AC Motors: Magnetism and the DC Motor


Written by:
Joe Evans, Ph.D

Figure 1 shows the components and magnetic field relationships of a simple two pole, brushed DC motor. The blue and pink stationary objects located on the periphery represent the north and south poles of a permanent magnet and give rise to the two-pole designation. The rotating “armature,” located in the center, contains two sets of windings that are 180 degrees apart and connected in series. When DC power is applied, they become an electromagnet and produce north and south poles.

Again a blue coil represents a north pole and a pink coil represents a south pole. Located on the motor shaft, just forward of the coils, is a split ring commutator that feeds the
two coils. At the nine and three o'clock positions are carbon brushes that apply DC power to the two rings of the commutator. The commutator functions as a switch that reverses the flow of current in the armature coils during rotation. The left hand figure shows the armature in the vertical position. You can also see that the splits in the commutator are similarly aligned. Current flows from the negative brush through the right commutator, into the coils and back to the positive brush through the left commutator. In doing so, it creates a north pole in the upper coil and a south pole in the lower coil. This causes the armature to rotate clockwise due to the opposing and attracting forces between the coils and the permanent magnets.

The middle figure shows the armature approaching one quarter of a rotation. The same forces are still at play, and the split areas of the commutator are approaching each brush. At exactly one quarter rotation, the forces cancel one another, but rotation continues due to the inertia of the armature. Just as it passes the one quarter mark the brushes come into contact with the opposite commutator ring, and the current flowing through the armature reverses direction.

The result is shown by the figure on the right in which the coils of the armature have reversed their polarity and the interactive magnetic forces arise again. In a two coil motor, this reversal occurs twice during each rotation. Motor speed is directly proportional to voltage and inversely proportional to the magnetic flux produced.

Three-Coil DC Motors

Brushed motors can be designed with any number of poles and coils. Also, the poles can consist of electromagnets rather than permanent magnets. Unlike the example in Figure 1, most two pole motors will have a minimum of three coils and a commutator ring that is split into three separate sections. This eliminates two basic problems.

I mentioned that there is an armature position at which no rotational force is created. If the motor were to stop there, it would not restart on its own. Also when the split portion of the commutator passes the brushes, a short circuit will occurthat can waste energy and cause damage if the current is high. A minimum of three coils solves both these problems.

The Semiconductor

With the advent of the semiconductor, another DC design became available in the early 1960s. Brushless DC (BLDC)motors are synchronous motors that are electronically commutated and overcome many of the limitations of the brushed motor. In this design, the components do a complete flip flop. The outer magnet poles are replaced with a stator that consists of a group of stationary coils installed in a circle, and the armature is replaced with a rotor that uses permanent magnets rather than coils.

Some electronic controllers that operate these motors use Hall Effect sensors to monitor the position of the rotor and determine when a particular stator coil should be energized. Other controllers use sensorless control and monitor the back electromotive force (EMF) that arises in the uncharged coils and eliminate the need for Hall Effect sensors. BLDC motors are popular in the electronics industry and tend to dominate many applications including computer hard drives, CD/DVD players and cooling fans. They are also used to power cooling fans in the HVAC industry as well as hybrid vehicles.

Resources

For an excellent pictorial presentation on how brush and brushless DC motors work, visit http://www.stefanv.com/rcstuff/qf200212.html. It was written for model airplane
enthusiasts but applies to all of us.

Finally, if you happen to be one of those who missed out on the opportunity to enjoy the benefits of simple physics you still have a chance. Paul Hewitt is the author of Conceptual Physics (http://conceptualphysics.com/). This high school text book first appeared in 1987 and is by far the best I have ever read.

The current 11th edition is pretty pricy, but older editions are available, inexpensively, at Amazon and Barnes & Noble. Don't be embarrassed that it is a high school text. I have a copy of the 7th edition and use it whenever I want to make a complex topic more understandable. There are also a couple links to free, online physics books on my website. Go to the “Other Educational Sites” section of www.PumpEd101.com.

Next month, I will discuss the operation of an AC motor and compare it to the DC motor.

Pumps & Systems, March 2011

Click the links below for the rest of the AC Motors series:

AC Motors Part Two - Three Phase Operation

AC Motors Part 3 - Single Phase Operations

AC Motors Part 4: Frame Size, Enclosures & Nameplate Data

AC Motors Part 5: AC Motor Life

AC Motor Torque

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