Auto ctl for 2 fan motors

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Designer69

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Gents and ladies, I am a young engineer and need to get a better understanding of this....

We have 2 fan motors 25HP each and currently they are powered off 2 different MCC starters with very simple control, local on/off switch for each fan. These fans are for a cooling system and there are 2 sensors for temperature and air flow. If they pick up low air flow/tempereture all they do is send a signal to a nearby annunciator and turn on a light. It's all manual.

what we want to do is automate the system, so if either alarm goes off, the stand-by fan should turn on and if the first fan fails the second should again turn on.

I know you can do this using a PLC in a local control panel but they want to stay away from PLC'S and software, how else would you do it?
Thanks
 
IMHO

You will spend more money on design labor / discrete compents (relays) / installation labor trying to do this without a PLC. During start-up and testing, when you find it isn't quite what the customer is expecting, you will be re-wiring in the field (more labor).

It looks like you are about (4-6) Inputs and (2-4) Outputs, there are many vendors that sell very small PLC's, (or even a "Smart Relay). Some of the vendors have FREE software for them. They can be aquired for less that $300.

A small PLC also accomodates future needs. Realys are inflexible.
 
Doc thanks for the reply, this is actually for a nuke power plant so it's not the cost they are concerned with, it is cyber security and other crap.

so the way you would basically do it with relays, would you have a new local control panel for the fans with relays and also would you have to move the motor starters from the MCC'S and put them in the local control panel then replace them with fused disconnect switches or something?
 
Well, I for one would also do this with a Smart Relay, but it can be done with hard wired relay logic as well. Unfortunately, there is a lot more to automating it than you may realize and you have not provided nearly enough information to tell you anything useful. For example:
  1. What is going to turn in On and Off in Auto? Are you still planning on doing so manually?
  2. If so, then do you want the other fan to come on even if it's switch is in the "Off" position? Probably a bad idea by the way, but if you don't do it that way, how will you do it?.
  3. Is the Flow switch NO held closed by flow? if so, how would you start in Auto mode without that switch already being made? (Hint, it requires a timer if you need to do it that way).
  4. Or is the Flow Switch NC, opening on high flow? Different scenario altogether.
  5. Same issues for the Temp Switches; NO closing on temp rise, or closing closing on temp fall? or NC opening on temp rise or fall?
  6. If one motors trips on OL, do you want the other to then ALWAYS run? Or only if it or the tripped one is commanded to Start? (And we are back to what will control starting the sequence again).
This is why it's sometimes better to use something like a Smart Relay or PLC for something like this. The hard wired logic can get overwhelming as you discover these and other unasked questions as you go along.
 
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Designer69 said:
Gents and ladies, I am a young engineer and need to get a better understanding of this....

We have 2 fan motors 25HP each and currently they are powered off 2 different MCC starters with very simple control, local on/off switch for each fan. These fans are for a cooling system and there are 2 sensors for temperature and air flow. If they pick up low air flow/tempereture all they do is send a signal to a nearby annunciator and turn on a light. It's all manual.

what we want to do is automate the system, so if either alarm goes off, the stand-by fan should turn on and if the first fan fails the second should again turn on.

I know you can do this using a PLC in a local control panel but they want to stay away from PLC'S and software, how else would you do it?
Thanks
Click to expand...

I imagine a switch turns on the annunciator light. Use that switch paralleled with your local on/off switch. Make sure the ratings and conditions are appropriate for usage. You may be dealing with two power sources, so that is another consideration. If so, perhaps use the indicator light circuit to energize a miniature relay (Tyco/Potter & Brumfield K10 series would probably work nicely) and parallel the NO contact(s) with the local on/off switch.

Note this will only take the human interaction out of the process and will not provided any better one-fan failure control than you have now.
 
Designer69 said:
Doc thanks for the reply, this is actually for a nuke power plant so it's not the cost they are concerned with, it is cyber security and other crap.

As long as the smart relay or small PLC is not networked, it can't be hacked from the outside world. If they are concerened about being hacked from the inside, (that scares me, who is working there?), but you can password protect it. There are some controllers that you can actually inhibit the COM port once it is runing, and it can no longer be accesed, viewed, or modified. It becomes a FIXED conroller with no connectivity. The only way to modify would be to factory reset to deafault and download a new program.


so the way you would basically do it with relays, would you have a new local control panel for the fans with relays and also would you have to move the motor starters from the MCC'S and put them in the local control panel then replace them with fused disconnect switches or something?
Click to expand...


There are two options:
1. Leave the Motor Starters where they are. Make an new "all-encompasing electrical drawing. NOTE: You will have (3) Seperate Control Voltage Sources in one control panel enclosure.
A. MCC1 Control circuit
B. MCC2 Control circuit
C. New additonal control circuit (supervisory) this allow the sytems to stay alive if you have to shut off one of the MMCs and maintain the other MCC.

You will want to use yellow wire, and warning signage idicating all outside sources, if you opt this route.

Option B:
Build one new single source panel that has enough power to drive both fans, and have one control source. Disadvantage, total loss of service on one fault.

It is very difficult to assess which is better from a few sentences in a post. If it is a NUKE plant, I have NO Qualifications whatsover. I am guesing there is very rigid specifications, design (fail-safe) criteria, and codes well above and beyond NFPA-70 or NFPA-79.
 
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