Inrush vs Running Current - Practical Considerations for a 1.5 hp 480V motor

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I have a fairly oddball situation and have racked my brain over it for a while now. The situation is thus. 1.5 hp motor. It feeds a pump. This pump in turn feeds water to a waterjet cutter. The purpose of the 1.5 hp pump is to feed clean water into the waterjet. However, the waterjet does not always need the 1gpm of water the pump supplies - for instance when it is traversing it will shut off the high pressure valve, so water use drops to 0.

The decision tree is thus:

Option 1 - Leave the 1.5 hp motor running constantly. Do nothing. Constantly run water from the pump and at times water would not flow. This is bad for seals, etc. There is a chiller in play to avoid excess heat build up but it still seems like a bad idea.

Option 2 - Leave the 1.5 hp motor running constantly. Flow restriction via a pressure compensating orifice on the outlet. This would feed up to 1 gpm to the high pressure pump and then dump the rest back to recirculate. This is obviously a fairly easy solution.

Option 3 - Do not run the 1.5 hp motor constantly. Tap into the solenoid that triggers the nozzle (thus starting a demand for water) and also tie in one of those 20+ gallon well water tanks that supplies pressurized water and allows well pumps to "rest" and not run constantly when you only need a half gallon of water. When the pressure of this tank drops to a set amount, run the 1.5 hp pump and cut it off when the pressure reaches another set level. Simple, right?

Here is the question. I know inrush currents can be 3 - 5 times (or more) of the FLA. This is a 3 phase 480V motor (although I don't know if that makes a difference here). Would it be wiser from a total cost to run perspective to turn the motor off when not needed and then turn it back on to recharge the holding tank or would it be wiser to just run the pump constantly, probably under option 2 so the seal life is maximized? I suppose the size of the pressure holding tank comes into play. Assume the waterjet cuts 80% of the time and traverses 20% of the time. It may be as high as 90/10.

We have some large 5 hp blowers that when you close the blast gate on, pull nearly no amperage draw. My understanding is that it is more economical to leave it running but not under load than to start the entire motor again (momentum - a motor already started needs a lot of power to start up but not much to maintain running).

Anybody care to chime in with some practical considerations or issues I have possibly not addressed? Short of putting an amp meter on the motor, I don't have a clear answer what is "best" in terms of lowest cost to run over time.
 
I like option 3 but how much pressure is required from the pump? What type of pump? Off is the cheapest but if you require 3000 gallons in a day you will pay for 3000 be it a pressure tank or cycling the motor more rapidly. Using a bypass would add another 10 or 20 percent depending on your cut /traverse times and just how much water you have to bypass to reduce pressure to an acceptible level.
 
Yes, it totally depends on the type of pump. Most centrifugal pumps will not be harmed by cutting flow off for short periods of time and the energy consumption will drop to minimal amounts. But if it's a positive displacement pump you cannot do that.

You also must concern yourself with the duty cycle of the motor, on and off too often can shorten the motor life significantly. A modified version of option 3 to consider would be to have a minimum run timer so that the pump does not short cycle.

Don't be too concerned with energy consumption though, "inrush" (really starting current) does not affect your energy bill in any significant way. If you have a centrifugal pump that can be dead-headed (no flow), the difference between running a short cycle with no flow and turning it all the way off will be relatively insignificant.
 
The pump only needs 30 - 50 psi (it has a booster pump). Centrifugal pump.

Water use isn't an issue - this is a closed loop system. Plus, even if it were not, you would still use the same amount - you only use water when the big intensifier pump is actually cutting. If it is off (as in the 10 - 20% of the time when it is traversing) then water isn't flowing at all. So that isn't a factor here.

In fact, eliminating the closed loop system would render this moot because you would just ensure backpressure from the water supply and it would use what it needed. But this is a closed loop system and thus we have the considerations as laid out above.
 
Personally, I would put a VFD on it and have a pre-set slow speed tied to the traverse signal so that whenever the cutter gets a traverse command, the VFD drops the motor speed. That way the motor doesn't need as much time to spin back up to full output when needed, and you avoid the duty cycle issues.
 
I thought about the VFD option but am not sure how great of a solution it would be. The "pressure tank" would act like a capacitor anyway and help to smooth out pressure spikes. As long as the high pressure pump gets 1 gpm it is happy. It takes 30 - 100 psi and turns it into 60,000 PSI. I don't think it cares if it is 30 or 50 or 70 or 100 psi incoming as long as it has the required flow rates.
 
This is not new technology is it ,,how do proven systems vary from what you want? I guess because I don't really know how it works I should remain mute but just curious,,,,,,are you trying to improve the system technology or just going for better economics?

I'm with Jraef on this, I too do not think economics is an issue espcially in an industrial complex.

dick
 
There is not, as far as I know, a "best" practice. The best practice would combine longevity of the pump with economic operation. No sense in having it run and run and run and burn out seals if it was left on and the waterjet was off for days. It is splitting hairs a bit perhaps in that there may be two or more "good enough" solutions.

But I was hoping to determine really if the inrush/starting current would cost more if the motor started frequently and ran for a bit vs leaving it running constantly. Basically, is it true that starting a motor a lot is more costly than leaving it running under little to no load. Although in this case, it would have a load. The load would be shunted though so it would do no productive work - it would just alleviate wear on the pump by avoiding dead heading it.
 
Recirculate the water?

Recirculate the water?

You could put a recirculation loop in the pipe and let the pump run. You might put a timer in the control circuit to shut it down after a time.
Running the pump with no flow will heat the water and may cause cavationion on the impeller. It can also happen with a recirc piping loop- you have more time than you would running a deadhead pump.
 
to me having a pressure reservoir type system would be the one to consider, and with the newer variable speed constance pressure drives out there, can reduce the electric demand as the motor is not starting at full power, as water usage is use the pump only runs at the speed needed to maintain the pressure at the set amount. but the cost to change over to one of these could be high, but maybe something to consider?
 
Most industrial settings have utility rates that are billed on kw usage. With motor starting although inrush current is high, the power factor is extremely low during starting, so most of starting current is reactive power (kVARs) as opposed to kW's. Since you are not billed according to the kVAR usage (unless there is a p.f. penalty) then this starting current is not really effecting the utility bill.
 
Rep said:
Short of putting an amp meter on the motor, I don't have a clear answer what is "best" in terms of lowest cost to run over time.
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On this piece of your question, amps through the motor does not correlate with watts consumed, especially below half load. Many motors, shaft uncoupled, will pull 35-50% of FLA, usually consuming 1-3% of full load kW.

Now for the other (your main) question. I'm involved in industrial HYDRAULICS. We use "charge pumps" which are similar in concept. With these, the power is a small portion of the total and we want to maintain circulation. They operate at (commonly) maybe 10% of system pressure and 10% of system flow. They stay on. We also have the situation where flow is not needed all of he time. We typically "unload" a fixed displacement pump back to the reservoir via a valve. With a centrifugal pump, this is more potentially a problem requiring pump curve study.

We rarely stop and restart unless the off time period is large (certainly at least multiple minutes).
 
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