applications are printing machines, extruders, paper machines, cranes, elevators, etc.

There also are many machines and conveyors where speed control between sections driven by separate motors have to be synchronized. Instead of over sizing the motors to eliminate the speed error caused by slip, it can be better to use sectional drive line-ups with separate inverters for each single 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 utilize the braking energy from decelerating sections (regeneration).

Slip compensation can be added to AC drives, to 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 cannot be 100 per cent of the slip because of rotor temperature variations that may cause over-compensation and unstable control. But the compensation can achieve accuracies up to 80 per cent, meaning slip can be reduced from 2.4 per cent to about 0.5 per cent.

 

Figure 5 - The Effect Of Slip Compensation.jpg (46786 bytes) Figure 5. The effect of the slip compensation.

Click on image to enlarge.

 

Contact The Author At: Mauri.Peltola @fi.abb.com

For more information on ABB Drives & Power Electronics, contact: Becky Nethery, Manager, Marketing Communications, ABB Inc., Automation Technology Products Division, Drives & Power Electronics, 16250 West Glendale Drive New Berlin, WI 53151-2840, Tel: (262) 785-8363, Fax: (262) 780-5120

e-mail: becky.nethery@ us.abb.com

 

Check these Engineering Books, a fantastic resource!

 

 

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