Motor Run Confirm

[controlnovice]

We have a critical application with an Agitator that we are controlling through our DCS.

We recently lost the run confirm contact (only the contact....the starter was still closed) which shut down the agitator (due to the way we configured it in the DCS).

Is there any reason to have the Run Confirm used as a feedback to keep the agitator running, or just use the Run Confirm as indication only?


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This is normally the space where people post something insightful.

 

[waross]

It depends.
I don't think that it is safe to answer that without knowing a lot more about your DCS scheme and the control philosophy behind the original design.
This may also be controversial. See thread830-151978
I have seen DCS schemes were a change such as you are contemplating would create safety issues. In other DCS systems it may not be a serious issue.
It depends.
yours

 

[jraef]

Depends on a lot of factors. If it shut down when you lost that contact, that means you probably have a "3-wire control" system, probably a Start and a Stop momentary push button set. The main reason for using this kind of control is for safety. That "run confirm" contact as you put it is what is referred to as a "Seal-In" contact around (parallel to) the Start button so that once the contactor is closed by pushing the Start button, it stays in until you push the Stop. If power should fail, the contactor drops out and you also drop out the Seal-In contact, which means that your motor will not restart when power is restored until someone physically presses the Start button again.

The alternative is called "2-wire control" where there is some sort of maintained control device, such as a selector switch that stays in a "Run" position. If power fails and is restored, the motor will re-energize immediately, which may involve a serious safety risk.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[controlnovice]

jraef,

Thanks for the refresher course!

Not only is a 2-wire control possibly a safety risk, but if there are multiple large motors (in our case), the electrical system may not be able to restart all of these at once after a power outage!



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This is normally the space where people post something insightful.

 

[BJC]

These guys make a current switch.

http://www.veris.com/

Check out their Hawkeye series.
I have used them in aggitator applications. They can be set to operate at a current level greater than no load ( so you don't get a positive indication with a broken belt or drive shaft). I have had them installed in MCC buckets by the MCC vendor.
I am prettys sure any of the solid state motor starters can sense the same thing. If your running Device Net it should be a snap.

 

[ScottyUK]

BJC,

They look interesting - I am always very wary of direct sensing relays, so a fully isolated type looks good. How do they compare cost-wise to a standard sensing relay with a cheap CT?

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I don't suffer from insanity. I enjoy it...

 

[resqcapt19]

For agitators that are critical to the process, we use a speed switch on the shaft of the agitator itself.
Don