Loading versus slip
Loading versus slip
(OP)
Hi - I hope you all can help me out. I'm a civil, trying to understand a centrifugal pump's operation - it isn't operating on the published curve. I'm wondering if I have an accurate curve, or if I have a motor problem. Mechanically, the pump is in good condition - we've inspected and measured the impeller. The motor, on the other hand, is a bit overloaded. The motor is a Reliance "Type P" 30 HP NEMA B with a 1.1 SF and nameplate rpm of 3530. The motor was installed in 1979. Based on the curve efficiencies, the motor is producing on the order of 30-32 HP whenever it is run. Typically it has one start per day, and operates continuously for 1 to 3 hours per day.
So here are my questions: For the pump curves to make sense, this motor is operating at about 3380 rpm (6.1% slip). Do you think this much slip is likely, or should I suspect my pump curves? Or is my issue a combination of motor speed and inaccurate curve? When I get my maintenance people back out there, what should they measure?
Thanks for your time.
So here are my questions: For the pump curves to make sense, this motor is operating at about 3380 rpm (6.1% slip). Do you think this much slip is likely, or should I suspect my pump curves? Or is my issue a combination of motor speed and inaccurate curve? When I get my maintenance people back out there, what should they measure?
Thanks for your time.





RE: Loading versus slip
1 – overload. However if that much overloaded, you'd probably trip.
2 – low or unbalanced voltage. Slip is proportional to V^2 / Rrotor
3 – mis-connection causes the winding to see a low voltage even though system voltage is normal. For example connecting delta motor in wye.
4 – Rotor defect. Causes Rrotor increase. Again Slip is proportional to V^2 / Rrotor
fwiw, My gut feel is that you will find none of the above 3 applies to your situation.In similar scenarios at our plant, the problem usually lies on the side of understanding the fluid / pump characteristics, not the motor. My gut feel is none of the 3 applies
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(2B)+(2B)' ?
RE: Loading versus slip
The motor can run at higher slip than "expected" due to...
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(2B)+(2B)' ?
RE: Loading versus slip
The slip increase with load and is necessary for torque production.
So I think the slip is normal if the motor is a bit overloaded like you sayd.
Regards
Carlos
RE: Loading versus slip
RE: Loading versus slip
I agree. Also, if the cause is overload and your motor is powered from 60hz (not vfd), then your (op's) motor is operating at about 3 times full load. Check current using clamp-on and observing applicable electrical safety precautions.
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(2B)+(2B)' ?
RE: Loading versus slip
Electricpete - what do you mean operating at 3 times full load? Yes, we are operating at 60 Hz.
RE: Loading versus slip
EstimatedLoad = (3600 - 3380) / (3600 -3530) = 314%
This is a rough estimate, subject to some uncertainties errors mentioned above (voltage, slight rotor resistance variation with temperature, uncertainty in knowing true line frequency, possible measurement errors, possible errors in manufacturer labeling the nameplate etc).
I would not believe this until I had verified with currrent clamp-on that the motor is drawing approx 3 times full load amps.
If you have developed this estimate of slip by trying to tweak your pump curve to match your operating point using DP~N^2, Q~N.... I think the "error" likely lies on the fluid side of that calculation.
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(2B)+(2B)' ?
RE: Loading versus slip
RE: Loading versus slip
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(2B)+(2B)' ?
RE: Loading versus slip
Something to look at is are you using tested curves for both the motor and the pump, or just the typical curves? Most typical curves are generated purely by theoretical calculations. Some typical curves are created by making a perfect part and doing everything to make it operate the best, no matter the cost (then production is just slapped together).
Has the pump/motor been rebuilt since the curves were published?
One other thing, how long has the operation been off the curve?
RE: Loading versus slip
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RE: Loading versus slip
T=2*Tbr/(slipmax/slip+slip/slipmax) I got :
slipmax=sqrt(Tbr/T*slip^2-slip^2)-Tbr/T*slip [Tbr=Breakdown torque]
Taken Tbr=2[*Trated] slipmax=7.26% and returning to the first equation T=1.97*Trated. That means 97% overload and it could not work in this condition.
You have to check the no-load slip and the actual current under load. I agree with jraef: if nothing unusual is happened with your motor -no overheating, no overload trip-then only a wrong measurement could be. Check also the hydraulic data: flow [GPM], head [FT] and specific gravity. BHP= (GPM*H*Sp.Gr.)/3960/eff.
If you take the pump velocity from v^2=Head/2/g and v=rpm*D[ft]/229 [ft/sec] D=diameter of pump rotor.
Then rpm=229*v/D. The error from all this [and mainly from sp.gr depending on temperature also!] could be more than 6%.Pump efficiency could be a source of error, too.
RE: Loading versus slip
Sounds like we are all in violent agreement regarding the conclusion.
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(2B)+(2B)' ?
RE: Loading versus slip
RE: Loading versus slip
how are you measuring power?
if your motor nameplate indicates 3530rpm at rated frequency and rated nameplate voltage then the current drawn 'should be' the rated amps on the nameplate if the motor is running at 3530rpm. If the motor is operating at rated frequency on rated voltage and running at 3380rpm, the current will be higher due to the motor running in overload. What amps is it drawing?
I assume the pump is DOL (Direct on Line) i.e. no VFD controlling or electronic softstart or fluid coupling.
RE: Loading versus slip
So the power was estimated from fluid parameters.
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(2B)+(2B)' ?
RE: Loading versus slip
A slip of 6.1% is unrealistic. This equates to an overload of more than 300% by simple calculations. No need to get into motor torque curves for a more exact figure. At 6.1% slip, the motor would be so overloaded that it would quickly be shut down or destroyed.
Check your pump, dynamic head, specific gravity, impeller damage. Anything else you can think of.
Bill
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