Misleading Terminology

Power electronics (including motion control) has an unusually high occurrence of misleading terminology. When taken at face value, misnomers result in confusion for neophytes and conceptual abstruseness in the literature of the field. Misleading language is slowly being replaced but plenty of it is still around.

Language and its components – words – matter. Words themselves don't mean anything, but we mean something by them when we use them. To define is to "show limits," to limit the range of meanings that a word can stand for. When the range of meanings is wide, such words label ideas. Narrow, precisely demarcated meanings, are concepts instead. If a car were defined to be a vehicle with four wheels and an engine, then some trucks and tractors would be included in the definition. Such a definition is conceptually too broad to be useful in distinguishing between such vehicles.

In the tradition of science and engineering, meanings labeled by key words are made sufficiently narrow in scope to avoid ambiguity. Key concepts have a one-to-one correspondence with the words that label them. But this standard use of language in science is not always maintained, and confusion can result.

Magnetic field intensity, H, in SI units of A/m, is alternatively referred to as magnetic field strength. Having two expressions for the same quantity, while not necessary, is not confusing. The two expressions accurately name the quantity. The related quantity, magnetic field density, B, in units of V× s/m², is sometimes called induction. Two labels for a single concept, while not parsimonious, are not confusing. So far, so good.

In introductory physics textbooks, it is not uncommon to encounter the expressions EMF and MMF, abbreviations for "electromotive force" and "magnetomotive force," respectively. These quantities are more properly named the electric and magnetic potential, respectively. The electric potential, in most cases found in electronic circuits is equal to voltage, due to the ohmic drop of current across an impedance. Voltage is the more general quantity and includes not only electric potential (which appears as differences in circuits) caused by ohmic drop and magnetically-induced voltage (as in a transformer), but also due to the movement of closed conductive paths in a magnetic field (as in motors), where closed wire loops "cut through" magnetic flux, described by the familiar "flux-cutting" equation:

v = B×l×u

where u is speed. These three phenomena produce voltage, and we commonly use the word "voltage" instead of "potential difference" because it includes all the causes of the quantity.

But voltage is not force. The quantities are distinct and require distinct labels. The unit of voltage is the volt (V), which is not convertible to any unit of force, such as the SI unit of the newton (N). To refer to voltage as "force" is to mislabel the kind of quantity that voltage is. Historically, discovery of forces involving electric and magnetic fields led to a confusion of the familiar quantity of force from Newtonian mechanics with hitherto unknown electrical quantities. The confusion is understandable; experimenters try to relate new phenomena to known physical quantities. But voltage, and its magnetic counterpart, MMF, are not forces.

MMF has units of amperes, A, not newtons. The expression, "MMF," used without referring to it as an abbreviation of "magnetomotive force" is more resilient than "EMF," and is still found in motor textbooks. It's proper name is "scalar magnetic potential" with symbol f^*. (See, for instance, Basic Electromagnetic Theory, by Paris and Hurd, McGraw-Hill, 1969, p. 229.) Personally, I prefer to symbolize magnetic potential (the magnetic Ohm's Law equivalent of voltage) as N×i, or "ampere-turns" instead of MMF or f^* when working out motor and transformer theory. (The turn, like the radian is not a unit as such, but a scaling factor which is often mixed in with units.) N×i is the terminal current, referred to the field reference-frame of the circuit, where N turns effectively make the current appear N times as great. Similarly, induced voltage, referred to the field side of the circuit is v/N, but lacks a name.

There are three basic means of producing torque with magnetic fields. For one of them, a stationary field orientation, at right angles to the stator field, is maintained by switching current through commutating brushes to the rotor winding that produces the oriented field. Motors based on this principle are called "dc motors" because dc can be applied to their terminals.

The permanent-magnet synchronous (PMS) motor (the second means) requires ac excitation of its windings, so that the rotating magnets' field orientation is at right angles to the rotating field produced by the stator windings. The alternating magnetic polarity (N/S) of the rotor magnets requires alternating current to rotate the stator vector. Like induction motors (the third means), synchronous motors are therefore ac motors. But PMS motors are commonly referred to as "brushless dc motors" when they are not dc motors in method of torque production. The "dc" means that, when combined with a motor drive that produces an ac excitation of the PMS electric machine, the user can supply a dc power source and the motor-plus-drive will work. The ambiguity in the expression "dc motor" results from its multiple meanings: 1) a means of torque production of the electric machine, or 2) the kind of power applied by the user to the motor as a system. Despite the possible confusion (which confused me when I started studying motors), the expression is common in the motor business. Hopefully, it will be replaced by PMS motor, the expression General Electric, for instance, uses.

Another source of confusion is in the use of the same mathematical symbols for different quantities. Because electronics, E&M, mechanics, and thermal processes are all modeled in power electronics, the same symbols from these different fields can appear in the same calculations. For instance, both voltage and speed commonly use v as a symbol. To avoid confusion, speed is often relabeled as u, as in the equation above.

The preferred symbols used to label the main concepts of motor theory have been set out in newer books such as Electromechanical Motion Devices, by Purdue U. professors Paul C. Kraus and Oleg Wasynczuk (MacGraw-Hill, 1989). For one, the "torque constant" K_T is the direct-axis synchronous-reference-frame flux-linkage, l_ds of the rotor magnets, referred to the mechanical (rotor) reference frame, symbolized as l' ^r_m. The prime refers the magnets to the stator windings.

I have been using the more manageable symbol, L, for the familiar quantity

where L is referred to the mechanical reference-frame of the shaft. Similarly, the "voltage constant," K_V, equals K_T for PMS motors when both are expressed in the mechanical rotating reference frame. Then

T = L× i and v_w  = L×w _me

High-energy particle physicists have averted confusion in naming new forces, fields, and their mediating particles by assiduously avoiding the re-use of already defined words (such as force). Instead, existing words from common experience, such as color and charm, which are not readily confused with existing physics language, are used. Ideally, new words, or words associated with purely fictional settings, (such as mathematician Lewis Carroll's jabberwocky language from Alice in Wonderland) would avoid any possible confusion. For instance, color relates to the wavelength of visible electromagnetic energy, and to talk of quarks having a certain color could reintroduce confusion due to conceptual coupling. Historically, new words (or neologisms) usually were invented, such as induction, for something new requiring a name. This is the best approach for avoiding confusion.

In mathematics, conflation of meanings is evident in calling numbers "imaginary." They are no more imaginary than real numbers, but since we learn early that the expression is not to be taken literally, confusion is avoided, as it is with the colors of quarks.

For both clear communication and cognition, conceptually refined language – especially the meanings labeled by key words – requires distinct words for distinct meanings. E&M and motor theory suffers particularly from linguistic abuse. By knowing it, and avoiding it, the corpus of talk in the field is both simplified and clarified.

Ó Dennis L. Feucht, 2000