Transformers in parallel

• To: tesla-at-pupman-dot-com
• Subject: Transformers in parallel
• From: "Eric Davidson" <edavidson-at-icva.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Wed, 25 Aug 1999 11:40:36 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Matus et al,
 
Proper phasing is but one concern when connecting transformers in parallel.
 They must also have the same exactly the same ratio and the same
impedance.  This may seem obvious, but two transformers with the same
secondary voltage may have different primary voltages.  I don't mean 120 or
240 volts, I mean 110, 115, 117 or 120 volts.  If the secondary current
ratings are identical, the impedances will be close enough. For the best
and safest results, I'd shoot for finding identical transformers (make,
part# etc.) to connect in parallel.  Connecting NSTs in series is not easy
or safe.  The secondary midpoint is connected to the case of the
transformer internally, as this cuts the potential between the secondary
bushing and the case in half.  In order to connect them in series you have
to unpot them and remove the connection to the case, or run them with their
cases ungrounded and floating.  Either of these options makes for an unsafe
situation and puts extra stress on the insulation (especially where the 120
volt connection is).  Since voltage is not the sought after quantity, place
a string of 9kV 30 mA units in parallel and you will be just fine. Hope
this helps!
 
Eric
edavidson-at-icva.gov
 

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