variac high current and heating 2

[DmanD]

hi all guys,
whenever we are testing a variac at no load, a specific turn of the coil is getting heated very quickly. this is happening with the specific turn on which the carbon brush is placed. also this does not happen with the whole winding, only at one or two turns.

first we thought that it was problem with insulating coating. but it was not the cause. then we considered skew in the winding but this was also not present.

please help me with this problem.

d'man

 

[busbar]

Might look at millivolt readings from brush to turn at and away from the problem area.

 

[CarlPugh]

Variac is the brand name for a variable transformer. These comments applly to most variable transforms using carbon brushes.

Tip of carbon brush should only be width of conductor. (Manufacture should have some instructions on brush width)

You should be able to set carbon brush so that it is only contacting one conductor.

It is normal for there to be some heating of the turn when the carbon brush shorts a turn.

If you are building variable transformers, try a different type of carbon for the brushes if the shorted turn temperature rise is excessive.

If you are testing a variable transformer built by someone else, contact them about the temperature rise.

Just for your information: If you hold an open AC clamp on ammeter near the shorted turn, the meter will indicate when the turns are shorted.

 

[hacksaw]

To chime in with the above, your wiper is shorting two adjacent turns.

 

[nbucska]

Old Soviet patent use a separate multipole switch/selector
connected to the xformer's taps. The connecting wires are
routed alternatingly through the two windows of a thick
stack of EI core, so the inductance of a full turn is in
series with any shorted section (between adjacent taps).
The thickness i.e. the inductance can be arbitrary large.



<nbucska@pcperipherals.com>

 

[jbartos]

Question to / comment on the original posting:
1. Please which industry standard are you using for the variac testing?
2. There is a possibility that the variac is tested improperly.

 

[DmanD]

thanks to all guys for enlightening me with all the facts. we did consider all the points specified by you. but too many discripancies were found on all aspects.

suggested by busbar; the millivolts from brush to turn at and away from the problem area was checked and found to be uniform.

as suggested by CarlPugh and Hacksaw the the brush was checked to be shorting two adjacent turns at the same time. it was found to be the case in some of them.

that still left some for which we are considering the quality of carbon brush supplied to us, as suggested by CarlPugh.

dear jbartos, we are manufacturing and testing the variable transformaers according Indian Standards which is identical to European Standards. to the best of my knowledge the procedure of tesing is correct.

special thanks to nbucska for imparting me with something i did not know till date. i will try to find such variable transformer in the electrical market because we have been approached for a light control application which requires such an arangement.

yours sincerely
d'man

 

[jbartos]

Suggestion: The pressure on the carbon brush and consequently on the tested turn should not be excessive. It may be causing the lower insulation resistance with respect to the neighboring turns or possibly against the grounded frame, depending on the variac design.

 

[CarlPugh]

Are you designing variable transformers?

If you are designing variable transformers, the volt/turn of the variable transformer has to be matched to the carbon type used for the brushes.

Lower volts/turn will of course cause lower heating.

30+ years ago when we were designing variable transformers, we used 0.8 volts/turn and for brushes Union Carbide Corporation-Carbon Products Division-National Electric Carbon type AY.

If you are building high power variable transformers, you should get copy of USA patents 3,268,842 and 3,489,973.
This approach was used by Glen Pacific to make MANY high power variable transformers with good operating characteristics and long life. Due to management problems, Glenn Pacific and Electro Engineering Works went out of business and no one has used this approach. (To the best of my knowledge.)

Good Luck
Carl Pugh

 

[jbartos]

Suggestion: A current limit may be implemented by adding Z, e.g. visit

http://www.pupman.com/listarchives/2000/February/msg00226.html

for variac basics

 

[DmanD]

hi all guys,
a great many thanks for all the suggestions. i am working on three factors currently which are:
carbon quality
carbon pressure
quality of the enamel coating of the copper wire used

i will continue to find the reason till i discover it.
if anyone can give me ideas to guage the insulation resistance of the wire for a paticular current rating at full load, please.

thanks to all
d'man

 

[CarlPugh]

Electrical characteristics of wire coating shouldn't affect transformer operation. Mechanical characteristics are important. Belden Armored Poly-Thermaleze has been used with good results.

No offense intended, but you are re-inventing the wheel.
If you will contact me, I should be able to answer most of your questions.

Carl Pugh
USA (510) 793-7675

My son in Brazil calls using the internet and a phone. The quality is poor, but the price is right.