Self-Excited Electromagnetic Drive for a Resonant Fatigue Machine

This paper describes the development of low frequency and self-synchronous electric drive equipment for fatigue life testing machines. These test machines simulate the vibrating loads that result from reciprocating engines, variable air loads, and so forth. The self-synchronous feature is important because the inherent high mechanical Q of the system amplifies some 10 to 50 watts of drive power into 2,000 to 5,000 pounds of vibrating load at frequencies from 7 to 40 cycles per second. Mechanically driven types lack sufficient speed stability to maintain resonance, and this results in poor control of the specimen test load. The power requirements for large loads at high speeds are excessive. An experimental 5,000-pound machine with self-excited electromagnetic drive is described. This electric drive is low in both original and maintenance costs. The specimen load is controlled as simply as the volume on a radio receiver. A 100,000-pound machine is nearing completion and is discussed briefly.