For the control of the pump power Figure 1 shows a circuit that is used by Eaton for this purpose. This incorporates a pressure feedback from the hydraulic potentiometer that is varied by the pump displacement the force from which is opposed by that created by the pump outlet pressure as can be seen from Figure 26. If the pump pressure increases to too high a value its displacement is reduced by the action of the spool valve to a lower level that corresponds to that demanded for a constant power characteristic.

Figure 2 shows the operating envelope for a pump that has all three controls so that for a given flow demand, determined by the valve setting, the maximum available pressure will correspond to that at point A. As the flow demand is al­tered the maximum available pressure will be controlled to the constant power line. Thus point A traverses the maximum power line the maximum pressure be­ing limited by the compensator setting.

Should the pump outlet pressure exceed this value the compensator will re­duce the pump stroke and, hence, the flow. This control is able to reduce the pump displacement to zero. With a blocked outlet port the pump stroke reduces to a level when the output flow is just sufficient to make up the pump leakage at the set pressure.