This is part two of a two-part post explaining how to determine the best size for a hydraulic pump motor and how to scale the size and cost with RMS loading and Hp limiting.
In Part 1 of this post, we discussed the correct way to size an electric motor for a hydraulic pump. Now we’re going to take things a step further by explaining how to safely scale down the size of your motor for increased efficiency and cost-savings.
There are two methods that you can use to safely put a smaller motor to work in your hydraulic pump. One is the Root Mean Square (RMS) method and the other is Hp limiting. Which you choose is based on how the hydraulic pump will be utilized.
The RMS method
Most hydraulic power units do not continuously operate using the same power load, and their flow and pressure levels are constantly changing as various actuators move during the machine cycle.
As an example, during a single cycle, a hydraulic pump might shift from 10Hp for ten seconds to 15Hp for five seconds, 4Hp for thirty seconds, 12Hp for ten seconds, and 5Hp for 20 secs. Although the pump reaches 15 Hp during the cycle, that is not its continuous operating zone. Rather, it is the upper range of the power demand.
In the chart below, you will see that the RMS value in this example is well below 10Hp. This means that as long as the power demand doesn’t exceed 150% of the motor’s rating—and the RMS value doesn’t exceed the motor nameplate rating—a 10Hp motor could be safely used in this application.
How to calculate RMS power
We arrived at the above solution by calculating the varying amounts of power needed throughout the cycle as well as the amount of time that power is used. In short, RMS or root mean squared power represents the integral of the squares of the instantaneous values during a cycle. The mathematical calculation is as follows:
If we take the numbers from the example above and apply them to this equation, the resulting calculation would look like this:
Power (RMS) = SQ.RT. ((10Hp^2x10s + 15Hp^2x5s + 4Hp^2x30s + 12Hp^2x10s + 5Hp^2x20s)/(10s+5s+30s+10s+20s)) = 7.78Hp
NEMA motors can be sized using this technique, IEC motors typically cannot. If in doubt, contact your motor vendor. When using a VFD make sure the drive can handle occasional overload current.
Another instance in which a smaller motor may be appropriate is with applications that require high flow at low pressure and high pressure at low flow. In such a case, you can utilize a variable volume pump that is capable of limiting its own power requirements, thus enabling a smaller motor to be used.
Consider the following scenario:
Let’s say your system requires a 20 GPM @ 500 PSI during a rapid advance and 3000 PSI at 0.5 GPM (clamping). Using the basic (flow x pressure)/(1714 x eff.) formula for sizing that we discussed in Part 1 of this post, you would probably consider selecting a 40Hp electric motor. But wait! Because this application requires high flow at low pressure and high pressure at low flow, you can use a Hp limiting pump and safely scale down to a 25Hp motor.
Pumps that offer horsepower limiting or other control options can help make your hydraulic system much more efficient while enabling you to conserve energy. Contact your local Parker Hannifin distributor for more information on Hp limiting pumps or for help deciding whether or not choosing a smaller motor is a smart choice for your hydraulic pump. And, in case you missed it, check out Part 1 of this post for more detailed information on determining the best motor size for your hydraulic pump.
For more information on Choosing an Efficient Electric Motor for a Hydraulic Pump, contact Parker's Hydraulic Pump and Power Systems Division.
This article contributed by Tim Beck, manager - system design and application, Parker Hannifin Corporation Hydraulic Pump and Power Systems Division.