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Discussion thread for: Start/Stop 3 wire AC motor control

Feel free to post comments here!!!

Scott SET.

ok,
looks like it's probably 208 or 480v , we could poke around part F of 430....some funky stuff there ( T430-72(b)??)

Interestingly, I recently had a discussion with someone about tapping L1 & L3 for a control ckt. I seem to remember seeing it done that way a while back. Now it is common practice to tap L1 & L2 to feed control transformers. Any comments?

Redsy, would there be any difference?

Here's another. On starters using grounded control circuits (120V), it's standard operating procedure for the grounded side to be run through the O/L contact (switching the grounded conductor). Isn't this a violation?

See this thread. This is how far I got.
http://www.mikeholt.com/forum/Forum1/HTML/000926.html

I always tap L1 & L3 because where I started my career at we had a "wild leg" on L2 and it reaked havoc on coils if we connected to it. Yep, I smoked a coil before I knew better.

Nick,
I brought this subject up a few months ago in one of the forums on this site (I'll search around for it). I didn't get any further than you did. There was a "because that's the way it is" theme to some of the posts.
It's my contention that this could cause damage to machinery (& people) in the case of multiple starters using a common control circuit if it goes to ground between the O/L contact and the coil.
These can be changed with a short jumper in most cases. I always change them.
What do you think.

Hi everyone, thanks for the comments!!!

I tapped L1 and L3 on these drawings only because it looks a little more clean, along with being quicker to trim a line in AutoCAD.

Not too sure if there's any law against connecting the control circuit to L1-L2 or even L2-L3, since the Line to Line voltage is the same no matter which way it's connected.

spkjpr, you must have been driving the coils from L-N [Line to Ground] in order for the high leg to burn out coils. If they were 240 VAC coils, the high leg to another Line would still be 240 VAC.
That's a good example, though, of when a person should be aware of which Line to choose from. If the control circuit was one Line to ground [neutral or common], and the system was a 4 wire Delta, there's a 1/3 chance in letting the smoke out of said contactor coils.

I made this set of schematics as simple as possible. The only reason for choosing 460 volts is to keep the motor connections simple! [these are series connected coils, rather than parallel coils for lower voltages]. Also, to keep things simple, I did not include a control transformer.
For the next series of schematics, I'll include a control transformer.

Thanks again for the comments!!! Keep 'em coming!!!

Scott SET

Probably not, just a matter of conventional procedures, or so I thought.

I would say that the true switching is done on the ungrounded side of control xfmr. by the control devices. The switching done by opening the OL relay is really not switching, per se. And besides, on 3 wire control, this drops out the coil, which will open the aux. contacts, and disconnect the ungrounded side, as well.