I have a older Trane RTU with a 30 hp supply blower motor that is changing speed and amp draw every few minutes. It is a constant volume system and I have ruled out building static and the makeup/return damper as the cause. The motor is 208/3/60, it is powered via a breaker and contactor (no VFD). It is wired across the line, the contactor and volts supplied are good. What I've observed is that when the motor has been off and somewhat cool it seems to run fine for about 30 min. Normal operation is at 80 amps (motor fla is 83 amps). After about 30 min. the amp draw drops to 50 and there is a significant cfm drop. After running at 50 amps and reduced cfm for about 5 min. it returns to 80 amps and full cfm. About 5 min. later to reverts back to the 50 amp low cfm condition. This cycle in amp draw and cfm is continuous. The motor is rated for 40°c over and is not running unduly hot. I've checked other 3 phase motors in the building and they are running fine (so it's not some weird power supply issue from the utility). I'm assuming this is a motor winding defect but I've never seen a motor failure like this. If anyone has experienced something like this before and knows what might be causing it, your insight would be appreciated. I'd like to understand the cause before condemning the motor.
Take total static pressure readings & motor rpm reading during both conditions. This will verify if air side.
Check voltage, voltage imbalance, amp imbalance. Megg the motor & supply line components Issues with Any of these would cause higher amps. Ohm motor windings for balance and ohms law calculated value.
It sounds like a winding is dropping out. If a winding drops before the motor starts (like a bug in 1 pole of the contractor) it will make the motor pull LRA but I’m thinking that if a leg drops while the motor is spinning then it will probably have enough momentum to get past that dead winding and on to the next. 50 amps is about 2/3 of 80 so it seems you are missing a leg somewhere. But that’s just a theory that I’m not entirely sure about. When it is at 50 amps what is your amperage and voltage on each leg?
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Either the voltage is droping to the motor, IE power supply, or contactor, or fuses/breakers inside the unit.
I would have a few meters going watching voltage when the amperage drops.
Id also be looking for voltage drop across fuses, breakers, motor starters/contactors.
Itd also be looking real close at the connections inside the motors field wiring interface. Both the line voltage tie ins, and any field made voltage selection connections.
There is a whole lot of “all is good” in your post. Clearly, it isn’t. Post all of your data. Fan sheave sizes, belt sizes, motor sheave sizes, static pressure, static pressure across the fan, full unit model number, motor nameplate, etc. you can not post too much data.
I have seen a similar issue like this before. My guess (because that is all we have to go on) is that you are riding the fan curve at the low end. It can appear to be a motor issue, however, it isn’t. My guess is that your static pressure across the fan is too great.
There is a whole lot of “all is good” in your post. Clearly, it isn’t. Post all of your data. Fan sheave sizes, belt sizes, motor sheave sizes, static pressure, static pressure across the fan, full unit model number, motor nameplate, etc. you can not post too much data.
I have seen a similar issue like this before. My guess (because that is all we have to go on) is that you are riding the fan curve at the low end. It can appear to be a motor issue, however, it isn’t. My guess is that your static pressure across the fan is too great.
Need mfg fan performance data. Model and selections?
This was a Saturday evening emergency call and I only took it far enough to know I wasn't going to get it resolved right away. I'll be going back on Monday, do more diagnostic tests and post more info... jayguy, this is a 20+ year old system that has design flaws (mostly load requirement verances) but it has never had air flow issues and nothing that would affect building static has changed. But interesting diagnostic puzzles are few and far between so I will get all that info and cover all the bases... just in case.
...jayguy, this is a 20+ year old system that has design flaws (mostly load requirement verances) but it has never had air flow issues and nothing that would affect building static has changed.....
this means nothing. just because they didn't know any better or notice a problem doesn't mean that there wasn't an issue. i have come across units that were running for many, many years that were suffering and nobody noticed (or cared, or wanted to understand what was going on) before i arrived.
if you look at the attachment, my thought is that you are operating on both sides of the 'hump'. while the drawn lines may be exaggerated, if you look at the line indicated by the red arrow, you will see how the 2 HP power line crosses it. with a slight increase in static pressure, you will follow the blue line and then the unit will drop CFM and power onto the other side of the hump. you can actually move to the other side of the 'hump' and hang out there with effectively 0 CFM. then after the pressure is relieved and the static is lowered, you can move to the other side of the hump and your power will rise in correspondence. the difficult thing to measure in the field is the line with the red arrow. that line represents static pressure and it is the exact same static pressure reading on both sides of the hump. without a power measurement (or a cfm measurement) you can not determine it from static pressure. you will also need the fan curve for this unit to determine what you are seeing.
this sounds to me like what you are experiencing. in chillers we call this "surge" (although the curve looks a little different in chillers). i have witnessed this in airflow many times...you just cant hear it like you can in chillers due to the density of the fluid and the nature of the design.
"Right" is not the same as "Wise".
Don't step on my favorite part of the Constitution just to point out your favorite part.
Just because you can measure it, doesn't mean it is important. Just because you can't measure it, doesn't mean it isn't important.
Thanks jayguy, I'm starting to think you might be right. I can get differential static pressure and amp draw easily enough but a accurate cfm measurement might be difficult. As a test, am I correct that if I manually keep the discharge static at or slightly below where the blower is working correctly (on the lower side of the hump) and the surging stops, this would indicate that you are correct and the problem is static pressure related? I can do this simply by shifting an exterior panel on blower discharge (aprox. a 3'x5' panel that I can shift on the bolt holes, about every 6"). I'm just looking for quick way to insure I'm not barking up the wrong tree so I don't end up having justify a lot of time spent on a diagnostic deadend.
This is a constant volume system, zones are electric reheats, no dampers on the supply side, no vfd. It does have return/make-up air dampers, but the air flow/amp draw surging took place with no position change.
This is a CV system. I'm not sure why we're talking about static pressure causing the issue unless you have a broken damper at the unit itself. Honestly, throw a megger on the motor first. It takes 5 minutes.
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I am not saying you are wrong. I am not saying I am right. But constant volume is a myth. It only exists on paper. Improper duct design, duct changes, changes in filter designs (especially lately with everyone wanting to change to more efficient and higher pressure drop air filters), blower blades getting dirty, balancing damper movement (intentional and unintentional), etc cause changes in airflow.
Thanks jayguy, I'm starting to think you might be right. I can get differential static pressure and amp draw easily enough but a accurate cfm measurement might be difficult. As a test, am I correct that if I manually keep the discharge static at or slightly below where the blower is working correctly (on the lower side of the hump) and the surging stops, this would indicate that you are correct and the problem is static pressure related? I can do this simply by shifting an exterior panel on blower discharge (aprox. a 3'x5' panel that I can shift on the bolt holes, about every 6")....
Sounds easy enough. You should be able to adjust that panel to a few places and test each time and see how the static changes and how the motor reacts. We know how the motor is going to react, it is just a matter of how close to the hump you are running currently. Sheave, belt, motor nameplate and unit model number will tell us pretty quickly if I am on the right track or barking up the wrong tree. Of course, this is all speculation at this point.
I'm well aware of the myth, I'm an HVAC engineer who spends most of their time in the field. I'm purely trying to help this person by going through the easiest and least time consuming diagnosis. It could be anything from changed ductwork, light fixtures being updated (old light fixtures could have had return air slots in them), or the new MERV 13 issue. The thing I am however most aware of is watching field technician's waste hours of a customer's time not throwing a motor on a megger.
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While I m a big fan of megging motors, I am curious as to what you think he will find. FLA is 83 and he is running 80 amps on the high side and then dropping to 50-ish amps. He doesn’t seem to have a high amperage issue. If his situation was reversed (50 with occasional runs to 80) I would have gone to motor megging first as well.
In all fairness, my theory is somewhat unlikely as well given that he is running very close to the motor FLA unless someone changed the sheaves “to get more air” or some other such scenario.
Also not sure what a Megger would tell you in this situation.
If it was shorting to ground on a winding, then that would trip a breaker or overload.
A winding to winding short wont show up with a megger.
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I'm in agreement, but too often I've seen problems overlooked due to incomplete diagnostics. Will already be the second trip.
There should already be a complete summary of reading and conditions including motor data and air data. Megg , resistance, voltages, amp readings would take less than 30minutes. External statics, rpms, & visuals another 30 minutes. Already plenty of time spent on job to collect this data. Performance curves could be reviewed later off site if not available initially.
Techs, especially new techs should use log sheets and cross references and not just assume the job is fixed when they find one problem, most jobs have more than one issue even if some are minor. When working tech support I reduced call backs by > 80% when I sent out log sheets before responding to inquiries.