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motor thermal overload

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jbudd

jbudd

Marine/Ocean
May 26, 2009
6
I have a 5hp ac motor. It trips off on thermal overload at light load but runs at a heavier load. At light load it draws 14 amps at a heavier load only draws 8 amps. Anyone know the cause?
 
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Have you tried measuring the voltage at light load? If the voltage is too high, the motor core may saturate and higher currents may result.




"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
 
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this motor is on a small ship with its own Generator. There is an exact duplicate of this motor/pump set and the other one works fine. The gen. voltage is constant.
 
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Can you define "light" and "heavier" load? Higher speed, for instance, doesn't necessary mean higher torque and current. Also, there can be a lower power factor at lower load, and overcompensate a load difference, drawing higher current RMS at lower kW(hp) load.
 
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light load is letting the pump discharge water overboard with only 5psi back pressure, heavier load is shutting discharge valve to bring pressure up to 20 psi.
 
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You might want to consult a pump curve to see if what you describe as "light" load really is light load.

Many centrifugal pumps will load the shaft more based on flow rate than back pressure.

Running the water over the edge at 5psi may increase the flow rate to such an extent that the motor is really overloaded. Restricting the flow so there is 20psi back pressure may reduce the flow to the extent that the shaft load goes down.
 
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If you can shut the valve without stalling the motor, then it is definitely a centrifugal type pump (as opposed to a deplacement pump) and then the load is at its highest when valve is fully open. Just as Dick says.

Same thing with a vacuum cleaner. Close the air intake (or outlet) and speed goes up as a result of lighter load, open fully and speed goes down because of high load. Contra-intuitive to many, but true nonetheless.

Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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What they are saying is that some types of pumps actually work in the opposite way you might think with regards to "loading". For several types of centrifugal pumps, load = flow, not pressure.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
 
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If the voltage is constant, I'd say you are mistaken about light vs heavy loading. The motor is drawing more amps because it is producing more power at the shaft.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
 
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All good comments above, and certainly correct that the behavior for your pump is increasing bhp requirement with increasing flow. It is the case for most but not all pumps commonly called centrifugal. It is the case for radial flow pumps and mixed flow pumps over portions of the range.

Your expectation of different behavior may come from experience with axial flow pumps which behave the opposite (decreasing bhp with increasing flow). Axial flow pumps are often used in high-flow low-dp applications - we have them in our power plant circ water pumps.


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Here is the FAQ I was trying to link:
faq237-1543

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I agree with electricpete. If the motor drives an axial centrifugal pump following the fig.7 of the indicated in the above FAQ
then closing the pump valve [throttling] actually we increase the Head in a way more than the flow and pump efficiency decreases and then the pump required bhp will increase too.
From Fig.no. 7 if the Head [Total Dynamic Head] will grow-up to 245%, efficiency will decrease to 29% level and the flow to 21%.
From bhp=Q*TDH*Sp.Gr./3960/Pump Efficiency
Since Sp.Gr does not change [the same temperature and the same liquid] :
bhp%=Q%*TDH%/Efficiency%=0.21*2.45/.29= 1.77[177%]
The motor required current will be I=bhp*.746*1000/voltage/motor efficiency/pf/sqrt(3)
We don’t expect dramatic change in pf and motor efficiency in the limits of 100% up to 177%
Then I%=bhp%
So, if I=8A is for rated flow [100%] then I= 14 A [175%] will be for only 21% flow.
 
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all the above info was great but still does not answer the original question. There are 2 motor/pump combinations, both worked fine but now one keeps tripping off on its thermal overload while the other does not. One draws 13 amps and the other only 8 amps.
 
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So the question is apparently now why you have a difference in current.

Have you compared the flow or d/p of the two pumps?
Have you compared the voltages?

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flow rate, pressure and voltage are all the same
 
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Hmmm, strange.

Are the currents balanced?

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Undo the two pumps at the unions on the suction and discharge side and swap them, pump and motor. Does the problem remain at the same location or does it move with the pump?
 
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There may be a restriction in the piping of one of the pumps. A piping restriction will lessen the load on many centrifugal pumps. Try davidbeache's suggestion.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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