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Pump Output Power isn’t the same as Pump Motor Size

January 18th, 2010

There is always a lot of confusion about the true power output of ultra-high pressure pumps. I guess it is only natural for people to assume that the pump with the bigger drive motor is the pump that generates the most power at the waterjet nozzle. The problem is that this just isn’t the case. There are 2 basic reasons that a large motor may produce only a jet with lower power:

1. The pump is of low efficiency and a big motor is really required.
2. The motor is larger than required for marketing or manufacturing reasons.

For the first case, I have written some fairly detailed articles about the difference in efficiency between direct-drive pumps and hydraulic intensifier pumps and how a direct-drive pump with a smaller motor can often outperform a hydraulic intensifier with a larger motor. (Refer to such an article at www.omax.com under “products”/ “direct-drive pumps” if you really want to get into it!). However the source of confusion goes even deeper than just the difference in these two types of pumps. Indeed, pumps of the same basic type can have different levels of efficiency depending on the details of how they are designed and manufactured. When the pump designer determined what size motor he would need to drive his pump he had to take that efficiency into account. He (or she!) started by knowing the desired effective power at the nozzle, based on pressure and flow rate. Then he had to add the power wasted by the inefficiency of his pump design and drive system (mechanical or hydraulic). Finally he had to select a motor that would do the job.

For the second case, the Manufacturing Manager has some input on motor selection because he wants a motor size that is readily available from suppliers or perhaps one that is already stocked for some other purpose. It is not uncommon for a pump manufacturer to use a drive motor that is actually larger than really needed. Moreover, the Marketing folks love it because it gives them “power bragging rights” over other pump manufacturers. Unfortunately the actual user may pay for those bragging rights with added electric utility demand charges and no real performance benefit.

The end result of all of this is that the final selected motor size may be substantially larger than the net effective power delivered to the nozzle. As I mentioned in my last entry, output pressure and flow rate are the true indicators of pump power, not the size of the motor.

Best regards,

John Olsen

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