MCC Start/Stop

[petersonra]

I take it all starting at the same time would blow the OCPD feeding the MCC?

I have seen MCCs where the start contact actually goes to a timer and the timer output actually starts the motor. If you have big motors you don't want starting at the same time you just set the first timer for 0 seconds, the second for 10 seconds, etc. The little motors don't usually matter much.

 

[mbrooke]

I have seen MCCs where the start contact actually goes to a timer and the timer output actually starts the motor. If you have big motors you don't want starting at the same time you just set the first timer for 0 seconds, the second for 10 seconds, etc. The little motors don't usually matter much.

Easy! Time delay relays

 

[Jraef]

What does your experience typically show for supply, return and exhaust fans? HVAC pumps and water pumps?

Most HVAC loads are controlled by some sort of BMS (Building Management System) and are going to be 2 wire control as a result. The BMS will be deciding how equipment restarts after a power failure, because there are pressure balancing, staggered restart and proper sequencing issue to consider. I did a project once on an office building where they were starting and stopping manually because the BMS was not ready yet. They got the balancing wrong between supply and return fans and created a vacuum in the building which trapped everyone inside because you couldn't open the doors.

 

[kwired]

Most HVAC loads are controlled by some sort of BMS (Building Management System) and are going to be 2 wire control as a result. The BMS will be deciding how equipment restarts after a power failure, because there are pressure balancing, staggered restart and proper sequencing issue to consider. I did a project once on an office building where they were starting and stopping manually because the BMS was not ready yet. They got the balancing wrong between supply and return fans and created a vacuum in the building which trapped everyone inside because you couldn't open the doors.

And similar with industrial processing line. Presuming it would automatically restart - there is likely an orderly procedure that would be followed just because the process demands it. If not automatically restarted manual restart would still likely be in certain orderly manner and not everything all at once.

Now if you had a lot of refrigeration or something that is nearly "always on" you might need some management system or staggered delay relays for such restarting situations.

 

[garbo]

I retired from a large Hospital/research center with 5 million square feet and everything under the sun was controlled by a BSO. We had 21 generators [ most being 3 megawatts ] and UPS on everything and still had problems during monthly testing of over 70 ATS'S & service single ending. Some air handlers took a few minutes just to receive a start command and up to 15 minutes to ramp up to full speed. we had over a dozen 125 & 150 air handlers with a 75 HP return fan that would often to fits on restart. Had to turn dual 125 or 150 HP supply fans to manual then ramp them down to say 20% to get return fan that kept tripping out due to running backwards due to supply fans. The 2 big companies that supplied the BSO never could find a remedy for this. At least once a month had doors in 3 buildings that either stayed open or could not open due to a problem with air handlers. In the winter freeze stats would always trip out and I was the first guy to be called even though it was under BSO guys or building systems department. They would not allow me to install indicating lights to tell when freeze stats were tripped.

 

[paulengr]

I retired from a large Hospital/research center with 5 million square feet and everything under the sun was controlled by a BSO. We had 21 generators [ most being 3 megawatts ] and UPS on everything and still had problems during monthly testing of over 70 ATS'S & service single ending. Some air handlers took a few minutes just to receive a start command and up to 15 minutes to ramp up to full speed. we had over a dozen 125 & 150 air handlers with a 75 HP return fan that would often to fits on restart. Had to turn dual 125 or 150 HP supply fans to manual then ramp them down to say 20% to get return fan that kept tripping out due to running backwards due to supply fans. The 2 big companies that supplied the BSO never could find a remedy for this. At least once a month had doors in 3 buildings that either stayed open or could not open due to a problem with air handlers. In the winter freeze stats would always trip out and I was the first guy to be called even though it was under BSO guys or building systems department. They would not allow me to install indicating lights to tell when freeze stats were tripped.

The VFDs need to be regenerative types with flying start. Not cheap HVAC 6 pulse diode front ends. Because they need to both detect speed and direction correctly on startup and to be able to act as generators/brakes during shutdown. Or program a damper to close first if it has one. But most HVAC drive people don’t know this.

 

[petersonra]

The VFDs need to be regenerative types with flying start. Not cheap HVAC 6 pulse diode front ends. Because they need to both detect speed and direction correctly on startup and to be able to act as generators/brakes during shutdown. Or program a damper to close first if it has one. But most HVAC drive people don’t know this.

or you can just put brakes on the fans so when they stop they don't run backwards. or enable the dc brake that many drives have available.

but everyone tries to do things cheap so none of the multiple solutions to this problem gets done because it would add cost.

these days even the cheap drives often have flying start because a lot of drives are used on air handling units. often the people starting up the drives don't even know the option is available and just has to be turned on in the drive. but that is a problem with being cheap too. no one wants to pay someone who knows how to setup the drive to do it so they make it the problem of the EC who rarely has anyone who specializes in such things so they do the best they can to get it running and call it good.

 

[Jraef]

or you can just put brakes on the fans so when they stop they don't run backwards. or enable the dc brake that many drives have available.

but everyone tries to do things cheap so none of the multiple solutions to this problem gets done because it would add cost.

these days even the cheap drives often have flying start because a lot of drives are used on air handling units. often the people starting up the drives don't even know the option is available and just has to be turned on in the drive. but that is a problem with being cheap too. no one wants to pay someone who knows how to setup the drive to do it so they make it the problem of the EC who rarely has anyone who specializes in such things so they do the best they can to get it running and call it good.

And you are assuming an EC is even involved. A lot of HVAC systems never see an EC... the duct monkeys do it all themselves, ad RARELY know what they are doing with the electrical part beyond what they can learn on YouTube.

 

[mbrooke]

I retired from a large Hospital/research center with 5 million square feet and everything under the sun was controlled by a BSO. We had 21 generators [ most being 3 megawatts ] and UPS on everything and still had problems during monthly testing of over 70 ATS'S & service single ending. Some air handlers took a few minutes just to receive a start command and up to 15 minutes to ramp up to full speed. we had over a dozen 125 & 150 air handlers with a 75 HP return fan that would often to fits on restart. Had to turn dual 125 or 150 HP supply fans to manual then ramp them down to say 20% to get return fan that kept tripping out due to running backwards due to supply fans. The 2 big companies that supplied the BSO never could find a remedy for this. At least once a month had doors in 3 buildings that either stayed open or could not open due to a problem with air handlers. In the winter freeze stats would always trip out and I was the first guy to be called even though it was under BSO guys or building systems department. They would not allow me to install indicating lights to tell when freeze stats were tripped.

Don't forget about those dampers


Basically I'm dealing with the same thing. Anyway, can I ask a few questions?

1) Why aren't you restarting the return fans starting first?

2) Why are return fan motors always smaller than supply fan motors?

3) Why do large buildings need both supply and return fans when residential and commercial (outside of resturants) always get away with one blower or a set of single blowers?

4) What is a coil freeze pump for? Are the spill air plenum dampers open such that the intake air dips below 32*F?

5) What is a "re-heat" pump for?

6) In a large system of supply and return fans- can't simple delays be added such that after a power failure return fan 1A starts after 5 seconds, supply 1B after 20 seconds, return 2A after 45 seconds, supply 2B after 60 seconds, return 3A after 75 seconds, ect? Alternate each fan over a 20 minute period? Same with exaust fans?

 

[paulengr]

Don't forget about those dampers


Basically I'm dealing with the same thing. Anyway, can I ask a few questions?

1) Why aren't you restarting the return fans starting first?

2) Why are return fan motors always smaller than supply fan motors?

3) Why do large buildings need both supply and return fans when residential and commercial (outside of resturants) always get away with one blower or a set of single blowers?

4) What is a coil freeze pump for? Are the spill air plenum dampers open such that the intake air dips below 32*F?

5) What is a "re-heat" pump for?

6) In a large system of supply and return fans- can't simple delays be added such that after a power failure return fan 1A starts after 5 seconds, supply 1B after 20 seconds, return 2A after 45 seconds, supply 2B after 60 seconds, return 3A after 75 seconds, ect? Alternate each fan over a 20 minute period? Same with exaust fans?

Q1: That is a question for the building management people but as a general rule with a two fan system the big fan sets the air flow (CFM) and the little one sets the pressure (in H2O). If they are both on drives the control loops should just manage it if tuned properly but close to zero pressure there is nothing to do it naturally bypasses the fan at set point. The issue many ECs don’t understand is that in a fan there is a minimum speed usually around. 6 Hz below which the motor will burn up from no cooling and that the fan itself has a minimum speed where it doesn’t draw air. The EC should set minimum speed at the higher of those two points. I would say that 90% of the configurations I see do not do this one simple step. Which is entirely acceptable if I did not set it up because I can just sell more motors. This eliminates the need to start return fans first. Second and again this is often an understanding thing main induced draft fans often have high inertia and will trip during starting on overload especially if not VFD controlled. You have to either close the damper first then start then enable control or at least shut off/go to minimum speed on the second fan.
Q2: They’re not always big/small but pressure control isn’t perfect. So with a two fan system the bigger fan sets CFM. It can be done with an air flow sensor and a temperature sensor and possibly humidity (measuring SCFM, not ACFM) then you can literally just set speed to match the fan curve. Then you adjust the static pressure with the small fan. It is usually negative at the fan and neutral somewhere else in a building. This is good because if we are going to leak air we want to leak conditioned air so we are not pulling moisture or hot/cold air in the doors. Plus blowing doors out is easier to open if the return air fan acts up over pulling doors closed. In industrial heaters (ovens, kilns) it’s just the opposite. We don’t want to leak the extremely hot air out so we want to be neutral to a slight vacuum. So we have a big pull/small push fan.
Q3: Volume. In a house we do not try to control static pressure. It at least used to leak enough this was a nonissue. If you put over 1/8” of static pressure on a door it either can’t be closed or can’t be opened (forced open/shut). At a little more pressure/vacuum everyone’s ears start popping. This is quite easy to do with larger fans. This is why we need two of them so we can control the static pressure. This is at the extreme. Otherwise (see Q2) it’s also an efficiency thing. Finally we could just use one fan and a lot of dampers but there is another issue. At a given speed and static pressure a fan moves a certain number of pounds of air per hour. But (a hint from Q1) we are dealing with ACFM not SCFM. Air density changes and so do all of the air flows as a consequence. So we need to account for these pressure changes so in a larger system we need a second fan to control pressure even with perfect damper settings. You can do it all with damper control but then the neutral point is always in the wrong spot, in the return ducts. We want it in the building space.
Q4, 5: no idea.
Q6: Sequenced starting is often done. If you truly wanted to get fancy the VFDs have an up to speed signal which could be interlocked into the return air start sequence so that it starts after the main fan stops accelerating assuming that much pressure is acceptable. Otherwise they should be started slow together then switch to PID control or perhaps use two starting profiles or something like that. I’ve seen all kinds of crazy schemes in this situation. Again see Q3.

 

[kwired]

Q1: That is a question for the building management people but as a general rule with a two fan system the big fan sets the air flow (CFM) and the little one sets the pressure (in H2O). If they are both on drives the control loops should just manage it if tuned properly but close to zero pressure there is nothing to do it naturally bypasses the fan at set point. The issue many ECs don’t understand is that in a fan there is a minimum speed usually around. 6 Hz below which the motor will burn up from no cooling and that the fan itself has a minimum speed where it doesn’t draw air. The EC should set minimum speed at the higher of those two points. I would say that 90% of the configurations I see do not do this one simple step. Which is entirely acceptable if I did not set it up because I can just sell more motors. This eliminates the need to start return fans first. Second and again this is often an understanding thing main induced draft fans often have high inertia and will trip during starting on overload especially if not VFD controlled. You have to either close the damper first then start then enable control or at least shut off/go to minimum speed on the second fan.
Q2: They’re not always big/small but pressure control isn’t perfect. So with a two fan system the bigger fan sets CFM. It can be done with an air flow sensor and a temperature sensor and possibly humidity (measuring SCFM, not ACFM) then you can literally just set speed to match the fan curve. Then you adjust the static pressure with the small fan. It is usually negative at the fan and neutral somewhere else in a building. This is good because if we are going to leak air we want to leak conditioned air so we are not pulling moisture or hot/cold air in the doors. Plus blowing doors out is easier to open if the return air fan acts up over pulling doors closed. In industrial heaters (ovens, kilns) it’s just the opposite. We don’t want to leak the extremely hot air out so we want to be neutral to a slight vacuum. So we have a big pull/small push fan.
Q3: Volume. In a house we do not try to control static pressure. It at least used to leak enough this was a nonissue. If you put over 1/8” of static pressure on a door it either can’t be closed or can’t be opened (forced open/shut). At a little more pressure/vacuum everyone’s ears start popping. This is quite easy to do with larger fans. This is why we need two of them so we can control the static pressure. This is at the extreme. Otherwise (see Q2) it’s also an efficiency thing. Finally we could just use one fan and a lot of dampers but there is another issue. At a given speed and static pressure a fan moves a certain number of pounds of air per hour. But (a hint from Q1) we are dealing with ACFM not SCFM. Air density changes and so do all of the air flows as a consequence. So we need to account for these pressure changes so in a larger system we need a second fan to control pressure even with perfect damper settings. You can do it all with damper control but then the neutral point is always in the wrong spot, in the return ducts. We want it in the building space.
Q4, 5: no idea.
Q6: Sequenced starting is often done. If you truly wanted to get fancy the VFDs have an up to speed signal which could be interlocked into the return air start sequence so that it starts after the main fan stops accelerating assuming that much pressure is acceptable. Otherwise they should be started slow together then switch to PID control or perhaps use two starting profiles or something like that. I’ve seen all kinds of crazy schemes in this situation. Again see Q3.

Plant I do work for has a "dryer" that makes (food grade) powdered product out of liquid product.

Same concept on that dryer, inlet fan is 40 hp outlet fan is 100 hp. The bigger outlet fan basically sets the CFM, the smaller inlet fan sets the static pressure inside the dryer. This particular application they generally want just a small amount of vacuum during normal operation, but adjustments to the inlet fan can make it go either side of zero pressure. Positive pressure usually results in more loss of product through the exhaust