Jump of EF motor fla

[CanuckEngineer]

Hey guys,

Great forum here.

I am usually on the electrical side but I recently was posed with some issues on the Operations side of one of our buildings.

Please feel free to read the sequence below and provide any comments. I am working on gathering supplemental data like BAS shops and mechanical schematics to help out.

Thanks,

RK.

Stage 1: If EF-1 or EF-5 or EF-8 runs, then EF-20 runs at 40% to match the capacity of 1 fan.
Stage 2: If EF-1 & EF-5 or EF-1 & EF-8 or EF-5 & EF-8 run, then EF-20 runs at 70% to match the capacity of 2 fans.

Here is where we start having problem.

When EF-20 switch from stage 1 of 40% to stage 2 of 70%, the amp draw would jump up to 42 amp and stay there. However, if we was to have EF-20 goes to 0% then ramp up to 70%, it would run just fine and not overload at all, and the amp draw was only 20 amp.
 What we also noticed was that when the fan switch from 40% to 70% without the full shutdown reset the amp draw would jump up to 42 amp then stay at 42 amp at 70%, 80%, 90%, & 100%.
Stage 3: If EF-1, EF-5 & EF-8 all run, then EF-20 runs at 95% to match the capacity of 3 fans. We found only 2 ways to get EF-20 to 95%.
 By having a full shutdown reset and then bring the fan from 0% to 70% then 95%. The idea here is to get the fan to stage 2 first then to Stage 3.
 By having a full shutdown reset then bring the fan from 0% to 95%.
 The amp draw at 95% is 30 amp or so using the 2 methods above. Without the full shutdown reset between the stage the fan would just go to 42 amp and stay there when ramping up from 60-100%.
We also notice that the ramping up time had no effect. As we had tried to have the fan ramping up really slow and this 42 amp jump still happened.
If anyone else had any other comment or observation please feel free to add in.

 

[CanuckEngineer]

EF-20 is a booster exhaust fan and is controlled by a Danfoss VFD model no: FC-102P30T6. 575V/3 phase/ 40hp.

Thanks,

RK.

 

[CanuckEngineer]

The BAS supplier thinks there is turbulence before the fan (EF-20) because there is 50,000 CFM + coming up through an 'S' bend before the fan.

Not sure if that would 'lock' the VFD current at 42A as mentioned above.

Thanks for all your input.

Regards,

RK.

 

[dbill74]

First thought is something screwy going on in the VFD. Has someone from Danfoss been out to inspect the VFD?
It's hard to diagnose with the information provided. Here are some questions that need to be considered in diagnosing the issue (in no particular order other than the order in which I thought of them).

What type of fan is EF-20? Centrifugal, vane-axial, etc.
Is there a time delay between second/third fan (EF-1,5,8) starting and EF-20 adjusting to new speed?
In your sequence to "reset" the fan, you are just talking about restarting EF-20, other fans are not being restarted are they?
Do you have static pressure sensors in the ductwork on either side of EF-20?
How close is the 'S' bend to the fan intake?
Are there any dampers (gravity or motorized) in the duct?
Has test and balance verified air flows, rotation speeds and pressure differentials across EF-20 at the various stages of operation? Even under conditions when VFD is pulling 42A.
Are you seeing the 42A going from 70%-95%? Or is it just from 40% to either 70% or 95%?
Is fan motor actually rated for use with VFD (I believe the term for VFD rated is 'inverter duty rated' if memory serves)? Older motors and some new fan motors are not designed to handle changing speeds relying on inlet dampers or adjustable pitch blades to control changing air volumes.
Have you reviewed this with the mechanical designer of record? What are his thoughts?

Bill

 

[CanuckEngineer]

Thanks Bill, excellent questions.

I will get you answers asap.

Regards,

RK.

 

[willard3]

I keep reading this and thinking that sooooooo many things could cause this.

What you need is to hire an Engineer familiar with air systems.

This is not the kind of problem you can solve on a web forum.

 

[dbill74]

Agreed Willard. Getting the designer of record involved and looking at the situation will be the best person to contact as he/she will be most familiar with the system and its intended operation.

 

[pckaakee]

Sounds like software issues or the vfd is locking up. Can you monitor the actual analog input to the vfd during control changes