Ok, I know there has to be a simpler way of accomplishing what I want to accomplish, but I can't really see it right now. I have 4 5HP 3PH 208V exhaust fans, each controlled by a VFD, I want all exhaust fans to turn on when the temperature exceeds 90 degrees Fahrenheit, and turn off when the temperature drops below 90 degrees Fahrenheit. Between 90 and 125 degrees I want all 4 motors to ramp up to 25% or around 525 rpm, between 120 - 130 degrees to 50% or 1050 RPM, from 130 - 140 degrees to 75% or 1575 RPM, from 140 - 150 degrees to 100% or 2100 RPM, and 160+ degrees, I want the motor to ramp up to 125% or 2625 RPM. I also have a CO Monitor with 24V control relays that I want to turn the fans on to 25% when 22 ppm is reached, and then to 100% if 28+ ppm is reached.

My only thoughts would be to use small PLC controller to control all 4 VFDs, and then have a digital temperature probe as an input and the CO monitor as two on/off inputs for warning and danger conditions. I realize that a setup like this would be quite expensive, so I am trying to think if there is anything less that would do the same thing, or if this would be my best bet.

Please define "quite expensive." Four 5 HP VFDs are going to cost a pretty fair chunk of change; I don't imagine that a small PLC is all that much in comparison to the rest of project.

I'm at a loss as to what heat load could give you the heat profile you're targeting.

Are these VFD's already installed and running?

As to the stair step increases in power... why not a linear run-up?

Carbon Monoxide monitor...? Are you exhausting air from some confined diesel-electric set?

I'd throw the whole problem over to an engineer.

For such a costly system, any PLC expense will be dwarfed by the design effort.

The design is actually more of a theoretical at this point in time, and is part of a project I am working on designing for one of my old professors classes. Kind of an overkill I know, but something that "might" be seen in the real world, trying to give them a somewhat real world example of how motor controls and such would be used in everyday applications. In the plans I am using, which are for a 12000 sq. ft night club with a 3000 person occupancy, the plans call for 4 5HP exhaust fans which are to be controlled by a simple start stop station connected with a CO monitor (not quite sure the exact purpose of the CO monitor as of yet)... so my thoughts were to go a little further and allow the fans to do more than simply turn on or off at full speed according to a single set temperature, thus I am thinking that a PLC controller running 4 5HP VFD's would be best suited for running the system. Cost.. is and isn't really a consideration.... I want a system that someone would be proud of designing in its simplicity and effectiveness, and not a Rube Goldberg of a motor control.

I appreciate all your input.

Wow,
A night-club with REAL ventilation!
This is ground-breaking.

Just make sure that the ventilation system doesn't cause a fire started in another part of the building to spread.

You need to prove to a Fire Marshall or the like to make sure that what you install is not going to "link" fire compartments, especially with an occupancy rating of 3000 people, some of whom may have taken illict drugs and may not be able to evacuate like the other drunken ones.

This is NOT a wind-up, I have been there before as a Fire Officer.

If you're looking for an example, a real-world application you might also consider is emergency ventilation in a data center. If the AC fails, the equipment will quickly overheat, and is frequently fitted with emergency ventilation fans so the data center can maintain operation.

I've never seen these fitting with VFD, it's either required because the AC failed, or it's not, there's very little in-between.

As in everything else, it must be tied into the fire protection system to not spread smoke and fan the flames. Also, FM200 requires the space be absolutely sealed.

Back in the olden days when there really were data centers they had redundant HVAC systems so you lose a whole system and still have enough HVAC to keep the DP system going. Some even had huge storage tanks on the water chillers so you could lose all the A/C and still have time for an orderly shutdown/transfer to the backup site.
I am not sure we even have that kind of data center anymore. When I was leaving they were turning those "glass house" operations into a small room with few racks that replaced a half acre of raised floor. (most of the reason I left). Some could live in the "office" ambient air.
When the bad card has a red blinking light on it and you can hot swap in a new one without taking down the system you don't need me.

If the VFD's can accept an analog input, use a 4-20 mA output or 1-5 V transducer for the temperature based speed control.

If the units can accept a Master / Slave relationship or a follower input, set one unit as the master and slave the other three off of it.

Larry C

Yes, data centers still do have this, and in fact it's gotten even more obscene for some of those commercial data centers where every outage means millions of dollars lost. 2(N+1) redundant AC, with wells and/or water tanks for the chillers, and emergency ventilation as backup. The emergency ventilation uses little power and is sometimes connected to the UPS, as well- if there is a multiple generator failure, a datacenter running for 2+ hours on UPS power without cooling can get miiighty hot. (sure, the UPS is only designed for 15 minutes, but with 2(N+1) units for redundancy upon redundancy (you wouldn't feed those redundant power supplies from the same N+1 UPS bus, would you?), each with a 15+ minute battery powering a system that probably isn't anywhere close to UPS capacity...

And yes, this is what it takes to go from 99.999% to 99.9999%. It's really really hard to get that extra 9!

I have been out of this for over a decade but the plan IBM was selling was redundant sites. You would have a mirror site 1000 miles away that could handle all of your critical applications with a moment's notice. That way you could switch over in the few minutes a basic UPS gives you and avoid all that redundancy on site. Money wise it really may end up cheaper, or at least that was what the selling point was. With national broad band capability, it is not really important where you are. I know IBM maintained mirrors in several places for our internal operations as far back as the 70s so they had experience.
The redundant site might actually be backing up dozens of sites but as long as they are not all down at once a minimal amount of hardware will do. Even if there is a bottleneck, slow is better than nothing at all.

Thats right. Contact your favorite VFD manufacturer and check with them. The VFD is no longer just a speed controller, I have seen them with PLC's on board (ABB)and will accept every imaginable input and can output too, on some models. They can work alone or with other VFD's and equipment as needed.
In fact, contact ABB through their web site. Then locate a local vendor. They are great about touting their products and will answer any questions you might have.
Note: Do not confuse ABB with AB!!!! Totally different.

I wonder if the CO monitor is actually meant to be a CO2 monitor? Lots of hot bodies dancing in a club will give off a lot of CO2 and create a "stale air" effect. A CO2 monitor might be intended to ramp up the ventilation if the atmosphere inside the club got too stale.

I've seen CO2 monitors/controls installed on HVAC systems in meeting rooms for the same reason...if the atmosphere gets stuffy with everybody breathing out CO2, then the system ramps up and brings in more outside air, to freshen up the room.

In response to Trumpy's question, the system according to the plans is to be interconnected with the FCP, and all ducts are to have electronically controlled fire dampers that failsafe to close by means of a heat sensitive link in the event of a power failure. My question is... if they are vented directly outside from the room with no other rooms or floors in between, do they really need fire dampers and such, or would you just want them connected to the FCP to shutdown in case of fire?

And in response to Beachboy's last question, I do believe that the CO sensor is actually supposed to be a CO2 sensor, as there would be very little CO present in this particular situation.

These units are pretty standard equipment for agricultural barns housing confined animals - hogs, chickens etc. Most of the control circuitry can be bought off the shelf with inputs for various activation triggers like CO2 levels, Temperature, ammonia, humidity etc. In operation the worse the condition the more ventilation needed etc (or reverse). They also have the ability to incorporate outside sensors so you dont open up the throttle for full blast ventilation in the middle of winter.

Here's an example:
http://www.qcsupply.com/Products/10555.aspx?ref=fp