August 2007 Electrical Apparatus

August 2007 Electrical Apparatus

This is a summary of the Electrical Apparatus August 2007 featured technical article,  by Richard L. Nailen, P.E.    

Once the rating and availability of a motor has been decided upon through study of the application requirements, the purchaser will benefit from reviewing a number of common alternatives that will influence the overall economics of the installation. Unless a user is certain that a completely standard motor will be suitable, careful attention should be given to special features, accessories (such as oil mist lubrication or space heaters), and a specification detailing what is required. That should include a request for factory test data that may be desired as a benchmark for condition monitoring in service.

A useful checklist includes such items as:

1, Voltage rating. Standard motor ratings are normally slightly less than the standard power supply system ratings, to allow for circuit voltage drop. Avoid a mismatch greater than 10 percent between motor nameplate voltage and the extreme variation expected on the system.

2. Enclosure. This must be appropriate for the environment, but the totally-enclosed machine cannot be considered watertight or dust-tight, and no motor should be expected to provide such capability.

3. Interchangeability. Although industry standards may dictate use of the same major dimensions for motors of different manufacturer, some dimensions can vary enough to cause mounting conflicts.

4. Starting duty: Whether a motor can safely start its load, and how often, can only be confirmed by the motor designer, given full knowledge of the load characteristics. The motor user cannot expct to make that calculation.

5. Variable speed: If a variable-frequency drive is to be used, that needs to be made clear when the motor is ordered. The best practice is to purchase both motor and power supply from the same vendor, to assure single-source responsibility for the drive.

6. Cost. Not only price, but long-term energy cost needs careful study. If a new motor is to replace an existing one, removal and replacement costs are also important.

To order a back issue with the full article, “Before you buy – a motor purchasing checklist,” call 312-321-9440 or visit our online webstore.

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