Ordered a centrifigal pump and put a vfd on it at customers request. They had a gasoline powered version but it was smokey and noisy inside the shop. We ordered same brand. Pump motor is rated at 11.6 amps with instruction to not restrict flow of water in or out. Pump pulls well over 14. We had to drop the freq to about 55 to prevent overload. Is it to much to ask that equipment perform as suggested?
No. But don't expect the mfg to give any extra.
Is the flow rate higher than it should be? Look at all of the specs as to pipe sizing, length, suction lift, discharge lift and things like that. Are they all within spec?
Yes. If the install is not exactly per spec, see above
... Now the voltage might play into it a bit. 208 vs 240. Never worried much about that issue before.
Problem
Graph on GPM at 0 head shows about 219. I would consider that open discharge. We might have 6 ft of lift through 30 ft of 2" flex hose. 55 hz gives us 125. ....
I don't have anything really bright. Troubleshooting recalcitrant pump installations is never easy.
However, here is some physics about why the install is not performing.
Pump affinity laws:
Volume Capacity
q1 / q2 = (n1 / n2) (1a)
Head or Pressure
dp1 / dp2 = (n1 / n2)2 (2a)
Power
P1 / P2 = (n1 / n2)3 (3a)
q = volume flow capacity (m3/s, gpm, cfm, ..)
n = wheel velocity - revolution per minute - (rpm)
dp = head or pressure (m, ft, Pa, psi, ..)
(assumes the wheel diameter doesn't change)
Decreasing the rpm 8% (60Hz to 55Hz) drops the power to 78%. Which appears to be exactly what you are seeing.
You don't have a lot of options:
Check the pump data. Possibly there are other pump models referenced. The difference between them will be the wheel diameter. Maybe they inadvertently put the 15HP wheel on the pump and a 15Hp motor. Power goes up with the cube of the wheel diameter - doesn't take much change.
P1 / P2 = (n1 / n2)3 (d1 / d2)3
d = wheel diameter
Model number on the pump (or a call to the mfg) should tell this. (Not likely - but i have seen this a couple of times.
And, as noted, the voltage of course. That's not difficult - just money. And you look pretty dumb if that isn't it.
The mfg restriction on outflow is odd. Most centrifugals don't mind higher outlet pressures, just reduces the flow rate following the pump curve. Some don't like shut-off head - makes them heat up. Generally speaking, high NPSH coupled with low outlet restriction causes the pump to run off the end of the curve and require high driver power. Might consider restricting the outlet, pushing up the discharge pressure. I would expect the current (power) to go down.
Generally, restricting the suction is never good. Just makes the pump prone to cavitate.
Good luck.
ice