fine art of load testing d-c motors, The

Electrical Apparatus, Jul 2004 by Nailen, Richard L

They have survived all predictions that they were about to disappear and remain a significant minority of industrial drives

"BLACK BAND" TESTING, SETTING THE neutral, checking interpole polarity, bar-to-bar testing-these and most other test procedures described in d-c machine literature are "diagnostic" (Figure 1). Their purpose is to search out defects-shorts and opens in the armature winding, improper field connections, or commutator problems.

Those considerations are all important. Occasionally, however, the need arises to determine or verify motor capability. Can it carry rated load without overheating? Is a specific overload permissible? How does one machine's efficiency compare with another's? Can a redesign be made to suit new operating conditions (Figure 2)? Such questions must be answered not by diagnostic tests, but by performance tests.

Such procedures can be simpler than for a-c motors because power factor, reactive volt-amperes, phase balance, and frequency are not involved. On the other hand, a d-c motor exhibits some added losses not present in an induction machine. And because the d-c motor is not an essentially constant speed device, more than one operating point may need to be examined. Other variables for consideration are brush grades and spring pressures. (This article will be concerned with brush-type machines. Many smaller d-c motors today are of the brushless type, lacking wound fields, which must be tested in different ways. Efficiency per se is of less importance than speed regulation or torque characteristics.)

No current IEEE standard deals with performance testing of d-c machines. The IEEE 113 Guide: Test Procedures for Direct-Current Machines (latest edition 1985) was withdrawn some years ago and is no longer endorsed by the IEEE (despite references to it in the latest edition of NEMA MG1). Therefore, acceptability of a particular test as proof of d-c motor performance is strictly between user and manufacturer.

Efficiency of d-c machines

Whereas higher efficiency is usually among the virtues claimed for any new line of polyphase a-c motors, that's not necessarily true for d-c machines. A sales bulletin may use terms like "optimum operating efficiency," but only after listing such other features as "more active material," "more ventilation area," "improved commutation," or "reduced heating." Catalogs do not include efficiency tables.

Is d-c motor efficiency of any concern? What constitutes a "high efficiency" d-c machine? Little attention has been paid to those issues, for several reasons. First, although d-c motors have survived all predictions that they were about to disappear in favor of a-c inverter drives, they remain a significant minority of all industrial drives. Second, the most widely used sizes and types of d-c motors are the small brushless types for which such performance capabilities as speed regulation are more important than efficiency. Finally, of course, unlike an induction motor, the d-c machine inherently offers a range of operating speed, determined by field control settings, throughout which efficiency is a variable.

Nevertheless, at any operating point, d-c motor efficiency has the same significance as in an a-c machine. Efficiency equals output divided by input, or (more accurately) output divided by the sum of output plus losses. Some of those losses correspond to those in a-c motors; others do not. In general, the full-load efficiency of a d-c motor will be several percentage points lower than for the corresponding a-c rating.

Two values of efficiency may be encountered in d-c literature. For the common shunt-connected motor, power input at any load equals applied voltage times the sum of field and armature current. Once the shunt field current is adjusted for rated load and speed, and remains at that value, load variation results in a range of armature currents with little change in armature voltage. An "armature efficiency" can then be calculated as:

(Armature input power-armature losses)/armature input power

This is readily calculated for each load point.

Efficiency of a d-c motor can be increased through the same kinds of redesign available for the induction motor. Increasing the cross-section of conductors (particularly in the armature) will reduce motor losses, as will a lower-loss steel in armature laminations (and in the field as well, for operation on rectified power with its "a-c ripple"). Predicting the exact effect at a given operating point is not difficult. However, users may be unable to define the hours of motor operation at specific load, hampering an economic evaluation of efficiency improvement.

Methods of determining efficiency

As with a-c motors, three basic test methods are usable to determine efficiency: "direct loading" (Figure 3), opposition or "pump-back" testing (also called "back-to-back" or "loading back"), or "loss segregation."

Dynamometer testing is the simplest method. However, small errors in either measured input or measured output can result in large errors in the ratio of one to the other. More important, providing a power source of sufficient rating to supply full motor load may be impractical, and the energy cost may be prohibitive.