Pumps and 50 or 60 cycles

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I am new here and I am not really sure where to post this question. I am also not an Engineer, so general language would be appreciated.

I sell Water Well Pumps at my location and on the Internet among other things. Today I had a call from a Man who wanted a Pump that was going to Africa. He needed 170 gallons per minute at 360 feet of head. No problem, a 30 hp motor and the right liquid end will do the job. Problem is, my book had pump curves for motors that are 60 cycle. In Africa I believe they have 50 cycles like most places around the globe. This means my motor which will spin at 3450 rpm's at 60 htz; and will spin somewhere around 2700 rpm's at 50 htz. Now the pump can't do what my curve said it would.

My question: How would you figure out which pump and motor to sell this man with only the 60 cycle pump curve?

Thanks in advance
bob...
 
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Answers and Replies

  • #2
Q_Goest
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Pump http://www.engineeringtoolbox.com/affinity-laws-d_408.html" [Broken] apply. Using the simple equations found on this web page should provide you enouh information to make pump curves for when the RPM changes.
 
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  • #3
Mech_Engineer
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Just find a pump/motor combo that is designed to operate at 220v/50Hz...
 
  • #4
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Just find a pump/motor combo that is designed to operate at 220v/50Hz...
That's the problem. All the curves I have are at 60 cycles. The manufacturers I buy from don't offer anything in 50 cycles.
 
  • #5
Mech_Engineer
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That's the problem. All the curves I have are at 60 cycles. The manufacturers I buy from don't offer anything in 50 cycles.
What about finding a pump which you can replace the motor with a 220v/50Hz one?
 
  • #6
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What about finding a pump which you can replace the motor with a 220v/50Hz one?
I would imagine most pumps you a simple synchronous motor in which the speed is determined by the frequency of voltage powering it. So if you used a 50Hz motor, its still going to spin at 50Hz unless you have some kind of speed controller.

Like Goest said, pump affinity laws apply and can easily be used to calculate new curves.
 
  • #7
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I checked out the affinity laws and they are over my head. Remember I'm a pumpman not a mathematician.

I have no way of getting my hands on a 50 cycle pump curve for the brands of pumps I sell. So that's totally not an option.

I was hoping there was some equation that would take what I found in my book at 60 cycles and basically bump it up to ? horse power and number of impellers to do the same job at 50 cycles. There probably isn't any such thing.

Thanks for trying guys.

bob...
 
  • #8
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The affinity laws are the equations you're looking for.

Start with flow rate:
you want 170 gpm at 50 hertz. What would that pump make at 60 hertz?

you need this:
q1/q2 = n1/n2

q1 is flow rate at pump speed n1 and q2 is flow rate at pump speed n2.

q1 is 170 gpm. n1/n2 = 50/60 = 0.83
so 170/q2 = 0.83, and
then q2 is 204 gpm

now head:
h1/h2 = (n1/n2)^2 (that's squared
h1 = 360 feet
(n1/n2)^2 = 0.83^2 = 0.69
so 360/h2 = 0.69, and
then h2 is 518 feet

so you want a pump that (at 60 hertz) would deliver 204 gpm at 518 feet. If you ran that pump at 50 hertz, it would deliver 170 gpm at 360 feet.

The power though, would be based on operating at 170 gpm at 360 feet, so the 30 bhp shouldn’t change.

Hope that helps.
 
  • #9
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Thanks Gmax, that's what I think I need. But like I said, I'm no mathematician, so if I tried to do all that math, I would certainly come up with the wrong answer. Algebra was not my strong suit.

If you could do that once more, with my numbers, that may be all I need.
GPM = 170
Head = 490
HP = 30hp
At 60 htz of coarse.

Thanks,

bob...
 
  • #10
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You also need to be sure that the motor used will safely tolerate operation at 50Hz at the highest possible voltage for your client's intended application.

AC motors can be overstressed by operation below their intended frequency, particularly if the supply voltage is above nominal. Bear in mind that the supply voltage may not be so predictable as it would be in your country.
 

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