Standard Motor Catalog

Section TR Technical Reference Guide

MOTOR TESTING Good motor application practice requires that uniform and meaningful terminology be used to define efficiency values which are determined by accurate test procedures uniformly applied by the motor manufacturers. Testing to accurately determine the value of motor losses can be time consuming and expensive. Also, wide variations in results can be experienced when different test methods are used. In order to maintain consistency, NEMA has established the following guidelines: Efficiency and losses shall be determined in accordance with latest revision of IEEE standard 112 or CSA C390. Polyphase squirrel-cage motors rated 1-500 Hp shall be tested by dynamometer. The efficiency will be determined using segregated losses in which stray load loss is obtained from linear regress ion analysis to reduce the effect of random errors in the test measurements. This procedure focuses on stray load losses where the greatest variance can occur from one motor to the next. It also clearly defines the procedure and requires testing either at full operating temperature or by making corrections for temperature differences. Finally, the test method requires use of a dynamometer to provide the most accurate data as a basis for accuracy improvement and stray load loss measurement. Motor efficiency is not an absolute or constant for all motors of the same design. Rather, the efficiencies of a large number of motors will fit a distribution as shown in Figure 32. Though Figure 32 illustrates a normal distribution, in reality the distribution need not be normal.

It is to be noted that there is difference in calculation between NEMA and IEC, the same is illustrated by the table below: Table 31: Comparison of Methodology for Efficiency Calculation between IEC and NEMA Parameters IEC NEMA Standards IEC 60034-2 IEEE-112 Method B

Temperature correction to Winding temperature Measured **

Rotor I²R loss or slip loss

No temperature correction

Stray load loss

0.5% of input*

Input

Measured

Measured

Calculated by subtracting the losses from input

Calculated from measured Torque

Output

* 0.5% of input; for a motor ** Calculated stray loss is smoothed by regression analysis; output is corrected. PLUG REVERSAL TESTING This is an accelerated life test to evaluate the integrity of the construction and materials for the insulation system of the motor. “Plugging” is rapid braking of the motor brought about by suddenly reversing two of the three phases of the supply voltage while it is rotating in any direction. This ‘plugging’ action subjects the motor winding to severe electromagnetic forces and consequent mechanical stresses. The test consists of subjecting the motor to several thousands of plugging and reversals. At the end of the test, the motor must be operable and successfully pass the standard insulation inspecting requirements and has exceeded the compiled historical data. Figure 33, show the Plug-reversal historical data for a 4-pole motor.

NOMINAL OR AVERAGE EXPECTED

MINIMUM OR GUARANTEED

MAXIMUM EXPECTED

Figure 32: Efficiency distribution

The nominal efficiency which appears on the motor nameplate corresponds to the nominal, or average expected efficiency on the curve. The guaranteed minimum efficiency appearing on GE energy efficient motor nameplates corresponds to the minimum, or guaranteed on the curve.

Figure 33: 4-pole GE motor test results data for minimum number of plug-reversals.

TR.38

Data subject to change without notice. 12/23 • www.wolongamerica.com