Automation Canada - Cables & Connectors Issue - 9

Add ringing and normal voltage fluctuations on the incoming line and
the cable may see more than 1,500V. With a 575V system, that number
jumps up to more than 1,900V. Using a 600V-rated cable in these
applica- tions makes about as much sense as using a 2,000V-rated
cable in a 4,160V application. Personally, I believe that the cable's rated
voltage should be higher than the voltage the cable sees. I would hope
you agree.
Thicker insulation is a good thing but it alone will not combat the
destructive effects of corona discharge. Corona discharge is simply an
electrical discharge. The Corona Inception Voltage (CIV) is the voltage
required to initiate a visible corona discharge from a conductor. Once
the CIV is reached, corona discharge will occur, and that discharge will
affect the insulation. As corona discharge produces heat, thermoplastic
insulations (like the PVC insulation used in THHN cables) are melted by
this discharge. Melted insulation does not work very well and cables
with melted insulation will experience a short circuit when voltage
breakdown occurs in the area of damage. There are other insulation
materials that will not melt called thermoset insulations (like
Cross-Linked Polyethylene, XLPE). Thermoset insulations are not
susceptible to premature failure from corona discharge like
thermoplastic insulations are. Most VFD cables have a thermoset
insulation like XLPE. Some VFD cables actually use PVC insulation which
not only can be damaged by corona discharge but also has other
problems like increased capacitance. But why is capacitance a
problem? Read on to find out!
In addition to allowing the cable to be rated for the voltage it actually
sees, a thicker insulation will also help reduce the cable's capacitance.
The main factors in calculating cable capacitance are the insulation's
thickness and its dielectric constant. Thermoplastic High Heat-resistant
Nylon coated (THHN) cable has a great price because it uses a thin PVC
insulation rated at 600V, but PVC insulation has a high dielectric
constant compared with thermoset materials. A 600V THHN cable may
have about 10 times the capacitance of a 2,000V XLPE insulated cable.

C A N A D I A N A U TO M AT I O N

" Why do I care about capacitance? " You say.
" Why did you buy a drive system in the first place? " I ask.
" To save energy and money! " You gladly respond.
This is where I start shaking my head.
Think of what a capacitor does. It doesn't let low-frequency current
pass, but it does allow high-frequency cur- rents to flow freely.
Remember the 60-Hz vs. multiple MHz discussion? At high frequency, a
high-capacitance cable is leaking lots of current to ground and thereby
is increasing energy usage and costs. In fact, enough current can leak to
ground that a drive may shut down.
Ironically, some people " fix " this drive tripping problem by up-sizing
the drive. They must be thinking the best way to reduce energy usage is
to use more energy. A better, less costly, green energy solution would
be to simply use a VFD cable that is not throwing so much of your drive's
energy away through capacitance coupling to ground.
WHY THHN IS NOT YOUR FRIEND
THHN cables not only have high capacitance, their PVC insulation is
only rated for 600V and can melt when exposed to corona discharge.
THHN, despite the fact that it is commonly used in drive applications,
meets none of the three criteria set forth above for a robust insulation.
Why do so many people use THHN? It's inexpensive and people don't
know about the downsides of using it in drive systems. There are a few
other reasons not to use PVC insulated cables, PVC is hydroscopic and
it can cold flow but we don't need to discuss those negatives in detail. I
think you already get the point.

VOLUME 3, ISSUE 2

9



Automation Canada - Cables & Connectors Issue

Table of Contents for the Digital Edition of Automation Canada - Cables & Connectors Issue

Automation Canada - Cables & Connectors Issue - 1
Automation Canada - Cables & Connectors Issue - 2
Automation Canada - Cables & Connectors Issue - 3
Automation Canada - Cables & Connectors Issue - 4
Automation Canada - Cables & Connectors Issue - 5
Automation Canada - Cables & Connectors Issue - 6
Automation Canada - Cables & Connectors Issue - 7
Automation Canada - Cables & Connectors Issue - 8
Automation Canada - Cables & Connectors Issue - 9
Automation Canada - Cables & Connectors Issue - 10
Automation Canada - Cables & Connectors Issue - 11
Automation Canada - Cables & Connectors Issue - 12
Automation Canada - Cables & Connectors Issue - 13
Automation Canada - Cables & Connectors Issue - 14
Automation Canada - Cables & Connectors Issue - 15
Automation Canada - Cables & Connectors Issue - 16
Automation Canada - Cables & Connectors Issue - 17
Automation Canada - Cables & Connectors Issue - 18
Automation Canada - Cables & Connectors Issue - 19
Automation Canada - Cables & Connectors Issue - 20
Automation Canada - Cables & Connectors Issue - 21
Automation Canada - Cables & Connectors Issue - 22
Automation Canada - Cables & Connectors Issue - 23
Automation Canada - Cables & Connectors Issue - 24
Automation Canada - Cables & Connectors Issue - 25
Automation Canada - Cables & Connectors Issue - 26
Automation Canada - Cables & Connectors Issue - 27
Automation Canada - Cables & Connectors Issue - 28
Automation Canada - Cables & Connectors Issue - 29
Automation Canada - Cables & Connectors Issue - 30
Automation Canada - Cables & Connectors Issue - 31
Automation Canada - Cables & Connectors Issue - 32
Automation Canada - Cables & Connectors Issue - 33
Automation Canada - Cables & Connectors Issue - 34
Automation Canada - Cables & Connectors Issue - 35
Automation Canada - Cables & Connectors Issue - 36
Automation Canada - Cables & Connectors Issue - 37
Automation Canada - Cables & Connectors Issue - 38
Automation Canada - Cables & Connectors Issue - 39
Automation Canada - Cables & Connectors Issue - 40
Automation Canada - Cables & Connectors Issue - 41
Automation Canada - Cables & Connectors Issue - 42
https://www.nxtbook.com/kerrwil/AutomationCanada/automation-canada-machine-safety
https://www.nxtbook.com/kerrwil/AutomationCanada/auto-tuning-and-variable-frequency-drives
https://www.nxtbook.com/kerrwil/AutomationCanada/automation-canada-automation-in-2021
https://www.nxtbook.com/kerrwil/AutomationCanada/AutomationCanadaMachineSafetyMay2021
https://www.nxtbook.com/kerrwil/automationcanadaquarterly/AutomationCanadaV2I5
https://www.nxtbookmedia.com