High voltage Plate Transformer

[Ron Reeland]

Hi:
My son is experimenting with vacuum tube operated Tesla Coils. So he has purchased a few very large plate transformers with 2950 AC output volts and higher. One in particular is a Stancor P-8035( weighs 80 lbs). A listing of it in an old Stancor catalog says it "May be used with secondary C.T. ungrounded."

However, a transformer just above, P-2126 ( weighs 95lbs), says "Secondary center tap must be grounded." This transformer rated at 2900 AC ouput volts.

These transformers likely were to be used in full wave center-tap rectifier circuits.

Why does one require center tap be grounded while the other does not? We are concerned that other transformers without reference data available could be damaged if not properly grounded. That would negate the use of transformers with a full-wave bridge rectifier circuit.

Thanks,
Ron reeland

[tautech]

Grounding the CT supplies 2 windings that are likely to be 180⁰ out of phase whereas without the CT grounded it'll be effectively 1 winding @ twice the voltage and that's where the limitation most likely is: isolation/insulation. 6KV is not a trivial voltage to insulate, transformers of this caliber are normally dipped and/or potted to prevent breakdown.

Be bloody careful with it as it's weight (lots of copper) would suggest it's capable of more than just a few mA's at the secondary. 

[David Hess]

Many of these plate transformers had two versions with one requiring the center tap to be grounded and the other allowing either end to be grounded.  If the center tap is grounded, then the voltage on either leg to ground is half the voltage compared to if a full wave bridge rectifier is used with one leg of the rectifier grounded.  The difference in transformer construction is the insulation.