Practical distance limit for 3-wire starter control? 2

[tgreif]

Many times, we've used a DC control relay for long distance control of motor starters. At some distance, a 3-wire control (start-stop) with an AC magnetic starter will not be acceptable due to the capacitive coupling in the Stop circuit. Even though the NC stop button is pressed, the capacitive coupling between the wires keeps the starter energized and will not allow the starter to release.

My question is, what is the recommended distance limit where this needs to be accounted for. Nobody's ever been able to define it for me in the past.

 

[nbucska]

For DC relay the line resistance is the critical parameter.
I would think that a few thousand miles should be possible.
( with multistage relays )

What do you want to do?


<nbucska@pcperipherals.com>

 

[dpc]

If your question is about ac control:

Assuming control wiring in water-filled conduit as a worst-case, maximum control wiring distance in feet due to shunt capacitance:

Three-wire control:

120V ac - 60 hz
#12 awg 460 feet
#10 awg 365 feet

240V ac - 60 hz
#12 awg 115 feet
#10 awg 90 feet

Two-wire control:

120V ac - 60 hz
#12 awg 920 feet
#10 awg 730 feet

240V ac - 60 hz
#12 awg 230 feet
#10 awg 180 feet

Source is an old tech note for Furnas Class 8501 starters - I doubt that much has changed. Use at your own risk. Note that these figures are for 60 Hz only. 50 Hz systems should be able to handle longer runs.

If your question was about dc control.... never mind. You can go a **long** way. Limited really only by series resistance.

 

[nbucska]

dpc:
I don't believe you can determine the distance without knowing the relay's data.

For DC you can use solid state switch, FET or multistage
relay etc.

You can switch it by phone from the other side of the Earth

<nbucska@pcperipherals.com>

 

[dpc]

I agree that the holding voltage required is a determining factor. I was only trying to provide actual data for an actual device. If specific device data is available, the distance can be calculated.

The question referred to motor starters and this is what the data is for. NEMA contactors have fairly similar characteristics across manufacturers.

 

[rhatcher]

A star for dpc. That is useful data. I agree that for starters manufactured to NEMA specs that similar characteristics should be expected across manufacurers and that this info provides a good generic guideline. However, for situations at or near any of the distance limits listed above, specific device characterstics (ie. normal variations between manufacturers and designs but within NEMA limits) will be a factor that will require individual consideration.

 

[REDDOG]

try this for info

http://www.squared.com/us/products/...29214c232a6a852568390055a15d/$FILE/M-379F.pdf

 

[dpc]

Thanks, Reddog. Now I can through away this old dog-eared piece of paper I've been saving for the past thirty years.

 

[advidana]

Are you sure it is due to the capacitive coupling in the Stop circuit or is it the voltage drop of the circuit that is giving you problems. I have run dc circuit for about a mile or more but I size the conductors for the voltage drop and use a distance relay to allow for the low voltage as part of the circuit.

 

[jbartos]

Suggestion: Wireless remote control technology can also be considered. It saves copper, labor, etc.

 

[jraef]

tgreif,
I too am having problems with your descrition of &quot;capacitive coupling effect&quot; preventing a contactor coil from dropping out, as you described it. I think the other posters here were describing the effects of voltage drop preventing the circuit from being able to energize or hold the circuit in, but I can't see how you could ever have enough capacitance in 2 control wires to hold a contactor in when the power has been removed by opening a PB contact. There might be a brief, and I mean millisecond, delay in interrupting the circuit, but surely it would have to be one hell of a capacitor to maintain enough energy on the contactor coil to keep it energized, and even then it would not be continuous!

What is a common occurrance however is an INDUCTIVE coupling effect on control circuits when proper attention was not givien to the wiring system design. If you run the control wires in the same conduit as the 480V motor power wires, and the distance is long enough, and the contactor coil seal-in power is low enough, and the control conductor resistance is low enough, you may have the 480V induce enough voltage on the control wires to maintain the contactor coil closed even if you open the source voltage with the PB contact. I have seen this happen a lot. Inductive coupling is essentially a transformer effect from the higher to the lower AC voltages. The right solution is to keep them in separate conduits, the magnetic fields can't penetrate very far. Switching to a DC control relay circuit as you have done will also do the trick because you can't induce DC voltage.

To that end, there is no hard and fast rule as to how long the distance is before it becomes a problem. As I noted above there are many factors that go into it. For instance if you have a large contactor coil with a high sealed current draw, it would be virtually impossible to induce enough energy on the control wires to keep it in. If however you run the 3 wire start/stop circuit through a small 120V pilot relay controlling a large motor circuit, it is very easy to have that problem.

Safe bet? Don't combine the wires.

Quando Omni Flunkus Moritati