Your VFD Cable Could Be Costing You Thousands in Equipment Failures
Key Highlights
- Improperly terminated VFD cables can generate up to 60 amps of common mode current spikes through facility ground grids, damaging unconnected control and communication equipment.
- Most electricians terminate VFD cable shields incorrectly by treating them like communication cables — floating one end or using drain wires instead of bonding both the drive and motor ends for low-impedance connections.
- Three recommended cable types include continuously corrugated and welded aluminum armor for hazardous locations, free conductor with three grounds and a copper tape shield, and flexible copper braid shield configurations.
Variable frequency drives (VFDs) have significantly impacted how motors are controlled in industrial settings since the 1980s. They save energy and improve process control. But when it comes to VFDs, one thing that’s often overlooked is the cable connecting a VFD to a motor. Lack of attention to your choices here might be costing you thousands of dollars in premature motor failures, unexpected downtime and interference with other equipment.
To get more insight on this, we connected with Steve Wetzel (SW), a member of IEEE and principal applications engineer with Southwire Company, one of North America's largest wire and cable manufacturers, for a recent episode of the Automation World Gets Your Questions Answered podcast.
Wetzel recently participated in a research project that tested different cable types and four termination methods to answer a critical question: Does the cable between your VFD and motor really matter? This research led to a forthcoming IEEE paper on the topic.
AW: To help manufacturers understand the importance of industrial cabling investments, what would you say is the main factor in how improper VFD cable selection or installation impacts motor life and performance?
SW: The top way that improper VFD cable selection or installation can impact motor life is through premature motor failure caused by bearing fluting that can significantly increase with the use of improper cable or improper termination techniques.
But the thing is that some people may not understand is that improper cabling between your drive and your motor can affect unconnected equipment. It can affect control and communication devices leading to shorter mean time between failure than the manufacturers recommend, and it can also create intermediate issues with plant equipment. And all that leads to downtime and decreased productivity.
AW: The IEEE paper you helped write based on your research shows that properly terminated VFD cables reduce certain current significantly compared to standard cables. Can you explain what that means for equipment uptime and maintenance costs?
SW: A properly terminated VFD cable will contain something called common mode current within the cable shield and that current is a creation of the drive. It's the net current flow down the cable. By properly terminating your VFD cable you turn that very high frequency current back to the drive through the cable shield. If it doesn't go through the cable shield, it's got to go back through some other method, which is typically through the ground grid of your facility — and the currents here can be significant. We've measured up to 60 amps common mode current spikes through the ground.
And if you think about it, if you've got a sensitive controller communication device and that current is going to go back because it's uncontrolled, then you don't know where it's going to end up.
AW: The IEEE paper also notes a distinction between standard cables and VFD cables. Can you explain when it's acceptable to use a standard cable and when a VFD cable is absolutely necessary?
SW: That is the $64,000 question. Unfortunately, it's very difficult to answer because each facility is kind of like its own electrical circuit. They're all unique in terms of how things are laid out. Even the drive manufacturers kind of dance around this issue. One drive manufacturer says in its installation guide that you don't need to use a shielded cable, which would be a VFD cable, if electromagnetic interference (EMI) is not going to be a problem. While that’s a true statement, it’s about as useful as saying you don't need to wear a seat belt if you're not going to get in a car accident.
You don't know until the installation is complete if you're having problems with electromagnetic interference and then it becomes very expensive to retrofit something like that.
AW: Considering all the VFD cabling research you’ve done, what would you say is the most common mistake facility managers and installers make when selecting and installing VFD cables?
SW: I haven’t seen much of a problem with cable selection, but the installation method is a big deal. Most electricians and installers don't know how to terminate that VFD cable properly because most have never worked with a shielded, low-voltage power cable. Therefore, what they'll do is reference a control cable installation, which they’re more familiar with. They understand you’ll likely bond that shield with a little drain wire at the source and float it at the other end, which is perfectly acceptable for mitigating all the radiated EMI.
When somebody installs a non-VFD cable (a non-shielded cable) and then they have issues and want to go fix it, if they're running conduit a lot of times the conduit is not sized to allow for this. They'd have to re-pull the conduit and that can turn into a real nightmare.
But we're not just dealing with radiated EMI because of the VFD’s high frequencies. To control the path of a conducted current, it has to be terminated on both ends or it's not going to go through correctly. That, I would say, is the biggest issue I see with VFD cable installation problems.
AW: In the research for this IEEE paper, you tested eight different cable types. For a typical manufacturing plant, are there two or three types you would recommend from among those and why?
SW: First, I would recommend a continuously corrugated and welded aluminum armor, which typically has a PVC jacket over the top of it. That's really a requirement for any Class 1, Division 1 hazardous location. In general, this is a very good VFD cable construction because that armor acts as the overall electrical shield in addition to providing mechanical protection.
The other two cable types I would recommend are a free conductor with three grounds and a copper tape shield. This type of cable is not really flexible, but it's great for industrial installations. And the third type I’d recommend would be a more flexible cable with a copper braid shield.
AW: Speaking of cable shields, you’ve emphasized the need to terminate the cable shield at both ends. This led me to wonder, is this something installers often skip? And is there a simple way to ensure that it's done correctly?
SW: This is one area I would emphasize that people pay attention to because, for some reason, there hasn’t been a lot of training done around this. Drive manufacturers all pretty much agree it needs to be terminated at both the drive and the motor, but the word just hasn't gotten out about this.
I've done informal surveys of electricians and a lot of them think you should terminate at one end — at the drive — and float the motor. I’ve also seen where people cut the shield off at both ends and use a drain wire to terminate because that's how you do a communications cable. The problem is, with a VFD, you want low impedance at high frequency to make sure that high frequency flows freely. A drain wire does not allow that to happen.
By properly terminating your VFD cable you turn that very high frequency common mode current back to the drive through the cable shield. If it doesn't go through the cable shield, it's got to go back through some other method, which is typically through the ground grid of your facility — and the currents here can be significant. We've measured up to 60 amps common mode current spikes through the ground.
Fortunately, there are some accessories you can use to make sure you get a low impedance and high frequency bond at both the cable and the motor. I strongly recommend using a drive manufacturer’s EMC (electromagnetic compatibility) plates, which many drive manufacturers are shipping on some if not all of their drives. I’ve seen these EMC plates thrown into boxes to be thrown away because, a lot of times, people just don't bother to put them on. But it bonds the PE (protective earth) bus with a very large surface area metal plate, which is very easy to attach the shield to at the motor.
And if you're running conduit, you can't use a VFD cable gland, but you can use VFD cable term kits, which bonds that shield to the motor ground with a very effective low impedance at high frequency connection.
AW: If a manufacturer discovers they have a VFD installation where the shield wasn't properly terminated, how difficult is it to retrofit these existing installations that weren't shielded properly and what type of improvement can be expected from this?
SW: Well, if they've run shielded cable but just haven’t terminated it correctly, we can likely help them with accessories to address that. For instance, installing an EMC plate at the drive is going to take up very little extra room. And hopefully they have room inside their cabinet to use a standard cable gland versus an EMC cable gland, which would take up no additional space at all.
The more common problem I see, however, is when somebody installs a non-VFD cable (a non-shielded cable) and then they have issues and want to go fix it, if they're running conduit a lot of times the conduit is not sized to allow for this. They'd have to re-pull the conduit and that can turn into a real nightmare.
This article captures a few highlights from our podcast conversation with Weztel. Listen to full podcast episode to catch all of his insights on VFD cabling best practices.
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David Greenfield, editor in chief
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