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A Typical Problem Of Vfd


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Hai all,


I am facing a typical problem.


I am presently designning a 3-phase i/P, 3 phase o/p drive of upto 10KW power rating.


The problem is,



Condition I when I give the input supply by varying the input 3-phase variac, I can able to run the motor.


Condition II If I apply the volatge through a MCB, the motor is not running.


Details are as follows.


I am using ST TD350 IGBT driver IC, which having the feature of Desat protection output also.

When I applied the voltage through variac,the driver IC is not giving any thing as desat fault.

But when I started directly through MCB, the fault pin of driver IC is coming as low...


When I apply the volatge through MCB,first time motor won't run, if u stop it and again start it within 2sec's time, that time motor is running.


Can anybody please help me in this issue.coz, I want submit the product for customer trails..


very uregent....



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Might be able to help you, IF you can tell me what I'm holding in my two hands.



Can't tell me ???



I gues I didn't give you enough information.



Well, neither did you.


Drawing would be nice so we could SEE what you are doing wrong .....


Hmmmmmm !



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I guess I will make some guesses:


I guess there is probably a rectifier and capacitive filter on the front end of your VFD and that you turn up the variac slowly to charge the capacitors without blowing them up destroying the rectifier and variac in the process.


I guess that when you apply full voltage by closing the CB, you have some mechanism that automatically limits the capacitor charging rate. I guess that mechanism may consistently allow a faster charge than you provide manually with the variac.


I guess something might happen with the faster charging or with the charge control mechanism that causes the output devices to fire in some uncontrolled way. I guess that may cause excessive current and activate the desat protection. I guess that the second time you close the CB, the capacitors may not have discharged completely and the problem doesn't occur upon recharging under that condition.


I guess that another possibility might be that there is a problem in the way the power supply for the control circuitry comes up when you apply power. I guess that could cause uncontrolled firing of the output devices as described above or an erroneous activation of the desat protection.



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