Understanding ac induction motor basics

Knowing how magnetism and electromagnetic induction work are crucial to understanding how an ac induction motor works and functions.


Image courtesy: CFE MediaTo understand how an alternating current (ac) induction motor works, it helps to look into the basics of magnetism and electromagnetic induction and how they impact the functions of ac induction motors. 

Basic magnetism

Magnetism refers to the force that attracts or repels two objects from each other. This action takes place because of a magnetic field that exists with the movement of electrically charged particles. Magnets, for instance, have a strong magnetic field which attracts certain ferrous materials to them such as nickel, steel, cobalt, and iron. Identical poles of two magnets repel, while opposite poles create an attraction.

A magnetic field surrounds the space surrounding a magnet. Placing a magnet in this field creates a magnetic force. This magnetic force still exists even after removing the magnet. Moreover, the magnetic field of an object creates a magnetic force on another component that has a magnetic field.

Law of electromagnetic induction

The law of electromagnetic induction explains how the magnetic field and electric circuit relate to each other. The physicist Michael Faraday played a huge role determining the law through his experiment using a coil and magnet, he was able to discover the induction of electromotive force (EMF) in the coil due to the changes existing to the flux linked to it.

According to Faraday, rapid changes taking place in the magnetic field results in a greater voltage or induced EMF in the coil. He also came up with two laws that further explain how electromagnetic induction works. One of these laws presents the fact that changes in the coil of wire's magnetic field lead to an EMF induced in the coil. Changing the magnetic field may take place through by moving the magnet:

  • Away from or closer to the coil
  • Rotation of the coil surrounding the magnet
  • Creating changes in positioning a coil in the magnetic field.

In another law, Faraday explained that the magnitude of the EMF induced in the coil relates to the change of flux's rate linked with the coil. The coil's flux linkage is the result of the number of turns made in the coil, as well as the flux related to the coil. Faraday's law is an essential law of electromagnetic induction. His law is applicable in various electrical machines and industries.

Rotating magnetic field

When it comes to operating an ac motor, it is important to understand the principle of a rotating magnetic field. This field affects all electromechanical applications including electric generators and induction motors. In simple terms, a rotating magnetic field pertains to the magnetic field with moving polarities. The opposite poles rotate around an axis or central point. There is a constant angular rate with the changes in direction during the rotation.

Induction motors do not use magnets as the latter tend to degrade over time. Instead, these motors use rotors which follow a rotating magnetic field of a stator. The rotor's short-circuited turns form eddy currents in the stator's rotating field. As a result, the rotor moves in an efficient manner. Since these are not synchronous motors, there is a particular degree of slip necessary for creating the current due to the rotor and the relative movement of the field.

How an ac induction motor works

Some pieces of equipment and motors rely on ac for power. When it comes to an ac motor, power is produced by the ring of electromagnets positioned around the exterior that produce a rotating magnetic field. This ring makes up the stator.

In the stator, various components conduct electricity. These components include a solid metal axle, coil, and freely-rotating metal pieces. To activate an ac motor, power is sent to the outer coils energizing these components in sequence. This sequence generates a magnetic field that rotates around the exterior of the motor.

The rotor is an electrical conductor suspended within the magnetic field. There is a constant change taking place in the magnetic field due to the rotation. This constant change leads to the production of an electric current situated inside the rotor. An induced current generates its magnetic field and rotates at the same time.

It is electromagnetic induction that makes it possible for such a motor to spin. Therefore, it is referred to as an induction motor due to the manner in which it rotates and operates.

David Manney is a marketing administrator at L&S Electric. This article originally appeared on L&S Electric Watts New BlogL&S Electric is a CFE Media content partner.

Top Plant
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2017 Top Plant.
Product of the Year
The Product of the Year program recognizes products newly released in the manufacturing industries.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
May 2018
Electrical standards, robots and Lean manufacturing, and how an aluminum packaging plant is helping community growth.
April 2018
2017 Product of the Year winners, retrofitting a press, IMTS and Hannover Messe preview, natural refrigerants, testing steam traps
March 2018
SCCR, 2018 Maintenance study, and VFDs in a washdown environment.
April 2018
ROVs, rigs, and the real time; wellsite valve manifolds; AI on a chip; analytics use for pipelines
February 2018
Focus on power systems, process safety, electrical and power systems, edge computing in the oil & gas industry
December 2017
Product of the Year winners, Pattern recognition, Engineering analytics, Revitalize older pump installations
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
April 2018
Implementing a DCS, stepper motors, intelligent motion control, remote monitoring of irrigation systems
February 2018
Setting internal automation standards

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

The Maintenance and Reliability Coach's blog
Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
One Voice for Manufacturing
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Maintenance and Reliability Professionals Blog
The Society for Maintenance and Reliability Professionals an organization devoted...
Machine Safety
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
Research Analyst Blog
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Marshall on Maintenance
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
Lachance on CMMS
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
Maintenance & Safety
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Industrial Analytics
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
IIoT: Operations & IT
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Randy Steele
Maintenance Manager; California Oils Corp.
Matthew J. Woo, PE, RCDD, LEED AP BD+C
Associate, Electrical Engineering; Wood Harbinger
Randy Oliver
Control Systems Engineer; Robert Bosch Corp.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me