Motor drives are a well-known technology for transforming the constant voltage from the main ac power supply into a voltage that varies to control motor torque and speed ideal for motors that are driving mechanical equipment loads. Motor drives provide higher efficiency than simple online motors and a degree of control not available on simple directly driven motors. These factors result in energy cost saving, higher production performance and extend the life of the motor. Let me explain how it works.

Motor systems are critical to the operation of almost every plant, accounting for 60% to 70% of all electricity used.  Also, we can identify variable frequency drives (VFD) as a source to provide plants with significant cost savings. Not surprisingly, motor drives are commonly used in many industries and facilities. To ensure uptime in these motor systems, maintenance and troubleshooting is a top priority.

Challenges to testing motor drives

Troubleshooting and testing motor drives, also known as variable frequency drives (VFD), variable speed drives (VSD) or adjustable speed drives (ASD), is often performed by specialists. They use several test instruments, including oscilloscopes, multimeters or other tools. Such testing can involve a degree of trial and error, using the age-old process of elimination. Due to the complexity of the motor systems, testing often occurs annually unless a system begins to fail. Determining where to begin testing can be problematic considering there is usually a lack of, or incomplete, work history for the equipment. This includes documentation of specific tests and measurements performed previously, work completed or the as-left condition of individual components.

Advances in testing technology have eliminated some of the challenges. Newer instruments, such as the Fluke Motor Drive Analyzers (MDA-510 and MDA-550) are designed to make motor drive testing much more efficient and insightful since they have the capability to document the process at each step along the way. These reports can then be stored and compared against subsequent tests to get a bigger picture of motor drive maintenance history.

An easier way to troubleshoot VFDs

Combining the functions of a meter, handheld oscilloscope and recorder with the guidance of a skilled instructor, advanced motor drive analyzers employ on-screen prompts, clear setup diagrams, and step-by- step instructions written by motor drive experts to guide you through the essential tests. This new method to break down and simplify complex testing enables an experienced motor drive specialist to work quickly and with confidence to get the detail they demand. It also provides a quicker path for less experienced technicians to start motor drive analyses.

Getting to the root cause of a motor drive system failure or performing a routine preventive maintenance check is best done with a set of standard tests and measurements at key points within the system. Beginning with the power input, we can perform five key tests with different measurement techniques and evaluation criteria:

1. Drive Input
Analyzing the power going into the motor drive is an excellent first step to determine if a feeder circuit to the drive has distortion, disturbance or noise that may be affecting power ground.
Drive input step by step guided measurement connections
2. DC Bus
The conversion of AC to DC inside the drive is critical. Having the correct voltage and adequate smoothing with low ripple is required for the best drive performance. High ripple voltage may be an indicator of failed capacitors or incorrect sizing of the connected motor. The record function of a Fluke MDA-500 Series Motor Drive Analyzer can be used to check DC bus performance dynamically in the operating mode while a load is applied.
3. Drive Output
Testing the drive output is critical to proper motor operation and can offer clues to problems within the drive circuits.
Voltage and current on the drive output
4. Motor Input
Voltage supplied at the motor input terminals is key, and cable selection from drive to motor is critical. Incorrect cabling selection can result in both drive and motor damage due to excessive reflected voltage peaks.
5. Motor Shaft Voltage
Voltage pulses from a motor drive can couple from a motor’s stator to its rotor, causing a voltage to appear on the rotor shaft. When this rotor shaft voltage exceeds the insulating capacity currents (sparking) can occur, causing pitting and fluting of the motor bearing race, damage that can cause a motor to fail prematurely.





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