View Full Version : Soft Stator Problem

22-07-2011, 03:34 PM
Dear Forum

I am using Soft stator for my Bitzer screw Semihermetic Compressor of 125HP
After 7 Months it was burnted {Picture Enclosed}

Can any help , how can motor will burnt out with Soft Stator.


Yuri B.
22-07-2011, 04:17 PM
Was oil return sufficient at all (at most) of the working time of the compressor?
Was windings temperature controlled with the help of the proper control devices?
Reliable overcurrent protection was also installed?

22-07-2011, 08:27 PM
Dear Sir

Everything is fine ,we checked it .

We came to know that if semi Conductor is Failed ,The motor burnt will takes place
is that true .


Yuri B.
23-07-2011, 09:43 AM
Everithing is possible, especially if the starter was of low quality, or insufficient functionality and you relied on it as a sole protection of your motor.

23-07-2011, 10:07 PM
Did you specified to Bitzer your compressor should be controlled by a softstarter?
Arcing is very likely to happen with a soft starter.

Was the softstarter also the and the only protection of the motor?

24-07-2011, 02:48 AM
Soft starter should have a parallel line contactor that takes load, I have heard of lots of failures with starters without them The SCR's fail and set up protection is not quick enough to stop winding failures, usually at max loads.

30-07-2011, 08:33 AM
Their is no by-pass contactor , The complete one part winding is damaged ,My doubt was, the winding got bunted
when the motor was in off .

When the burnt has taken place when it is in running the carbon deposit will be in whole compressor and the oil will be in complete black color
but it is in green color .

The compressor has a saftey of High Winding Temperature Trip , even the thermistor was completly melted

still we are finding for the exact cause

30-07-2011, 07:15 PM
what was the use of this bitzer ?was it instaled in a chiller?what was the brand?
was your soft starter a " siemens" made ?
we encounterd a simiilar burnout that was covered by warrnty!!

31-07-2011, 09:51 PM
"Starting Time" configuration is critical on Soft Starters and should be strictly configured according manufacturer specifications.

A too long starting time can generate a thermal "stress" of motor winding that can easily ends in a burnout.

This can be easily verified stripping down the compressor and inspecting the defective motor (you should find a motor winding completely overheated).

Anyway keep yourself "open minded" and consider also most "classic" reasons for burnout....

01-08-2011, 04:02 AM
the winding configuration and terminations in the compressor terminal box. Were the winding star connected or delta connected. That is to say low voltage or high voltage connections
One winding failure of three windings, big blow hole in a winding or three windings cooked. What was the result of failure

01-08-2011, 12:47 PM
Hi , the winding is delta connected, one winding is failure ,
below the terminal plate out of six leads , three leads are melted.

When the compressor is running at -18Deg Temperature this burnout has taken place
at discharge side on suction side the winding is good.

its a bitzer make

I dint understand one thing , the motor burnout has taken exactly below the terminal plate , there is
a high winding temperature thermistor is there to trip in high temperature ,even though it get failure .

04-08-2011, 03:38 AM
Hi ssk,
interesting failure result, flash over happened at the terminal block and one winding failed. Generally the flash over would happen at the weakest point, in your case the terminal block. The initial problem was in a winding failure, probably in the winding slots, possible cause would be contaminents,breaking down insulation on windings, creating high current draw on one winding and that same contaminate created the flash over at the terminal block. Suggest moisture was the intital problem. Winding thermistors would not react fast enough, and shock outage of failure would not be protected by starter

04-08-2011, 12:01 PM
Their is no by-pass contactor , The complete one part winding is damaged

There you go. The soft starter should be connected to both windings at once, as in direct on line start. You soft started one winding, which is a no no.

04-08-2011, 08:49 PM
as i stated above, we had a similar problem with a bitzer ,the soft starter that was used, only has two thyristors,one phase is always energized,for some crazy reason the soft starter malfunctioned and the entire compressor overheated until it shorted out ,allthough it had all the protectiones instaled like the criwan module etc.
a intresting anomally!
we still dont know what really happened ,the warranty covered this one !!

07-08-2011, 06:09 AM
Warranty was not covered , but even bitzer people are saying that it was burnted because of thyristors fail.
its get malfuncoined.
But this a crazy thing .. i faced for a first time

I like to know the exact working cylce of soft-stator , can any one suggest me.
There is any website for this.


08-08-2011, 03:29 AM
I have to admit that I have never come across a soft starter that functions on only two phases. Leaving one winding live. What brand of soft starter did you install and I will keep away from them.
But I have seen installed two phase contactors, scary stuff.
Check Danfoss Drives for a reputable brand. All the bells and whistles protection. The HVAC series are basically designed for what you want.

08-08-2011, 07:17 AM
It was not a brand make . But i like to know , What chances are there for Motor Burnt-Out , When it is running with soft starter .
It is a Dol , there is no by pass line conductor .
We connected all the six leads to the stater - where the stater has three bridges .
Out of this One part winding has damaged.

09-08-2011, 02:50 AM
please clarify if part wind motor, if so you will have 12 terminals in the terminal box. Should be wired for high voltage start arrangement, or DOL / delta configuration. Which means you will have 6 links , or three per winding. The links should go top to bottom on each winding. Then connect supply from starter to each set of windings in the same patton . ie.,
Can you post a picture of terminal box, this would make things a lot clearer for comments

09-08-2011, 06:41 AM
Yes it is part winding motor 50:50 , its a Dol Connection , on terminal plate it has six links , three links per winding.

09-08-2011, 07:20 AM
one more picture please see it.6971

10-08-2011, 02:47 AM
The back of terminal block has six terminals. Interesting winding failure, half the stator cooked / slow burn and coil tails blown off. Possible loss of one phase started failure.

DAC josep
17-08-2011, 05:13 PM

This type of failure has got in part winding normal DOL (contactor ) Two contactor starter type due to contactor hold on problem and the shift timer problem. But here it is soft starter ,by soft starters winding burning like this , is very rare.The picture shows that it has been run few with high winding temperature(even if there is thermistor) .If the moisture presence inside the system the same type of problem has been observed . In screw compressors the terminal box inside getting moisture condensation and thus it gose to ground fault.In this situation if the ground fault protection configuaration is not perfect , it leads to winding failre.intresting analysis***** Keep on discussion.