Motor drives are a widely used technology that converts a constant voltage from the main AC power supply into a variable voltage for controlling the torque and speed of a motor, making them ideal for driving mechanical loads. Motor drives offer higher efficiency than simple in-line motors, as well as a level of control not available with simple direct-drive motors. These factors can save energy costs, improve productivity, and extend motor life.
According to the U.S. Department of Energy (DOE), motor systems are critical to the operation of almost all plants, accounting for 60% to 70% of total electricity consumption. The DOE also identifies variable frequency drives (VFDs) as a source of significant cost savings for plants. Unsurprisingly, motor drives are widely used in many industries and facilities. To ensure the uptime of these motor systems, performing maintenance and troubleshooting is a top priority.
Challenges in testing motor drives
Troubleshooting and testing of motor drives (also known as variable frequency drives (VFDs), variable speed drives (VSDs), or adjustable speed drives (ASDs)) are typically performed by experts using a variety of test instruments, including oscilloscopes, multimeters, or other tools. These tests may involve a degree of trial and error using age-old elimination methods. Due to the complexity of motor systems, testing is usually performed annually unless the system begins to fail. Determining where to begin testing can be challenging, given that equipment often lacks a work history or has an incomplete one. This includes documenting specific tests and measurements previously performed, work completed, or the condition of individual components after adjustments. Advances in testing technology have eliminated some of these challenges. Newer instruments such as the Fluke MotorDriveAnalyzer (MDA-510 and MDA-550) are designed to make motor drive testing more efficient, insightful, and capable of documenting the entire process step-by-step. These reports can be saved and compared with subsequent test results for a more comprehensive understanding of the motor drive's maintenance history.
Simpler VFD troubleshooting methods
These advanced motor drive analyzers combine the functionality of instrumentation, handheld oscilloscopes, and recorders with guidance from skilled instructors, using on-screen prompts, clear setup diagrams, and step-by-step instructions written by motor drive experts to guide you through basic tests. This new approach to breaking down and simplifying complex tests enables experienced motor drive professionals to work quickly and confidently to obtain the detailed information they need. It also opens up a faster path for less experienced technicians to begin motor drive analysis.
By implementing a set of standard tests and measurements at key points within the system, the root cause of motor drive system failures can be identified, or routine preventative maintenance checks can be performed. Starting at the power input, key tests are completed throughout the system using different measurement techniques and evaluation criteria, ending at the output.
The following are basic tests for troubleshooting motor drive malfunctions:
Please note that FlukeMotorDriveAnalyzer guides you through these tests and automates many of the necessary calculations, giving you confidence in the results. Additionally, you can save data to reports at virtually any test point, allowing you to upload documentation to a Computerized Maintenance Management System (CMMS) or share it with colleagues or consulting experts.
Safety Precautions: Please read the product safety instructions before starting testing. Do not work alone and comply with local and national safety regulations. Wear personal protective equipment (approved rubber gloves, face shield, and flame-retardant clothing, etc.) to protect against electric shock and arcing when hazardous live conductors are exposed.
To begin each test using FlukeMotorDriveAnalyzer, simply connect the test probes according to the graph and then press "Next".
Step-by-step measurement connection guide for driver input testing
1. Driver Input
To determine whether there is distortion, interference, or noise in the driver's power supply circuit that could affect the power grounding, the power supply of the motor driver should first be analyzed.
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● Comparing the driver's nominal rated voltage with the actual supply voltage allows for a quick check if the value is within acceptable limits. A deviation of more than 10% indicates a power supply voltage problem. Determine if the input current is within its maximum rated range and if the wiring specifications are appropriate. Compare the measured frequency with the specified frequency. A frequency difference exceeding 0.5Hz may cause problems.
● Check if the harmonic distortion is within acceptable limits. Visually inspect the waveform shape, or view a harmonic spectrum screen that displays both total harmonic distortion (THD) and individual harmonics. For example, a flat-topped waveform may indicate a non-linear load connected to the same power supply circuit. If the total harmonic distortion (THD) exceeds 6%, there may be a problem.
● Check for voltage imbalance at the input terminals to ensure that the phase imbalance is not excessive (less than 6%-8%) and that the phase rotation is correct. A high voltage imbalance reading indicates a phase loss. A reading exceeding 2% may cause a voltage gap, triggering the driver overload fault protection device to trip or interfering with other equipment.
● Current imbalance test. Excessive imbalance may indicate a problem with the driver rectifier. A current imbalance reading exceeding 6% may indicate a problem with the motor driver's inverter, and could cause issues.
2. DC bus
The internal AC-DC conversion of the driver is crucial. For optimal driver performance, proper voltage, adequate filtering, and low ripple levels are essential. High ripple voltage may indicate a faulty capacitor or an incorrectly connected motor. The recording function of the Fluke MDA-500 Series MotorDriveAnalyzer allows for dynamic monitoring of the DC bus performance under load. This test can also be performed using the FlukeScopeMeter® TestTool or an advanced multimeter.
Ripple on DC bus
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● Determine if the DC bus voltage is proportional to the peak value of the input line voltage. Except for the controlled rectifier, the voltage should be approximately 1.31 to 1.41 times the RMS line voltage. Low DC voltage readings will trip the driver, which may be caused by low input supply voltage or input voltage distortion (such as flat-top).
● Check for any distortion or error in the peak line voltage amplitude. This could lead to overvoltage or undervoltage errors. A DC voltage reading that differs from the nominal voltage by +/- 10% may indicate a problem.
● Determine if the peak values of the AC ripple have different repetition levels. After AC-DC conversion, a small AC ripple component will remain on the DC bus. Ripple voltages exceeding 40V may be caused by capacitor failure or an underrated driver for the connected motor or load.
Voltage and current at the driver output
3. Driver output
Testing the driver output is crucial for the proper functioning of the motor and can provide clues to problems in the driver circuitry.
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● Determine if the voltage and current are within limits. High output current can cause the current to heat up, thus reducing the insulation life of the stator.
● Check the voltage/frequency ratio (V/Hz) to ensure it is within the motor's specified limits. A high ratio may cause the motor to overheat; a low ratio may cause the motor to lose torque. A stable frequency and unstable voltage may indicate a DC bus problem; an unstable frequency and stable voltage may indicate an IGBT (Inverter-Guided Block) problem. Unstable frequency and voltage indicate a potential problem with the speed control circuitry.
● When checking the driver output, focus on measuring the voltage-to-frequency ratio (V/F) and voltage modulation. A high V/F ratio may indicate motor overheating. A low V/F ratio may prevent the connected motor from providing the required torque to the load, resulting in incomplete execution of the intended process.
● Use phase-to-phase measurements to check for voltage modulation. High voltage spikes can damage motor winding insulation and cause the drive to trip. Voltage spikes exceeding 50% of the nominal voltage will cause problems.
● Check the steepness of the switching pulse indicated by the driver reading. The rise time or steepness of the pulse is represented by the dV/dt reading (the rate of change of voltage over time), which should be compared with the specified insulation value of the motor.
● Test the switching frequency of the phase against DC. Identify potential problems with electronic switches or grounding; fluctuations in the signal may indicate a problem.
● Voltage imbalance should ideally be measured under full load conditions. Voltage imbalance should not exceed 2%. Voltage imbalance causes current imbalance, which can lead to overheating of the motor windings. Causes of imbalance may include driver circuit failure. If one phase fails, it is called "single-phase operation," which can cause the motor to overheat, fail to restart after shutdown, significantly reduce efficiency, and potentially damage the motor and connected loads.
● Measure current imbalance. For three-phase motors, current imbalance should not exceed 10%. Larger imbalances at low voltage levels may indicate a short circuit in the motor windings or a phase-to-ground short circuit. Significant imbalances can also lead to driver tripping, excessive motor temperature, and winding burnout.
4. Motor input
Providing voltage to the motor input terminals is crucial, and equally important is selecting the correct wiring between the driver and the motor. Incorrect wiring can damage both the driver and the motor due to excessive voltage spikes. These tests are essentially the same as the driver output tests described above.
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● Check if the current at the terminals is within the motor's rated range. Excessive current can cause the motor to overheat and shorten the life of the stator insulation, leading to premature motor failure.
● Voltage modulation helps identify high voltage peaks to ground that may damage motor insulation.
● Voltage imbalance can severely affect the lifespan of a motor and may be a sign of inverter failure. This can lead to voltage gaps and cause overload fault protection devices to trip.
● Current imbalance may indicate voltage imbalance or a problem with the driver rectifier.
5. Motor shaft voltage
Voltage pulses from the motor driver can couple between the motor stator and rotor, resulting in voltage on the rotor shaft. When this rotor shaft voltage exceeds the insulation voltage of the bearing grease, flashover current (sparking) may occur, leading to pitting and groove corrosion of the motor bearing races. This damage can cause premature motor failure.
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●Measure the voltage between the motor frame and drive shaft. For example, the MDA-550 offers a carbon fiber brush probe for this purpose. This test easily detects the presence of destructive flashover currents, while pulse amplitude measurement and event counting functions allow you to take action before a failure occurs.
