Author archives: Steve Wetzel

Proper termination of variable frequency drive (VFD) cable is essential to realizing the benefits that can be achieved from using this special cable. These benefits include reducing electromagnetic interference (EMI), minimizing ground currents, controlling common mode current (which if left uncontrolled can damage motor bearings), and more. These benefits result in reduced downtime, fewer drive trips, and improved system performance. These benefits, however, can only be achieved through proper termination of the cable.

Southwire Company, LLC has a great application note on VFD cable termination, titled Begin with the End in Mind – Proper VFD Cable Termination. While that application note discusses the basics, it only addresses the simple system involving a drive, a motor, and a cable connecting them. If that’s the system you have, that application note is all you need. But many systems are more complex. Some systems involve a quick disconnect between the drive and the motor. Other systems may have a junction box between the drive and the motor. And still other systems might have both. In cases like this, it is important to know how to handle cable termination into and out of these additional components.

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Beginning with the end in mind is not only one of the 7 habits of highly effective people, it’s also a great philosophy to have when working with variable frequency drive (VFD) cable! If you have variable frequency drives at your facility you probably have heard of variable frequency drive cable. VFD cable has been shown to improve system performance by reducing electromagnetic interference (EMI), minimizing ground currents, controlling common mode current (which if left uncontrolled can damage motor bearings) and more. You may know a lot or a little about this specially designed cable that runs from your drive’s inverter to the motor. A lot of companies make VFD cable and a lot of salespeople from these companies will tell you that you should be running this cable. They may be right, but that is only half the story. The other half of the story is if you don’t properly terminate this cable, you lose most of the benefits it can provide.

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In applications where multiple motors are each powered by a separate VFD, care must be taken regarding the selection of the inverter to motor cables. Cable selection is even more critical if the cables are to be run any distance together in a raceway. Single conductor cables, while commonly used for some drive applications, can cause issues in such an installation. In addition to safety issues (see Southwire application note number 2012, VFD Cables – A Safe Bet), electromagnetic coupling can cause issues with drive performance.

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Introduction: One significant change to UL 44 (Standard for Safety for Thermoset-Insulated Wires and Cable) in the 2018 release is the addition of the 1000 Volt rating of US type designations. Now XHHW, in addition to having a 600 Volt rating, can be rated 1000 Volt. RHH and RHW cables, which had 600 V and 2000 V ratings, now can be rated 1000 Volts.

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Purpose: Insulation resistance testing is a non-destructive test procedure. The test measures the insulation resistance between the phases and/or between phase and ground. It is commonly used in the industry for acceptance testing prior to energizing the cable and for maintenance testing programs. General Testing Information • For single conductor non-shielded cable on a reel, insulation resistance testing cannot be performed due to the fact that low voltage single conductors do not have a grounding conductor, shield or ground plane. • For other cable on a reel, insulation resistance testing can be performed provided the sealing caps are removed. The procedure to test these cables is outlined below. • NOTE: It is important to remove sealing caps from both ends of the cable to be tested. Residue inside the sealing cap can be conductive and lead to false readings.

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Did you know that if you run cables that connect your variable frequency drives (VFDs) to your motors you could have a significant safety risk in your plant or factory? Fear not, there is a simple solution to this potential problem. It’s a fact. Non-shielded cables emit noise. In many cases, this is not a significant prob- lem. Most of us have heard that 60 Hz hum that happens when a phone line is run too close to a standard 600 Volt power cable. It’s really nothing more than a nuisance with standard power. But the same physics behind that hum may be creating a safety issue in your facility. VFDs change standard 60 Hz power in to variable frequency power that allow us to ex- perience significant energy savings, better control of our equipment, and reduced main- tenance costs. However, like most things in life, there are trade-offs. The down-side of a drive system is that it generates lots of high frequency voltage components that can cause problems with motors, drives, and other plant equipment. These same high frequency waveform components can also cause safety issues. Let’s look at how.

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Correctly sizing a VFD Cable for your drive and motor is really not difficult if you know where to look. By knowing what sections of the National Electrical Code (NEC) to ref- erence, you can correctly size cable conductor size for your system. Just follow these five simple steps to size cables for low voltage drive systems with operating voltages not greater than 575 volts. STEP ONE: Determine the minimum temperature rating of your equipment. Temperature ratings are important to know when derating the cable for the application as higher temperature ratings allow cables to handle more current. The NEC tables for ca- ble ampacity for low voltage cables have columns for 60°C, 75°C and 90°C. The column you use will be based on the minimum temperature rating of your drive terminals, your motor terminals, and your VFD Cable. Most drive terminals are rated for 75°C. All Southwire VFD Cables carry a 90°C conduc- tor temperature rating but this is not true of all VFD Cables from other manufactures. Motor terminal temperature ratings can vary from 60°C to 90°C. Each of these temperature ratings needs to be verified with the manufacturer’s datasheets or user manuals. If other equipment is being used that is in the cable’s path, like a quick disconnect, collect that devices temperature raring too. Once you have all the temperature ratings, record the minimum value.

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DC Hi-Pot Testing is used for proof testing shielded cables (5kV to 46kV) in the field. The test can be done at various times such as acceptance of new cable installation, maintenance testing to track insulation degradation and as a pre and post test for splicing existing cables to new ones. The test will expose gross imperfections that are caused by improper handling, installation techniques or termination workmanship. A DC Hi-Pot test is not capable of locating the point of failure, rather it gives you an assessment of the whole system.

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