Ac inverter problem

hesham

Member
Join Date
Jan 2003
Location
cairo
Posts
328
hi guys
i have just finished a supervision for installing a control system for BAF( Batch annealing furneces) the supplier installed allen bradly Ac inverters in each furnes 1336 plus series 22kw.
after starting up and working for few days the inverters start to blow up one after one without any reason.when i chech for that i found a difference in Amps between the phases in the inverter input specially the first one is much lower than the other two.every thing else is alright the motor the cables,the motor terminator ....etc
Can any one help me and explain why this difference in input Amps. is this could be a reason for damaging the inverters.
N.B this inverters was in the stores 3 years ago may be the capacitors is not in a good condition.
Any comment
 
Hi neighbor

I when you say blow away what you mean any particular part damaged ? or the VFDs stoped to work.?

Its normal that the current in the input and output is not equal and not make sens unlless you useing True RMS current tester.
Use the VFDs disply to measure the motor current and compare to motor rate current.
The Variable Frequency Drive VFD Take the AC voltage convert to DC
and create new sinus with a new Frequency and voltage.
the current you measured in the input is not exactly related to motor current.which is OK.

By the way I half Egyptian my father born there.
I still have famely in Alexandria.

If you need more help just writh it down
 
hesham, it's a little hard to help you because you haven't told us what went wrong with the inverters and especially whether you are measuring the current imbalance on a good or a failed unit.

Assuming you are seeing the current imbalance on a good unit, I don't conclude that there is any capacitor problem. My main concern would be whether the incoming power supply is properly balanced. Is the voltage between the three phases within a few volts of being equal. And, is the voltage from each phase to neutral within 10-20 volts of being equal.

I don't know what the Egyptian practice is but inverters are not really designed to operate on three phase power that is far out of balance to neutral or phase to phase. Worst would be running with one phase grounded and the other two running full voltage above ground.

You might want to check this and report back what you find.

I'll do what I can to help.
 
Half Egyptian Airkby
Its normal that the current in the input and output is not equal
The VFD output current and voltage is 100% balanced there isn't any problems in the output.the only remarek which i have is that the inout current is much less than the values mentioned by the manufacture .
hesham, it's a little hard to help you because you haven't told us what went wrong with the inverters and especially whether you are measuring the current imbalance on a good or a failed unit.
DickDV
After the failure of the first two units i take this measurments on a 2 Working VFD.
the 2 malfunctions unit is sent to the supplier he mentioned that the internal choke coil is totally damaged we are still waiting for the complete report to know all the failures.
I already measure the 3 phase input voltag it's balanced and within the limits specified by the manufacture it was 370-370-373 volt
also the phase to nutral measurment was 217-217-217 V
thanks for help. :(
 
DickDV


I dont think The VFD running with one phase grounded and the other two running full voltage above ground.
The VFD should not run at all.
What I can think is that the voltage in the input is low.above the 10% which allowed if the voltage go behind 360V you might have problem.
But you suppose to get alrm of low voltage.
 
hesham

I all most sure that the problem is low voltage 1336 made for
400-480 volt.
may be during the day its go below 373V.
Try to add 3Phase capacitor it will incrice your voltage in this panel.
 
Most drives will run for a while with one phase grounded and the others at full voltage above ground. It adds needless stress to the drive's lead-to-ground insulation and often results in premature failure. I will not start a drive on a grounded lead supply.

Based on hesham's data, two new drives with blown bus inductors makes me suspect a defective batch of inductors. While I am not particularly fond of Allen-Bradley products, they are a responsible supplier of quality equipment and their drives are no exception. That they may have a pattern of failure here is not unheard of and can happen to any supplier. I'd let them try to take care of the problem. If they treat you right, good. If not, it's time to look for something else.

While the 1336's that I see are usually 460V 60Hz rated, I can hardly imagine them selling drives in Egypt with anything but 380V 50Hz ratings. If these drives are 460V drives, they should be changed out to something closer to the actual input voltage. Adding capacitors is not going to get you a workable solution.
 
I checked in A-B manuals 1336 suppose to hendel 373V
the definition 380-460v +-10%.
So I take my word back.
But if you install capacitor for improve Cos Pi you will increase your
voltage.
373v is lower for the industry.

By the way Hesham.did you measure the voltage when the motor was running or not?
 
hesham,

is the line voltage "clean" ?
High-voltage spikes can do a lot of damage.

You write that you have annealing furnaces. Are there some kind of melting furnaces with its own power electronics system (thyristor choppers) ?

I ask, because these can generate a lot of high-powered electrical noise. I have seen just this at a plant where there was only one transformer to supply everything, including some melting furnaces. The noise generated by the melting furnaces caused a lot of trouble, including burnt electronics everywhere else. The nominal voltage was 400V, but I think that in the spikes there could be peaks up to 1000V.
You cant catch these peaks with a standard AC volt-meter.

The solution was to install a new transformer, so that the melting furnaces could be separated from everything else.
 
Thanks guys very much for your replaies
I already measure the input voltage to the drive while the motor is running the supply is stable and more i operate more than on motor in the same time to chech for this i found also the supply is good and within the permisible values.I agree with Dick that it could be a failure in the inductors i am thinking to added another external chock coil to the units and i will cancel the internal one and let us see what happen
the rating of the nverters is 100% compatible with the network in Egypt 380V 50Hz
The transformer suuplying the BAf also supply the pikling line but the rolling mill is supplied from a seperate transformer.
Can any one provide a formula to calculate the inductance of the chock coils.
thanks
 

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