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Considerations for Purchasing a Large Electric Motor

04/07/2020

 

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For some reason, you've found yourself in a position where you need to purchase a large electric motor -- as in a motor that is not a standard "NEMA" electric motor. It may be that some new equipment is being installed at your location, you may be in search of a surplus motor, or the time has come to replace one. Regardless of why you need a large electric motor, there are some key things to keep in mind. But first, let's define what we mean by large.

Large Electric Motors

These are not your everyday, on-the-shelf motors that every PT (Power Train) house and motor shop has in stock.

The kind of motors under discussion here are typically built specifically for your application and although some large motors may provide hundreds of horsepower (400+ hp), the kind I'm talking provide thousands of horsepower. These are typically referred to Above-NEMA or A-NEMA motors and they can be far more challenging to purchase than a standard motor.

The Basics of Bidding Out an ANEMA Motor

First, start with the basics which include information such as ...

  • Horsepower
  • Speed
  • Voltage
  • Full load amps
  • Enclosure
  • Frame
  • Mounting
  • Bearing type
  • NEMA design
  • KVA code
  • and quite a few more!

You also want to include any other information you have about the original motor and its application. This can include frame dimensions, speed vs. torque curves, data packets, and schematics.

But There's More!

However, that still isn't enough. Common sense tells us that if we order fruit, we might get anything from an apple to a raspberry -- but what if we had an orange in mind? Believe it or not, this relates to obtaining quotes for large motors.

If you go to a motor manufacturer and simply tell them you need a large motor, there is a good chance that you won't get what you need. This is especially true if you are requesting bids from different manufacturers. You need to make sure that you give all of them the exact same set of parameters and be as specific as possible or you will not get what you need.

Let's talk about what I mean by specific. You may require something like this:

  • Class F insulation system or better
  • Sleeve bearings to be split for ease of maintenance
  • Vibration levels to be within API 541
  • Witness testing of certain tests by plant personnel
  • Screens/filters to be re-useable and washable

You can also require that the manufacturer meet certain specifications, such as IEEE 522 (Surge Comparison Testing), NEMA MG1 (Standards for Motors & Generator), ANSI C50.41 (Polyphase Induction Motors for Generating Stations) or API 541 (Form Wound SCIM’s 350hp+) -- just to name a few.

In short, you can be as specific as you need to be in order to get the right motor -- and you're going to need to be very specific.

Considerations when Purchasing a Large Motor

There are some additional considerations to take into account when you are in the market for an A-NEMA motor. For example, if you need a large motor quickly, consider a surplus motor or adapting to a motor that is more readily available. You often can purchase a surplus Above NEMA motor, completely rebuilt and 100% warrantied for 20-40% less than the cost of a brand new one and it may be at a fraction of the lead time. Surplus motors have been known to save the day in an emergency, besides saving time and money.

Also keep in mind that every manufacturer makes their large motors a little different. If you go direct to the manufacturer, you may not see/hear the differences -- but an experienced supplier of large motors can help you see the whole picture and all the options. We just happen to know a really good supplier of large electric motors! 

Conclusion

One of the best places to purchase a large motor is through an EASA repair shop. Their experience in troubleshooting and repair will give them insight into how motors work in different applications and how they can differ from manufacturer to manufacturer. Just remember there are many different options -- and HECO can help you with them all.

 

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