DriveRx Cable for VFD Systems
by: D.S. Reith, C.E.T., Applications Specialist at Nexans Canada Inc.
Several years ago a drive manufacturer conducted a study on which cable constructions were the best overall to use with VFDs. They considered various technical concerns associated with the use of VFD systems as well as economic issues such as cable and connector
cost, installation factors, and cable availability. Their study included cables that were commonly used for these applications as well as similar constructions with modifications to minimize or eliminate some of the unwanted side effects associated with the use of these modern VFD
drives with their fast rise time outputs. Results of the study were presented in an IEEE paper at the 1996 Annual Pulp and Paper Industry
Eight different cable constructions were set up to run the same motor from the same VFD. This set-up was constructed to simulate a real
industrial environment, but also had test equipment installed to measure the various electrical data to be used for the comparison. Five major electrical objectives of the tests were to select a cable that would:
a) minimize net induced ground currents into the drive system ground.
b) minimize common mode current.
c) minimize motor frame standing voltage.
d) minimize cross talk between adjacent cables.
e) provide the best electrical connection of sheath/shield and ground for optimum performance.
The results of tests for these five objectives, as well as the economic considerations, were given different “weighting factors” based on their importance to arrive at an overall rating comparing the eight different cable constructions. No one cable rated best in all categories, but the best overall cable outperformed the other seven in all of the most critical areas.
STUDY RECOMMENDS DriveRx® TYPE CABLE
Of the eight cables studied, the best overall construction was one with three insulated copper conductors, three bare copper grounding conductors, a continuous aluminum sheath and an overall jacket. DriveRx® VFD cable stocked by Nexans incorporates all these recommended design features, and it is CSA approved.
Insulated copper conductors should be at least 600-volts rated. This will enable the insulation to withstand high voltage spikes and reflected voltage. CSA has a 600-volt insulation level, then a 1000-volt level. A 1000-volt cable is recommended for a 575-volt system.
Nexans DriveRx® cable has 1000-volt rated RW90 XLPE insulated copper conductors.
Three bare copper bonding (grounding) conductors ensure a balanced low resistance path to ground for minimizing or eliminating common
mode currents and motor frame standing voltage. In addition three copper grounds in combination with the aluminum sheath ensure that this grounding path retains a low resistance as the cable ages. The total cross-sectional area of the three grounds must meet Canadian Electrical
Code (Table #16) requirements.
Nexans VFD Cable has three bare copper bonding conductors with total area meeting Table #16 requirements.
Cable with a continuous aluminum sheath provides an excellent shield for the high frequency pulses being transmitted to the motor. This will reduce cross talk to and from other cables running adjacent to it. In addition this sheath retains its good shielding capability over its life. Metal
tape shielded and interlocked armoured cables lose some of their shielding quality as they age.
The sheath in DriveRx® cable, in combination with the sectored copper bonding conductors, provides a long term, low resistance path to ground.
The testing conducted on the cable also included connectors. The IEEE paper outlined the requirements for connectors that were found to
provide the best electrical connection of sheath/shield and ground for optimum performance. These include 360-degree contact to the sheath,
the ability to accept a ground bushing, and to have good high frequency techniques (i.e., shielding). The VFD connectors, as well as the cable sheath, were originally designed to meet CSA requirements but also to be capable of handling sheath circulating and fault currents in normal
60 Hz feeder circuits. It turns out that just like the cable sheath, the connectors are ideal for use on VFD feeds as well.
Nexans DriveRx® cable has a heavy wall continuous aluminum sheath that on its own meets Table #16 requirements. Nexans
VFD cable connectors meet all the suggested requirements.
A jacketed cable helps to prevent the sheath or armour from coming in contact with ground along its route to the motor. An overall jacket helps
to ensure that this sheath is grounded at the ends only, and in that way avoids stray currents being picked up by contact with building steel or
the cable support structure.
Nexans DriveRx® cable has a sunlight resistant, black PVC jacket that is FT4 and AG14 rated to CSA requirements.
Nexans DriveRx® cable is CSA approved as RA90. It has 1000-volt rated RW90 XLPE insulated compact copper conductors, three bare
copper bonding conductors, a heavy wall continuous aluminum sheath, and a black, sunlight resistant overall PVC jacket. It is also CSA FT4 approved for hazardous locations (HL), and the outer jacket is sunlight resistant and low acid gas emission (AG14) PVC. Specialized VFD cable
is the preferred choice of several of the major drive manufacturers for cable connecting the drive unit to the motor.
Specialized connectors are another critical element in the modern drive system. Nexans VFD Cable Connectors are CSA approved in both the
wet (type “W”) and dry (type “D”) styles. In addition the type W is CSA approved for Class II, Groups E, F and G hazardous locations.