[Boatanchors] Parallel Operation of HV Power Upplies

[Carl]

Using a FWB on a vintage CT transformer often causes unwanted sounds and 
smoke.
Transformer insulation does not have an infinite life span.

In Byrons case with identical transformers the KISS principle works 
well. Evaluation in use will tell if there is any unbalance and then a 
remedy can be applied.

Of course one can go into lengthy discourses and break out Termans but 
by the time its said and done many of us will be worm food.

Carl
KM1H


----- Original Message ----- 
From: "J. Forster" <jfor at quik.com>
To: "Byron Tatum" <bjtatum1 at att.net>
Cc: <boatanchors at mailman.qth.net>
Sent: Saturday, February 14, 2009 9:04 AM
Subject: Re: [Boatanchors] Parallel Operation of HV Power Upplies


> The problem with just paralleling the transformer secondary leads is 
> that
> you will get circulating currents in the secondary windings, if the
> transformers are not exactly identical. This increases their internal
> dissipation and heating. The amount depends on how unequal the units 
> are.
> It essentially shorted turns.
>
> Possible solutions:
>
> General Radio made special "balancing chokes" for use when paralleling
> Variacs. They were essentially a choke with a center tap. The outer 
> legs
> went to the transformers, the center leg was the output. The 
> cleverness of
> this is that it has a high impedance to circulating currents, but a 
> low
> impedance to the wanted power (because the magnetic fields buck in the
> choke). You could easily use an old trandformer this way.
>
> A second solution is to use two series chokes, one per transformer, 
> but
> this is lossier than the mothod suggested above.
>
> A third way is to use separate diodes for each transformer (two per, 
> if
> it's center tapped, four per if you're using a FW bridge) and connect 
> the
> unfiltered DC outputs in parallel. While this will not guarantee 
> complete
> equal load sharing, it will prevent circulating currents.
>
> A fourth way is to parallel the filtered DC outputs. I see no real
> advantage to that, unless you have two fully built, unregulated, power
> supplies.
>
> If the supplies have active regulation, all bets are off.
>
> FWIW,
> -John
>
>
>
>
>> A Quick Question-
>>         I need about 800 mA at 2KV. I do not have a single 
>> transformer to
>> supply this. I do have two identical transformers that will supply
>> 400 mA at 2 KV.
>>         I have been studying parallel operation of transformers, but 
>> had
>> these questions:
>>                 Would just paralleling the transformers be 
>> sufficient, or
>> should there be low value resistors isolating the HV
>> windings?
>>                 Would it be better to use two identical power 
>> supplies
>> (transformers, rectifiers, filters) and connect the HV DC
>> leads together and parallel them in this fashion? (I have
>> full power supplies already containg the 400 mA at 2 KV
>> transformers)
>>                 If I went the complete power supply paralleling route
>> would there be any benefit in passing each HV DC lead
>> through a HV diode before actually tying leads together?
>> 
>> Thanks,
>> Byron
>> WA5THJ
>> 
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