Energy audit ( What percentage of HP is actually being used )


Senior Member
We have been changing all our lighting out to LED's to cut on energy cost's . Our plant is wanting to look at other means to save energy . We have quite a few hydraulic test stands where the operator installs the part on the stand and runs various test's at different pressure's and flow . Some times the stand run in bypass without being fully loaded . I was asked ( If a pump is running unloaded or very minimum loading , what percentage of HP is being used ) . What's the best way to answer this question ?


Electron manager
There is no universal answer that will fit every motor driven application.

Your mentioned hydraulic pump very likely draws less power when in bypass than when there is loading.

Reading current alone isn't the correct approach in determining things either, as power factor is likely different loaded vs unloaded.


Senior Member
The question you were asked cannot be answered because they don’t tell you the mechanical load.
If you can find an efficiency curve of the motor, hand it to them and tell them to pick the load point and read the answer!

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Senior Member
Induction motors consume significant reactive power even when lightly loaded. So if you're currently paying PF charges then adding PF correction could provide savings in the long run.


Senior Member
190913-1059 EDT

Most industrial hydraulic pumps operate with a hydraulic pressure regulator. The motor runs at approximately full speed, and the pump is pumping approximately full volume capacity, and if no hydraulic load, then all that oil flow at the regulated pressure is dumped to the tank. Thus, lots of heat and wasted power into the dumped oil.

Pressure times flow is power. No flow to a hydraulic load, and efficiency is very low. Input power is simply going into waste heat.

This is why electric servo systems provide a more efficient means to move a machine slide. When there is no slide motion, then there is little electric power input to the servo.

Put a watt meter on the pump input, and observe power input. Not a an ammeter.



Staff member
Yes, a watt meter is the only way to get any meaningful information. Everything else is a guess based on many other guesses, so could be off by a very large margin of error.

And just to add add on further to what gar said, a typical hydraulic system that uses an accumulator (the tank he mentioned) is rarely more than about 60% efficient. So one good way to save energy, if they can’t change that test stand to be servo actuated, is to have their operators turn it off between tests rather than leave it running idle for periods of times. Then to ameliorate the effects of starting the motor over and over, use a soft starter.


Senior Member
If the plant is big enough, you can justify the cost of a three phase power analyzer.( ~$5K)
That will tell you exactly what machines or areas your savings are and where you can get your the best bang for the buck.