Electrical Apparatus magazine, 
July 2004 The fine art of load testing D-C motors

Most of the many tests for brush-type d-c motors are intended to diagnose motor condition. In addition to the methods of evaluating insulation integrity, common to all electrical machinery, procedures exist to properly position brushes, ensure correct field polarity, and evaluate commutation.

However, to check temperature rise (particularly in the armature), establish torque capability, or determine efficiency, a load test is required. As with a-c motors, three basic methods exist. The most straightforward is a dynamometer test. Besides the need for a loading device, this method has two disadvantages. First, it requires a d-c power source capable of supplying full motor load. Second, efficiency calculated only as the ratio of input to output is subject to considerable error.

A second method is the "pump-back" or "opposition" test. This requires two identical machines with their fields adjusted so that one operates as a motor, the other as a generator. External power (either electrical or mechanical) supplies only the machine losses. The simplest of three commonly-used test circuits requires the least equipment, but also yields the least precise value of efficiency.

Loss segregation (the remaining method of evaluating full-load performance) gives the best efficiency result. However, it does not permit direct measurement of armature temperature or evaluation of commutation under full load. D-C motor losses differ in several ways from those in an induction machine. Since the armature magnetic field is alternating, d-c motor core loss occurs in the rotor rather than in the stator. The brush-commutator interface involves a constant friction loss plus a "contact loss" that varies with armature current. Losses occur in the field supply circuit. Copper loss is present in both the shunt field winding (constant with field excitation) and in series field or interpole windings (variable with load).

Although ways of evaluating those losses have been widely published, no current IEEE test standard exists for d-c machines. The applicable document, IEEE 113-1985, was withdrawn some years ago. Loss segregation is dealt with in detail by IEC 60034-2, which says little about pump-back testing.

Industry news...

Featured Technical Article
July 2004

On this page is a summary of the Electrical Apparatus July 2004 featured technical article, by Richard L. Nailen, P.E. , "The fine art of load testing D-C motors" Trilingual summary titles are not available online for this issue...

To order backissues which contain the foreign language summaries and the full article, call 312-321-9440 or visit our online webstore. See also links to additional technical summaries, below.

For our international readers, summaries of the technical articles are provided in German, French and Spanish. (Summaries do not appear when the article is U.S.-specific.)

search engine by freefind advanced

Advertising for the January Electrical Apparatus closes December 15. Classified ads for the December issue close December 15. (Space is sometimes available after the closing date - call 312-321-9440.)