HP change calc

[ptonsparky]

	Pump Data	Revised Data @:
Original	60 HZ	58.0	56.0	54.0	52.0	50.0	48.0	46.0	44.0
Rpm/dia	1770	1711.0	1652.0	1593.0	1534.0	1475.0	1416.0	1357.0	1298.0
Gpm	1050	1015.0	980.0	945.0	910.0	875.0	840.0	805.0	770.0
Hd	116	108.4	101.0	94.0	87.1	80.6	74.2	68.2	62.4
Equiv psi	50.228	46.9	43.8	40.7	37.7	34.9	32.1	29.5	27.0
Torque	3561	ft/lbs
hp	100
rpm	1770
Increments	-2	Revised
Hz	60	58.0	56.0	54.0	52.0	50.0	48.0	46.0	44.0
Rpm/dia	1770	1711.0	1652.0	1593.0	1534.0	1475.0	1416.0	1357.0	1298.0
ratio	29.5
Fan/Pump Power
Original RPM	1770
Original Power	74570
2nd RPM		1711.0	1652.0	1593.0	1534.0	1475.0	1416.0	1357.0	1298.0
Power Rqd - KW	74570	67358.8	60628.2	54361.5	48542.3	43153.9	38179.8	33603.5	29408.2
% Change		-9.67%	-18.70%	-27.10%	-34.90%	-42.13%	-48.80%	-54.94%	-60.56%
RPM		96.67%	93.33%	90.00%	86.67%	83.33%	80.00%	76.67%	73.33%
of original KW		90.33%	81.30%	72.90%	65.10%	57.87%	51.20%	45.06%	39.44%
2nd HP reqd		100	100	75	75	60	60	50	40

Customer is wanting to put a vfd on an existing well that feeds two pivots. He is the new owner and will not be running both pivots at the same time. One larger than the other. Does the reduction in KW and HP required at the different HZ look right? 50HZ will deliver enough water for the smaller pivot.

 

[Ingenieur]

yes

pump power ~ GPM x Head
so a ratio of new/60 Hz can be used
74570 (60 Hz power) x (new Hz gpm x hd)/(60 Hz GPM x hd)

50 Hz
74570 x (875 x 80.6)/(1050 x 116) = 43178 W or 58 HP, use 60 HP (your table 43153.9 W)
matches your chart

44 Hz
74570 x (770 x 62.4)/(1050 x 116) = 29228 W or 39 HP, use 40 HP (your table 29408.2 W)

 

[ptonsparky]

yes

pump power ~ GPM x Head
so a ratio of new/60 Hz can be used
74570 (60 Hz power) x (new Hz gpm x hd)/(60 Hz GPM x hd)

50 Hz
74570 x (875 x 80.6)/(1050 x 116) = 43178 W or 58 HP, use 60 HP (your table 43153.9 W)
matches your chart

44 Hz
74570 x (770 x 62.4)/(1050 x 116) = 29228 W or 39 HP, use 40 HP (your table 29408.2 W)

Thank you.

It's the first I've tried my hand at the Power reduction portion. GPM vs HZ, I've done.

 

[Besoeker]

Thank you.

It's the first I've tried my hand at the Power reduction portion. GPM vs HZ, I've done.

Looks like a centrifugal pump.
There is a simpler way. Mulyiply the power by the ratioratio of the speeds cubed.

P2 = P1*(n2/n1)^3

So, for 50Hz you get P2 = 74570*(1475/1770)^3 = 43153.9

BTW, your second table staates kW.
I assume you meant W.

 

[Ingenieur]

Looks like a centrifugal pump.
There is a simpler way. Mulyiply the power by the ratioratio of the speeds cubed.

P2 = P1*(n2/n1)^3

So, for 50Hz you get P2 = 74570*(1475/1770)^3 = 43153.9

BTW, your second table staates kW.
I assume you meant W.

yep
meant W
ahhh....the affinity rule

 

[Besoeker]

yep
meant W
ahhh....the affinity rule

Just using cube law for centrifugal pumps.

 

[ptonsparky]

Looks like a centrifugal pump.
There is a simpler way. Mulyiply the power by the ratioratio of the speeds cubed.

P2 = P1*(n2/n1)^3

So, for 50Hz you get P2 = 74570*(1475/1770)^3 = 43153.9

BTW, your second table staates kW.
I assume you meant W.

I think they tried to teach us how to simplify back in grade school. Obviously I was sick that week.

Yes, W.

 

[Besoeker]

I think they tried to teach us how to simplify back in grade school. Obviously I was sick that week.

Yes, W.

And obviously I can't spell ratio.........
Or staates...........
Or maybe it's just phat phingers..........

Ever so slightly more seriously, I like to try to keep things as simple as they can be. It reduces the probability of errors. Occam's razor comes to mind.
Maybe I should have asked you why you wanted to convert from HP to kW. I don't disagree with your conversions BTW. I was just curious about the why....

Another small point. You have expressed some of the calculations to six digits of resolution. That doesn't make them incorrect as calculations of course.
I'd have made the 43153.9W 43kW for example. You don't need to know it to the lat tenth of a watt. We wouldn't normally be able to measure to that degree of resolution.

Just my two pence worth.

 

[Ingenieur]

I think they tried to teach us how to simplify back in grade school. Obviously I was sick that week.

Yes, W.

since the pump speed is fixed by freq/poles

new power = (new f/base f)^3 x base power
(50/60)^3 x 74570 W = 43154 W

the pimp hydraulic power is the one I'm used to
power new/power base ~ the ratio of (gpm x ft head)

no right or wrong, whatever is easier for you to wrap you noggin' around lol

 

[Besoeker]

the pimp hydraulic power is the one I'm used to

Glad I'm not the only one who does typos..........

 

[ptonsparky]

Ever so slightly more seriously, I like to try to keep things as simple as they can be. It reduces the probability of errors. Occam's razor comes to mind.
Maybe I should have asked you why you wanted to convert from HP to kW. I did not think about using HP to directly replace KW. I don't disagree with your conversions BTW. I was just curious about the why....

Another small point. You have expressed some of the calculations to six digits of resolution. That doesn't make them incorrect as calculations of course.
I'd have made the 43153.9W 43kW for example. You don't need to know it to the lat tenth of a watt. We wouldn't normally be able to measure to that degree of resolution.
I let my spreadsheet determine the resolution and was curious as to how close they would come to my Lookup table. I changed the decimal places on some, others I did not.
Just my two pence worth.

This appears to work for the few I tried.

		p2=p1*(n2/n1)^3		hp2=hp1*(n2/n1)^3
p1	75	KW		HP1	100
rpm1	1770				1770
rpm2	1475				1593
p2	43	KW		HP2	72.9

Thank you.

 

[Besoeker]

This appears to work for the few I tried.

		p2=p1*(n2/n1)^3		hp2=hp1*(n2/n1)^3
p1	75	KW		HP1	100
rpm1	1770				1770
rpm2	1475				1593
p2	43	KW		HP2	72.9

Thank you.

One ought to use rpm as you have done but for this particular motor it works with frequency also. That's because the slip is given the same value at all duty points. Normally, the slip would reduce as the load reduces.
In the grand scheme of things, it doesn't make a huge difference.

 

[ptonsparky]

This came in surprisingly close to SEWAG.

63 HP vs the 100 to maintain 50 PSI at the older short pivot and the newer one without the swing arm.