Vacuum Contactor Peak Current And Motors Insulation Damages

If failure appear at switching on, then the cause is most probably the successive prestrikes that appear at closing the Vacuum contactor , and may lead to overvoltages (first prestrike burns a micro spike on the contacts surface and increases their dielectric withstand, high frequency oscillation, ...).

The solution efficiency is improved if it includes a capacitor which slows down the frequency of oscillations, thus the number of successive prestrikes.

More data on prestrikes :

http://www2.schneider-electric.com/corporate/en/products-services/technical-publications/technical-publication-knowhow.page?c_filepath=/templatedata/Content/Technical_Publication/data/en/shared/electrical-engineering/electrical-know-how/high-voltage-plus-1kv/ect151.xml

you use the contactor to turn the engines on or of? it is strange that the winding´s do not withstand the peaks produced... you could change the the way those motor´s start in other to ask less current in the start, or even, use a autotransformer to regulate the voltage... there are many ways to deal with one same problem! but the pocket usually is the ruling factor. hope that this comment were usefull.

Dear colleagues, I must apologize with all of your because in my first statement of the problem I said that the motor failure occurred during the starting whend I should say during the contacts breaking off which is quite different the anlysis, really under this last condition and also inching, unforeseen stalling starting can produce high peak of voltages damaging the winding insulation.

zorry it was my fault

Bye

Like I told you before, there are many way´s to look at one same problem. If it occurred on stop you could put the engine in -of load- and with a triac for such powers reduce gradually the speed to finally stop it safely. but it seams to me that the engine just need maintenance to is windings.

It seems that this one keeps cropping up as the generations go by. I first saw this one some 35 years ago when vacuum contactors first came into favour and were being used for starting smaller and smaller motors D.O.L. down to say(100kW at 6600volts) MV.

Then the problem was that if the motor was tripped the vacuum contactors had relatively high values of chopping current. This chopping action produced multiple restrikes and very fast rates of rise of voltage. This was made worse by the capacitance of the motor feed cable and the higher Z of the smaller size motor stator windings causing resonance.

The net result was that a large proportion of the voltage was impressed across the first few turns of the stator winding, with subsiquent very early failure.

The solution was achieved by work done in three main areas.

The main solution was the "invention" of the Zorc transient suppressor which was designed to solve this problem and was (and still is today) fitted across the motor terminals as a standard.

The chopping current of the vacuum contactor was reduced by contact material re-design.

The type of insulation of the motor winding was changed and more attention was paid to the design of the winding of the motors.

The length of cable to motor and its capacitance are important and determine if a Zorc should be used.

Bob

So you were also in the BP oil rig failures story 35 years ago .... at this time I advocated using SF6 sontactors, which are free from most of these transients .( proved by comparative tests done at UMIST.. but although these contactors still exist , it it not the only solution any more !

For vacuum contactor switching both the magnitude and the rate of rise of surge voltages are detrimental parameters for motor insulation. To mitigate this problem ZORC, which is a combination of Z_nO type arrester and R-C snubber, can be installed conveniently at the cable chamber of the vacuum contactor panel. This will limit both these parameters to acceptable level and will protect the motor against possible damages due to switching surges. However, it is recommended to install ZORC as near to the motor as possible.

Nowadays, vacuum contactors are being manufactured which have very low chopping current. Hence switching surges will not be an issue with these contactors.

If new motors are procured with vacuum contactors as starters, they may be specified for vacuum switching application and will be provided with strengthened insulation at the end turns to withstand the switching surges.

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If new motors are procured with vacuum contactors as starters, they may be specified for vacuum switching application and will be provided with strengthened insulation at the end turns to withstand the switching surges."

Yes, that would be the best thing to do in first place! Those motor´s are not for the price of 1 buck for Kg, and are usually used for major industrial processes.

Yes, I agree!