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