Motor Contactor and PLC operation 3

[JBUDA54]

I have been told that there is a distance limitation between the PLC output and the Motor Contactor coil. I have a PLC output located apprx 1800 feet from the motor starter. Is there a special relay that is required to take into account the excessive voltage drop that is present when trying to pull in the contactor? Also their is a good amount of capacitance that may cause the contactor to stay energized when trying to shutdown the motor. I have tried to research this and have come up short in finding any pertinent information. Any advise from those that have faced this issue I am very much eager to hear past solutions. Thanks in advance.

 

[dpc]

An interposing relay with a dc coil should generally allow much longer distances than ac. But this problem depends on your circuit configuration and source of 120 V ac. And voltage drop is not the main problem - that you can solve by using larger wire.

If the 120 V ac power source is at the PLC and the contactor is 1800 feet from these, you might be OK (although 1800 feet is a long run for 120 V ac control)

If the 120 V ac power is coming from the starter, running 1800 feet to the PLC output and back, then you have a problem. The capacitance in the control circuit may provide enough voltage to prevent the contactor from dropping out when the PLC contact opens.

Is this a 120 V ac control circuit?

 

[JBUDA54]

Yes, I have a freedom Cutler Hammer contactor with a 120Vac 60Hz Ac Coil. I ran a calc to see if I had a problem and determined from the Coil Data the following:

P.U. VA 230
Coil Voltage 120
P.U Amps 1.916667

AC Resistance of #14 AWG per 1000 ft 2.7
AC Resistance of #14 AWG per ft 0.0027
Total Distance 1800
Total AC Resistance 9.72 ohms

Voltage Dropped 18.63

Percentage of VD 0.84475

Operation Limits 85% to 110% of Rated Voltage

I am at the threshold of this Coil's operation.

This assumes that a #14 fits in the Controllers terminal. If I had to use a #16 even worse.
Coil data found out htis site.

http://www.eatonelectrical.com/unsecure/cms1/PG03301014E.PDF

Can someone explain to me the difference in this Freedom Starter Coil Chart the difference in operation limits and Hot and Cold P.U Volts?

 

[dpc]

As I already said, the voltage drop problem is just one issue. You can solve that by using larger wire or by doubling up the conductors. You do not have to run the larger wire all the way to the coil terminal - just to the starter. The 6-inch segment of smaller wire will not have much impact on voltage drop.

But you still need to be concerned about the shunt capacitance of this long control circuit.

The best solution all around at this distance would be to use a dc powered interposing relay.

 

[JBUDA54]

Thanks dpc for the recommendation on the DC relay. This recommendation was also stated by one of my co-workers. I believe I have a handle on dissertaining the Voltage Pick up implications, but how do you figure out if you will have problems with the capacitance looking like a short circuit when you have opened up the PLC output to de-energize the motor? What kind of calculation can be done? Am I supposed to get capacitance values of the cable manfacturer? If I do then I have to determine from the coil data sheet if the coil will drop out based on the Hot or Cold D.O. Volts in the chart, right? Please explain hot and Cold P.U. or D.O Volts. I am assuming that the hotter the coil the more resistance the coil will have causing the P.U./D.O to be less sensitive. I have made an inquiry to the MCC Manufacturer, but would like to know any thoughts from this forum. TIA.

 

[dpc]

Yes, you can obtain approximate data on the distributed shunt capacitance in the cable (assume it is wet or even submerged).

This might help: thread248-128558, although the Square D link doesn't seem to work for me.

 

[dpc]

I found it. - I can't get a URL, but go to Square D US Website and search for:

Distant Control of AC Relays, Contactors, and Starters, Document Number M-379

Good luck,
dpc

 

[JBUDA54]

Found it! This is exactly the literature I was looking for! Four Stars for dpc!!!

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=0900892680079349

 

[controlsdude]

1800 feet
You might consider using a field bus communications with a drop to this IO block. Then from the IO block have the contactor triggered off of this IO block. Then do a local power drop to this box and the power losses are a mute point.

 

[SteveWag]

Try using a Warrick 16HM relay. The supply voltage to the relay is 24, 120 or 240 VAC, the operating voltage (over the 1800 feet) is 12 VDC. I am looking at page E-9 of the GEMS Sensor cat. You should find it on line. Voltage drop is not a concern and with DC capacitance is not an issue.

Steve Wagner

 

[JBUDA54]

It was decided that we would use a solid state control relay from B/W Controls for this application.

http://preview.ametek.com/content-manager/files/stp/5200 Service Bulletin.pdf