Motor per NEMA MG-1 Electrical, Dimensions per IEC

[MechEng Dan]

Let me start off by mentioning that I am a mechanical engineer and do not have in-depth knowledge of motors.
I am reviewing a bid for a large boiler feedwater pump that will require a 7,500 hp motor and will be installed in the United States. My specification requires compliance with NEMA MG-1. (My specification specifically indicates IEC motors are not acceptable.) The prospective pump supplier informs me the proposed motor will be fully compliant with NEMA insofar as electrical design and performance is concerned. However, I am told the dimensions of the motor will comply with IEC, not NEMA. I was further informed NEMA does not provide any dimensional guidance/requirements at this size motor.

Is NEMA MG-1 electrical & performance adherence with IEC dimensions an typical or expected arrangement for this motor size?
I am concerned about taking on a hybrid NEMA/IEC motor for installation in the United States.

 

[GroovyGuy]

Hi Dan,
At 7500hp, you are well above the limits of NEMA MG-1. I am a tad bit tainted as I work in the oil & gas industries, and therefore I would recommend that you consider AP1-541 spec for a motor of this size.
The following links will give you an introduction to motors > NEMA MG-1;

https://www.baldor.com/mvc/DownloadCenter/Files/BR435

https://www.industry.usa.siemens.co...ic-motor/anema-motors/Pages/anema-motors.aspx

GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[ScottyUK]

At that output you'll be looking at a custom or semi-custom design, so you get to pick a lot of the features.

API 541 is almost a pro-forma spec from which you choose your options. You can't simply order an API541 motor and have any certainty what you will receive because there are a myriad of options from which you need to select and state your preferences.

As an end-user, on a weekly basis I curse the engineers of 40-odd years ago who allowed a few dozen NEMA motors onto our site, not because they are bad motors but because they are incompatible with pretty much everything in this part of the world: Imperial fasteners, Imperial dimensions, non-recognised certification, split bolt cable connections... the list of crimes is endless. You may find that putting an IEC motor into a NEMA country will have much the same effect on your customer.

 

[BrianPetersen]

Disclaimer: no personal experience with motors of that size. BUT. While the ability to pick and choose options might sound like a good thing to do, be mindful of the possibility that the supplier has already done this to their preferences, and overruling their preferences CAN change what would otherwise have been as close to an off-the-shelf and in-stock motor as you are apt to get (with something of that size), into a special-order that takes longer to get and with a price increase to suit.

Motors in a size range that I'm more familiar with (much smaller) and which happen to be direct-coupled to the input of (let's say) a SEW Eurodrive gearbox, are commonly IEC-dimensioned motors built to US or Canadian electrical standards.

Talk to the supplier and identify what issues you have and what flexibilities you have.

A motor of that size is not something that you are going to walk into an industrial supply store to buy an in-stock replacement anyhow.

 

[rhatcher]

Dan,

It is not unusual for a motor of this size to be built to meet NEMA MG1 electrical standards with an IEC (metric dimensioned) frame. The question is what difference it makes once you are above the standard NEMA and IEC frame sizes. The answer is that it makes very little difference, literally.

NEMA frames will use inch dimensions and IEC frames will use millimeter dimensions and, for either use, the dimensions will be easy to measure ‘round’ numbers. For example, the NEMA 10840 frame has a shaft height of 23in/584.2mm while the comparable IEC frames are 560mm/22.05in and 630mm/24.08in. The reputable large pump OEMs can build to either standard.

I am in agreement with GG that the quality of the motor is much more important and the best way to assure this is to have the motor built to specification. API 541 for the Oil and Gas industry is a great specification but it adds a lot of cost to the motor and may be more than you need. An example of what I mean by “more than you need” is API’s heavy emphasis on meeting Division 2 hazardous area location requirements. Many industries outside of Oil and Gas have no need for this requirement.

A good specification for above MEMA motors outside of the Oil and Gas industry is ANSI/NEMA C50.41-2012 “Polyphase Induction Motors for Power Generating Stations”. This may be more appropriate for your boiler feed pump application.

 

[ScottyUK]

Although withdrawn in 2002, BS5000-40 "Motors for driving power station auxiliaries" isn't a bad starting point. It's not worth paying for now it's been retired as there is only one page of any real value, but among the key requirements are that the motors must be capable of:
[ul]
[li]Starting and running at 0.8 pu rated voltage for at least twice the normal starting time;[/li]
[li]Continuous operation between 0.96 and 1.02 pu frequency, with voltage and load falling pro-rata below 0.96 pu.[/li]
[/ul]

I'm surprised that there aren't detailed specifications issued by the utility building the plant, a 7500HP BFW pump must be going into a decent-sized station and I doubt this is the first one they've built. Client specs are generally a lot more prescriptive than the minimum set by the standards, certainly the specs issued by the CEGB and successor companies were very prescriptive.

 

[GroovyGuy]

I will agree with you in that specifying an API-541, with all of it's options, is not for the faint of heart. In particular I would recommend that you carefully review the level of (factory) tests required, as this can add a lot of additional cost to your motor.

Dan: Is this service configured as stand-alone or will it be spared (ie n+1)? Will the motor be DOL started, ASD driven, soft-start, etc? ie You should be able to lower the motor's cost and improve the motor's efficiency by 1% if you intend never to DOL start the motor, but rather have the motor ASD started/controlled.

rhatcher: I wouldn't think that a API-541 motor's cost is affected too much by the fact that it is Zone 2 approved, although I don't know this for a fact as I have never spec'd an API-541 motor that was not to be Zone 2 approved.
I was not aware of the specifcation ANSI/NEMA C50.41-2012 “Polyphase Induction Motors for Power Generating Stations”. I would certainly reccomend to Dan that he aquire a copy of this spec -- perhaps it is all that he might need.
GG



"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[rhatcher]

GG,

The Division 2 requirements do not add much cost unless the motor frame has to be oversized to meet the temperature code. This was given as an example of a requirement that many industries do not need. As you stated, the degree of factory testing as well as other requirements do make an API 541 motor more expensive than it's non-API counterpart.