Unit 29: Three - Phase Transformers
Three phase transformer connections are often used to supply ____ which tends to ___ the system
single phase loads / unbalance
The total power output of a three phase transformer bank is the ____ of the three transformers
sum of the rating
Three phase transformers are constructed by winding three separate transformers on ___
the same core material
The center connection of a wye is often tapped to provide a neutral conductor for three phase loads. This produces a ____ system. Common voltages produced by this type of connection are ____
three phase four wire / 208/120, 480/277
____ value of voltage and current must be used when calculating the values associated with the transformer
Phase
How do you get PHASE CURRENT from line current in a WYE connection?
Phase and line current are the same in a WYE
What is dielectric oil?
Provides electric insulation between the windings and the case, help provide cooling, and prevent moisture buildup.
The ____ connection is used to change three phase power into two phase
Scott
____ transformers provide better phase balance then open delta connection
T - Connection
Proper phasing can be checked with a ___. If the phasing is correct the voltmeter will show __ volts. If phasing is wrong, the voltmeter will show____
voltmeter
Transformers should not be connected as a ____ unless the incoming powerline contains a neutral conductor
wye - wye
The ___ connection is primarily used for grounding purposes
zig - zag
WYE: (E)phase =
(E)line/1.732
DELTA: (E)line =
(E)phase
WYE: (E)line =
(E)phase x1.732
DELTA: (I)phase =
(I)line / 1.732
WYE: (I)line =
(I)phase
DELTA: (I)line =
(I)phase x 1.732
What happens if a phase winding of one transformer is reversed?
An extremely high current will flow when power is applied
How do you get PHASE VOLTAGE from line voltage in a WYE connection?
Divide line voltage by 1.732
What schematics are used to illustrate a transformer connection?
One - line diagram
The T - Connection can be used to provide a three phase four wire connection with ____
two transformers
If a transformer is powered by a three phase three wire system, the primary winding must be connected in a ____ connection
delta
How do you get PHASE CURRENT from line current in a DELTA connection?
divide line current by 1.732
An ____ connection can be made with the use of only two transformers
open - delta
The NEC requires that a high leg be identified by an ___
orange wire or by tagging
When calculating three phase transformer value, the __ for three phase circuits must be followed as well as the ___ for transformers
rules
When closing a delta, what must be checked before final connection and applying power?
should be checked for proper polarity
When an open - delta connection is used, the total output power is ____ of the sum of the power rating of the ____
86.6% / two transformers
How do you get PHASE VOLTAGE from line voltage is a DELTA connection?
Phase and line voltage are the same in a DELTA
____ can be used as a three phase transformers bank by connecting their primary and secondary windings as either wyes or deltas
Single phase transformers
It is common to center tap one of the transformers in a delta connection to provide power for __ loads. When this is done, the remaining phase connection becomes a ____
single phase / high leg