Piston pump efficiency at high temperatures 1

[frans]

I am using a variable displacement pump to supply two variable displacement motors, driving via a splitter box. Recently I saw as temperature increases the electrical drive motor current increases. The system final output torque and speed is kept constant. The electrical motor is running at constant speed. A dramatic increase in motor current occurred around 80 deg C oil temp. Why? I know the oil is thinner and more leakage occurs and efficiency goes down as the pump has to theoretically work harder to supply the same power. But how physically does this translate into higher motor current. Does the pumpautomatically compensate? Or is something unrelated happening in my system?

 

[budt]

Any pump must pump more oil as viscosity decreases due to temperature increase. The extra flow in the case of your Variable Displacement pump goes into the pump housing and out the Case Drain. I always install a Case Drain Flow Meter which tells how much wasted energy the pump is producing. This Case Drain flow had to be at system pressure before bypassing so extra HP was required even though system output work did not increase.

The bad news is this wasted energy all turns into heat and the oil gets hotter which causes more bypass which makes more heat which lowers the viscosity which waste's more energy----

Your problem could be the pump or motors are worn and are bypassing excess oil with cool fluid. This can be determined with Flow Meters at the Case Drains. It also could be the oil has broken down and all the Viscosity Modifiers have been lost. I had the oil changed on a Pressure Compensated piston pump and lowered Case Drain flow by 80%. The oil had been over heated for some time due to a relief valve bypassing since it was set below compensator pressure.

Get some Flow Meters on the Case Drains and check for bypass flow with cool oil (AT STARTUP AFTER EXTENDE SHUT DOWN). Have the oil analyzed to see if it is bad due to over heating. I could tell the oil was bad in the above scenario since it had a very bad odor. If Case Drain flow is more than 10% of pump maximum flow send the pump out for repair or replace it. The motors Case Drain flow should be less than 1-3 GPM with cool oil and at high pressure.

Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[frans]

Thanks Bud, your reply was very helpfull. I checked with the supplier of the pump & motors and they would expect an up to 3% loss of efficiency from 70 to 80 deg oil temp rise. This would only translate into higher electrical energy usage if the pump stroke was changed to keep up the speed. By the way, the system (& oil) is brand new.

The system speed and torque is controlled via setting the stroke on the pump and motors by sending a signal to the "stroking valves". Is it possible that the stroking would change due to the thinner oil and hence we see a rise in electrical energy used without having changed the signal to the "stroking valves"?