Dan Patterson 
Flanders Electric Motor Service, Inc. 
Evansville, Indiana 
Technical Services Committee Member 

Few things in your business life can be more annoying than a large envelope marked CONFIDENTIAL that contains repair specifications from a potential “large” customer. Right? Well things may not be as bad as they seem. Vibration standards, properly written, not only benefit the customer by adding longevity to their equipment but also can equally benefit your business by reducing warranty claims. Vibration analysis has been determined to be the best indicator of rotating equipment mechanical faults. Identification of frequency components can be a valuable tool in determining if faults exist before returning the finished product to the customer.

The law as it is written 

NEMA MG1 Part 7 does a wonderful job of covering mechanical vibration measurement, evaluation and limits as it applies to new motors. So how then can it be applied to rebuilt ones? I will not attempt to cover all of MG1 Part 7 in this article but will extract some of the more important procedures that can be applied to rebuilt motors tested in the service center. 

Mounting 
Part 7 allows the manufacturer to select either a resilient or rigid mounting. Resiliency is achieved by suspending the machine on an elastic support whose natural frequency is 33 percent or less than the lowest rated speed. 

Rigid mounting is achieved by fastening the machine directly to a massive founda­tion. The foundation must have less than 0.02 in/sec peak background vibrations and its natural frequency must be at least 10% away from shaft speed. Furthermore, it must not be within 5% of either two times shaft speed or two times electrical-line frequency. If the rigid method is selected, vibration limits are multiplied by 0.8. 

Shaft key 
Hopefully by the time the motor reaches the test run portion of repair, shaft key compensations have been addressed. Part 7 states: “For the balancing and measurement of vibration on machines provided with a shaft extension keyway, the keyway shall contain a half key.” I would like to add, this means from end to end. 
Unlike NEMA standards, IEC requires the motor to contain a full key for balance and vibration evalua­tion, while the coupling, pulley, or other shaft accessory be balanced without compensation to its keyway. 

Measurement points 
Measurements apply to each bearing housing in the Horizontal, Vertical, and Axial positions as close to the bearing as possible. Generally, measurements taken on a fan cover are unacceptable. For vertical motors, the vertical and horizontal position readings are both taken radially, and are displaced by 90 degrees. 

Test speed 
Motors should be tested at the highest rated nameplate speed. Additionally, this applies to inverter fed and DC motors. For bi-directional motors, vibration limits apply in both directions. 

Vibration limits 
Vibrations levels for speeds 1200 rpm and above are based on the peak velocity of 0.15 inches per second (3.8 mm/sec). Vibration levels for speeds below 1200 rpm are based on the peak velocity equivalent of 2.5 mils (63.5 microns) p-p displacement. Acceleration limits are constant at 1 G peak between 400 and 10,000 Hz. 

Part 7’s unfiltered vibration limits differ for motors whose speed is below 1200 rpm. But why? Velocity is a measurement of distance divided by time, e.g., in/sec. Probably the person who coined the phrase “It’s mils that kill” still carries some weight when it comes to writing standards. When judging reliability, a 300-rpm machine with .2 in/sec (5.1 mm/sec) sinusoidal vibration will have the same longevity as a 3600-rpm machine with the same .2 in/sec (5.1 mm/sec) vibration. 

A practical approach 
Strictly speaking, it is not necessary to be compliant with NEMA MG 1 Part 7 because it applies to new motor manufacturing. Nevertheless, Part 7 provides the basis for vibration measurement of electric motors. Since we are repairing motors rather than building new ones, latitude in testing methods is acceptable.

Resilient mounting reduces system mass and therefore changes natural frequency. Rigid mounting provides a fair assessment as long as the foundation is not too “massive” in relationship to motor weight. For example, if a 25 hp motor were bolted to a massive foundation the motor - base combination would be highly damped and result in an “unfair” vibration test. In-service 
Table 2. Vibration limits for repaired electric motors and generators. motor foundations generally range from 3 to 10 times motor weight, most at the lower end of this range. So which method is better, and best represents internally produced vibrations? Neither. Both are subject to dampening or the lack thereof. One preferred and well-accepted method is to rest the motor on a machined surface, preferably one embedded in concrete and in which background vibrations are less than 0.02 in/sec peak. The motor’s mounting surfaces are cleaned and shimmed to accommodate any soft-foot conditions. This method requires reduced voltage starting of the motor. Application of this method offers the best simulation of the motor’s natural environment. 

The vibration limits in NEMA MG1 Table 7-1 (Table 1) are then applied when the motor reaches full speed with full voltage applied. As an alternative to Table 7-1, consider using the “Patterson Chart” of vibration limits shown in Table 2, where velocity acceptance values remain constant regardless of service speed. 

References
NEMA Standards Publication MG 1-2006. 

Categories: Vibration

Tags: Vibration Motor testing

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