reverse contactor theory & application

[sarbot]

Any help appreciated.

I have two different Hz frequencies at same 220 voltage, 40 amps each leg.

I need to be able to automate the selection of one source - - or the other - - but never BOTH.

If one source is the "default" source, always connected by reverse contactor, when I need to switch to source 2 then I assume I need source one contactor to open first before source 2 contacts close.

Same in the reverse, if I am running off source 2, then I send a signal to switch, then I need source 2 contacts to open before contacts for source 1 close energizing source 1.

I have heard of mechanical and electrical interlocks.

Does mechanical interlock when properly functioning ALWAYS open the currently powered contacts before it closes the contacts for the new desired source of power ?

Should I also specify electrical interlock as well for "double backup safety" ?

 

[waross]

A mechanical interlock does not open any contacts or do anything electrical. When one contactor is energized the mechanical interlock mechanically blocks the other contactor from closing. The simplest form is a small teeter-totter between and under the contactors. However most mechanical interlocks are a little more sophisticated than a teeter-totter. Always use electrical interlocks also.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

So you have two separate power sources, one at 50Hz, the other at 60Hz, then you ALSO have motors that need to reverse? Not sure why you need the isolation on the two power sources, I can't envision where you would have BOTH available in the same place, so if your equipment (motor) is rated for the other speed, then why bother isolating? Unless you have a portable generator that only puts out one frequency or the other and your equipment gets installed all over the world? If that's the case, I think you need to find another generator mfr.

Be that as it may, to answer your specific question, no, a mechanical interlock doesn't DO anything. It functions to PREVENT having both sets of contactors being closed at the same time. The purpose is to provide a back-up of electrical interlocking of the coil circuits. Let's say for instance that your coil circuit and aux. contact arrangement is correct, and you de-energize one contactor before energizing the other. What if the armature fails to move, i.e. the contacts are welded? If the electrical interlock circuit is also shorted, it might be possible to pull in contactor #2 while #1 is still closed. With a mechanical interlock, that would not be possible. The resulting high air gap on the 2nd contactor should overload the control circuit and blow the control fuse, but at least you did not cause a bolted fault (phase to phase short circuit).

As waross said (while I was typing this), you need both systems.


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[sarbot]

Gentlemen:

Thanks for your advice.

Today I spoke at length with an guy who said what I needed was to purchase a magnetic starter with dual contactors and the mechanical interlock in place, plus it comes with one overload relay.

He said to add another overload contactor/relay for about $20 so both power feeds are protected from each other.

Essentially, what I need is a starter set up for a two speed motor - - which uses a standard reversing starter set, but is wired slightly different.

He said that the coil terminals for both NC / NO are already present on each contactor in the box so I can wire up an electrical interlock system without adding anything.

I just wire my three phases of one power feed into the top terminals T1, T2, and T3 of the left contactor. Next, wire my three phases of the second power feed into the top terminals T1, T2, and T3 of the right contactor.

Next, jumper the output T1 on the bottom of the left overload contactor to the T1 on the bottom of the right overload contactor. Do the same for T2 and T3.

Next, connect each of the three load wires to their respective T1, T2, and T3 on the bottom of one overload contactor.

The wiring of the electrical lockout scheme using the existing coil relays is a bit more interesting.

As I understand, run a first set of control leads to make the left contactor coils NO, then continue on with those same leads to the right contactor coils to make it NC.

Then run a second set of control leads to the right contactor to make it NO and then continue on those same second set of leads to the left coil to make it NC.

In this manner the "upstream" or 1st coil to receive juice would be energized NO before the "downstream" NC coil (the 2nd coil) which allows power to flow in that particular contactor.

 

[waross]

The electrical interlocks or NC contacts will be in place and pre-wired on a reversing contactor. See the diagram that will be included with the starter. I would pass on the overload relays. They are meant for motor overload protection, and are not suitable for general protection. Run each supply through a breaker ahead of the reversing contactor. You will have good, code approved protection and will be able to isolate both contactors for service work.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Sarbot,

There is really not much to this. The fact that you "spoke at length" with someone about it makes me wonder if you shouldn't just go to the nearest electrical shop and ask them to wire up these two contactors for you. That is something you need to do anyway, because you are simple not allowed to do power work without having the necessary skills.

But do not confuse the guys with your speculations. Just ask them to wire up two contactors with electrical interlock. It is then your resonsability to make sure that contactor 1 is off before energising contactor 2 - and vice versa. And that seems to be a very minor problem. No problem at all, actually.

It would be interesting to know what kind of application this is. Do you have two frequencies available at the same time? And need to switch between them, using a computer to control the switching?

Are these voltage sources three-phase? And why are you also bringing "reversing" into the discussion? If you really need to reverse AND switch between two different voltage sources, then you need another set of contactors for the reversing function. Or are you just using the term "reverse" as a name for one of the contactors? Please clarify.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

...
It would be interesting to know what kind of application this is. Do you have two frequencies available at the same time? And need to switch between them, using a computer to control the switching?

Are these voltage sources three-phase? And why are you also bringing "reversing" into the discussion? If you really need to reverse AND switch between two different voltage sources, then you need another set of contactors for the reversing function. Or are you just using the term "reverse" as a name for one of the contactors? Please clarify.

That's what I asked early on. I am still concerned that this is being over designed / over thought. If the two frequencies are not at the same site at the same time, there is no real need for isolation at all. The only concern is with motor speeds / power etc. and maybe control circuit compatibility. isolation will not solve those issues.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376