Uh-oh?

[220wire]

I have problem with two 230/460v 15 kw 30hp (56amps) pumps. The service is 208/120Y 3 phase and the two pumps are 230v 3 phase. I installed a 75kva transformer to step up the voltage to 230v, and two VFD's to control the pumps and amperage draw. The transformer I got was an Acme transformer (TP-53364-4S) which I backfed and didn't not ground the XO terminal. One pump motor that was rebuilt recently burned up which may have been due to the bearings that wern't replaced. The VFD's have 230-234 volts on line and load sides. The amperage which should be 56amps per VFD is going waaay down to around 28 amps when both pumps are running at the same time. They are set up for lead lag and usualy don't run together. My question is why would the ampacity drop down to half with both pumps running? The tech said that the voltage isn't changing when they are on together.

 

[ohmhead]

I have problem with two 230/460v 15 kw 30hp (56amps) pumps. The service is 208/120Y 3 phase and the two pumps are 230v 3 phase. I installed a 75kva transformer to step up the voltage to 230v, and two VFD's to control the pumps and amperage draw. The transformer I got was an Acme transformer (TP-53364-4S) which I backfed and didn't not ground the XO terminal. One pump motor that was rebuilt recently burned up which may have been due to the bearings that wern't replaced. The VFD's have 230-234 volts on line and load sides. The amperage which should be 56amps per VFD is going waaay down to around 28 amps when both pumps are running at the same time. They are set up for lead lag and usualy don't run together. My question is why would the ampacity drop down to half with both pumps running? The tech said that the voltage isn't changing when they are on together.

Well check your table in code book for 230 volt 3 phase motors ot might help they pull 80 amps but what type of motors ?

Even a synchronous pulls over 56 amps .

I would say thats your voltage problem

 

[hardworkingstiff]

If a pump is dead-heading, the amperage will drop. I don't know if it will drop to what you are reading, but the less work they are doing, the lower the amperage. I don't know much about VFDs, but I'd re-check to make sure they are all programmed correctly.

 

[ptonsparky]

Are they set up with pressure transducers to control speed and pressure?

Could be just normal operation for two pumps running.

 

[ptonsparky]

In your case one pump may normally supply all the pressure that is needed. When use causes the pressure to drop below what one pump can supplpy the second will come on providing the needed boost in volume. When this happens both pumps will then run at reduced speed and amperage until the pressure has been reached for ___ amount of time. Depending on how they are set up via the VFD the 30HP pumps may only pull 58 amps.

 

[hillbilly1]

Is it a sewage lift station? Usually on those if the product overwelms the chosen pump, the other kicks on for more volume, Maybe the output piping is too small. Just guessing.

 

[Doug S.]

Dumb question?

Dumb question?

Are the pumps helping each other, IE each does less work when they are both running?

 

[220wire]

The pumps are on a lead/lag/alternate primary setup, so when the effluent level reaches a certain level one pump kicks on, if that pump isn't keeping up with the flow into the liftstation then the second pump kicks on. There is a transducer in the pit but it is used just to sense the level and not to control the speed of the drives. The VFD is set to ramp the pumps to full power within 3 or 4 seconds. There is a check valve that also opens to prevent sewage backflow. The nameplate and the pump manufacturer (Mody) both state 56A @ 230v. We thought that the transformer was having issues, but think that is ruled out now.

 

[Smart $]

Could be the effluent inlet is restricted enough that neither pump motor is fully loaded when both are running. Do you have any means to check each pump's output volume? If yes, compare volume of one pump running to volume of both pumps running and if not near double, the inlet is the problem.

 

[masterinbama]

Are your pumps drawing full current when running solo, then dropping in amp draw when both are running? This would be normal for impeller style pumps. Their amp load is proportional to volume of product moved not pressure produced. I.E. a pump designed to move 1000 GPM will draw FLC @ 1000 GPM but will draw significantly less current @ 500GPM. You can actually have a motor draw more than nameplate if it is operated above rated flow and below rated pressure.

 

[Smart $]

Could be the effluent inlet is restricted enough that neither pump motor is fully loaded when both are running. Do you have any means to check each pump's output volume? If yes, compare volume of one pump running to volume of both pumps running and if not near double, the inlet is the problem.

Ran out of time before I could change the "is"... should be "may". My brain isn't exactly geared for fluid dynamics at present :roll:

Another possiblitiy is the discharge piping. Are the discharges run separately or is the effluent recombined into one process pipe via a tee. If the latter, is the process piping larger after the tee? If not, you probably have a bottleneck effect at the tee. Aside from peculiararities of fluid dynamics, most centrifigal pumps work by creating differential pressure. In electrical terms, somewhat similar to the voltage of a battery. Two batteries connected in parallel will provide the same voltage, i.e. pressure. And a connected load will still receive the same amount of current, i.e. flow. Half from each battery, given they have identical characteristics.

 

[LarryFine]

I wonder, too, if both pumps share a single intake pipe.

 

[220wire]

The effluent is pumped into a pit approx. 8x8x15 and from there these two pumps work to control the level in the pit. I believe that both pumps are bottlenecked into one 6" output line which runs about 3/4 of a mile to the waste treatment ponds. They have grinders/macerators on them to assist with the larger items that pass through. I thought that a pumps amperage was supposed to be somewhat close to the nameplate not no matter what the situation. The Poco has a 75kva transformer feeding the pump station building and another building off to the side. I also used a 75kva thinking that I needed to give the pumps some extra room if they needed it and not installing the minimum size transformer, also recognizing that transformers are most effecient near their max kva rating. The pumps calc'd out to be about 44.3kva, so I am not sure if something is up with the way I designed this. I didn't hook up the new replacement pump but it sounded like it was a wye/delta pump. Reason being the tech told me that there were two sets of wires in the cable, one bigger set and one smaller set. I also thought that a pump motor weather it is tapped on the high or low side doesn't care if its power supply is wye or delta just that the voltage is within the manufucturer's specs. I am still lost on this and appreciate the responses from you all.
Cheers-

 

[hillbilly1]

I didn't hook up the new replacement pump but it sounded like it was a wye/delta pump. Reason being the tech told me that there were two sets of wires in the cable, one bigger set and one smaller set.
Cheers-

The smaller set may be thermal cut out wires, some motors have internal temp sensors to monitor or shut down the controls if the motor overheats. How small are they, and how many?

 

[Besoeker]

The effluent is pumped into a pit approx. 8x8x15 and from there these two pumps work to control the level in the pit. I believe that both pumps are bottlenecked into one 6" output line which runs about 3/4 of a mile to the waste treatment ponds.

That hydraulic set-up may explain a lot about what's happening. If the two pumps are running in parallel at the same speed they will produce no more pressure (head) with two running than with one. The pressure drop along that long pipe will largely determine the flow rate. And flow times pressure gives you power.
Same flow, same pressure, same total power with one pump running or two.
So it is entirely reasonable to see the current drop in each motor when you run two of them together.

I thought that a pumps amperage was supposed to be somewhat close to the nameplate not no matter what the situation.

No. At full speed full power the current should match the nameplate figure.
At no load, the current might be around 30% of nameplate but it can vary for motors of differing makes and ratings.