Linear Dynamometer With Variable Stroke and Frequency

John H. Glenn Research Center

An improved linear dynamometer has been developed for testing linear alternators that are to be used to convert mechanical power to electrical power in free-piston Stirlingcycle engines. Both the frequency and the length of the stroke of this dynamometer can be varied continuously, even during operation; consequently, the dynamometer can be used to fully map the capabilities of a linear alternator throughout its service envelope (its operational range as defined on a plot of limiting stroke length versus frequency) in a single test.

The dynamometer includes a balanced twin-throw eccentric crank mechanism that converts the rotary motion of a drive motor to the desired linear motion of a plunger that constitutes the moving part of the linear alternator under test. To eliminate vibration on the alternator stator, the crank mechanism also imparts an equal and opposite linear motion to a balance mass that is equal to the mass of the plunger. The position of the drive-motor-and-crank assembly along a line perpendicular to the linear-stroke axis can be adjusted hydraulically to vary the eccentricity of the crank and thereby vary the length of the stroke. A compensating mechanism maintains the center of the plunger stroke with the center position of the stator. The frequency of the stroke is controlled simply by controlling the speed of the drive motor.

The dynamometer comprises four main interconnected modules: the crankcase with drive mechanism (see figure), the power pack, the operator console, and the load bank. The crankcase is an assembly of steel plates formed into a closed box enclosing the drive mechanism described above. A scavenging pump maintains a negative gauge pressure in the crankcase to minimize leakage of oil at the joints. The power pack contains a 125-horsepower (93-kW) motor that drives a compound pump that provides (1) pressurized oil for controlling the stroke and driving the hydraulic motor and (2) pressurized oil from a separate supply for lubrication.

A magnetostrictive sensor measures the displacement of the plunger. An encoder on the shaft of the hydraulic motor measures the speed. A programmable-logic controller compares the displacement and speed readings with set-point values of speed and stroke commanded via the operator console and actuates solenoid- operated servo-valves to adjust the flows of oil accordingly.

The power generated by the linear alternator is dissipated in the load bank, which is a commercially available unit that presents a purely resistive load of 3 ohms and up, with selectable resistance to absorb up to 30 kW at up to 300 V. The load resistors in the load module are cooled by a fan. The load module is equipped with meters that read potential, current, frequency, and power, and with fault indicators and lockouts for fan failure, over-temperature, and over-voltage. The potential and current readings from the load bank are displayed on the operator console along with the frequency, the stroke length, the stroke-control position, and the reaction force of the stator in the alternator under test.

This work was done by George Yarr of Clever Fellows Innovation Consortium, Inc., for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4–8
21000 Brookpark Road
Cleveland
Ohio 44135.

Refer to LEW-16951.