SECTION
DRY-TYPE DISTRIBUTION TRANSFORMERS
ACME ELECTRIC
•
MILWAUKEE, WI
•
800.334.5214
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acmetransformer.com
10
36. Can 4-Winding Single Phase Transformer be auto-
connected?
Yes.
There are occasions where 480 volts single
phase can be stepped down to 240 volts single phase by
autoconnecting a standard 4-winding isolating transformer
as shown in Figure 1. If connected in this manner, the
nameplate kVA is doubled. For example: A 10 kVA load can
be applied to a 5 kVA 4-winding transformer if connected per
Figure 1.
37. What about balanced loading on Three Phases?
Each phase of a three phase transformer must be considered
as a single phase transformer when determining loading. For
example: A 45 kVA three phase transformer with a 208Y/120
volt secondary is to service 4 loads at 120 volts single phase
each. These loads are 10 kVA, 5 kVA, 8 kVA,and 4 kVA.
Note: that maximum loading on any phase does not exceed
10 kVA. Each phase has a 15 kVA capacity.
45 kVA
3 phase
= 15 kVA per phase
If incorrect method is used, phase B will have an 18 kVA
load which is 3 kVA above its normal capacity of 15 kVA and
failure will result even though we only have a total load of 27
kVA on a 45 kVA transformer.
38. What is meant by “Balanced Loading” on Single Phase
Transformer applications?
Since most single phase
transformers have a secondary voltage of 120/240, they will
be operated as a three wire system. Care must be taken
in properly distributing the load as the transformer secondary
consists of 2 separate 120 volt windings. Each 120 volt
winding is rated at one-half the nameplate kVA rating. For
example: A 10 kVA transformer, 120/240 volt secondary is to
service an 8 kVA load at 240 volts and two 1 kVA loads at
120 volts each.
If the incorrect method is used, winding A will be loaded
at 6 kVA, and winding B will be loaded at 4 kVA. These
do total 10 kVA but, since each winding is only rated at
5 kVA (1/2 of nameplate rating), we have an overloaded
transformer and a certain failure.
39. What are typical applications for transfomers?
ACME transformers should be specified to:
(1)
Distribute power at high voltage.
(2)
Eliminate double wiring.
(3)
Operate 120 volt equipment from power circuits.
(4)
Insulate circuits/establish separately derived circuits.
(5)
Provide 3-wire secondary circuits.
(6)
Buck and Boost (See Section VII).
(7)
Provide electrostatic shielding for transient noise protection.
Figure 1
H1
480V
H3 H2 H4 X1 X3 X2 X4
240V
Enclosure Definitions
Type 1 Enclosures
— are intended for indoor use,
primarily to provide a degree of protection against
contact with the enclosed equipment.
Type 2 Enclosures
— are intended for indoor use,
primarily to provide a degree of protection against
limited amounts of falling water and dirt.
Type 3R Enclosures
— are intended for outdoor use,
primarily to provide a degree of protection against fall-
ing rain, sleet and external ice formation.
Definitions Pertaining to Enclosures
Ventilated
— means constructed to provide for
circulation of external air through the enclosure to
remove excess heat, fumes or vapors.
Non-Ventilated
—means constructed to provide no
intentional circulation of external air through the
enclosure.
Indoor Locations
— are those areas protected from
exposure to the weather.
Outdoor Locations
— are those areas exposed to
the weather.
Hazardous (Classified) Locations
— are those
areas, which may contain hazardous (classified)
materials in sufficient quantity to create an explosion.
See Article 500 of The National Electrical Code.
(NEUTRAL)
10 KVA
8 KVA
4 KVA
5 KVA
A
B
C
CORRECT WAY:
(NEUTRAL)
10 KVA
5 KVA
4 KVA
A
B
C
INCORRECT WAY:
8 KVA
CORRECT WAY:
1 KVA
120V
120V
240V 8 KVA
1 KVA
A
B
INCORRECT WAY:
1 KVA 1 KVA
120V
120V
240V 8 KVA
