Standardized Medium-Voltage Induction Motor Tests Ensure Quality

Manufacturers and operators produce, purchase and use better motors with regulated testing.

Written by:
Dennis Bogh, GE Motors – Power Conversion
Published:
July 14, 2014

Efficiency tests are a series of tests made according to IEEE 112. Efficiency testing normally measures the losses of less than 5 percent of the motor rating. To measure such a small value accurately, the losses are measured individually.

Friction, windage and core losses are measured without load on the motor. Stator losses are a function of the stator resistance, which is accurately measured during the routine test. Rotor losses are calculated from the locked-rotor current.

Stray load losses are more difficult to measure and are frequently accepted as a tabulated value from IEEE 112. Overall losses are compared with customer specification. Data collected during the efficiency test are used to confirm the motor model and the speed-torque curve.

The motor is loaded either electrically or mechanically to rated current, and the temperature rise is measured. Electrical loading is accomplished by the dual frequency method from IEEE 112, method F. Mechanical loading is executed by dynamometer or brake. Motor vibration is measured cold before the temperature rise test and hot after the test. The vibration magnitude and angle are compared at the hot and cold temperatures to ensure rotor stability.

The vibration spectrum is recorded, and the magnitude is presented as a function of frequency. Vibration specifications typically include separate limits for overall vibration and harmonic vibration.

Noise is measured at no load in a quiet area. ANSI/NEMA MG-1 advises the measurement standards in Section 9.

Special Tests

Additional special testing is frequently required for motors with 3,600 rotations per minute (rpm) or slower motors with less than 5,000 HP.

Additional special testing is often required for motors operating at 3,600 rpm and those with more than 5,000 HP operating at 1,800 rpm to verify the dynamic characteristics of the rotor. These data confirm design values and are used for baseline data during later maintenance testing.

Conclusion

Motor testing covers a broad range of processes from incoming material tests to final heat runs. Tests provide quality control in the factory to identify and correct any errors before they affect the manufacturing schedule.

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