Two transistors break down frequently

[Signode]

Dear friends;

I am new in this Forum.

I have an Automatic voltage stabilizer 5KVA 220V +/- 3% with 24VDC servomotor control for fans in a school. I have been facing a problem for one year that two power transistors D401 and B546 installed on its PCB break down frequently. I replace but after one or two months, one of them or both break down. For me, when ever local power supply shut down and then again comes and all the fans were already switched on, the failure of transistor(s) then appear. May be I am completely wrong as I could not find the real cause.

This is a local designed stab which worked perfectly one years after purchasing. I have replaced its servo motor but fault did not remove. Replacing the trans again and again also damaged the PCB. Now I have been thinking to replace the both transistors with doubled capacity. Any tip from anyone? Can anyone help me out which transistors would be better for replacing the old?

 

[Resqueline]

It would help if you supplied the servomotor specs.. I guess the supply on the PCB is +/- 24V? Are there protective diodes across the transistors? Are their base and emitter leads connected together?

 

[Signode]

Dear Friend;

Thanks for replying.
There is nothing write on servomotor, but i think it 24V, 20W.
Yes, the supply on PCB is +/-24V.
There is only one diode connected in the series of base cicuit.
No, base and emi is not connected.

 

[Resqueline]

If you can measure the dc resistance of the servomotor it will tell us the stall/startup current. It won't take much to overload those transistors, they're quite puny.
If the bases and the emitters are not connected together then I'm not sure how the circuit is made, and there can be other issues blowing the transistors - other than them being under-rated.
But you might want to just replace the transistors with beefier types. They seem to not be anything special (TO220, 150V, 2A, 20W), and there would be MANY suitable replacements to choose from (up to 20A, 200W). Which shops would be convenient for you to buy from?

 

[Signode]

How i can install the Fly back diod?

Please see the taached picture for power circuit control diagram of Servo motor and suggest me where should i install the fly back diod.

 

[Resqueline]

Ok, that makes a lot more sense to me, but does the base connections come from the same pin on the same 741 or are there separate circuits?
It's common practice to put diodes across C-E (kathode to C on the D401, & kathode to E on the B546) in order to protect the transistors from inductive kickback.
Another way of saying/doing this is to put two diodes in series, kathodes pointing towards +, and connecting the midpoint to the motor.
You can use any of the 1N4000 series diodes for this purpose.

 

[Signode]

Thanks a lot.
Can you draw on the attached picture what you told?