Dear Sirs,

I need some information about Capacitance Effect which is created in the end of transmission lines carrying AC power. My problem is as below:
The Stop/Start Switch (starter) is located in Master Control room located about 1100m. far away the electric motor. The level of the voltage is
440VAC. When we push the Stop button, the motor will not be stopped because of capacitance effect.
One way to solve the problem is to change the level of the voltage to DC.

I would greatly appreciate if someone can advise me any technical solution to resolve the problem.

Best regards,
Zare

 

Responding to zare’s 21-Aug (14:12) query… see Control List Thread #10262363 “Controlling from Far Away” which provides some information.

Regards, Phil Corso ([email protected])

 

Correction to my 21-Aug (22:22) response:

Thread is #1026233638!

Phil Corso

 

If this is the standard start/stop contactor lashup where a contact from the contactor seals the coil voltage and the stop switch in series breaks the seal, then have your stop switch ground the wire to the coil as well as opening it. Obviously, it has to be break before make. But without a really good idea of what you have, it's just a guess. The idea is to remove any induced or coupled voltage from the coil.

Regards

cww

 

Unless the motor is very small, or the cable very unusual, no significant energy should be stored in the cable. Additionally, stored capacitance won't power an AC motor. It won't even spin a very small DC motor for very long.

Capacitance may affect the slow down of the AC motor. Properly sized resistor/capacitor networks should be able to slow down an AC motor quicker. You might want to check if you have phase correction capacitors sitting somewhere in the circuit. This could alter the stopping characteristic of the motor.

Some reactive transmission line effects are present with long-distance runs. Hydro companies spend much time worrying about them. Typically, the distances involved are on the order of 10,000 km, well in excess of your 1 km wiring run. Additionally, for a synchronized grid network like a hydro company, time delays and synchronization in the transmission lines can become significant. This isn't a factor in a simple motor application.

If this problem is happening in a real circuit, check the starter. It may have welded contacts, arcing contacts, or be malfunctioning in some manner.

 

Responding to Curt Wuollet’s 22-Aug (22:33) solution to the problem... what you propose violate the NEC!

Now, I know that on occasion you condone stepping over the line to “get’erdun” (as one celebrity puts it!) But, your proposal to 'short' the stop-circuit conductor to ground, clearly violates Art 250. Furthermore, there are legal solutions!

Regards, Phil

 

One reason why DC is used on many emergency stop circuits is because of this exact problem.

You said "the level of the voltage is 440VAC", but I assume that is the motor, and not the control circuit. Some motor contactors have replaceable coils where you can replace the AC coil with a DC coil.

In one system design I worked on we used contactors which had what the vendor called "economy DC" coils. These were coils which had a nominal voltage which was designed to be fed from a rectified AC source. We used 120VAC through an inexpensive full wave rectifier (purchased from an electronics supplier). This allowed the use of DC (which we needed for some reason which I can't recall) without excessively large currents (or a large power supply).

If you can use a similar solution, you might be able to just change the coil on your motor contactor and add a rectifier.

 

Hi Phil I don't have my copy handy, It's pretty old if I can find it anyway. What's the problem? Would they prefer I said Neutral? Grounded conductor? In the absence of the specifics of his control voltage scheme and country and other details, I used the word ground conceptually.

What I perhaps should have said is to switch the wire to the coil to the other wire to the coil. That should give you a fairly good chance of stopping the motor. If it would still upset the NEC somehow, perhaps you could quote the passage and we could all learn. I know I'd think it was a pretty good Idea if I was that far away and the damn motor wouldn't stop.

Regards
cww

 

Curt, thanks for giving me the segue into my “bear” story:

A group of hunters was seated around a campfire, swapping stories. They asked the old-timer in their midst to give some hunting tips. He said, “While hunting for bear, I was confronted by the largest grizzly I ever saw. It stood 10-ft tall, and had a 10-ft arm-spread! But, I couldn’t get off a clean shot, because the bear was standing behind a tree. But, I noticed a rock wall to the bear’s left and my right. I figured I could ricochet a bullet off a flat spot on the wall and hit the bear! I estimated our respective distances to the wall. I also knew the rifle’s muzzle velocity and its side and drop drift-rates. Taking into account wind speed and direction, then used trigonometry to calculate the angle of reflection with which I’d get the bear square between the eyes! I checked all calculations and fired!” One of the young’uns said, “Get the bear?” The old-timer responded, “Nope, missed the wall!”

Curt, my earlier response gave you the “wall!” Now, you get the “bear!”

 

I have already sent a message offering a suggestion on this subject, so I won't repeat it here. However, for those who are interested in what previous message Mr. Corso is referring to, the gist if it is quoted below:

> I strongly oppose the use of AC power for the circuit length you
> mentioned. Such circuits fail because of the phenomenon called
> "long-wire capacitance!" While possible to energize (close) such a
> circuit, wire-to-wire or wire-to-ground capacitance introduces
> de-energization or drop-out problems!

The message was in "Controlling from far", with the original question dated 7 April, 2007 - 12:40 am, and a URL of
http://www.control.com/thread/1026233638#65533

P.S. To Mr. Corso - as I have mentioned before, whatever client software you have been using recently (web interface or mail client) seems to have a character encoding problem. The control.com web site seems to be able to filter the extra characters out OK, but the mailing list passes them through in all their unprintable ASCII glory. If your own messages look OK to you, then likely the problem is in the character set configuration on your PC. There are three or four dozen unprintable characters (ASCII characters above 127) in
your last message.