Motor Pulling Less Current?

[ryanm]

How and why would a motor pull less current than is stated on its name please?

The motor is attached to a pump, which is underground and pumps water. The flow from underground/the pump is very poor, and is only pulling 10A instead of 15A.

How is this possible? In my very limited experience I've never seen a motor pull less current than the name plate says.

Inspecting the pump needs contractors to pull it up, so anything which I could possible check? The wiring is going through a contactor/overload, then to the pump.

Thanks

 

[ProDave]

The rating on the name plate is the maximum current when the motor is loaded to it's maximum mechanical power.

Put a small mechanical load on it, and it will draw a lot less power.

If you are saying it's drawing less power than it used to, then perhaps one of the impellors has fallen off or got damaged so the pump is less effective?

 

[ryanm]

Thanks for the replies.

The pump use to pull 15A when it was installed, and gave a good flow (no measurements though). It hasn't been used for a few months, and now it is only pulling 10A, with very poor flow.

I think you are correct saying some of the impellars may have become disconnected etc.

 

[poni]

more likely that there is crud in the impeller(normally just one fitted with fins) or the holes that let the water in are blocked   

 

[ryanm]

Poni,

If it was blocked though, surely the motor would pull more current?

 

[ProDave]

Surprisingly no.

Either way, I think the pump has to come up for inspection.

 

[ryanm]

Yeah, I think we need to get the contractors in to lift the pump out.

Out of interest, and for my education, why wouldn't the pump pull more current if it was blocked? I just presumed it would, and in my head it sort of makes sense, as the motor would be pushing against something which it wasn't designed for.

 

[ProDave]

Cavitation

 

[Sidewinder]

It's not necessarily "pushing / pumping" against a blockage.

It could be "sucking" against a blockage which will reduce the current.

Cavitation

Not quite PD, but nearly,

Cavitation in centrifugal pumps is a specific phenomena that occurs across a narrow pressure/flow band, and can cause major internal damage due to cavitation erosion.

 

[ryanm]

Brilliant, thanks for the help guys, makes more sense now.

The pump is 50m underground, and just has a 2" pipe attached to it, which feeds a tank. Quite simple, but not too accessible. Due to it not being used for a long time, it maybe is blocked on the suction side!

Thanks

 

[poni]

50m down, is it a borehole pump?

 

[poni]

as others have said yes blocked inlet holes will cause low flow and low current

if its a borehole then lime scale can build up inside the pump, most borehole pumps are multi stage pumps so just needs a few blocked veins, could it have run dry? this will burnout the stages(manly plastic )    

 

[Marvo]

Multistage centrif borehole pumps are best left alone unless you're familiar with them. Low run current could be due to blocked suction or blocked discharge as well in some cases. Also if there's insufficient head it can cause impellor lift which will affect the run current. Often when they drill the borehole they and they hit good water shallower than expected they install a pump where the hole depth doesn't fulfill the minimum head pressure requirement. They often throttle the pump on a valve on the discharge pipe to artificially introduce higher head. If someone has opened all the valves and taps completely this could cause your unexpected run current figures and could also result in pump damage.

 

[ryanm]

Hi Guys,

Just to update on this. I spoke to the company who installed our borehole (you were right ) and instantly they said it was running backwards! After some investigating, it turned out one of the shift techs had replaced 'a chatttering contactor' as he put it!! He obviously had swapped a phase over by mistake, and not tested it....good job it didn't damage the pump really.

Thanks for all the help again!