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#198951 - 02/08/11 07:26 PM Long control circuit  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
I have a motor contactor control circuit that I will protect properly leaving the controller where it also receives its power.

Is there cross talk problems on a 208 VAC contactor coil circuit (311VA surge, 37VA sealed) when run about 2100 feet one-way? The far end is just a dry contact (24 VAC coil and not as practical). Planning on 16 gage (18% and 2.2% voltage drop respectively without looking at power factor effects) and just trying to avoid requiring a specialized circuit that I know will work, but just may complicate things without providing any real benefit. I can also surge protect the control line without the special circuitry.


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#198953 - 02/08/11 08:39 PM Re: Long control circuit [Re: sabrown]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,142
Estero,Fl,usa
You are not going to pick a relay with noise or induced voltage. How are you driving it from the source end? High frequency spikes will be invisible to the coil so if you have noise sensitive control circuits you may want some suppression on the controller end. That may be as easy as a ferrite bead and/or a small disc capacitor.


Greg Fretwell

#198958 - 02/08/11 09:06 PM Re: Long control circuit [Re: sabrown]  
Tesla  Offline
Member
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
I'd spec #14 off hand.

Is the 208VAC in any way linked to the load?

It had better be isolated from back EMF.

You may need parallel runs depending on the coil. Super long runs bring with themselves impedances that may surprise you. At 208VAC your run must go up and back 4,200 feet. That's 1,400 yards! Further, can I assume that the control circuit must hold firmly?

You may benefit by using a PLC/ compounded relay at the motor to reduce the control current. The master relay could be DC which makes for crisp logical states.



Tesla

#198974 - 02/09/11 01:43 PM Re: Long control circuit [Re: sabrown]  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
The control circuit is just a pump mag, the 208 VAC is not isolated from the load at all. 14 gage gives a surge voltage drop of 9.2 to 9.4% (results from two different programs, slightly different wire assumptions). Droping the power factor as expected in a coil to an assumed 0.5 bring the drop t0 4.9% (beyond the capability of one program). I other words I am crossing the line between threory and practicality.

Using loose math, we take the coil and assume that its X/R ratio is 11 (my first likely bad assumption):
thus R = 11X, Z=V*V/VA=(208)(208)/311=139
Z=sqrt(X*X + R*R)= sqrt(121(X*X) + X*X), so
X=12.6 (reactance)
R=138


The 14 gage wire (from Table 9) for 2100 feet
X=.3
R=13

for a final
X=13
R=151

Z= 152

This gives a circuit flow of 1.36 amps when compared with the 1.5 amps of the mag at its rating, is a 10% drop in current. The mag should be rated for proper operation from -20% to +10% thus we are with in the operational parameters even during the initial surge with 14 gage.

But none of this takes in account the capacitve component of the 2100(*2) of wire.
C=0.225AK/tE12 where A is the area, K is the dielectric constant (Vacuum K=1, air greater than 1, solids and liquids may be 2 to 14000), and t is the distance in inches between plates, E12 means 10 to the 12 power.

Making a boat load of bad assumptions:
K=2, t=0.133 (the diameter of XHHW-2), A=(diameter of 14ga)(one way length in inches)=(0.024)(25,200)=604.8

so
C=2 nanofarads (and could be off by a factor of 1000 larger)
reactance of wire would be X=-1/2(3.14)fC = -0.2 to 200 micro which is insignificant.

I am probably over analysing this, but it seems with all my bad assumptions that this would not be an issue, but in the real world??? Such as I did not take into account the power factor effect, and all this may be just pulling the wool over your eyes making it seem like an EE is needed. Besides, I may have mistakes in here besides the poor assumptions.

The compound relay situation is what I want to avoid, for I would go beyond that to likely a Gems Sensor such as the series 26M. No issues there other than surge protecting the line if desired, they can sometimes see the surge protection as a closed contact.

Shane


#198975 - 02/09/11 01:52 PM Re: Long control circuit [Re: sabrown]  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
Tesla,

Back EMF? I have encountered that only when a coil was placed line to neutral improperly so it allowed a line to line feed back through the coil via the motor and pilot lights. I have not encountered it single phase line-to-line, nor three phase though I can see how it could be an issue again most likely through a pilot light.

What were the issues that created this in your experience?

Thanks, Shane


#198976 - 02/09/11 01:54 PM Re: Long control circuit [Re: sabrown]  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
Thanks for both of your replies above. I greatly value your opinions as they are from real world experience.

Shane


#198985 - 02/09/11 08:40 PM Re: Long control circuit [Re: sabrown]  
Tesla  Offline
Member
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
When a large motor -- especially distant from the distribution board -- kicks out it's still creating a huge back EMF that has to find a home -- until nature bleeds it off.

When ever the contactor is close to the motor and uses the same power taps for its own coil you can run into troubles on line-to-line coils. Line to neutral coils ( 24, 120 & 277 ) have the ability to bleed off enough of the distortion to resist chatter during start-up or excess transitory voltages during cut-off.

Collapsing fields can produce voltages ever higher until discharge is achieved. Hence, the electrodynamics of the old-fashioned DC coil and spark-plug. Imagine a 12V coil collapsing to produce 25,000 VDC - for an instant. Yeah, that's what happened to fire each and every spark plug in a 1960's engine.

When your motor is dropped-out by the contactor -- where will the rotational energy go? Too often it will test the contactor if it's using the same taps.

BTW, what size of contactor are we talking about #2, #4 or just a stinking #0?



Tesla

#198987 - 02/09/11 09:29 PM Re: Long control circuit [Re: Tesla]  
jay8  Offline
Member
Joined: Jul 2005
Posts: 183
Vancouver, BC
I've never installed a control circuit with this kind of length involved,so just throwing this out there but wouldnt some kind of networked control strategy be more reliable in this application? This would allow a data line to take up the distance instead of the voltage drop on a 208 volt control circuit.


#199003 - 02/10/11 03:08 AM Re: Long control circuit [Re: sabrown]  
Tesla  Offline
Member
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
Agreed...

When the distances are that great...

I'd rather go entirely away from any co-dependency.

That means compound contactors of either digital or analog form.

The WHOLE idea behind control circuits is to separate their dynamics from the controlled load: no chatter no over-voltage - no cross talk at all.

Last edited by Tesla; 02/10/11 03:09 AM.

Tesla

#199011 - 02/10/11 12:33 PM Re: Long control circuit [Re: sabrown]  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
The contactor is Nema size 2 and has operated for years without issues with the on/off located right there. But now for convienence we need to provide a dry contact the 2100 feet away. The pump is about 500-750 feet from the controller.

I will go over the replies closer in a little bit. I have just been informed of and we have found 3 corrupt files on the server after just a simple reboot last night. Got to see how many more. 3 different projects, 3 different users! It not sounding good. Backup from Tuesday night is available. I don't have the hair to pull out anymore


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