VFD for pump

[BrianEngPME]

Hello,

I am an engineer w/mechanical & electrical background, reviewing a VFD submittal for some pumps. The VFD the contractor is proposing doesn't meet the specs, and I was wondering if anyone could tell me how important it is to have the ability to produce a lot of torque in order to get the pump started. The specs on the job call for the VFDs to provide a minimum current of up to 1.8 times the load for 3 seconds. The VFDs they are submitting can only provide up to 1.35 times the load current for only .5 seconds.

I want to be able to understand a little better, so I can make the judgement call as to let these VFDs pass, or just reject them outright. In addition to serving pumps, some will also probably serve HVAC equipment.

Thanks in advance,

Brian Meagher, EIT

 

[Mobius44]

It sounds to me like the supplier is trying to substitute smaller VFD's to save $$.

Depending on what is being pumped and how the pump is loaded, the pump may be very lightly or very heavily loaded at starting. Whether the smaller VFD's work in your application or not depends on factor's you didn't post and I don't know. In such situations I never feel lucky enough to roll the dice.

Why not just request the supplier either meet the spec, or have them explain why the equipment they want to substitute is acceptable in the application and ask them to accept responsibility (penalty $$) if the equipment doesn't perform?

 

[ScottyUK]

They 'might' be ok for a centrfugal or mixed flow pump, if it's a reciprocating type then your chances are slim-to-none. VFD manufacturers play some cute games with marketing: they sell essentially the same drive as a 'variable-torque' drive and rate it at, say, 100HP with very little overload capability and also sell it as a 'heavy-duty' drive rated at maybe 70HP and with a large overload capability.

I agree with Mobius44's opening statement: unless this is stupendously large pump then there will be a drive available which meets your spec, they are just trying to sell you something with an inferior specification which will be running closer to its thermal limit, on which basis it is unlikely to last as long before it fails.


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If we learn from our mistakes I'm getting a great education!

 

[BrianEngPME]

Well, it is a centrifugal pump.

Specs: "Description: Factory-assembled and -tested, centrifugal, overhung-impeller, close-coupled, end-suction pump as defined in HI 1.1-1.2 and HI 1.3; designed for installation with pump and motor shafts mounted horizontally. Rate pump for 125-psig minimum working pressure and a continuous water temperature of 225 deg F."

This is the only spec section on pumps, so that bit about temperature would be for the HW pumps, which serve piping that runs throughout the whole building to serve a VAV system. So far the VFDs submitted have only been for the CW pump & its backup. The CW piping goes to the AHU, which is in the same room as the pump and has a 30ft water head pres drop, as well as a chiller that is probably 120 feet away from the building and has a 4.7ft water head pres drop. So, with 3" pipe over all that distance, I would guess the pressure drop is appreciable? In which case, I should go with you guys' advice about asking for a better VFD?

 

[ScottyUK]

It will probably work, especially if the vendor is selling this as a packaged unit. Either drive will only deliver power demanded by the load, so the larger drive won't make a better job of spinning the pump, it just won't be working as hard to do so and the internal components of the drive such as DC bus capacitors, which have a relatively short life, will typically last longer in a drive which is not working hard. You decision is probably more down to weighing up the cost v's longevity balance than a pure 'will it work / won't it work' choice.


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If we learn from our mistakes I'm getting a great education!

 

[ozmosis]

I guess you need to look at the spec and determine if it is being used to meet the actual requirement of the site conditions or it is a generic specification written by a VFD vendor looking to get their product only specified, whether you need that extra starting current or not.

Also, look at the current rating of the two VFD's and determine if 1.8 x FLC of VFD1 is actually higher than 1.35 x FLC of VFD2 and then compare this to the motor and determine if it is relevant.

 

[BrianEngPME]

Well, Scotty, the VFD is not a packaged unit with the pump. They are separate vendors. BTW, I was misled by some ambiguous labeling in the submittal; it turns out they were providing VFDs for both the HW and CW pumps. Also, Oz, it is our project (US gov't) spec we're using, not one of those overly-specific mfgr specs that are basically marketing gags. But anyway, to follow your suggestion...

HW pump - 16.7 FLA
CW pump - 24.2 FLA

Per project spec:
HW VFD: 16.7 x 1.8 = 30.1 A
CW VFD: 24.2 x 1.8 = 43.6 A

Per equip't from vendor:
HW VFD:

Ok, I can't even do that calculation because they submitted VFDs for pumps of the wrong voltage. Either somebody else in our office approved a change of what panel is serving the pumps (not likely) or this is just a really bad submittal.

Thank you guys for your help. I reckon that if they get their act together for the next submittal, and they submit from the same manufacturer, I'll use you guys' advice to review the equipment at that time.

Thanks again

Brian Meagher, EIT

 

[BrianEngPME]

Sorry Oz, I didn't mean any offense about the comment I made about the manufacturer specs. Maybe you work for an equipment manufacturer... Um, I know that's just standard business practice, and I guess I was being harsh. But I really do appreciate the advice from everyone.

Brian Meagher, EIT

 

[ozmosis]

Brian
No offense taken. Yes, I do work for one of those VFD manufacturers but we do try and ensure our tender documents reflect what is really required and then follow up with the right information..
I was meaning with the current rating that you should look at the FLC of the motor at the rated voltage and the rated Amps of the VFD. You may find that there are differences between VFD manufacturers, and the % overloads of the apparent lower rated VFD may either be higher than the other or at least adequate for the rated motor.

 

[DickDV]

As a nearly universal rule, drive/motor packages running centrifugal pumps and fans are specified for full motor hp and amps with 10% short-term overload.

To specify 80% short-term overload (1.8) is generally a sign that the spec writer didn't know what he/she was doing. As a result, the people quoting have to add a little judgement, given the application, and quote what would appear to be "normal" sized equipment.

To choose to drop from 1.8 times nameplate to 1.35 times nameplate is hardly irresponsible under the circumstances. I would be tempted to drop to 1.1 if I had the drive that was sized for that ratio of overload.

What is irresponsible is to do that and not point it out. So, I would exercise my best judgement and then take exception with an explanation.

While there are surely good specs out there, I am sure that anyone having to deal with specs these days, especially government specs, will agree that many are so poorly written that it almost becomes impossible to figure out what to quote. Sadly, it ends up that the vendor that is willing to "cheat" the most, wins the bid. Making the situation even more difficult is the ugly reception a vendor typically gets if he/she questions inconsistencies or absurdities in the spec. I've been so poorly treated when trying to bring some reasonableness to a spec that often we just walk away. It isn't worth the hassle.

When I see a well-written spec, it practically glows in the dark!