HST Case Study
The example machine used for this study is a hydrostatic transmission (HST) with a pressure-compensated pump and motor, Figure 1. The compensators—simple pressure-area/spring-force mechanical devices—were designed and adjusted so that the motor compensator activates at a lower pressure. Then, when it reaches a pressure that causes the motor to go to full displacement, the pump compensator becomes active. It’s referred to as “sequential compensation,” because one compensator activates immediately after saturation of the other.
This configuration results in a motor speed-torque characteristic known as a constant output power drive. The aim of such a transmission and compensation method is to prevent prime mover lugging.
A series of graphs helps illustrate the power performance of the transmission under study, Figure 2. These curves aren’t vital to the current analysis, but do offer some insight into the nature of the doubly compensated transmission. In this case, it’s clear that output power is not constant, while input power shows the opposite. Also, the lost power can be seen to be nothing more than the input power minus the output power. Only the lost power curve is important to the following discussion.
The lost-power diagram gives clues as to how the application may be adjusted in order to get more useful work done for fewer dollars, Figure 3. Typically, lost power for continuous rotational pumps and motors, as well as transmissions, is low at low output speeds and increases with speed.
Some other key points are that all four individual efficiencies (two volumetric and two mechanical) were set to 97%. Also, the pump was operated by a speed-regulated prime mover that maintains a constant 1800 rpm. Peak input power was 102 hp.
Similar to what was stated above, this transmission’s losses are lowest when operating is low speed, which is also the region of high pressure, and they increase exponentially with increasing speed. This is usually indicative of a machine that is heavy in mechanical (frictional) losses , but “tight” in terms of volumetric losses. And yet, the instant model has all four individual rated efficiencies set to 97%.
Therefore, it begs the question: “How can this speed-dependent rise in losses be possible?” It is an interesting question, and is worthy of further investigation, but it will not be undertaken here.
Lost Power Vs. Efficiency
Lost power in the hydrostatic transmission is the sum total of power losses that support internal leakages and internal frictions. It’s better than conventional efficiency for measuring the efficacy of converting prime-mover power into useful variable output shaft power. That’s because lost power measures energy flowing out of the heat exchanger and potentially helps save money otherwise allocated for what amounts to wasted fuel. Efficiency is merely the percent of power at a given operating point, that makes its way to the load.