Standardized Medium-Voltage Induction Motor Tests Ensure Quality

The term “testing” covers a range of processes. Parts supplied for motor manufacturing are tested as they are manufactured and when they arrive at the plant. These tests make sure that the parts’ quality is consistent with the motor manufacturer’s internal standards. Discovering that the copper or steel is to the wrong standard after assembly can be expensive. As manufacturing progresses, hundreds of inspection and test points ensure that assemblies are correct and function properly together. Deviations can delay the manufacturing schedule if not corrected early in the process.

Medium-Voltage Motors

Medium-voltage induction motors are used worldwide to drive pumps, compressors and other rotating loads. Medium voltage is defined as more than 600 volts and less than 15,000 volts. At more than 600 volts, motors are typically at least 250 horsepower (HP). Motors for adjustable-speed services are often tested at 60 hertz, unless rated for another frequency. The American National Standards Institute (ANSI), National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC) provide standards for motors. This article focuses on ANSI/NEMA standard motor construction. NEMA recognizes standard tests from the International Organization for Standardization, IEC, the Institute of Electrical and Electronics Engineers (IEEE), and others. The ANSI/NEMA standard for motors and generators (MG-1) includes four pages of reference standards. API and other organizations have overlays that enhance the ANSI/NEMA motor requirements and provide additional test requirements.

Specialized Manufacturing Tests

Frequently performed specialized manufacturing tests include the winding surge, stator core and, occasionally, destructive coil tests. The winding surge test is performed once the motor core is wound but before it is sealed with Vacuum Pressure Impregnation. This test locates shorted turns or anomalies in the insulation that need to be repaired before manufacturing proceeds. The core test locates shorts between the laminations. Sometimes, extra stator coils are manufactured for motors with stator voltages at or above 6 kilovolts. These coils are later tested to destruction to ensure that the insulation system has been properly manufactured. ANSI/NEMA MG-1 references IEEE 112, IEEE Standard Test Procedure for Polyphase Induction Motor and Generators, for most electrical tests. This standard provides only the procedures and processes for testing. The limits for pass/fail are available in the ANSI/NEMA MG-1 and overlay documents that are called by contract.

Qualification Tests

Qualification tests are used to evaluate motor design. These tests include IEEE 1776 insulation tests. A complete insulation system test can require more than a year to complete. It would include accelerated life tests designed to simulate motor temperature and vibration. Once finished, the test would not be required again until the insulation system was changed. Motor temperature rise is a qualification test for each design. Once a design is qualified by temperature rise tests, repeated testing is unnecessary for motors of the same design.

Acceptance Tests

Acceptance tests confirm that a motor has been correctly manufactured according to the design. Generally, these tests are considered routine and are applied to all motors. Routine tests per ANSI/NEMA MG-1, paragraph 20.16, include:

• Measurement of winding resistance for each phase (values compared with each other and the design value)
• Measurement of no-load current, power and speed
• Hi-pot at double the rated voltage plus 1,000 volts for one minute

These routine tests indicate any issues with the motor. Several other tests are often included as routine:

• Insulation resistance, commonly referred to as a meggar test, is completed before the motor is subjected to a hi-pot test to assure no grounds exist.
• Air gap measurement avoids variations in the air gap between the stator and rotor, which cause vibration.
• No-load vibration is assessed. Many errors in the manufacturing of a motor cause vibration. Any windings that are not correctly connected cause electrical vibration. Bearings that are not correctly seated or seals that rub will also cause vibration.
• No-load losses are measured according to IEEE 112 and require that the voltage to the motor be varied. Testing provides the values of friction and windage losses separately from the core losses.
• A bearing temperature rise test ensures that the bearings are correctly seated and that the lubrication system operates properly.
• The Polarization Index (PI) test is performed using a meggar and is a relative measure of the insulation quality. All new motors will pass the PI test, but the results are necessary for future reference.
• Instrumentation (temperature probes, vibration probes and space heaters) is tested.

Routine test results are compared with internal motor manufacturer standards, design data and customer requirements to determine pass or fail.

Complete Tests

Complete tests usually refer to temperature rise, efficiency, locked rotor current, vibration spectrum and noise tests. They are design qualification tests and normally completed on one motor of a given design. Motors of the same design only require routine tests to verify successful manufacturing. The rotor is restrained, and less-than-rated voltage is applied to one phase while the current is measured. Ratio calculations provide the full-voltage locked-rotor current. 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.