Electric Motors
Reliance Electric Motors
presents a wonderful background look at electric motors. Here is some of it.
Click on Reliance Electric Motors to go to the full text.
Basic Motor Theory
Introduction
It has been said that if the Ancient Romans, with their advanced
civilization and knowledge of the sciences, had been able to develop a steam
motor, the course of history would have been much different. The development of
the electric motor in modern times has indicated the truth in this theory. The
development of the electric motor has given us the most efficient and effective
means to do work known to man. Because of the electric motor we have been able
to greatly reduce the painstaking toil of man's survival and have been able to
build a civilization which is now reaching to the stars. The electric motor is a
simple device in principle. It converts electric energy into mechanical energy.
Over the years, electric motors have changed substantially in design, however
the basic principles have remained the same. In this section of the Action Guide
we will discuss these basic motor principles. We will discuss the phenomena of
magnetism, AC current and basic motor operation.
Magnetism
Now, before we discuss basic motor operation a short review of magnetism
might be helpful to many of us. We all know that a permanent magnet will attract
and hold metal objects when the object is near or in contact with the magnet.
The permanent magnet is able to do this because of its inherent magnetic force
which is referred to as a "magnetic field". In Figure 1 , the magnetic
field of two permanent magnets are represented by "lines of flux".
These lines of flux help us to visualize the magnetic field of any magnet even
though they only represent an invisible phenomena. The number of lines of flux
vary from one magnetic field to another. The stronger the magnetic field, the
greater the number of lines of flux which are drawn to represent the magnetic
field. The lines of flux are drawn with a direction indicated since we should
visualize these lines and the magnetic field they represent as having a distinct
movement from a N-pole to a S-pole as shown in Figure 1. Another but similar
type of magnetic field is produced around an electrical conductor when an
electric current is passed through the conductor as shown in Figure 2-a. These
lines of flux define the magnetic field and are in the form of concentric
circles around the wire. Some of you may remember the old "Left Hand
Rule" as shown in Figure 2-b. The rule states that if you point the thumb
of your left hand in the direction of the current, your fingers will point in
the direction of the magnetic field.
Figure 1 - The lines of flux of a magnetic field travel from the N-pole to
the S-pole.
Figure 2 - The flow of electrical current in a conductor sets up concentric
lines of magnetic flux around the conductor.
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Figure 3 - The magnetic lines around a current carrying conductor
leave from the N-pole and re-enter at the S-pole.
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When the wire is shaped into a coil as shown in Figure 3, all the individual
flux lines produced by each section of wire join together to form one large
magnetic field around the total coil. As with the permanent magnet, these flux
lines leave the north of the coil and re-enter the coil at its south pole. The
magnetic field of a wire coil is much greater and more localized than the
magnetic field around the plain conductor before being formed into a coil. This
magnetic field around the coil can be strengthened even more by placing a core
of iron or similar metal in the center of the core. The metal core presents less
resistance to the lines of flux than the air, thereby causing the field strength
to increase. (This is exactly how a stator coil is made; a coil of wire with a
steel core.) The advantage of a magnetic field which is produced by a current
carrying coil of wire is that when the current is reversed in direction the
poles of the magnetic field will switch positions since the lines of flux have
changed direction. This phenomenon is illustrated in Figure 4. Without this
magnetic phenomenon existing, the AC motor as we know it today would not exist.
Figure 4 - The poles of an electro-magnetic coil change when the direction of
current flow changes.
Magnetic Propulsion Within A Motor
The basic principle of all motors can easily be shown using two
electromagnets and a permanent magnet. Current is passed through coil no. 1 in
such a direction that a north pole is established and through coil no. 2 in such
a direction that a south pole is established. A permanent magnet with a north
and south pole is the moving part of this simple motor. In Figure 5-a the north
pole of the permanent magnet is opposite the north pole of the electromagnet.
Similarly, the south poles are opposite each other. Like magnetic poles repel
each other, causing the movable permanent magnet to begin to turn. After it
turns part way around, the force of attraction between the unlike poles becomes
strong enough to keep the permanent magnet rotating. The rotating magnet
continues to turn until the unlike poles are lined up. At this point the rotor
would normally stop because of the attraction between the unlike poles. (Figure
5-b)
Figure 5
If, however, the direction of currents in the electromagnetic coils was
suddenly reversed, thereby reversing the polarity of the two coils, then the
poles would again be opposites and repel each other. (Figure 5-c). The movable
permanent magnet would then continue to rotate. If the current direction in the
electromagnetic coils was changed every time the magnet turned 180 degrees or
halfway around, then the magnet would continue to rotate. This simple device is a
motor in its simplest form. An actual motor is more complex than the simple
device shown above, but the principle is the same.
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Catch up on the latest Industrial Systems News
and Events.
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Welcome!
Motor-driven equipment accounts for 64 percent of the
electricity consumed in the U.S. industrial sector. Within the
nation's most energy-intensive industries—which are the focus
of ITP's Industries of the Future strategy—motor systems
consume
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approximately 290 billion kWh per year. In these industries
alone, improvements to motor systems could yield dramatic energy
and cost savings. The key to these savings is applying energy
efficiency equipment or implementing sound energy management
practices.
On this page, find BestPractices resources that are specific
to motor systems. These include publications, software tools,
and training information. Most can be downloaded from this site;
others can be ordered from the EERE Information Center. (Links
for ordering are provided). The following documents can be
viewed using Adobe Acrobat Reader. Download
Acrobat Reader.
Motor Tip Sheets
Eliminate Voltage Unbalance (PDF
446 KB)
McBroom Electric Teams with Cummins Engine Company to Save
an Estimated $200,000 Yearly in Energy Costs (PDF
480 KB)
Motor Efficiency Case Studies
Improving Efficiency of Tube Drawing Bench (PDF
103 KB)
Motor System Upgrades Smooth the Way to Savings of $700,000
at Chevron Refinery (PDF
73 KB)
New Water Booster Pump System Reduces Energy Consumption by
80% and Increases Reliability (PDF
208 KB)
Optimizing Electric Motor Systems at a Corporate Campus
Facility (PDF
192 KB)
Optimized Pump Systems Save Coal Preparation Plant Money and
Energy (PDF
190 KB)
Reduce Pumping Costs through Optimum Pipe Sizing (PDF
86 KB)
Replace V-Belts with Cogged or Synchronous Belt Drives (PDF
647 KB)
Technical Publications
Buying an Energy-Efficient Electric Motor
Publication Date: September 1996
Length: 8 pp.
(PDF
106 KB)
Energy Management for Motor Drive Systems
Publication Date: June 1997
Length: 100 pp.
Order
Number: DOE/MC-10021
Improving Fan System Performance: A Sourcebook for Industry
Publication Date: April 2003
Length: 92pp.
(PDF
1.17 MB)
National Transformation Strategies for Industrial Electric
Motor Systems, Volume I
Publication Date: May 1996
Length: 85 pp.
Order
Number: DOE/PO-0444 Vol. I
National Transformation Strategies for Industrial Electric
Motor Systems, Volume II: Market Assessment
Publication Date: May 1996
Length: 254 pp.
Order
Number: DOE/PO-0044 Vol. II
Optimizing Your Motor Drive System
Publication Date: September 1996
Length: 8 pp.
(PDF
87 KB)
Reducing Power Factor Cost
Publication Date: September 1996
Length: 4 pp.
(PDF
98 KB)
Replacing an Oversized and Underloaded Electric Motor
Length: 6pp.
(PDF
162 KB)
The Impacts of the Energy Policy Act of 1992 on Industrial
End Users of Electric Motor-Driven Systems
Publication Date: September 1996
Length: 4 pp.
(PDF
242 KB)
United States Industrial Motor Systems Market Opportunities
Assessment (PDF
6.61 MB)
United States Industrial Motor Systems Market Opportunities
Assessment: Executive Summary
Publication Date: December 1998
Length: 22 pages
(PDF
5.13 MB)
Variable Speed Pumping: A Guide to Successful Applications;
Executive Summary
Publication Date: May 2004
Length: 22 pages
(PDF
1.1 MB)
Software
Training and Events Calendar
Many of BestPractices' training programs and events focus on
motor system efficiency. Check out the training
and events calendar to see the current offerings.
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Have fun. That is the goal of many of the offerings here at ElectricianEducation.com.
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