De-rating motor nameplates to match utility load limits

[bentov]

Sorry if this is covered already, couldn't find it (likely didn't search correctly) . . .

Our local utility limits pole mount transformers to 300kva, thus 300hp max motor load. Deep well pumps often teeter on the edge of that (i.e. development testing might indicate "325hp max"). The service construction expense of exceeding 300hp (switch to pad mount transformers, more expensive meter mains, etc.) is significant, so users are motivated to understate potential load. Pump company contractors routinely re-stamp new motor nameplates (like, 350hp re-stamped to 300hp) to help users avoid that cost step-up. We understand (and practice when warranted) oversizing motors and controls within reason to improve reliability/longevity, but that practice seems problematic to me when nearing the utility supply limit threshold. Typically only the HP rating is altered, pretty obvious what's going on and would take a lot more effort (like an actual counterfeit nameplate) to hide the true motor rating - but apparently no one looks too closely.

When asked for an opinion I warn that regardless of pump bowl trim & actual load, across-the-line inrush current hinges on actual motor HP and rotor design. I caution that a utility transformer overload failure is likely to generate a big repair bill, and that responsibility for that cost might land on whoever yielded the stamp hammer if the cause is traced back to misrepresentation. We have to date declined(many)requests to do the actual physical restamping.

Now of course most are on soft starts or VFDs so starting currents (and in the case of VFDs full load currents) are adjustable/settable, and in good conscience we have performed those adjustments after the fact to keep everyone out of trouble. Still, if I were a utility engineer I would be thinking any settings turned down can be turned right back up.

What do you guys think? Is it ethical to install a new 350hp motor & VFD, stamp the nameplate and turn in the load on the utility application as 300hp, even though someone could come along later and override our programmed current limits?

 

[waross]

The Utility should be able to pick up potential problems by the peak demand readings on the revenue meters. Of course, in the real world this depends on the Utility policy and whether the Utility is flagging high peak demands for investigation.


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

 

[Parchie]

The Utility should be able to pick up potential problems by the peak demand readings on the revenue meters. Of course, in the real world this depends on the Utility policy and whether the Utility is flagging high peak demands for investigation.

Correct. Utilities install demand meters on industrial consumers aside from installing kWH meters. There is also this low PF penalty such that consumers keep a close watch on their load power factor/ use PF correction devices.

 

[edison123]

It's cheating, plain and simple. The utility peeps are not fools. They will catch it sooner than later.

Muthu

http://www.edison.co.in

 

[waross]

Companies with a good trusted relationship with their local utility and AHJ are, in many instances, allowed shortcuts in procedures. many of us have seen AHJs allow the utility to connect to a service before they have inspected the job, on the assurance of the contractor that the job is up to standard. Some less trusted contractors will have to wait until the AHJ can fit one more inspection into his schedule. The job will be inspected days or weeks later at the convenience of the AHJ.
Utilities have helped my to trouble shoot by supplying a dump of the meter data. Some of the newer meters are able to record a wealth of data which may be downloaded. Amps and Volts for each phase, kW demand, KVAR demand and a few other parameters at 15 minute intervals for the last month. This data was invaluable in trouble shooting several problems.
A good relationship and mutual trust with the utility and the AHJ is valuable to a business.
Some day there will be an issue that will be traced back to an oversized pump. The responsible contractor may lose that trust and relationship. Intangible but a business loss never the less.


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

 

[cranky108]

Something sounds strange here. Our company has 167 kVA pole mounted transformers single phase in stock, and if banked in a set of therr that would be 500 kVA. So this does not make math.
Is this an artificial limit being set by the utility?

There is a practical limit on how much weight can be put on a pole, and above that many companies would put in a platform between two poles.

We have a different problem with customers over stating there expected load, and the answer for that is simple, bill them for the service size. That usually brings the customer to a more realistic projection.

The only thing I can figure is the utility is trying to limit the growth on there overhead circuits.

If the concerns are well pumps, I am assuming this is a rural area, as very urban areas have big well pumps.

 

[bacon4life]

My utility has a limit on overhead bank transformers due to the weight on the pole for larger sizes. Although we have used platforms in the past, we now typically push for padmounted transformers. Part of it seems to be the increased transformer size as higher efficiency banks are mandated.

For across the line starting, we limit motor size due to flicker concerns. For VFDs with low harmonic distortion, we just evaluate the thermal loading of the transformer. One other consideration is how much engineering support the customer seems to have. If the customer owner is willing to acknowledge the risk having to later upsize the transformer, sizing smaller might be reasonable. If it is developer building something on spec, I would want make sure everything is sized large enough to make the final customer happy.

For a storm water lift station or other intermittent load, I wouldn't be too concerned. For a 24/7 load or summer peaking load, I would review the expected load factor. We would probably flag a transformer loaded to nameplate for more than a few weeks a year to have the sizing rechecked.

 

[bentov]

edison123, I'm with you (though we sugar coat the message a bit more to customers - still declining to wield the stamp ourselves). And I've always thought no one could actually "get away with" an actual cheat given standard demand metering, as pointed out by waross & Parchie.

I wonder if we're being unfair to some "stampers" though. I expect many projects require motor oversizing (for cooling considerations, etc.) so that installed equipment appears to exceed load reported to the utility. Routinely sizing utility supply for more than actual load in those cases would be inefficient though. But I bet inspectors (from local AHJ and/or utility) must check form boxes verifying installed vs. planned HP, going to be hard for those guys to accept anything other than physical rating marks. Rock and a hard place then . . .

We still have in the area some of those double pole platform mounted banks of 167s you mention, bacon4life (interesting handle!), but going forward the limit seems to be single can 300 ratings for pole mount. To further complicate things, the new incentive plan just rolled out: $40/hp for VFD "less than or equal to" 300hp, also $20/hp for premium efficiency motors "less than or equal to" 300hp, another $18k on the table. We would advise our farmer to invest in the 350 to future proof the well and avoid possible cooling issues in our intense summer heat. He's going to say, "OK, just be sure to restamp the nameplate so I can get the rebate". At least its not boring . . .

 

[cranky108]

Most wells have a rating on how much water they can provide, so sizing a pump larger than you need dosen't sound like a sound practice. Also most well permits have a maximum water permitted to pump, so a larger pump also isen't a good idea.

Have you consitered reduced voltage starting on your motor?

 

[jraef]

Load on a pump motor has to do how much pumping is being done, plus losses in the pump and motor. If you throttle the output, or trim the impeller, you are reducing the flow and thus reducing the HP and power draw of the machine. So if for example I have a 400HP motor and I throttle the output to 50%, I'm pulling significantly less kW from the source, maybe 250HP (lots of variables involved). Using a VFD on the pump has the same, more dramatic effect on power consumption.

One major obstacle to this is head, the ability to lift the column of water from the well. You can't beat the laws of physics. So TYPICALLY, a well pump is sized to get the water to the top or wherever it is being pumped, no more. Commercial off the shelf motors come in standard sizes, so do ESP (Electrical Submersible Pump) motors in that size range. So if they are using 350HP motors and trimming the impellers to provide 300HP of pumping, that's perfectly legit. Whether or not that's enough to get the water to where it needs to go is a hydraulic problem, not an electrical one.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington