There are various ways to construct a motor, and the properties of that motor will depend on the construction choice. The series motor configuration has some desirable properties, but it can become quite dangerous to use if proper safety precautions are overlooked.
“Motors,” per se, is a complex subject. Variations in motor designs abound and lie well outside the scope of this essay. Rather, the goal here is to focus on just one aspect of one particular type of motor. To pay proper homage, Figure 1 shows three basic motor designs.
Figure 1 The three basic DC motor types, this article focuses on DC series motor.
Readers may study the first two at their leisure, but we will focus on the DC series motor highlighted in green and begin with an examination of its basic structure.
The DC series motor
A magnetic field is required. That field is provided by current-carrying coils that are wound over steel structures called “poles”. The number of poles may vary from design to design. Simple-mindedly, Figure 2 shows three examples of pole design: two poles, four poles, and six poles. Note the alternation of north (N) and south (S) magnetic polarities.
The armature is shown as a setup for four (Figure 2) paralleled paths of wires that are insulated from each other but tied at their ends. In the example shown, there are twenty-four armature conductors arranged in six groups of four conductors, or in four parallel paths, each.
Figure 2 The DC series motor structure showing two, four, and six poles with alternating N and S polarities.
It is conventional to use the letter “Z” to represent the number of armature conductors (twenty-four as shown) and the letter “A” to represent the number of paralleled conductors (four as shown) in each path. Please do not be confused by the fact that this “Z” does NOT refer to an impedance and that this “A” does NOT refer to an area.
As shown in Figure 3, we now look at the circuit of this structure.
The field coils, wrapped around each pole, are connected in series to form the field coil.
The armature conductor groups are wired in series, with their returns being made through the center of the armature, where their wire movement is slowest. By contrast, the outermost sections of the armature conductor groups move quite rapidly as they cross the magnetic flux lines of the poles and since they are all connected in series, they generate a summation voltage called the “back electromotive force” or the “back EMF”.
Figure 3 The DC series motor equivalent circuit, the series connections of the outermost sections of the armature conductor generating back EMF.
The current flowing in the field coil and the current flowing in the armature is the same current. There is no other place for the current to flow. The available torque of a DC series motor is therefore proportional to the square of that current. By using really heavy and large conductors for both, that current can be made very large, and the available torque can be made very high. Such motors are used in high torque applications such as engine starters, in heavily loaded and slow-moving lifting cranes, commuter railroad cars, and other such applications.
The governing equation for generating back EMF is as follows in Figure 4.
Figure 4 The governing equation for back EMF, where the back EMF equals the total magnetic flux multiplied by the rotational speed multiplied by the number of series-connected armature groups.
The total magnetic flux equals the flux per pole times the number of poles. The back EMF equals the total magnetic flux multiplied by the rotational speed multiplied by the number of series-connected armature groups, which, for our present example, will be six for our six-pole magnetic structure.
Connect the load!
Now comes the crucial point to remember about DC series motors.
For safety’s sake, no DC series motor should ever be operated without a mechanical load. A DC shunt motor or a DC compound motor can be safely operated without a mechanical load (separate discussions), but a DC series motor CANNOT be safely operated that way.
When the DC series motor is operating, there will be some back EMF generated in the armature as shown in Figure 4. That back EMF will act in opposition to the input voltage in determining the field and armature current, as shown in Figure 3 and as follows:
However, suppose a DC series motor is allowed to run without a mechanical load as the DC series motor undergoes rotary acceleration and starts to gain rotational velocity. In that case, a current flow exists for which some measure of torque exists for which there will be some measure of angular acceleration. With no mechanical load, the rotor will always be rotationally accelerating and gaining in rotational velocity because there is then no load to take rotational energy away from that rotating armature.
As the armature accelerates, the back EMF tends to rise, which lowers the current flow, which lowers the magnetic flux, which lowers the torque, but the flux and the torque do not go to zero, and the rotational velocity will continue to rise. The rotational velocity will keep increasing, tending toward further raising the back EMF, which further reduces the current flow, which further reduces the magnetic field as the rotational velocity continues to increase, and so on and so on, but it is in a vicious cycle of rotary speed-up that constitutes a runaway condition. If there is no mechanical load on the armature, there will be no upper limit on the armature’s speed of rotation, and the DC series motor can and will destroy itself.
A story
It is stridently recommended that any mechanical load being driven by a DC series motor be coupled to that motor by a gear mechanism and never by a belt because a belt can break. If such a break occurs, the DC series motor will have no mechanical load, and as described, it will run away with itself.
This issue was taught to my class by my instructor, Dr. Sigfried Meyers, when I was in Brooklyn Technical High School in Brooklyn, NY. There was a motor lab area. Dr. Meyers told us of one day when there was no faculty supervision at hand, several students snuck into that lab and decided to hook up a lab motor in a series motor mode with no mechanical load. When they applied power, the motor did exactly as Dr. Meyers had warned that it would do, and the motor was destroyed.
As Mr. Spock would put it on Star Trek, that was “an undesirable outcome”.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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