Hi Rhymond
How do you know the compensator is actually set to 800 psi? In the absence of an indicator on the pump can you actually measure the flow (with a flow meter installed on the pump outlet) or can you calculate the flow approximately (from motor current and outlet pressure)?
If the compensator is actually set to 800 psi and is working properly but your minimum displacement is set to a significantly non-zero value then, as the outlet pressure increases, you will see this: full flow from the pump when the pressure is less than ~715 psi (slightly lower than this if your pump is larger than 45 cc/rev), then the flow will gradually decrease to a minimum value as the pressure rises to 800 psi, and then the flow will stay at this small value (ignoring losses in the pump) until the pressure reaches an upper limit - that being the setting of your unloading valve. If your pump IS changing displacement as the pressure increases then you will hear a difference in the noise level as you approach 800 psi and there will be a change in the relationship between motor current and pressure. [I'm assuming here that the pump isn't driven by anything other than an electric motor.]
If the compensator is broken (and jammed in the "not reducing the pump displacement" position), or if the pump displacement changing mechanism is jammed at full displacement, or if the compensator is set higher than the unloading valve, then, as the pressure increases, the flow will stay [nominally] the same - just as if you had a fixed displacement pump. There will be no noticeable change in noise (it will just get louder) and the motor current will go up steadily as the pressure goes up.
The link you sent doesn’t work where I live but I know which pump you mean. There is no adjustment of maximum or minimum displacement as standard, but it is possible you have a special version which has the adjustments available or has been set-up in a special way (with a spacer built in to limit the minimum displacement to some particular value).
Check the model number of the pump against the coding key in the datasheet. If there is a number at the end of your pump’s code like “SO xyz” (xyz standing for any number) then you have a “Special Order” or “Spezial Ordnung” version. With these SO numbers you have to contact Rexroth to find out what is special about the component: it could be something small (the label being stainless steel rather than aluminum) or it could be a huge list of major changes to the way the thing works. Rexroth might not even be able to tell you what your SO means - sometimes they work closely with an OEM to create a configuration that suits a particular machine and Rexroth are contractually constrained to only sell that version to the OEM and to keep the special formulation confidential.
I did once encounter a system where the variable displacement pump had a deliberate and significant positive setting for minimum displacement and had a pressure compensator setting well below the relief valve setting. The control philosophy was that at low pressures the pump worked as a variable displacement unit but at high pressure it became a [small] fixed displacement unit. It was a kind of “expensive HI-LO, but cheap horse-power limiter” system. I suppose it saved a few valves and gave an easily adjustable power limiter but I couldn’t help thinking it was odd. Anyway, is it possible that’s what you have - the significant non-zero minimum displacement might be deliberate. I’m also persuaded that this might be the case because you have an unloading valve in your circuit; a simple pressure compensated pump circuit wouldn’t have one of these.
Hope this helps.
DOL