motor thermal overload

[jbudd]

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?

 

[jbudd]

already tried swapping motor/pumps, seems like an electrical problem with the motor

 

[electricpete]

I still want to know if current is unbalanced. I assume this is three phase motor (is it?).

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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[7anoter4]

I am sorry, I didn't remark your post of 27.05.09 8:18 and I thought the flow was the "load" .If the load is the backpressure, in this case the pump is a radial flow pump and from the Fig no. 5[from the link of the electricpete FAQ] an explanation may be as following:
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.