Motor-driven loads like compressors, fans and pumps are at the heart of modern manufacturing. Many facilities have incorporated adjustable speed drives (ASDs) into their motors in the interest of saving energy and enabling greater control over the motor’s speed and torque. The result is a complex system that can pose challenges to any maintenance, repair and operations (MRO) team.
When motor and drive systems fail, it can be extremely difficult to determine whether the drive, the motor, or the load is causing the trouble. Fortunately, most motor issues are easier to identify and resolve when you have the right tools – like a power analyzer and a motor drive analyzer.
Relying on guesswork or an improvised diagnostic process makes the repair process needlessly time-consuming and expensive. A systematic approach, driven by fault tree analysis, saves time and resources. This is especially important for organizations facing labor shortages and a lack of experienced personnel.
This article will discuss some of the most effective ways to troubleshoot issues in a three-phase motor and adjustable speed drive. We’ll talk about how to pinpoint which part of the system is causing the issue and how to diagnose faults using a repeatable, standardized approach.
How to Assess Motor Health
When you’re faced with an undiagnosed motor-drive issue, your first step should be to assess the health of the motor by checking for current imbalance. The current drawn on each phase of the motor should be balanced (or at least, the imbalance should be less than 10 percent). Current imbalance leads to a wide range of problems including increased vibration levels, mechanical stress (including high operating temperatures) and torque pulsation.
An imbalanced current can indicate a few possible issues. It could mean that the motor has an internal fault, like wear on the stator insulation. Or, it could mean that there is a voltage imbalance. If you can rule out the voltage imbalance, then you can assume that the motor is the source of your problem.
Check for a Voltage Imbalance
Using your motor drive analyzer, you can check for a possible voltage imbalance at the output and the line side of the ASD. Bear in mind that even a relatively small voltage imbalance can create a high current imbalance — the disparity can be six or even ten-fold, so you’ll want to check for any level of voltage imbalance.
At this time, it’s also a good idea to rule out loose connections as the cause of the motor-drive trouble. You can do this with an infrared thermometer or thermal imager. Readings that are significantly higher than the ambient temperature can point to loose or bad connections.
Isolate Problems in Your Drive
Drives can trip because of undervoltage, overvoltage, or overload. Fault codes on your drive will tell you which issue tripped the drive. Once you know, it’s a question of figuring out the root cause of the issue.
Dealing with Undervoltage
Undervoltage is often the result of voltage sags on the drive input. You can check for this with a high-quality power quality analyzer. The best analyzers also come with a built-in sags and swells function.
If the issue isn’t external, the undervoltage could be caused by an issue with the DC link capacitors or the reactor. You can check your capacitors with a digital multimeter, or you can use the trend function of your power quality analyzer. You can also check your reactor by checking to see if the waveform has changed on either side.
What About Overvoltage?
Overvoltage can be traced back to a number of different root causes. It’s often the result of issues with your capacitors or your reactor, but it can also be caused by voltage transients.
Regenerative loads — like cranes and elevators — can also cause overvoltage, since they feed back power into the electrical system during their normal course of operation. Normally, dynamic braking circuits are installed to protect your drive from the excess energy produced by regenerative loads. But if the braking circuits aren’t installed correctly, they won’t prevent excess energy trips.
Finding the Cause of Overload
Overloading is the surest way to overheat the motor, leading to an automatic shutdown and, of course, unplanned downtime. A high overload will trip the drive and shut down the motor right away; a lower level of overload will take longer to trip the motor.
Overloading happens when a load demands more torque and current than the motor can supply. When checking for overload, be sure to measure the current drawn over a period of time. This is known as load profiling and you can measure it by using the power-record function on a power quality analyzer.
Check for periods of unusually high current. You can make this calculation by checking the full amp load information on the motor’s nameplate and comparing the current load against that. Motors operate most efficiently when they’re within 60 to 80 percent of their full load amps. A higher load will cause the motor to overheat; a lighter load may waste fuel and resources.
Variable Torque Vs. Constant Torque Loads
Variable torque loads, like fans and blowers, rarely cause overload-related problems for drives. These loads don’t operate at full speed most of the time, which means they draw less current.
If a variable torque load does trip the drive, it’s usually because the load was not correctly sized to the drive. It’s also possible that the load changed or was impacted in another way. For example, a mechanical issue could require more initial torque to get the load moving, which would cause it to trip.
Constant torque, or heavy-duty loads, require a consistent level of current, regardless of the speed they’re operating at. This can easily cause overheating. Unfortunately, many motors use a cooling system that only kicks into action when the motor speeds up. This kind of system won’t cool your motor when it’s running at a lower speed – even if it’s using a high level of current. If this is causing problems, it’s a good idea to use an external cooling system instead of relying solely on the motor’s built-in fan.
Taking Additional Motor Measurements
When you’re using an ASD-motor system, there are some complications that you need to be ready for. The ASD isn’t capable of filtering out all of the high-frequency elements of the voltage waveform and this can cause issues like overvoltage reflections, leakage currents, motor shaft voltages and bearing currents. With the right tools, you can diagnose these issues quickly.
To identify overvoltage reflections, use an oscilloscope to measure phase-to-phase at the motor terminals to check for peak values. Overvoltage reflections should be visible on the scope waveform. You can correct them by shortening the drive-motor cable length, using a motor with higher-grade insulation, or using filters.
To spot motor shaft voltages, measure voltage shaft-to-motor ground with a carbon brush or stranded wire. If you find voltages of eight to 15 volts between the shaft and the frame, you can usually solve the problem with a shaft grounding device.
To diagnose leakage current, measure with a current clamp around all three phase conductors. You can correct this issue using EMI suppression cables or a common mode choke.
Moving Forward
In this article, we’ve discussed some of the most common causes of motor-drive faults. Although this isn’t a comprehensive list, it’s an excellent starting point for maintenance teams working with three-phase motors and adjustable speed drives.
It’s hard to overstate the importance of a proactive maintenance approach, especially when it comes to maintaining motors and drives. Addressing issues early with standardized protocols is one of the best ways to see improvements fast when it comes to uptime, asset lifespans and maintenance costs. And that’s a powerful way to move forward.
Michael Crepps, Fluke Application Specialist, joined Fluke in 2016 as a Technical Support Engineer. Michael has been providing support and training on electrical test and measurement tools for the past 7 years. Prior to Fluke, Michael worked in the bio-tech industry providing technical expertise in manufacturing and development.