ArticleTesting/Inspection of Induction Motors
by TECO Westinghouse Motor Corporation
This is an excellent paper for the analyst or the engineer that wants a more thorough understanding of the testing necessary to ensure both the mechanical and electrical systems/components of a motor are in good condition without indications of deterioration; vibration analysis is one of the tools used to evaluate the overall condition of the motor. This paper was written as an introductory document for personnel not very familiar with Electrical Motor inspection and testing, but is a good reference for other personnel with more experience. Illustrations, pictures, and data plots are used throughout the document for examples of the items discussed. This is a practical paper intended for personnel that are responsible for reliable operation of a facilities motors. This paper is not written as a case history, but provides numerous examples and test and inspection results for reference. This is a very comprehensive paper that covers a large percentage of testing and inspections that are considered important for a thorough condition assessment of a motor.
“These tests and inspections are based on the requirements of American Petroleum Institute (API) Standard 541, Form-Wound Squirrel Cage Induction Motors — 500 Horsepower and Larger. Other references are made to NEMA MG1, and IEEE STD 112.
In general it is good to conduct complete engineering tests on new designs to evaluate all motor performance parameters, and commercial or routine tests on duplicate designs.
1) Routine — Testing usually performed by the vendor to ensure the motor operates to accepted specifications. Commonly called “commercial test” by most vendors.
2) Complete — More extensive testing which usually includes a full load heat run test, efficiency, power factor, full load and locked rotor currents, breakdown torque and percent slip, stator and bearing temperature rises, speed torque characteristics etc.
3) Optional — Special testing specified by the customer such as: surge comparison, stator winding immersion, dielectric absorption ratio, bearing housing natural frequency test, coastdown test etc.
4) Observed — Vendor will notify the customer of the date of inspection or testing; however, the test or inspection shall be performed as scheduled, and if the customer is not present, the vendor will continue so as not to impact the production schedule.
5) Witnessed — Testing or inspection that places a hold on the production schedule and that the test or inspection will not be carried out until the customer is present. Requires successful pretest of motor in most cases.
6) Inspection or Hold Points — A point specified by the customer to ensure various stages of the motor production meet their specifications. Such as rotor balancing, surge comparison testing, inspection of stator before vacuum pressure impregnation (VPI), or welding before final assembly.
The inspections will be discussed with respect to description and information obtained. The testing section will also include information concerning the benefits derived from each test.
Dimensions should be checked against the certified outline drawings to confirm the motor mounting hole locations, size and the location of the vertical and horizontal jacking bolts. The main and auxiliary terminal boxes should be checked to ensure proper location, proper conduit size hub, terminal block size and terminal identification. All accessories should be checked for their size, type and range.
All external items required to be stainless steel should be checked nameplates, screens and filters, all external fasteners, and lube oil piping. Series 300 stainless is normally specified and a check with a magnet is sufficient, as Series 300 is nonmagnetic.”
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