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Reversing Contactors Wiring Diagram

rimcanyon

Diamond
Joined
Sep 28, 2002
Location
Salinas, CA USA
Looking for a wiring diagram for a pair of contactors to support instant motor reverse (i.e. you don't have to press stop first). The diagrams I found from Siemens and Cutler Hammer were for a single reversing contactor, not a pair. This is for a drill press with a three phase motor.

The drill press is currently setup with a pair of 3TB4112 contactors, one of which is bad. The machine is wired with a 220->110 3 phase transfomer, so the coils are 110V, which also supplies the light. The current wiring requires you to press stop before changing direction.

I bought a pair of Siemens 3TB4122 contactors, which are similar to the 3TB4112's except they have several extra contacts, both NC and NO. I think that is exactly what I need to support instant reversing. I could wire the new contactors just like the old ones, but I would like to add the instant reverse functionality.
 
So why is the electrical interlock not sufficient? Assume the motor is running in fwd. If the reverse switch is actuated, the nc contact on the switch opens, and the fwd coil is de-energised. If it fails to release or it is slow, that is not a problem because the reverse control path goes through NC contacts on the fwd switch.
 
Because if one set of contacts hangs up mechanically, you have a condition where you have a dead short across the contactor. Electrical interlocks may prevent a coil from energizing but nothing in a contactor senses a hung up contact unless that's a new feature I'm unaware of. When that happens, its amazing how much carnage it can cause, including spraying molten copper or brass around the enclosure. We has a brand new Feeler 2nd op lathe (harding copy) where the factory did not see the need to use a mechanically interlocked pair of spindle contactors. One day while tapping some parts and using the spindle reverse, guess what happened?
 
For the original question, I think you would need an extra NC contact on the switch to break the power to the opposing contactor. More contacts on the contactor shouldn't be necessary.

I would absolutely want mechanically interlocked contactors. Most contactors do not guarantee that contacts open/close simultaneously or in a specified sequence, so it's absolutely possible that the NC 'interlocking' contact closes before the main power contacts open. The exception is special safety contactors.
 
I put Siemens contactors in when I redid the electrical on my Van Norman mill. Instant reversing was not an objective, but I still went with mechanically interlocked forward/reverse contactors. Peace of mind.
 
Looking for a wiring diagram for a pair of contactors to support instant motor reverse (i.e. you don't have to press stop first). The diagrams I found from Siemens and Cutler Hammer were for a single reversing contactor, not a pair. This is for a drill press with a three phase motor.

The drill press is currently setup with a pair of 3TB4112 contactors, one of which is bad. The machine is wired with a 220->110 3 phase transfomer, so the coils are 110V, which also supplies the light. The current wiring requires you to press stop before changing direction.

I bought a pair of Siemens 3TB4122 contactors, which are similar to the 3TB4112's except they have several extra contacts, both NC and NO. I think that is exactly what I need to support instant reversing. I could wire the new contactors just like the old ones, but I would like to add the instant reverse functionality.
Two contactors.
One wired forward rotation, one wired reverse.
With a selectable switch.
The schematic writes itself.
But, you wouldn't need one.
 
I drew one up, just for kicks. Still getting used to this drawing package.

Drum/rotary switches work, either for switching directly or to switch contactors, but they don't give you no-volt release.
 

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More optimised and readable drawing - less wiring to the controls if remote.
 

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If we're doing plug reversing it's already throwing caution to the wind, right? I'd use the existing good contactor to turn the motor on, and then use one of those sketchy looking $25 Chinese DPDT contactors on a separate switch for the bang-bang reversing of two leads. The downside (one of many) is a failure of the reversing mechanism keeps the motor running in one direction. Best to wire it so off is reverse, so a failure doesn't drive a tap through the part and beyond.

Leviton makes motor rated toggle switches that are actually quite beefy, that I like to use for quick and dirty projects. Unfortunately they don't seem to have them in DPDT or reversing or anything fun.

In my experience cheap drum switches just have sliding contacts and a ball detent and do not inspire confidence in their ability to survive hard use.
 
I bought a used Allen Bradley size 00 reversing contractor with mechanical interlock, 500TOD94. It has the same footprint as the Siemens, so it should make an easy swap. It has a couple of devices attached to the coil A1 terminals, looks like an inline resistor and capacitor from the label. What is the purpose? The coils are 110V AC which matches the control circuit.


IMG_3664.jpeg
 
Inductive surge/kick suppression, I would assume.
Yes. It's a commercial item from several manufacturers. , that's what they are made for.
I know my way around drum switches.
And Doubled-up drum switches.
Hey, they are simple, and they work.
Also chunky and clunky.
There are better ways these days.
Better? I like just slamming the lever over to reverse and getting reverse, no issues. Simple, cheap, works. Not so convenient if you have to do it remotely, though. Other methods may be better then.

But then, I have no objection to older methods that work.
 








 
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