Omron VFD Problem 6

[sparks22222]

Hey guys,

I came across this VFD problem in one of our locations.This plant had a problem with an Omron VFD where the drive (5.5kw) is operating a rooftile holder which goes up and down at low and fast speeds with a brake installed.
Well before i came to the location(3 y old only) the electrician had changed the braking resistor which was heating up(which couldnt be touched)after a few runs of the drive.He changed this braking resistor(resistance was low) even changing a few of them but with all heating up.At that stage the inverter had overloaded and eventually shorted out on the L1/L2 input terminals.
Once the electrician had changed the inverter with the same type(OMRON)and this worked for a while but same problem showed up again and eventually burned out the second inverter as well.(Shorted out on L2/L3)
At this stage i came on the scene to face this problem of no inverter in stock.So we had a UNIDRIVE VFD for 7.5kw and tried this to get us out of a hole.So while we waited for the next OMRON inverter we decided to check why both inverters burned out.So we changed the 5.5kw brake (sew)motor(couldnt check resistance,no megger) and replaced the motor cable and then tried the UNIDRIVE inverter.After some time sorting out the parameters in the new inverter,we eventually got this drive running.
Just wondering has anyone come across this problem or something similar.Why was the braking resistor heating up (i know probably from too much regenerative energy being supplied from the motor when braking hard) and the inverters damaged and the strange thing is we dont know where the problem was after all this.It may have been the motor,the cable,the resistor(which i dont think)or the inverters.
I have come across some inverters where the dc bus is damaged and they are replaced by me but this was an odd one.
I appreciate any help on this.

 

[jbartos]

Suggestion: It may be necessary to review the motor size and duty requirements for the motor given mechanical load.

 

[macmil]

There was another thread on a similar subject a month or so ago. I paid attention to it because I had a similar problem on an overhead crane hoist several years ago; we put a VFD on the hoist; its operation was great when it worked, but went through 3 drives in a matter of months and the manufacturer declined to keep supplying warranty replacements. One post in the previous thread pointed the finger at the effects of the regenerative braking. ( Disclosure; I am mechanical ) Sounds as though your problem could be along the same lines.

 

[Tumppi]

There are two things you should know,when selecting braking resistor.The minimum ohmic value for the drive and the power rating for the braking resistor.

Minimum ohmic value can be found from manufacturers manuals.
(What lower is resistance,that higher is current).

Power rating can be calculated,when braking duty and load is known.

When a drive/inverter is braking (motor is regenerating) ,
then drives braking transistor (chopper) will connect drive DC-link voltage across the braking resistor to keep DC-voltage below overvoltage-trip level.

If power rating of resistor is too low, then resistor overheats and it will be damage if voltage across it is not removed.
Thats why an overload/thermal protection for resistor is needed.
If there is no protection for resistor and it overheats ,and
goes e.g. shortcircuit, then the rectifire and braking transistor (chopper)of drive can be damaged.

In hoist/lift applications when load is coming down,the drive can be braking/regenerating all the time.(Depends of mechanical construction).
So,you may need to select resistor rating for continous power and that can be as high as the power rating of drive.

With UNIDRIVE you can see from parameter 10.10 when motor is
regenerating.Parameter 10.11 tells you wheter the braking
transistor (chopper) is on or off.
You can protect resistor by setting parameters 10.30 and
10.31 according your resistor,but external device is still
recommended,if braking transistor fails e.g. goes shorted.
From parameter 5.05 you can check DC-link voltage,if it goes over 830VDC,drive will trip OU-overvoltage.

 

[ScottyUK]

When sizing your braking resistor, you must consider that it must dissipate the regenerated energy from the motor, without letting the DC bus voltage rise above a safe level. Energy is dissipated according to:

P=V^2/R

where
P = power
V = DC bus voltage
R = brake resistance.

The maximum safe value of V should be known, and the peak regenerated power from the load should be be calculated. This will allow you to calculate the required value of R.

Many problems with drives failing under regenerative conditions are caused by DC bus overvoltage. Can you connect a meter across the brake resistor, and see what the voltage is rising to?

 

[jraef]

What they said

The VFD manufacturer will tell you the resistance value you need and a "recommended" power rating, but you must select a power rating appropriate for your application. I used some 20HP Electromotive VFDs (made by Yaskawa, the same as the Omron) for hoists on aircraft cranes (as in Boeing 747's) and had some initial trouble with regen until we increasesd the power rating of the resistors enough to handle the load AND duty cycle. After that, no more problems.

And yes, the resistors were always hot to touch when in use, that's the nature of the beast! However, if they are under-rated you may experience premature failure from excessive watt density, the amount of wattage trying to dissipate over the surface area of the resistor. We ended up using industrial heater strips as power resistors since they are designed for that purpose.

Quando Omni Flunkus Moritati

 

[sparks22222]

when all else fails play dead

thanks guys for all the help on that one..we have upgraded the braking resistor and the unidrive inverter looks more reliable than the omron inverter..looks ok for now after checking the above readings from your replies..will keep you all updated if it fails again
(ill play dead next time jraef)
anyway happy christmas guys and enjoy the holidays and dont work too hard for the new year..

sparks22222

 

[macmil]

Hi Sparks,
thanks for the feedback.
Merry Christmas; for 2004 work smarter not harder !!
mac

 

[KIMATEK]

There is connection link for DB Calculation Examples at

http://www.kimatek.com/technical_links___news

just in case you want to review.