Motor Rating

[steve66]

A 3 HP motor is labeled at 9.5 amps, 208V, 3 phase.

If the motor draws 10.9 amps, does that mean its being overloaded?

The thing that confuses me is that the NEC tables rate a 3 HP motor at 11 amps.

The motor is rated at a 1.15 Service factor, and I know that means it can be 25% overloaded without causing any real problems. But the project specs state that motors aren't allowed to be operated above a 1.0 service factor, and the owner wants something done to fix this percieved overload.

 

[Lectricbota]

A 3 HP motor is labeled at 9.5 amps, 208V, 3 phase.

If the motor draws 10.9 amps, does that mean its being overloaded?

Yes

The thing that confuses me is that the NEC tables rate a 3 HP motor at 11 amps.

The NEC tables are for branch circuit wiring and protection and seems to me they take the worst case scenario approach in case that motor has to be replaced with a different one down the line. It has no bearing on specific motor running amps.

The motor is rated at a 1.15 Service factor, and I know that means it can be 25% overloaded without causing any real problems. But the project specs state that motors aren't allowed to be operated above a 1.0 service factor, and the owner wants something done to fix this percieved overload.

I don't think it would be a good idea to overload a motor continuously but hopefully someone with more knowledge on this will chime in.

 

[kwired]

A 3 HP motor is labeled at 9.5 amps, 208V, 3 phase.

If the motor draws 10.9 amps, does that mean its being overloaded?

The thing that confuses me is that the NEC tables rate a 3 HP motor at 11 amps.

The motor is rated at a 1.15 Service factor, and I know that means it can be 25% overloaded without causing any real problems. But the project specs state that motors aren't allowed to be operated above a 1.0 service factor, and the owner wants something done to fix this percieved overload.

Fixing the overload condition means lessening the driven load or putting in a larger motor. When you tell owner he has to slow down the rate of work being done by this motor he likely will not like that. They always want more from equipment than what it is designed for, and can't understand why motors burn out if run in the service factor a lot.

 

[steve66]

This motor runs a sewage ejector pump. So it runs for 15 sec., and then sets for hours. So I have no concerns about it being overloaded (especially given the service factor). But the owner also has the right to enforce a spec, so I believe we may need to reduce the running amps.

The contractor has proposed choking off the inlet valve a little to reduce the flow rate.

 

[Besoeker]

This motor runs a sewage ejector pump. So it runs for 15 sec., and then sets for hours.
So I have no concerns about it being overloaded (especially given the service factor).

If the motor is continuously rated and runs at that duty then, like you, I don't see it as a problem. And presumably the motor has overload protection so if it did get overloaded for long enough to be a potential problem, it would trip anyway.

But the owner also has the right to enforce a spec, so I believe we may need to reduce the running amps.

What specification would he be enforcing?

The contractor has proposed choking off the inlet valve a little to reduce the flow rate.

So the pump will run a bit longer at a bit lower load? And the system would probably be less efficient given the wasted power with the pressure drop across the inlet valve.
What would that achieve?
Not a lot I suspect. If it was up to me, I'd argue the case for the motor being suitable for purpose.

 

[rbalex]

Steve, if you have access to a full version of NEMA MG-1, you should review Section 14.37 to see what "service factor" really means. Unfortunately the MG-1 Condensed version doesn't quite say much more than ?In those applications requiring an overload capacity, the use of a higher horsepower rating is recommended to avoid exceeding the temperature rises for the class of insulation system used and to provide adequate torque capacity.?

The NEC Tables are based on the absolute worst motor any NEMA manufacture makes. The Tables don?t really apply to overloads. See Section 430.6.

 

[ActionDave]

This motor runs a sewage ejector pump. So it runs for 15 sec., and then sets for hours. So I have no concerns about it being overloaded (especially given the service factor). But the owner also has the right to enforce a spec, so I believe we may need to reduce the running amps.

Is the motor running long enough to develop any backpressure?

The contractor has proposed choking off the inlet valve a little to reduce the flow rate.

That would work in principal, but then you go back to my question. Overall I agree with you, no worries. I think your contractor is being a bit anal. Not a terrible thing in the sewage business, but sometimes you gotta let it go.

 

[Besoeker]

I think your contractor is being a bit anal. Not a terrible thing in the sewage business, but sometimes you gotta let it go.

A producer of material that drives his business sector??

 

[ActionDave]

A producer of material that drives his business sector??

Quite right old chap.

On a related note - If you have not seen the movie "Kenny" I highly recommend it. It is an Aussie mockumentary from which I stole the phrase "Let it go".

 

[steve66]

What specification would he be enforcing?

The owners standard specification. Its a bit odd - the owner has a huge specification that they expect Architects and Engineers to start with for a project. Its probably 300 or 400 pages.

Some of the specs are contractor oriented, but some are more like requirments for Architects and Engineers to follow while designing a project. I think its fairly common for large Universities and public facilities.

Some of it makes sense, and some is just downright silly.

 

[steve66]

Steve, if you have access to a full version of NEMA MG-1, you should review Section 14.37 to see what "service factor" really means. Unfortunately the MG-1 Condensed version doesn't quite say much more than “In those applications requiring an overload capacity, the use of a higher horsepower rating is recommended to avoid exceeding the temperature rises for the class of insulation system used and to provide adequate torque capacity.”

The NEC Tables are based on the absolute worst motor any NEMA manufacture makes. The Tables don’t really apply to overloads. See Section 430.6.

I was just going by what the NEC handbook says which is basically that a motor with a 1.15 SF will tolerate a 25% overload indefinately without overheating. I realize its not good to push equipment to its limit, but we are only talking about a 15% overload for a few seconds. Then the motor shuts off completely.

Is the motor running long enough to develop any backpressure?

I think so - it wouldn't draw more than rated amps if the backpressure wasn't there.

That would work in principal, but then you go back to my question. Overall I agree with you, no worries. I think your contractor is being a bit anal. Not a terrible thing in the sewage business, but sometimes you gotta let it go.

It's not the contractor that has the issue with the 15% overload - its the owner. The contractor is just trying to find some way to resolve the situation without replacing an entire motor.

 

[ActionDave]

...I think so - it wouldn't draw more than rated amps if the backpressure wasn't there.

Funny thing about centrifugal pumps, they need backpressure or they will draw more amps. It is counter-intuitive but once you get your mind around it it makes sense. If the pump has no pressure against it it can move more effluent so it is doing more work.

 

[Besoeker]

It's not the contractor that has the issue with the 15% overload - its the owner. The contractor is just trying to find some way to resolve the situation without replacing an entire motor.

Maybe the contractor could offer a performance warranty on the installation?
The motor will do what it is required to do. Repair or replacement will be provided in the event of failure. How could the owner reasonably refuse that?
For that matter, does the owner understand the technical issues?
Is he/she/it qualified to dictate how the contractor he/she/it engaged to do the job how to do the job?

 

[Gregg Harris]

I was just going by what the NEC handbook says which is basically that a motor with a 1.15 SF will tolerate a 25% overload indefinately without overheating. I realize its not good to push equipment to its limit, but we are only talking about a 15% overload for a few seconds. Then the motor shuts off completely.


I think so - it wouldn't draw more than rated amps if the backpressure wasn't there.



It's not the contractor that has the issue with the 15% overload - its the owner. The contractor is just trying to find some way to resolve the situation without replacing an entire motor.

Do you have or can get the actual name plate information and the actual voltage at the pump?

 

[ActionDave]

...
For that matter, does the owner understand the technical issues?
Is he/she/it qualified to dictate how the contractor he/she/it engaged to do the job how to do the job?

How much would you wager in $ or lbs. the answer is, "No."

 

[Jraef]

You, as the electrician, have absolutely NO CONTROL over the amps being drawn by the motor. Let it go... (I like that).

Load = flow in a centrifugal pump; less flow = less load if the head remains the same, hence the concept of the reduction in the intake. He could also trim the impeller, reduce the size of the pipe, it really doesn't matter how he gets there. But once the flow is reduced, the load on the motor will be reduced and the current will drop. Most likely, the pump OEM looked at the pumping performance spec, i.e. head and flow, and decided to do the job by running a smaller motor into the Service Factor rather than a larger motor. Standard Operating Procedure for 99% of pump OEMS I'm affraid; trim the cost to take the job low. But as you point out, the owners can write their owns specs, silly or not, and enforce them. That's the risk the lowballer supplier or contractor takes in bidding based on liberal spec interpretation and taking a chance that nobody will notice; he lost that bet. Again, not your issue, the pump suppler or mechanical contractor needs to deal with it in a method acceptable to the owner.

By the way, a motor with a 1.15 Service factor can be run at 115% of rated load, not 125%. Running a motor into the service factor is expected to put the motor sevice life and some performace specifications at risk, but NEMA MG-1 doesn't say anything stronger than that any more (it used to). Most likely the 125% you are thinging of is the maximum pickup point of the thermal overload proterction; separate issue.

Paraphrased (to avoid copyright infringement) from MG-1-1998

"Sevice factor: a multiplier applied to the HP rating that indicates a permissible HP loading that can be carried out under the conditions specified for the service factor."

"A motor running continuously at a service factor greater than 1.0 will have a reduced life expectancy compared to running at its rated power. It may have efficiency, power factor and speed different from those at its rated load, but minimum locked rotor and breakdown torque values will remain the same."

End users have every right in my opinion to insist that motors NOT be sized so as to run into the Service Factor, because THEY are the ones who will bear the long term cost of premature replacement. If they CLEARLY state that up front in the project specifications, then like I said, the pump supplier rolled the dice and lost. If they did not however, and are trying to enforce apresumptive expectation that was NOT part of the original specs, then they should bear the burden of paying for a change order.

 

[rbalex]

FWIW MG-1-2006 says pretty much the same thing. Section 14.37 cross references several other specific effects of going past the 1.0 SF.

As a general rule, most refineries I've worked for specify designing at 1.0 SF.

 

[Besoeker]

The owners standard specification. Its a bit odd - the owner has a huge specification that they expect Architects and Engineers to start with for a project. Its probably 300 or 400 pages.

Some of the specs are contractor oriented, but some are more like requirments for Architects and Engineers to follow while designing a project. I think its fairly common for large Universities and public facilities.

Some of it makes sense, and some is just downright silly.

Yes, I come across things like that - mostly from consultants. And some of it is, as you say, downright silly. One that sticks in my mind was a specification for some fairly big variable speed drives. The actual performance requirement was given on about three pages. By way of contrast there was a 40-page document on how to build the wooden packing cases - including diagrams of the angle that the nails had to be hammered in at.

Another was for a bunch of heater control panels. No problem. Had done a couple of similar projects for the same customer - or rather the same end user. This time it came to us via a facilities management company (FMC). We'd bid the project much as we had done previously and were awarded the contract. A day or so later the project manager (FMC) came to see me loaded down with specifications. He had a box that we normally get five reams of copy paper in. That's already 2,500 sheets. There were further files taped to the outside of the box. All this after the project had been placed.
There was a specification on how manuals should be written - including font style and size and a page on the correct use of the comma....you couldn't make it up.........ironically, their specifications didn't abide by those rules.

Back to your motor. Does the specification require the motor to be continuously maxim rated or does it allow, or even could it be argued that it allows consideration of an intermittent duty cycle?

 

[kwired]

You, as the electrician, have absolutely NO CONTROL over the amps being drawn by the motor. Let it go... (I like that).

Load = flow in a centrifugal pump; less flow = less load if the head remains the same, hence the concept of the reduction in the intake. He could also trim the impeller, reduce the size of the pipe, it really doesn't matter how he gets there. But once the flow is reduced, the load on the motor will be reduced and the current will drop. Most likely, the pump OEM looked at the pumping performance spec, i.e. head and flow, and decided to do the job by running a smaller motor into the Service Factor rather than a larger motor. Standard Operating Procedure for 99% of pump OEMS I'm affraid; trim the cost to take the job low. But as you point out, the owners can write their owns specs, silly or not, and enforce them. That's the risk the lowballer supplier or contractor takes in bidding based on liberal spec interpretation and taking a chance that nobody will notice; he lost that bet. Again, not your issue, the pump suppler or mechanical contractor needs to deal with it in a method acceptable to the owner.

By the way, a motor with a 1.15 Service factor can be run at 115% of rated load, not 125%. Running a motor into the service factor is expected to put the motor sevice life and some performace specifications at risk, but NEMA MG-1 doesn't say anything stronger than that any more (it used to). Most likely the 125% you are thinging of is the maximum pickup point of the thermal overload proterction; separate issue.

Paraphrased (to avoid copyright infringement) from MG-1-1998


End users have every right in my opinion to insist that motors NOT be sized so as to run into the Service Factor, because THEY are the ones who will bear the long term cost of premature replacement. If they CLEARLY state that up front in the project specifications, then like I said, the pump supplier rolled the dice and lost. If they did not however, and are trying to enforce apresumptive expectation that was NOT part of the original specs, then they should bear the burden of paying for a change order.

My experience the owner never wants to lessen the load on the motor, they are always wanting me to turn up overload settings. Even when you explain to them the motor is doing all it is designed to do they still say things like "we need to be able to unload trucks faster than that" :slaphead: That is when you have to hold back from saying something like "if your cheap @#$ would have purchased equipment sized for your needs you wouldn't be having this issue now you are trying to blame the overload problems on me and my installation of equipment you selected that was too small for the job, or at least for your current expectations".