How does an operator know that the motor they are getting to drive a pump, or other equipment, is up to the challenge? Will it stand up to the test of time? Does the motor meet the basic performance requirements?
Does the motor meet stringent specifications? Motor tests can help answer these questions. Many electric motor standard tests are available and specified by end users. These tests are requested for varying
reasons and provide a different insight into the motor and its usage. It is therefore important to evaluate why a customer may request a certain test, the short-term and long-term value of the test, as well as what criteria are considered for passage or failure of the motor based on the testing outcome.
By Timothy Albers, Director of Product Management, Nidec Motor Corporation’s Commercial and Industrial Motor Division/US Motors
Types of Motor Tests
Standard Production or Short Commercial Test
The first test considered is the most common. It is typically referred to as a production test or a short commercial test. The NEMA (National Electrical Manufacturers Association) Standard MG-1 provides a standard production test for all electric motors. Although the test is specified by NEMA to occur, the standard does not call for a test report to be provided with the motor or to the end user.
Generally speaking, all completely assembled motors receive a production test prior to shipment from the factory. A short commercial test is conducted on every motor. This test confirms conformance to design, however no specific values are recorded. The exact nature of this test varies by motor type, but at a minimum, the motor is run at no load and visually inspected. There is no extra charge for a production test, and this test requirement does not need to be noted at order entry. The minimum requirements of the short commercial test in productionare also defined in NEMA and it is considered a fairly consistent test across NEMA manufacturers.
Short commercial test are used to determine that the motor meets NEMA MG1-12.55 requirements; or Part 20 requirements for Titan motors.
The typical tests conducted on assembled motors that are furnished with shaft and complete set of bearings are as follows:
- Measurement of winding resistance. Winding resistance primarily confirms that the length and size of the copper wire used in the motor is correct.
- No-load readings of current and speed at normal voltage and frequency. The no-load amps are also an indication that the motor meets the correct manufacturing specification. The check is to a design value of no-load amps.
- Current input at rated frequency with rotor at standstill (locked rotor current) for squirrel-cage motors. The locked rotor current is an important value for setting protection for the motor upon installation. The locked rotor value can be estimated based on the locked rotor code on the
motor nameplate, but a tested value will provide a more accurate measure for setting motor protection.
- High-potential (Hi-pot) test in accordance with NEMA MG-1 3.1 and 12.3. A Hi-pot test applies a voltage higher than the rated voltage of the motor for a specified time for the purpose of determining the motor’s adequacy against breakdown of insulatingmaterials and spacings under normal conditions per NEMA. The voltage and duration changes based on the voltage of the motor, but it is always at least 1500 volts. It is a stressful test for the motor and should not be repeated often as repeated Hi-pot testing will damage a motor winding. Passing the Hi-pot is an indication of a good integrity of the motor winding. The Hi-pot is also used to check the integrity of motors
that are installed.
An end user can also request to witness a short commercial test at the motor factory. Normally the motor is completed and set aside for the customer to travel to the factory. Scheduling of the test and end user travel time can delay motor shipment to accommodate the witnessing of the test. All testing can normally be witnessed with the same limitations of timing.
Complete Initial Test
The complete initial test is based on the IEEE Standard 112, method B, dynamometer test. The IEEE 112 Standard 112, method B is mostly harmonized with other international motor test standards that include CSA C390 and IEC 60034-2-1. One important characteristic of this test is that the motor is run until thermal equilibrium prior to taking final values; this simulates field operation.
This test consists of:
- Insulation resistance per IEEE Standard 43. The value can be used as a baseline for future readings after the motor is installed. If the resistance value has a steady or sudden reduction in value, that is an indication of winding degradation.
- Winding resistance and high potential tests have the same value as for the Short Commercial test.
- Full-load heat run, percent slip, no-load current, full-load current, locked rotor current, lock rotor torque, breakdown torque (calculated), efficiency and power factor at 100%, 75%, and 50% full load. There is significant value in the full load heat run while capturing speed (and the resulting slip), torque, efficiency, and power factor. The full motor performance is evaluated and categorized. The tested values can be used for setting end-use parameters such as flow, power, and other characteristics. The test also allows for an in-depth comparison of the design motor performance to the produced motor’s performance. This test is the test that is used for efficiency compliance by many governments as well as criteria for acceptance by many end users.
The complete initial test is one of the more commonly requested witness tests due to its comprehensive nature.
The Sound Test is a no-load test performed in accordance with ANSI S12.51 and NEMA MG-1 Part 9. The requirements for sound testing a very specific based on the room, the background noise level, the type of equipment, and the distance from the motor while taking measurements. NEMA MG-1 Part 9 is a good place to start to read to determine what has to occur for sound testing. Sound is measured as both sound pressure and sound power. NEMA specifications are all in sound power. Sound pressure can be calculated from the sound power readings but require some additional information. Nominal or expected sound levels are typically provided by a motor manufacturer based on sound testing of a typical motor design. Sound testing is requested by end users due to environmental or factory sound requirements.
Polarization Index (PI)
In accordance with IEEE Standard 43. The Polarization Index is also referred to as the Dielectric Absorption Ratio. The PI test takes the Insulation test that is listed as part of the Complete Initial test to the next level. The test uses a megger to take the insulation resistance value for one minute and then holds before continuing for an additional nine minutes to obtain a ten-minute resistance value. The PI is the ratio of those two values. A number higher than two indicates satisfactory winding. The PI can be used by the end user to set a baseline for future motor winding cleanliness and integrity. Tracking the value of the PI can also be an indication of the integrity of the winding.
Spray Test or Immersion Test
This is a test of a form wound stator with a sealed Insulation System.The test applies sprayed water or immersion of the stator into a water tank with specific chemistry. The form wound stator must maintain a level of resistance to pass the test. This is a very aggressive test that can damage the winding. It is typically an expensive test to specify and is normally only required on motors that will go into highly critical applications.
Inverter with Motor Test
This text involves running the production motor on an inverter. The complete initial test can also be conducted at the factory while running on an inverter. The test specification for this application is different from
most tests used in these applications. The most commonly accepted standard for a complete test with an inverter is IEC 60034-2-3. Any testing completed by a motor manufacturer with an inverter will also have incremental costs. IEC 60034-2-3 requires testing at multiple load and speed points, which adds to the test complexity and time.
This test is the same as the Complete Initial Test, but a robust set of curves are provided to the customer. The development of these curves through testing adds significant additional time to complete the initial test. To create the required curves, the performance of current, power factor, and speed (slip) are verified at many load points. Calibrated testing is normally requested by an equipment OEM such as a Pump OEM to utilize the motor in the OEM’s test lab. The curves are used to provide specific performance points to allow for pump performance calculations.
IEEE 841 Standard Test
Unlike most standard production motors, the IEEE 841 Standard Test requires that a form of the Initial Production Test be provided with each IEEE 841 motor. That test report includes the motor no-load current, power and speed, the Hi-pot test result, the Insulation resistance, and a vibration test. Motor manufacturers vary in the scope of additional data provided and the test format, but the amount of data included as standard on IEEE 841 motors is more extensive than any other standard NEMA motor. Those values can and are used by the end user to provide a baseline for maintenance and motor evaluation. Vibration and RCF (Reed Critical Frequency) testing is also available. Vibration testing can take multiple forms including filtered and unfiltered values in multiple directions. More information on vibration testing will be provided in a follow-up article.
These are some of the most common tests that can be performed on a motor to validate its performance and if it will meet the requirements to run in a specific environment. As always there needs to be an analysis of the required tests that need to be performed and the associated costs to perform them.