VFD insights

• This article covers how to maintain a variable frequency drive (VFD) by keeping it in proper operating shape with targeted preventive maintenance.
• By incorporating maintenance, heat and other problems can be reduced or eliminated in VFDs.

The dream when you buy industrial electronics is that they will work right out of the box, never need any attention and last forever. Unfortunately, sometimes the first one is somewhat true, but the next two wishes are just that — wishful thinking. Here is what things a conscientious variable frequency drive (VFD) owner should do to keep their equipment in good condition.

Heat is the No. 1 enemy of the drive. Drives, potentially, only have a couple of moving parts. Even motors that turn for a living only have a few parts that actually move; they are more noticeable in the motor than in the drive. Some drives don’t even have a single moving part.

Figure 1: Watts loss table. Courtesy: Yaskawa America Inc.

Motors have to worry about heat as well, and also need to worry about rapid physical motion that is dependent on healthy bearing operation. A VFD may only have cooling fans and relays that may have moving parts.

Heat issues come from two distinct sources: ambient temperature and heat caused by passing large enough amounts of current through metal conductors. See Figure 1 for a watts loss table. For more technical details, visit IEEE.

Calculating VFD heat loss

Because we are discussing a modern VFD, users can get a pretty good idea of watts loss at full load from the technical manual provided by the manufacturer.

The unit to measure heat loss is watts. On top of that, devices used in the manufacturing of modern VFDs feature temperature measuring devices built in. Most of the current that will go to creating heat will at some point pass through the VFD’s insulated gate bipolar transistors (IGBT).

IGBTs are the very fast, very efficient output switches that create the pulse width modulation waveform that is applied to the motor to create torque and power. Modern IGBT modules also have built-in thermistors measuring the heat that the module is experiencing. No formulas are required. At most, you will just have to check a keypad monitor for the exact drive temperature or maybe just read it over a network connection in real time.

Figure 2: Heatsink temperature monitor. Courtesy: Yaskawa America Inc.

Figure 3: Peak current monitor. Courtesy: Yaskawa America Inc.

The only issue with real time temperature monitoring is that it can’t differentiate between heat rise due to usage and the heat due to ambient temperature. So part of any installation and subsequent maintenance is to always control the ambient temperature around the VFD. Even when not being used, the drive can never cool down any lower than ambient. The higher the ambient, the less of the heat rise that is allowed before your VFD has to shut down on a temperature fault. It is safe to assume that if you trip on a high-temperature fault too often, it probably won’t be good for your drive components. Moral to the story: try not to cook your VFD.

VFD maintenance tasks

Engineers and experts who manage VFD upkeep should be seriously considering several typical preventive maintenance (PM) tasks:

• Clean the heatsink.

• Clean the filters (UL Type 12 enclosures especially).

• Check fan operation.

• Check the load.

• Check and retighten power and control connections.

• Replace fans as needed.

• Replace capacitors as needed.

• Replace soft charge contactor as needed.

Before we delve into these items, we should make clear that only qualified personnel with the proper knowledge of VFD operation should be performing these checks and maintenance. Capacitors retain energy for a while after lock-out/tag-out, and heatsinks take a while to cool down enough to touch.

Figure 4: proper and improper connections. Courtesy: Yaskawa America Inc.

Cleaning the VFD heatsink

Because the heatsink is essential for cooling the devices attached to it, the heatsink needs to be kept in optimal shape, which is an easy task. Simply keep the heatsink fins clean and unclogged and they will do their job to wick heat away from sensitive electronics like IGBTs and diode modules.

Generally, compressed air of sufficient pressure blown down through all of the fins will clear and dislodge any foreign matter that may be impeding the dissipation of heat. Because of the thermistors built into the output transistors, you can check the actual heatsink temperature with a keypad monitor or by reading the proper register contents over any kind of network connection to the drive.

If your environment involves oil mist or equivalent substance, the mist combined with the dirt and the dust in the air may make a substance that can cling to the heatsink fins and take more than just air to clean up. Consult with the manufacturer regarding proper cleaning techniques.

Check on the VFD filters

If the drive is of the UL Type 12 variety, there will be filters that need to be checked and kept clean. Filters will normally only be on the openings that are meant to draw cooler ambient air into the box. Either vacuuming or compressed air can be used to clean the filter media but be careful to not use too much pressure/suction or the filter media can be torn and damaged.

Any filter damage, even the smallest of tears, will require the filter to be replaced instead of cleaned. The frequency of the filter cleaning is very dependent on how dirty the environment is. Spare filters will aid in getting up and running quickly and many enclosures will allow safely changing the filter while running.

Learn to check the VFD fans

For the heatsink to do its job of cooling the drive devices adequately, it may require ambient air to be constantly blown across the fins for them to cool down more quickly. This, in turn, allows it to absorb more heat from the drive. This makes heatsink fans critical for tripless operation and long VFD life.

Some smaller drives don’t require heatsink fans while some larger drive might require three or four large fans. The proper amount of cubic feet per minute necessary was calculated by the drive designer based on heat loss of the current carrying devices running at the drive’s temporary overload ratings. Therefore, at normal operation (not in overload), there should be a slight excess of airflow. Still, all fans will need to be turning at their rated speed and in the proper direction.

Fans can be checked to be sure that they spin freely while unpowered and simple observation during a drive run state should show all fans spinning. Modern fans are probably hardier than some even a generation ago and most now include hall-effect sensors that will assure that the fan is spinning when it is supposed to be.

If the fans do have sensors, the VFD will either trip when a fan fails or will warn the user that at least one fan is not operating properly. Even if the drive doesn’t trip for a failed fan, if the temperature of the VFD cannot be maintained within the VFD specification, the thermistor in the IGBTs will report the temperature to the control board and the control board will shut down the drive to save the components in the drive.

Figure 5: Infrared camera image. Courtesy: Yaskawa America Inc.

Look at the VFD’s load (current)

It might seem like doing a load check is not part of a maintenance routine. VFDs have continuous and overload current ratings. The continuous rating is like it implies — all day, every day. The overload rating is just for short-term and intermittent usage.

The more time spent pumping overload levels of current through the VFD, the more likely it is that the drive won’t live as long as it was intended. Occasional overload, especially for accelerating large loads, is OK and what it is intended for.

But overuse of the overload capabilities can be detrimental. Some drives offer monitors that will track the peak current during each run state and inform the user at what frequency the drive was operating at when the peak was hit. If a VFD offers this kind of information, it may be a good part of vetting the health of the drive to run the application and let the drive tell you the peak current pushed through it during a typical operation.

Check the connections

Why check all of the power and control connections on a VFD? Weren’t they properly torqued down to exacting specifications during install? This is yet another reason to have personnel that are factory-vetted performing the VFD installation.

But due to vibration and usage, the control and power connections may become loose. What kind of issues can a loose control connection create? Most control connections are of low power types, like 24 volts direct current Vdc or 4 to 20 milliamps mA, for example. The loose connection can cause intermittent or inappropriate operation of the application.

Loose power connections are a whole different story. Due to the larger currents passed through power connections, loose connections can cause heat and even a combustion issue. When a power connection is loose, there is less connection area passing the current between conductor and the connection surface. This increases the heat produced by the current passing through, a type of bottle neck. Think of it as trying to pass too much current through an undersized conductor (thin wire).

Figure 6: Cooling fan monitors. Courtesy: Yaskawa America Inc.

The result is a level of heat that will cause thermal damage and possibly fire. A good PM program uses thermal scans of power connections to look for inappropriate levels of heat around power connections.

Again with the heat, albeit for a different reason than component wear and tear.

How to know when to replace VFD components

When it comes to an important application, it may be prudent to replace components that are nearing their service life expectancy during a yearly PM downtime or while changing something else about the application. Modern VFDs offer some maintenance monitors that count down the life expectancy of major components like fans, contactors, capacitors and output transistors.

Future drive improvements

Just like the rest of the plant engineer’s world, artificial intelligence (AI) may become a facet of PM. As with other uses of AI, the more information — in this case application, drive and motor performance information — the better AI will be in predicting component failure. Using all the information, like current, temperature or bus voltage, the VFD will be able to warn of impending issues.

PM is, of course, part of every good VFD installation. Mitigating heat issues by maintaining proper operation of heatsinks, fans and the application itself, will make it so that you only have to worry about a VFD once in a while.

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