Five test points to understand the efficiency and performance of the motor drive

Motor drives are a ubiquitous technology that converts a constant voltage from an autonomous AC power source into a variable voltage that controls the torque and speed of the motor, making it ideal for use in motors that drive loads on mechanical equipment. Motor drives offer higher efficiency than simple in-line motors, as well as having a degree of control not available with simple direct drive motors. These factors can save energy costs, improve production performance, and extend motor life.

According to the United States Department of Energy (DOE), motor systems are critical to the operation of almost all plants, accounting for 60 to 70 percent of all electricity consumption. The DOE also identified variable frequency drives (VFDs) as a source of significant cost savings for the plant. Not surprisingly, motor drives are commonly used in many industries and facilities. To ensure the uptime of these motor systems, performing maintenance and troubleshooting becomes the number one priority.

Challenges in testing motor drives

Troubleshooting and testing of motor drives, also known as variable frequency drives (VFDs), variable speed drives (VSDs), or variable speed drives (ASDs), is typically performed by experts using a variety of test instruments including oscilloscopes, multimeters, or other tools. These tests may involve a certain level of trial and error using the age-old method of elimination. Due to the complexity of the motor system, testing is usually performed once a year, unless the system starts to fail. Considering that devices often lack or have an incomplete working history, it can be problematic to determine where to start testing. This includes documenting specific tests and measurements previously performed, work done, or the adjusted condition of individual components. Advances in testing technology have eliminated some of the challenges. Newer instruments such as the Fluke Motor Drive Analyzer (MDA-510 and MDA-550) are designed to make motor drive testing more efficient, insightful, and provide step-by-step documentation of the entire process. These reports can be saved and compared with subsequent test results for a more complete picture of the motor drive's maintenance history.

Five test points to understand the efficiency and performance of the motor drive

An easier way to troubleshoot VFD

Combining the power of meters, handheld oscilloscopes, and recorders with the guidance of skilled instructors, these advanced motor drive analyzers use on-screen prompts, clear setup diagrams, and step-by-step instructions written by motor drive experts to guide you through basic testing. This new approach to breaking down and simplifying complex tests enables experienced motor drive experts to work quickly and confidently to get the details they need. It also opens up a path for less experienced technicians to get started with motor drive analysis faster.

By implementing a set of standard tests and measurements at critical points within the system, it is possible to find the root cause of a motor drive system failure or to perform routine preventive maintenance checks. Starting at the input of the power supply, the key tests are completed using different measurement techniques and evaluation criteria throughout the system, and ending at the output.

The following are the basic tests used for motor drive troubleshooting:

Note that Fluke Motor Drive Analyzer walks you through these tests and automates many of the required calculations, allowing you to be confident in your results. In addition, you can save data to reports at virtually any test point, so you can upload documents to a computerized maintenance management system (CMMS) or share them with colleagues or consulting experts.

Safety Tip: Be sure to read the product safety instructions before starting testing. Do not work alone, please follow local and national safety norms. Wear personal protective equipment (approved rubber gloves, respirators, flame retardant clothing, etc.) to protect against injuries from electric shock and arcing while dangerous live conductors are exposed.

To start each test with Fluke Motor Drive Analyzer, simply connect the test probes according to the diagram and press "Next".

1. Drive input

In order to determine if the driver's feed circuit is free of distortion, interference, or noise that could affect the power supply to ground, the power supply of the motor drive should first be analyzed.

Test

Comparing the nominal voltage rating of the drive to the actual supply voltage allows a quick check to see if the value is within acceptable limits. More than 10% outside of this range indicates a problem with the supply voltage. Determine if the input current is within the maximum rated range and if the wire size is appropriate.

The measured frequency is compared to the specified frequency. A frequency difference of more than 0.5 Hz can cause problems.

Check if the harmonic distortion is within acceptable limits. Visually inspect the waveform shape, or view the harmonic spectrum screen that shows both total harmonic distortion and individual harmonics. For example, a flat-top waveform can indicate a nonlinear load connected to the same feed circuit. If the total harmonic distortion (THD) is more than 6%, there may be a problem.

Check the voltage imbalance on the input terminals to ensure that the phase imbalance is not too high (less than 6% - 8%) and that the phase rotation is correct. A high reading of a voltage imbalance indicates a phase outage. Readings above 2% can cause a voltage gap and cause the drive to overload, the failsafe to trip or interfere with other equipment.

Current run-out balance test. An excessive imbalance may indicate a problem with the drive rectifier. A current balance reading of more than 6% may indicate a problem with the motor drive's inverter and will cause problems.

2. DC bus

AC-DC conversion inside the drive is critical. For optimal driver performance, it is essential to ensure that the voltage is correct, adequately filtered, and that ripple levels are kept low. A high ripple voltage may indicate a faulty capacitor or an incorrect specification of the connected motor. The recording feature of the Fluke MDA-500 Series Motor Drive Analyzer dynamically checks the performance of the DC bus in operating mode when subjected to load. You can also use the Fluke ScopeMeter® Test Tool or an advanced multimeter for this test.

Test

Driver DC bus DC level 1

Ripple on the DC bus

Determines if the DC bus voltage is proportional to the peak value of the input line voltage. With the exception of the controllable rectifier, the voltage should be approximately 1.31 to 1.41 times the RMS line voltage. Low DC voltage readings can trip the driver, which can be caused by low input supply voltage or input voltage distortion (e.g., flat top).

Check the line voltage peak amplitude for any distortion or error. This can lead to overvoltage or undervoltage errors. A difference of +/- 10% from the nominal DC voltage reading may indicate a problem.

Determine if the peak of the AC ripple has a different level of repetition. After AC-DC conversion, a tiny AC ripple component is retained on the DC bus. Ripple voltages in excess of 40 V can be caused by capacitor failure or the driver rating being too small for the connected motor or load.

3. Drive output

Testing the drive output is critical to the proper functioning of the motor and can provide clues to problems in the drive circuitry.

Test

Determine if the voltage and current are within the limit values. High output currents may heat up the current, reducing the insulation life of the stator.

Check the voltage/frequency ratio (V/Hz) to ensure that it is within the specified limits of the motor. A high ratio may overheat the motor; A low ratio causes the motor to lose torque. Stable frequencies and unstable voltages may indicate DC bus problems; Unstable frequencies and stable voltages may indicate switching (IGBT) problems. Unstable frequencies and voltages indicate that there may be a problem with the speed control circuitry.

When examining the driver output, it is important to measure the voltage-to-frequency ratio (V/F) and voltage modulation. When the measured value of the V/F ratio is on the high side, the motor may overheat. When the V/F ratio is low, the connected motor may not be able to provide the required torque under load, resulting in an inability to adequately execute the intended process.

Check the voltage modulation using phase-to-phase measurements. High voltage spikes can damage the motor winding insulation and cause the drive to trip. Voltage peaks that exceed the nominal voltage by more than 50% can cause problems.

Check the steepness of the switching pulse indicated by the drive reading. The rise time or steepness of a pulse is expressed by a dV/dt reading (the rate of change of voltage over time), which should be compared to the specified insulation value of the motor.

Test the switching frequency of phase versus DC. Identify if there is a potential problem with the electronic switch or ground, and if the signal fluctuates up or down, it may indicate a problem.

Voltage imbalances are measured, preferably at full load. The voltage imbalance should not exceed 2%. A voltage imbalance can cause a current imbalance, which can cause the motor windings to overheat. Causes of imbalance can include driver circuit failure. If one phase fails, it is referred to as "single-phase operation", which can cause the motor to heat up, fail to start after shutdown, greatly reduce efficiency, and can damage the motor and the connected load.

Measure the current loss balance, which should not exceed 10% for three-phase motors. A large imbalance at low voltage may indicate a short circuit in the motor windings or a short phase to ground. Large imbalances can also cause drives to trip, motor temperatures to become too high, and windings to burn out

4. Motor input

Supplying the voltage to the motor input terminals is critical, as is the proper choice of wiring between the driver and the motor. Incorrect wiring selection can cause damage to drives and motors due to high voltage peaks. These tests are essentially the same as the drive output tests described above.

Test

Check that the current on the terminals is within the rated range of the motor. Excessive current conditions can cause the motor to overheat and shorten the life of the stator insulation, which can lead to premature motor failure.

Voltage modulation helps identify high voltage spikes to ground that can damage the motor insulation.

Voltage imbalance can seriously affect the life of a motor and can be a sign of inverter failure. This can lead to a voltage gap and cause the overload failsafe to trip.

A runaway current balance can indicate a voltage imbalance or a problem with the driver rectifier.

MDA Engineer

5. Motor shaft voltage

Voltage pulses from the motor drive are coupled between the stator and the rotor, resulting in a voltage on the rotor shaft. When this rotor shaft voltage exceeds the insulation voltage of the bearing grease, flashover currents (sparks) may occur, which can lead to pitting and groove erosion of the motor bearing race, which can lead to premature motor failure.

Test

Measure the voltage between the motor frame and the drive shaft. For example, the MDA-550 offers a carbon fiber brush probe for this purpose. The test makes it easy to detect the presence of damaging flashover currents, while pulse amplitude measurement and event counting allow you to take action before a fault occurs.

Five test points to understand the efficiency and performance of the motor drive

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