Proportional and servovalves can be applied to variable-volume pump controls to accurately set flow from the pump in relation to feedback from an actuator. Most pressure-compensated pumps available today have this feature as an option.

An example might be a closed-loop hydrostatic pump and motor that must stay at a constant speed. Feedback from an encoder signals the pump to increase or decrease flow as the motor overspeeds or underspeeds.

Proportional coils make excellent infinitely variable flow controls that can be controlled electrically. They can act as simple throttle valves or perform as full-blown pressure-compensated types that adjust to pressure fluctuations.

Common circuits for proportional flow controls include feed-speed changes to deal with load fluctuations; hydraulic motor rpm changes to control product backup; or maintaining constant speed as loads vary. (Pressure-compensated proportional flow control valves are covered in Chapter 13.)

Proportional coils also make excellent infinitely variable pressure-control valves. Pressure settings from remote locations or a PLC at anytime from minimum to maximum almost instantaneously. Most of these pressure controls are for relief and reducing functions but could be used on any of the pressure controls. (Proportional pressure-relief valves were covered in Chapter 9.)