Motor Testing: An Overview

Jonathan Trout, Noria Corporation

Motor testing assesses the integrity of a motor through the use of computer-supported equipment or tools that monitor trends within the motor.

Motor Testing  

What Is Motor Testing?

Motor testing assesses the integrity of a motor through the use of computer-supported equipment or tools that monitor trends within the motor. The main objective of motor testing is to reveal hidden problems and prevent unnecessary failure. Specific to electric motors, motor testing evaluates static parameters like insulation, wire damage and electrical current leakage, as well as more dynamic parameters such as distortion, temperature fluctuations and balance.

Mechanical motor testing includes things like looking into the cracks of a motor's rotor and lamination sheet makeup. While each motor test applies to most alternating current (AC) or direct current (DC) motors, each testing method depends on the construction and application of the motor in question.

Motor testing is regulated by the Institute of Electrical and Electronics Engineers (IEEE) through standards such as IEEE 43 – Insulation Resistance and Polarization Index, IEEE 56 – Maintenance AC Hipot Test, IEEE 95 – DC Hipot Test, and IEEE 400-2001 – The Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems. These standards are constantly reviewed and improved when deemed necessary by the board.

Motor testing is often used in a preventive maintenance or reliability-centered maintenance program. Motor testing with a preventive maintenance program can test motors while they're operating in their normal environment under normal loads to confirm they're running at acceptable or optimum limits. Motor testing often alludes to issues before visual inspection makes them apparent.

Making motor testing a part of a maintenance program is important because once a motor endures damage, it is often irreversible (referred to as core damage); this leads to the motor not running with the same efficiency as it once did, if at all. Motor testing lends itself to multiple benefits, including:

  • Increased uptime. Identifying defective motors before they reach a point of failure ensures your system(s) remain up and running. This clears the path for more economically planned maintenance tasks to correct discovered issues.
  • Cost savings. Motor testing gives you a clear picture of the real-time condition of the motors within your assets, limiting potential collateral damage due to failures and reducing maintenance costs. What type of maintenance a motor requires or whether it needs to be replaced is a critical and potentially expensive decision.
  • Energy conservation. Motor current analysis (MCA) testing can help identify conditions within a motor that lead to increased power consumption. This could negatively impact the motor's overall power quality, accelerate asset wear, and present itself as costly increases in energy consumption and peak use.
  • Improved safety. Motor testing reduces the urgency and frequency of breakdowns, allowing maintenance teams to shift the majority of their work to scheduled downtimes. This gives personnel a de-energized working condition in which to make repairs. Motor testing can also detect faulty electrical connections in a circuit that may not be picked up with regular infrared testing, reducing the risk of a fire.

While the ins and outs of motor testing can be intimidating and a bit complex, knowing the basics along with using modern motor testing tools and equipment can greatly simplify the task of testing motors.

 

Motor Testing Tools

Modern motor testing tools make taking readings and analyzing a motor's current condition fairly simple once you understand how each tool works. Many testing tools are equipped with multifunction capabilities, incorporating more than one device into each tool. Having a basic toolkit with the following tools is a good place to start.

  • Digital multimeter (DMM) - A DMM measures multiple electrical quantities, such as voltage (volts), resistance (ohms) or current (amps). Some DMM models include special features that allow you to take minimum, maximum and relative measurements as well as test diodes and capacitors. DMMs are used for testing for power loss from blown fuses, excessive current levels from overloaded circuits and improper resistance from damaged insulation or equipment. Digital Multimeter

    DMMs are considered a multifunction tool because they combine multiple single-function tools such as a voltmeter, ammeter and ohmmeter. This tool includes a display where measurements can be read in real time, buttons for selecting a variety of functions (depending on the model), a dial for choosing primary measurement values (amps, ohms or volts), and input jacks where test leads are inserted.

  • Clamp-on ammeter - A clamp-on ammeter measures the current in a circuit by assessing the strength of the magnetic field around a conductor. The majority of clamp-on ammeters measure AC, but some assess both AC and DC. The hinged "jaws" on the meter allow technicians to clamp the jaws around a wire, cable or other conductor in an electrical system. This enables the technician to measure the current without disconnecting or de-energizing the system. Clamp-on Ammeter

    The jaws are made of ferrite iron (usually encased in plastic) and designed to detect, concentrate and measure a magnetic field that is generated by current as it flows through a conductor. Clamp-on ammeters have become multifunction testers, with some models having capabilities similar to a DMM. They are popular because they're safe and convenient, allowing technicians to forgo cutting wires to insert a meter's leads, since the clamp-on ammeter's jaws don't need to touch a conductor to take a measurement.

  • Megohmmeter - Often referred to as a megger, a megohmmeter is a type of ohmmeter used to measure the electrical resistance of insulators. In other words, megohmmeters are utilized to determine the condition of the insulation on wires and motor windings. They do this by introducing a high-voltage, low-current DC charge and by assessing the resistance to identify whether there is current leakage or damage to the insulation. Megohmmeter

    The amount of current depends on the applied voltage, the system's capacity, the total resistance and the temperature of the material. Generally, the higher the current, the lower the resistance. The insulation resistance value displayed on the screen is a function of three independent subcurrents: conductive leakage current, capacitive charge leakage current and polarization absorption leakage current. Routinely using a megger in your maintenance program is a good way to guarantee your circuits are safe.

  • Non-contact thermometer - A non-contact thermometer or spot thermometer is a motor testing tool that measures temperature at a single point from a safe distance. Resembling radar guns, these handheld thermometers are ideal for determining the temperature at a specific spot on a surface. They're used for measuring thermal radiation on hard-to-reach assets or assets operating under extreme conditions. Non-contact Thermometer

    Spot thermometers work by using field of view (FOV) and distance-to-spot ratio (D:S). The D:S is the ratio of the distance to the object you're measuring and the diameter of the temperature measurement area. The larger the ratio number, the better the instrument's resolution and the smaller the area that can be measured.

  • Power quality analyzer - Power quality analyzers are the ultimate multifunctional tools for motor testing. While they're more expensive ($1,000 to $8,000+) than most of the other tools discussed, they can include multiple features depending on the model. Power Quality AnalyzerIn addition to having DMM capabilities, some power quality analyzers can conduct energy studies and power quality loggings by capturing and recording a large number of power quality parameters. Other functions of power analyzers may include:
    • measuring all three phases and neutral;
    • capturing dips, swells and inrush currents; and
    • analyzing software integration and compatibility.
 

Types of Motor Tests

Motor Test Types

There are numerous motor testing techniques, especially when it comes to electrical motor testing. Most of these fall under one of two categories: online or offline testing, or static or dynamic testing. A good predictive maintenance program typically uses both.

Online dynamic testing is done while the motor is running. It gives technicians data on the power quality and operating condition of the motor. Dynamic testing equipment should be able to collect and trend all data essential to electric motors. This includes power condition, voltage level, voltage imbalance and harmonic distortions, current levels and imbalances, load levels, torque and rotor bar signatures, etc. Analyzing the collected data from online testing can reveal problems through indicators such as power condition, motor condition and performance, load assessment, and operating efficiency.

Offline static testing should be used on a regular basis to determine how the components within a motor (windings, rotor bar, etc.) are functioning as well as to perform a current and voltage analysis. Static testing often finds problems like broken or loose rotor bars, issues with end rings, an unequal air gap between the rotor and stator (eccentricity), and misalignment. As the name suggests, this type of motor testing is done when the machine is stopped. Static testing assesses things like resistance/insulation resistance, high-potential (HiPot) tests, polarization, surge tests and more.

Nearly half (48 percent) of all motor failures are due to electrical issues, according to a survey by the Electric Power Research Institute (EPRI). Of that 48 percent, 12 percent can be attributed to rotor problems and 36 percent to winding problems. To help mitigate these failures, a variety of motor tests can be performed on electric motors. Some of the most common include:

  • Electric motor impulse testing: Impulse testing helps you understand how an electrical system can withstand sudden overvoltage caused by weather (lightning strikes), regular duty situations like when low- or high-voltage equipment changes operations, or high-voltage variations in AC-DC inverter output.
  • Electric motor rotation testing: Testing for rotational direction is crucial before you connect a motor to its load so you don't damage the load or cause confusion for the operator. For example, a motor-driven impeller in a mixer is designed to be directional, so to get adequate mixing, it's important to maintain the intended direction.

    Proper rotational testing is done with a phase rotation meter. For instance, if you're installing a three-phase motor, the meter will have six leads on it — three on the motor side (lead side) and three on the line side (supply side).

  • Wound rotor electric motor testing: Testing with a wound rotor allows you to isolate the three basic components (stator, rotor and resistor bank) to identify the root cause quicker. Much like a primary to secondary relationship in a transformer, any variation in the rotor circuit (secondary) that includes the resistor bank is evident on the stator (primary). Conversely, any issues on the stator are reflected on the rotor circuit.
  • Insulation resistance testing: With electric motor insulation, as temperature increases, resistance decreases. This is known as a negative temperature coefficient. Testing the insulation helps ensure the insulation resistance of a de-energized motor decreases after starting the motor. It's not uncommon for the temperature to increase initially as moisture evaporates from the increasing temperature of the windings. Insulation resistance testing needs a temperature rectification to 104 degrees Fahrenheit (40 Celsius), according to the IEEE 43 standard.
  • Megger testing: One of the most popular tests thanks to its simplicity, the megohm test (megger test) is another way to test the insulation resistance of an electrical motor. A megohmmeter can provide high DC voltage (usually 500V to 15kV) at a predetermined current capacity to test insulation strength. It's best practice to use a megger test with other forms of testing, as it isn't capable of detecting all potential faults inside a motor's winding.
  • Winding resistance testing: Winding resistance testing brings to light dead shorts, loose connections and open circuits. Measuring the resistance of windings ensures all circuits are properly wired and all connections are secured. All coiled windings should have a predetermined resistance specified by the manufacturer for the motor to operate correctly. This resistance lets just the right amount of current to flow through the coil.

    This test is typically done using a digital multimeter. By touching the red (positive) lead of the multimeter to the positive end of the windings and the black (negative) lead of the multimeter to the negative end of the windings, a reading will appear on the screen in ohms. This is the resistance.

  • Polarization index (PI) testing: This motor test is used to determine the fitness of a motor. The index is made up of calculating the measurement of the winding insulation resistance. The PI gives you an idea of how much dirt or moisture buildup there is, the insulation integrity and how well the motor operates. For this test, the applied voltage should be kept constant for 10 minutes, with an insulation resistance reading taken at one minute and a second insulation resistance reading taken at 10 minutes. The ratio between the one-minute and 10-minute measurements gives you the polarization index.
  • DC step voltage test: Step voltage testing is another way to evaluate the insulation integrity of a motor or system. It's typically done after a successful PI test by starting with the same voltage used in the PI test. As the name implies, as the step voltage test progresses, the voltage applied to the insulation system increases every 60 seconds, which is predetermined by the technician. As the voltage is increased, the current is plotted on a graph. Upon completion of the test, if a non-linear graph presents itself, this usually alerts you to insulation issues. Step voltage testing is outlined under IEEE 95 standard.
  • HiPot test: Short for "high potential," a HiPot test checks for good isolation or that no current flows from one point to another point. Think of this as the opposite of a continuity test (where current flows easily from one point to another). The HiPot test verifies that insulation is adequate for the regularly occurring over-voltage transient. This test is ideal for identifying things like nicked or crushed insulation, stray wires, braided shielding, conductive or corrosive contaminants, and spacing problems, among others. The basic voltage for HiPot testing is 2X (operating voltage + 1,000V), according to the International Electrotechnical Commission (IEC) 60950 standard.
  • Automated tests: Most modern motor testing equipment uses automatic testing and fault diagnosis equipment to eliminate the chance for operator error when interpreting results. Automated testing can detect micro-arcs and stop the test automatically if needed. Automated testing equipment comes with software that holds all test output data, so historical readings can be built up over time and reports of that data can be generated. You can find automated testers that combine all static electrical tests in one portable device.

In addition to these electric motor-specific tests, other common motor testing methods can be used such as vibration analysis (especially for bearings), thermography and shaft alignment testing.

 

Motor Testing a Three-Phase AC Motor

Three-Phase Motor

Three-phase motors (induction motors) are designed to run on the three-phase alternating current (AC) power used in most industrial applications. AC electricity switches direction (from negative to positive) and back numerous times a second. For example, the electricity in your home alternates back and forth from negative to positive, 60 times per second. These changes in power occur via a smooth continuous wave called a sine wave. Three-phase AC has three sources of AC power which are all out of phase with each other, meaning no two AC waves are ever at the same point at one time.

Three-phase motors are commonly used in commercial and industrial settings because of their ease of operation, low cost, low maintenance, speed variation, durability and high starting torque. Ensuring the health of a three-phase motor puts into practice many of the testing methods mentioned above.

  • Earth continuity and resistance test: Using a multimeter, measure the resistance between the body of the motor and the ground. You're looking for a reading of 0.5 ohms or less. Some standards may specify 0.1 ohms.
  • Power supply test: For three-phase motors (in the United States), the expected voltage for a 230/400V system is 230V phase to neutral and 400V between each of the three-phase lines, according to the National Electric Manufacturers Association (NEMA). Using a multimeter, check to confirm the correct voltage is applied to the motor. Verify that the connection type is in good condition. For three-phase motors, the connection type is either star(Y) or delta.
  • AC motor winding continuity test: Use a multimeter to check the continuity of the motor winding from each phase. If any phase fails the continuity test, you could have a burned-out motor. Note that identifying windings will vary depending on where you are. According to the IEC, winding designations in the United States are as follows: High-voltage terminals show as L1, L2 or L3. Low-voltage terminals appear as 1, 2 or 3. In Europe, U, V or W would be used for high-voltage terminals and R, S or T for low-voltage terminals. In the United Kingdom, R, Y or B would be seen for high-voltage terminals and A, B or C for low-voltage terminals.
  • AC motor winding resistance test: Use a multimeter or ohmmeter for phase-to-phase terminal winding resistance testing. For the United States, this would be L1 to L2, L2 to L3 and L3 to L1. Ensure the ohms reading for each winding is the same (or close to the same).
  • Insulation resistance testing: In three-phase motors, insulation resistance typically is measured between each motor winding or phase and between each motor phase and the motor frame (earth). Using a megger or insulation tester, set the voltage of the tools to 500V and check from phase to phase and from phase to the motor frame (earth). Generally, a bad reading is anything less than 2 megohms, while an excellent reading would be 100 megohms or greater.
  • Running amps test: Finally, with the motor running, you can check the full load amps with a tool such as a clamp-on meter.

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