Motor Ventilation 4

[fishmax05]

All,

I am presently working with a tandem set of DC motors. On these DC motors there are 2 blowers each supplying approximately 7K CFM. What I was thinking about doing ... Rough Idea basically is removing the 2, 15 HP blowers and routing duct work to a single lets say for grins 150HP AC motor with a blower configuration. My question is this. I am not familiar with dampening but, could I put a pressure switch or dampeners that would adjust the pressure to each motor accordingly? I know this is slightly confusing but, I am wanting to maximize the efficiency of this system. Presently there are 4 15 HP blowers running during our run time. Thinking one motor supplying both motors.. Thanks! Fish

 

[MiketheEngineer]

Knowing little about EE - do you want to give up the redundancy of what you already have?? One motor goes down and maybe you can limp by with the other one. In your new scenario - if motor goes down you SOL!!

 

[MikeHalloran]

15 HP motors don't always consume 15 HP worth of electricity. You need to make accurate measurements so that you can tell when you are actually optimizing something.

You didn't mention the power rating of the cooled motors. What, 2000HP?
And you propose adding a single point of failure?
How much downtime and lost money would be associated with a failure of one of those monster motors?



Mike Halloran
Pembroke Pines, FL, USA

 

[fishmax05]

Ok, the cooled motors are 2000 to 2500 HP DC traction motors ran in tandem. The present set up to cool these is two 10 to 15 HP AC blowers running while the (DC motors run) at all times and 30 minutes after shut down (obviously to cool down the motors). To answer your question Mike if one of the 10 to 15 HP blowers shells I am down anyway. Money isn't my concern. I work for a systems integrator. Wanting to run a VFD on this proposed motor to run of the current feedback off of the two DC's. This would be fairly easy. Kinda like a throttle for the cooling. Also I am wanting a cleaner source of cooling air. These motors are in the shredding industry so the environment is defiantly filthy. I have filtration figured out so that's not an issue either. I will either run one AC motor or 2 motors one per motor. The goal is to get the 2 blower configuration off the top of the motor.

 

[waross]

Be aware that by going to a single cooling fan, you may find out what the designer of the present system already knows.

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

 

[jraef]

Ok, the cooled motors are 2000 to 2500 HP DC traction motors ran in tandem. The present set up to cool these is two 10 to 15 HP AC blowers running while the (DC motors run) at all times and 30 minutes after shut down (obviously to cool down the motors). To answer your question Mike if one of the 10 to 15 HP blowers shells I am down anyway. Money isn't my concern. I work for a systems integrator. Wanting to run a VFD on this proposed motor to run of the current feedback off of the two DC's. This would be fairly easy. Kinda like a throttle for the cooling. Also I am wanting a cleaner source of cooling air. These motors are in the shredding industry so the environment is defiantly filthy. I have filtration figured out so that's not an issue either. I will either run one AC motor or 2 motors one per motor. The goal is to get the 2 blower configuration off the top of the motor.

But as others keep pointing out, you STILL are giving up your n+1 redundancy in the system;
Operating mode A) 2 motors + 1 large blower.
Operating mode B) 1 motor + 1 large blower on VFD, motor can be #1 or #2, but this still means that if the SINGLE blower goes down, you lose ALL possible operations and what I know of the shredding business, no shredding = no revenue.

You goals are laudable and attainable: saving energy, increasing operating performance by improving cooling air quality. Aside from missing the redundancy issue, the other thing I see missing is that if you are a SYSTEMS integrator, you must consider the ENTIRE system. Right now you seem to be thinking of this strictly as a control system problem, but there are a number of mechanical issues involved that you cannot ignore. Right off the top, you seem to be thinking you can equate air flow to current monitoring of the DC motor. That is an immediate flaw, because unless you have a TON of empirical data to know the ACTUAL HEATING effects of cyclical loading and unloading, current monitoring alone is not going to be enough info to get you there. Then you also cannot just equate blower HP to air flow, there are ducting issues, filter loading issues, maintenance requirements etc. to consider. I think you are thinking that by over sizing the blower (" ...lets say for grins 150HP AC motor...") that will cover for it, but then you want to add in a variable speed drive, which implies Variable Air Volume. You don't KNOW the required air volume now, let alone after you add more duct work and a filtration system, so how can you even begin to size the blower?

Here's my suggestion, based on going from what you KNOW to what you NEED.
[ol 1]
[li]You know that 2 x 15HP blowers are doing the job. You cannot however ASSUME that a 30HP blower will work because you are adding duct work. Get an ME involved to MEASURE the air flow from the existing blower and then you KNOW that you need at least that much air flow AT the motor. Also in that study, add in a cross-ventilation ducting system so that Blower 1 can feed Motor 2 and vice versa, plus factoring in that your air filtration system will be 50% loaded at the worst case scenario. Have the ME then calculate the required BHP you will need at a SINGLE blower for THAT ONE MOTOR with the ducting necessary. This means you end up with 2 larger blowers, each one capable of the worst case scenario, cooling the OTHER motor running alone via the cross vent system.[/li]
[li]Once you know that, add in an air flow monitor at the motor so that you will be able to SEE that each individual motor is actually getting the minimum air flow it needs from the above (empirically determined now from the existing system).[/li]
[li]Also add in differential pressure monitoring across the filtration system to provide warnings to the user when the filter is getting more than 50% loaded, since that is the determining factor of your motor BHP study.[/li]
[li]Use the VFD on EACH blower motor in a PID loop control based on AIR FLOW AT THE MOTOR for now. At this point this is the ONLY thing that you know for sure will work. When the filter is clean, it will turn down the blower to maximize efficiency, but as the filter loads, it can still maintain that minimum require air flow up until the filter is 50% loaded.[/li]
[li]Put in some sort of simple vane control system to do the cross venting. This reclaims your n+1 operating conditions: motor 1 w/ blower 1, motor 1 w/ blower 2, motor 2 w/ blower 2, motor 2 w/ blower 1, motor 1+2 with blower 1+2.[/li]
[li]Explain to the user that FURTHER improvements in energy savings are only possible by adding TEMPERATURE monitoring EMBEDDED in the windings at each DC motor itself. That however should probably be done at the NEXT scheduled rewind as it will be difficult to justify the expense of taking it down now just to add them. But that is something open to discussion actually.[/li]
[li]Once you get there, use the air flow monitoring as a safety backup, but do a primary PID loop on each motor hottest RTD value to optimize air flow based on the thermal EFFECTS of cyclical loading / unloading of the DC motors.[/li]
[/ol]

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