Automation Canada - Cables & Connectors Issue - 10

DON'T SKIMP
Drives don't like to experience over-current trips, its not conducive to
their long term well-being and it's my guess you would prefer that all
your drives' energies get sent down the cable to the motor instead of
bypass- ing the motor and going to ground. You probably don't want to
be one of those people that will spend more money to buy drives to
save energy and then spend less money on cable so that you can throw
away some of that energy you paid more money to save in the first
place. I suggest you spend a little more money on the cable to save
energy - kind of like when you spent more money on a drive to save
energy.
DON'T CAUSE PROBLEMS
So far we have seen that using the incorrect cable between your
inverter and motor can lead to:
* Premature cable failure due to over-voltage and corona discharge.
* Operating issues with nearby equipment due to uncontrolled EMI.
* Wasting energy by sending it to ground and bypassing the motor,
via capacitive coupling.
* Unnecessary drive trips due to high cable capacitance.
A properly designed VFD cable will minimize all of these above
issues. However, the wrong cable (or an improperly designed VFD
cable) will not only NOT minimize all of these issues, an improperly
designed VFD cable can actually contribute to the severity of drive
related problems! It can do this by actually increasing the amount of
common mode current (CMC) in the cable itself. But before we look at
how a cable can actually increase the amount of CMC flowing let's look
at what CMC is and why it's important. CMC is defined as the total sum
of current flowing in the cable. Add up all the current flowing in all the
conductors, grounds and shields and if you get a number close to zero,
life is good. In traditional 60-Hz power systems the CMC flowing in the
cable is very close to zero. In today's high speed drive systems, CMCs of
100 amps have been mea- sured. That is a very big number.
100 amps is a problem and we will see why a little later but for now
let's see how we can have such a large amount of CMC flowing in the
cable in a VFD system. To simplify things, let's just look at the power
C A N A D I A N A U TO M AT I O N

conduc- tors (where all the current is supposed to be flowing). In
traditional 60-Hz power systems, each phase consists of a nice looking
sine wave that is out of phase by 120° from the other phases. When we
add up three equal amplitude sine waves that are 120° out of phase you
get zero thanks to the wonders of trigonometry.
In the world of variable frequency drive pulse width modulated
waveforms, things are not so nice. Imagine
a simple two state drive that outputs either +V, or -V on each phase.
Any way you look at it, you cannot add up the three phases to be
anything close to zero. The closest you can come is +V + +V + -V or +V
+ -V + -V in either case, the amplitude is V and that is not going to equal
zero unless V equals zero and if V equals zero we paid way too much
money for this drive that has no output!
GOOD CONSTRUCTION
Cable construction can make this situation even worse! To see how
let's go back to our simple sine waves and look at the current flowing in
the ground conductor of a 4 conductor cable.
There is no power being forced down the ground conductor but
according to Michael Faraday's law of induc- tion we will see a current
flowing in this ground conductor from each of the phase conductors.
The phases adjacent to the ground are closer to it than the phase
opposite the ground. When the three phases are not equidistant from
the ground we don't get complete cancellation of the induced sine
waves and we will have current flowing in the ground.
Compare that to the current we see flowing in a symmetrically
designed VFD cable, sometimes called a 3+3 design. In this cable
design we have three phase conductors and three grounds, one
ground in each of the three interstices between the phases. As this
construction is symmetrical, trigonometry magic occurs and we get a
complete cancellation of the induced currents. We end up with zero
current flowing in the ground.
So a " 3+3 " VFD cable will have less CMC flowing in it than a 4
conductor VFD cable. And less CMC is a good thing! So why do you
want less CMC flowing? Because the CMC flowing from the inverter to
the motor has to find a way back to the inverter.
VOLUME 3, ISSUE 2

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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
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Automation Canada - Cables & Connectors Issue - 42
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