motor thermal overload
motor thermal overload
(OP)
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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RE: motor thermal overload
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: motor thermal overload
RE: motor thermal overload
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor thermal overload
RE: motor thermal overload
RE: motor thermal overload
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.
RE: motor thermal overload
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: motor thermal overload
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RE: motor thermal overload
"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg
RE: motor thermal overload
thread237-242074: Current values for motor-pumps with outgoing not open 100%
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor thermal overload
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.
http://www.eng-tips.com/faqs.cfm?fid=1543
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RE: motor thermal overload
FAQ237-1543: How does hp change with flow for a "centrifugal pump"
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RE: motor thermal overload
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 http://www.gouldspumps.com/cpf_0004.html:
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.
RE: motor thermal overload
RE: motor thermal overload
Have you compared the flow or d/p of the two pumps?
Have you compared the voltages?
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RE: motor thermal overload
RE: motor thermal overload
Are the currents balanced?
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RE: motor thermal overload
RE: motor thermal overload
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: motor thermal overload
RE: motor thermal overload
Let's clarify what you mean by "swapping motors/pumps":
You took motorA/pumpA and put it into piping system B and power supply B? (And put the Bpump/motor into A piping/power?) And the higher current followed the pump/motor?
If so an additional possible check would be to swap motors only and not pumps. If the "problem" follows motor only and power supply/pump/system are not changed then that is proof of the motor. Additional checks on the motor would be windings resistance check and winding insulation check. Also rotate by hand checking for roughness.
Also we made an assumption that the higher current was the abnormal one. It is probably the case if you have overload tripping now which was not tripping before under similar conditions. Could be confirmed also if you have the pump curve.
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RE: motor thermal overload
If the Head%=backpressure% then if backpressure[5psi]%= 27.5% and backpressure[20psi]%=4*27.5= 110%
From Fig.5 diagram:
If the Head would 110% then bhp=75% if the head is 27.5% then bhp=120%.
If bhp%=I% then if 8A= 75% then 14 A is 120%.Other combination may be possible also.
By the way, in your post of 28.05.09 22:40 you said [quote:] "flow rate, pressure and voltage are all the same" .I think in no way the flow and the pressure could be the same for 5psi and for 20psi backpressure.