AM Contactor Wiring Question

[spinjector]

I was wondering something about the way AM contactors are wired.

It seems that in most cases, two of the switches are used as the tally for the switch position, and the other two are used to manage the coils.

I've seen the coil switches wired a few different ways:

1) They are cross-connected to prevent both coils from firing at the same time.

2) They act as an "active latch", so that if the contactor arm moves out of position, the power comes back on and zaps it back into place. This effectively locks it into position while power is applied, and I guess it would also help with old/sticky contactors that are moving a bit slowly and/or might need some grease.

3) A combination of 1 & 2.

4) Neither 1 or 2. In other words, not connected, such that 1 & 2 are handled in the switching controller.


So...

Are there any advantages/disadvantages to any of these?

Is there an accepted norm?

I like both 1 & 2, but I'm rather in favor of #4.

 

[NoTimeForSleep]

Yes, so that the contactor stays in a seated position and does not loosen the connection between the bar and fingerstock. If that were to happen the fingerstock will burn up fast and you are out of business. Also if you have constant voltage the coils may burn up. I have seen some with constantly energized coils and some that only energize when becoming loose or switching.

 

[spinjector]

Yes that's what I meant by #2. If the contactor arm moves out of position, the microswitch opens, the normally-closed contact sends power back through the coil, pulls the contactor arm back in, and the microswitch opens again. This is what I meant by it needing constant power. But the coil doesn't actually get power unless the arm moves out of place and the microswitch opens.

So this is the preferred method?

 

[Tom Wells]

It's preferred if the holding method works, if it doesn't you'll get a fine chatter at 60 Hz, or pulsed chatters at whatever the dropout duty cycle is.

A switching controller may get confused as easily as a mechanical interlock can age, break, gum up or whatever.

It's a good thing to test.

.. Seems to be contactor weather.
Last 3 days at work, contactors are dying.
Lost a Nema 4 contactor 100 hp 480 3-phase (westinghouse 20 years old) on a printing press dyer.
This one still worked, but began to chatter itself to death with a weak coil. I had to tear it apart,
repair it and nurse it along for 2 days, while it still chattered as the press ran, until a NOS Westinghouse NEMA 4 came.

and had to replace a Nema 3 for a 40 hp 480 3-ph suction/blower from a Square D to a Siemens.
Darn europeans who refuse accept any of US standard but make sure all control wires will be wrong length if originally dressed
neatly....different mounting base sizes..but oddly the compression allen screws were NOT metric...it was made in Mexico for a Siemens division with a (USA) Georgia address.

 

[spinjector]

Oh, I was referring to RF contactors, not AC contactors. As I'm not familiar with the NEMA variety of which you speak, can't even say if they have any similarities, other than the basic idea of an electromechanical device that uses a smaller voltage to switch a larger one.

 

[frankberry]

Yes. The preferred method is to use on pair of switches to stop the current to the coil when the contactor reaches the end of its travel and the Rf contacts are completely seated. The other pair of switches should be used for the Phasor logic to prevent the transmitter from powering up if the contactor isn't in the proper position.

 

[spinjector]

Frank - that much I was able to figure out, as having power in the coils 24/7/365 was obviously a bad idea.

The part that I wasn't sure about was wiring the two switches that control the coils so they have constant AC on the open contacts, so they close and kick the arm back into place if it moves.

 

[K4WRF]

Frank - that much I was able to figure out, as having power in the coils 24/7/365 was obviously a bad idea.

The part that I wasn't sure about was wiring the two switches that control the coils so they have constant AC on the open contacts, so they close and kick the arm back into place if it moves.

The most reliable connection scheme is to use a set of the micro switches contacts to de-energize the actuating solenoid; then the other micro switch to enable the opposite solenoid for when that mode is selected. As Frank said, use the others for tally and interlock signaling back to the phasor controller so that the status can be viewed and the transmitters muted during the switch. I prefer to extinguish the 220 volt solenoid control voltage going to the contactors and supply that voltage only when it is needed for switching. Leaving it on has caused me more problems in the form of burned up solenoids and erratic behavior not to mention just having 220 Vac floating on the system all the time. If those switches can’t move in three seconds, they ain’t going to move and in a very short time, are going to be on fire anyway.
w/

 

[boiseengineer]

Once there was a station with 2 towers DA-N that had the two-coil RF relay burn up in the phasor. So they replaced it with a single coil relay that required constant voltage to hold it in the night position. The home brew controller was thusly modified to keep voltage on all the time.

Someone forgot that there were no end of travel limit switches on the origional switches, nor any tally back to the controller. Now the origional relays were 240 V and the new single coil was 120 V so no immediate burn ups. But they were always having trouble changing pattern.

Top it off the electrolytics in the controller had dried up so the Power Rock wasn't muting. At all.

Finally the s### hit the fan with a couple burned up relays and a really broken Power Rock.

 

[spinjector]

Oopsie. You mean the Rock didn't just spew corona off what was left of the switch? =-)

That's the problem with non-standard, or multiple standards, or just plain home-brew. Sooner or later someone does an oopsie and something goes kerplooey.

 

[frankberry]

I have never kept voltage on the microswitches except during pattern change.
A good lightning strike can cause the microswitches to fail resulting in 220 volts on the coils.
This will burn up the solenoid coils quickly.

 

[ChiefEngineer]

Old men and their memories. Most AM stations don't even change pattern these days. Can you imagine explaining to a GM you need microswitches ($$$) to change pattern and meet FCC Rules. Mention the cost of FCC violations? As one operator said, "that's why we have an FCC attorney."

I spent a lot of time rebuilding several DA systems that had been neglected. After this the next guy just ignored them, and all. At pattern change time no indication of signal strength change. Certainly no audio silence audio.

I certainly enjoy reading of such things as this post... just the same.

On a side note I have heard reported SM sounds better from the transmitter site. When I was in nashville pre flood it sounded pretty good. Comments on this. I heard you had moved on Watt.

 

[boiseengineer]

You mean the Rock didn't just spew corona off what was left of the switch? =-)

Power Rock's controller was a mess too. What VSWR protection!?!
We first discovered the problem when at the studio we watched the DJ change pattern & it only made a "pop" on the air. Even he thought that was weird that it didn't go off for a second or so.

We were in town to fix other things and it just snowballed from there.

 

[spinjector]

Ah love those engineering snowballs. ;D

 

[littlejohn]

Back in tthe 80s, KVOO lunched a couple of contactors. The early MW-50 as installed didn't >quite< kill >all< of the pulse generator, so there was a bit of RFstill being produced when the contactors switched. Which of course made a very small spark and a pit on the finger stiock. 6 months of this and there was enough pitting that the roller fuinally hit pits for most of the contact area and when the 50KW came up, the contactor went away. Six months later, same same. At that point I believe it was Larry White who realized what was going on and reset up the MW 50.