variable torque VFD - IGBT over temperature problem 1

[mohpower]

Hi everyone,

I have an issue on 200 HP variable torque VFD driving a 250 HP fan. the issue is the phase B IGBT inverter that goes to overtemperature that causes a VFD tripping. at the beginning we thought the VFD was faulty then we replaced with a new one. the same problem appeared in the second one. so now we think we are getting this problem from the motor. could you please guide me what procedures should we follow to solve this issue?. by the way, we are getting an RPM metter to perform a power quality analysis.

Thank you for inputs

 

[itsmoked]

Phase "B" in likely in the center. It is likely then, the hottest running one. You are not allowing it to cool enough, or the VFD's ambient is too high. How, and where is the VFD mounted?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Marke]

Hello mohpower

I assume that you have ensured that the drive is operating within its current ratings and ambient temperature ratings.

Over temperature can be caused by a number of things, the first is in adequate cooling. If you have the VFD mounted inside an enclosure, you need to ensure that there is sufficient airflow through the enclosure or the temperature within the enclosure will get too high and there will be inadequate cooling.

Another cause of over temperature, is excessive losses in the VFD and these are usually: excess current, excess cable capacitance or too high a switching frequency.
Ensure that the length of screened cable is within the manufacturers specifications for the switching frequency used. It may be necessary to reduce the switching frequency to reduce the losses.

You could also role the output cables around one terminal to see if it is an issue with the motor and/or cables on that one phase. i.e. shift the cable from L4 to L5, the cable from L5 to L6 and the cable from L6 to L4. This will not alter the direction of rotation but will shift the problem to another phase if it is an external problem.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[gepman]

Although I think that the issues that Marke raised are more likely, I would also check the inertial load of the fan and your accel parameter. If you had a large inertial load on the fan and a short decel parameter setting I would expect a trip on bus overvoltage but if for some reason your VFD is hunting for a speed and is continuously accelerating a high intertial load at the upper capacity of your drive then it could cause a problem.

 

[jraef]

I'm very curious about the drive sizing, but maybe it's a typo? You have a 200HP VT drive on a 250HP fan motor? A typical 250HP motor is going to be rated at 302A FLC (assuming 460V). Looking at a few 200HP VT rated drives on the market, I get a FLC range of between 240 and 304A maximum. If your VFD is on the lower end of that spectrum, I'd be surprised if only one of the IGBTs is hot!

 

[DickDV]

jraef's observation is reason enough to size drives by amps, not hp. Look at the motor's continuous running amps and its peak amps under standard load conditions. The drive must be capable of sourcing both.

 

[mohpower]

Thanks for your answers,

The FLA of the motor is 270 amps (460 VAC, PF=0.9). the load is a fan, so it is a VT and 200 HP drive is supposed to handle it.
if the problem is due to cooling, then phase B is the one which has to get the overtemperature?

 

[mohpower]

the VFD is mounted inside an MCC.

 

[Marke]

Hello mohpower

What is the temperature inside the MCC?
If the temperature is over 40 degrees C, this could be a cause of your problem, and you will need to increase the ventilation.

Best regards

Mark Empson

http://www.lmphotonics.com

 

[mohpower]

I don't know the temperature inside the MCC. I will record it this week and letr you know then.

thank you

 

[jraef]

Makes sense that phase B would be the one with a problem if inadequate cooling is an issue. It's in the middle and getting radiant heating from the adjacent stacks (depending on the design of course).

MCC mounting of VFDs is always tricky at best. Heat stacking and air flow issues are a constant source of pain for those who do it. Is it a manufactured / tested package from the MCC supplier, or is it a VFD that was mounted in the MCC after the fact?

 

[DickDV]

With the motor FLA at 270 and most 200hp VT rated drives at 245-250amps continuous, you likely are in trouble with the drive sizing. The only possible relief would be if the fan hp at full speed is low enough for the motor to only be loaded to the 245-250amp range. Otherwise, the drive is simply too small for the load.

Instead of changing out the drive, why not limit the motor maximum speed to the point at which the amp draw just matches the drive output amp rating. That will be the most you can hope to get from the drive/motor package in its present form.

As already mentioned, if the drive is in an enclosure that has its interior temp over 40 degrees C, that would be an additional cause of trouble.

 

[ozmosis]

A lot of VFD's these days calculate temperature rise in IGBT's rather than physically measure them. The heatsinks are usually measured but this is not often a constant measurement but a thermastatic measure: i.e. once it reaches (typically) 90degC then the thermastat will come in and trip the drive. The fact that your VFD is picking up the Phase B pair of IGBT's would possibly indicate to me that the measurement is calculated. So, picking up on the point DickDV has said, it is possible that your drive sizing is incorrect and possibly the motor data you dial into the VFD during commissioning does not actually match the real motor data. This can impact the way the internal temperature measurements are defined and calcualted if the base current and voltage are not correct. Typically an IsqT calcualtion is used internally for devices in VFD's and if the base information on the current and voltage is wrong, this can have consequences on the whole performance of the drive.
Mohpower. This is just one of the items to check. Others have already been suggested. The fact you have had two drives with the same issue leads me to think it is an external issue (ambient temp) or, as I mention above, a setting issue with the VFD.
Common problems with VFD overtemp are internal fans not working but this is unlikely if two drives are having the same problem soon after each other.
Have you got enough space around the drive for adequate ventilation (top and bottom)?
A 200hp drive will generate approximately 7~8kw of heat on full load, this needs to be exhausted from around the drive quickly otherwise it will get very hot.

 

[itsmoked]

7kW cannot possibly get out of any enclosure without forced air running thru it. Or, huge missing areas like no bottom and no top.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Hence the problematic nature of mounting large VFDs in MCCs. Ditto what itsmoked said. If the fans are circulating the air already in the MCC, it is just blowing hot air around. You need serious ventilation out the front doors.

The FLA of the motor is 270 amps (460 VAC, PF=0.9). the load is a fan, so it is a VT and 200 HP drive is supposed to handle it.

I have an issue on 200 HP variable torque VFD driving a 250 HP fan

Caveat: if the VFD is a 200HP Constant Torque rated drive, it should be able to handle a 250HP VT load. But you said it was a 200HP VT drive, in which case it can only handle a 200HP VT load. You need to carefully investigate the CURRENT rating of your VFD; forget the HP claptrap.

 

[mohpower]

Hi everyone,

the VFD is a 200HP Contant Torque used to run a 250 HP fan (considered as variable torque).

Yesterday, i took the following measurements at full load(VFD running without tripping).
- currents : ph A: 255 Amps, Ph B: 252 Amps, ph C: 254 Amps
-motor power was at: 90.6 %
-motor torque was at: 90 %
-motor speed was : 1785 RPM
- unit temperature (the temperature read at the VFD keypad screen): 57 Deg. celsius.
-room temperature was around : 20 Deg. Celius
-the mounted fans on the MCC doors are running and with the right flow of air (three at the bottom sucking air inside the MCC and three at the top sucking out air from the MCC).

i would like also to inform you that the motor is programmed to run at two preset speeds (30hz and 60 hz). sometime the motor can run for weeks at 60hz. i am not sure if this will also affect the selection of the VFD.
the motor drives a gear box of a ratio 1:2 making the speed of the fan to be 3600 RPM.

hope this will help.
let me know if more infos are needed

thank you and happy new year for all

 

[DickDV]

The one piece of info still missing is the drive continuous current rating. It should be on the drive nameplate. Anything under about 270amps will be too small. Above that, the drive is large enough and the overheating problem is not due to sizing.

If the drive keypad is telling you the heatsink is at 60 degrees C with a 20 degree C ambient and all fans working, I would suggest that the air flow is inadequate or possible not passing over the heatsink properly. Sometimes, there are baffles or obstructions that keep the air from actually moving thru the drive heatsink. None of that bypassed air is doing any cooling. It might be helpful to check the air temperature exiting the drive cabinet. If the heatsink is at 60 degrees C, I would expect the discharge air to be somewhere in the 50-55 degree C range. If less than that, I would look for the cooling air bypassing the heatsink fins somewhere.

 

[gepman]

I am looking at an A-B catalog that I happen to have on my shelf. The 200hp Heavy Duty (Constant Torque) Powerflex 700 drive is rated at 300A continuous, 330A one minute, and 450A three second on Normal Duty (Variable Torque) loads and 245A continuous, 368A one minute, and 490A three second on heavy duty loads.

Did you get your measurements from the drive or from a handheld meter? Do you have a meter that can read the output of VFD correctly?

Again have you checked the moment of inertia on the fan? You are running it at over the amps for a heavy duty load for a long time and then perhaps you are asking this high moment of inertia load to accelerate. Moving the air is a variable torque load but accelerating the fan is not necessarily. When I size VFD's for centrifuges I have to upsize VFD's due to the long acceleration times. I have also got into some trouble with large boiler forced draft fans in the several hundred horsepower range and had to adjust the accel parameters.

 

[rawelk]

DickDV makes a good point - if heatsink temperature is right up into the 60°C range then you need more effective cooling.

Even if you have high enough flow through the MCC enclosure - I'm guessing something around 700 or 800 CFM should do - you could be 'short circuiting' some of it around the drive (if flow paths exists that don't go over the heat sinks then you are).

 

[ozmosis]

The VFD itself will have it's own fans to blow air over the heatsink but if the air being blown is already hot, then it's only going to get hotter.
Ambient temperature means the temperature directly around the drive, i.e. in this case inside the cubicle with doors closed.
mohpower; is the VFD showing any overtemperature alarms? If it is, then you need to do something about it or you will have another power transistor failure and likely compounded problems with capacitors. If your DC link capacitors are drying out then they can cause failure on your IGBT's.
Where is 57degC being measured? If it is actual heatsink temperature then this is not unusual.