Understanding Motor Drive Efficiency and Performance

Motor drives are commonly found technology that is used to transform constant voltage from the main AC power supply into a voltage that varies to control motor torque and speed. Motor drives have higher efficiency and have a degree of control that is not available on simple directly driven motors. These features result in energy cost saving, better production performance, and extended life of the motor.

Motor systems are vital to the operations of almost every plant. It accounts for 60% to 70% of all electricity used [according to the US Department of Energy]. Unsurprisingly, motor drives are frequently used in many industries and facilities. Make troubleshooting and maintenance a priority to ensure that motor drives are working efficiently.

CHALLENGES IN TESTING MOTOR DRIVES

Variable Frequency Drives (VFD), Variable Speed Drives (VSD), or Adjustable Speed Drives (ASD) are often performed by specialists. They use several test instruments like oscilloscopes, digital multimeters, and other test tools. The said testing may involve trial and error using elimination. Testing typically occurs annually, unless, there is a system malfunction. Due to a lack or incomplete history of the equipment, identifying where to begin such a test becomes problematic. Equipment history includes documentation of specified tests and previous measurements performed, completed work or as-left condition of individual components.

But newer instruments like the Fluke Motor Drive Analyzers MDA-510 and MDA-550, makes drive testing more efficient and insightful with the capability to document the process every step along the way. These reports can be stored and compared against subsequent tests to get a bigger picture of motor drive maintenance history.

AN EASIER WAT TO TROUBLESHOOT VFDs

Advanced motor drive analyzers combine the functions of a meter, handheld oscilloscope and recorded along with the guidance of a skilled instructor to employ on-screen prompts, clear setup diagrams, and step by step instructions written by motor drive experts to help you through the needed tests.

To get to the root cause of a motor drive system failure or when doing a preventive maintenance check is best done with a set of standard tests and measurements at key points within the system. Starting with the power input, key tests with different measurement techniques and evaluation criteria are completed throughout the system, ending at the output.

Here are the essentials tests for troubleshooting motor drives:

Safety notes: read the product safety information before you start. Comply with local and national safety codes. Use personal protective equipment to prevent shock and arc blast injury where hazardous live conductors are exposed.

To begin each test with a Fluke Motor Drive Analyzer simply connect the test probes according to the diagram, then select Next.

Drive Input

An effective first step in determining if a feeder circuit to the drive has a distortion/disturbance/noise that may be affecting power ground is to analyze the power going into the motor drive

• Differentiate the measured frequency against specified frequency. Exceeding 0.5Hz difference could cause problems
• Observe if whether the harmonic distortion is within an acceptable level. Look at the waveform shape or view the harmonics spectrum screen that shows both the total harmonic distortion and individual distortion. Flat-top waveforms can indicate a nonlinear load connected to the same feeder circuit. If total Harmonic Distortion exceeds 6% there is a potential problem
• Check the voltage unbalance at the input terminals to make sure that the phase unbalance is not too high (less than 6-8%) and that the phase rotation is correct. High voltage unbalance reading can indicate phase failure. Over 2% reading can lead to voltage notching and cause a trip of the drive’s overload fault protection or disturb other equipment.
• Check for current unbalance. Excessive unbalance may indicate a drive rectifier problem. A current unbalance reading over 6% may point to a problem within the motor drive and become problematic.

DC Bus

The conversion of AC to DC inside the drive is critical. It is required to have the correct voltage and adequate smoothing with low ripple to have the best drive performance. High ripple voltage may indicate failed capacitors or incorrect sizing of the connected motor. You can use Fluke MDA-500’s record function to check DC bus performance dynamically in the operating mode while a load is applied.

• Determine if the DC bus voltage is proportional to the peak of the input line voltage. The voltage should be about 1.31 to 1.41 times the RMS line voltage (except controlled rectifiers)
• Check for any distortion or error in peak amplitude of the line voltage. A DC voltage reading +/- 10% from the nominal voltage can be a sign of a problem.
• Identify if the peaks of the AC ripple have a different repetition level. Ripple voltages above 40V can be due to malfunctioning capacitors or a drive rating too small for the connected motor or load.

Drive Input

It is important to check the drive output. It can show clues to problems within the drive circuits.

• Check if the voltage and current are within limits. High output current decreases stator insulation life.
• Ensure that the voltage/frequency ratio is in specified limits. A high ratio may make the motor overheat; a low ratio will make the motor lose torque. Unstable frequency and voltage indicate potential issues with the speed control circuits
• Observe the voltage modulation using phase to phase measurements. Voltage peaks higher than 50% can be problematic.
• Check the steepness of the switching impulses as shown by the drive reading. It is the rate of the voltage over time. Compare it to the motor’s specified insulation.
• Measure voltage unbalances at full load. It shouldn’t exceed 2% as it can cause excessive heat in the motor winding.
• Measure current unbalance, which should not exceed 10% for three-phase motors. Large unbalance with low voltage can be caused by shorted motor windings or phases shorted to ground.

Motor Input

The voltage supplied at the motor input terminals = key. Cable selection from drive to motor = critical.

Improper cabling selection can result in both drive and motor damage due to excessive reflected voltage peaks.

• Check that the current present at the terminals is within the motor rating. Over-current conditions can cause the motor to get hot and lessen the life of the stator insulation.
• Voltage modulation can help identify possible problematic high voltage peaks to ground
• Voltage unbalance that affects the life of the motor can be a sign of damaged inverter.

Motor Shaft Voltage

When the rotor shaft exceeds the insulating capacity of the bearing grease, flashover currents or sparks can occur. It can result in pitting and fluting of the motor bearing race that can damage motor permanently.

• Measure the voltage between the motor chassis and the drive shaft. The Fluke MDA-550 provides a carbon fiber brush probe for this purpose. This test can detect the presence of destructive sparks and the impulse amplitude can help you prevent future failures.

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