Transformers: Voltage and Current Part II

Vocabulary and formulas regarding transformer ratios, KVA ratings, connections, and performance rules based on the Part II lecture.

Turns Ratio ($TR$)

The ratio of primary voltage to secondary voltage, calculated as $TR = V_p \div V_s$ or $TR = I_s \div I_p$.

Primary current ($I_p$)

The current in the primary windings, calculated as $I_p = I_s \div TR$.

Secondary Voltage ($E_s$)

The voltage produced at the secondary winding, calculated as $E_s = E_p \div TR$.

Primary Voltage ($E_p$)

The voltage applied to the primary winding, calculated as $E_p = E_s \times TR$.

KVA (Single Phase)

The thousand volt-amp rating for single phase transformers, calculated as $KVA = E_s \times I_s \div 1000$.

KVA (Three Phase)

The thousand volt-amp rating for three phase transformers, calculated as $KVA = E_s \times I_s \times 1.732 \div 1000$.

Impedance ($\text{\%Z}$)

A percentage value used to determine the maximum short circuit current delivered if secondary connections are shorted.

Max short circuit current ($I_{max}$)

The current delivered during a short circuit, calculated as $I_{max} = I_s \div \%Z$.

Primary windings

The transformer components that convert electrical energy into magnetic energy.

Secondary windings

The transformer components that convert magnetic energy into electrical energy.

Auto transformer

The least expensive transformer solution for providing a higher voltage for a new machine in a factory.

Potential transformer

A transformer commonly used with a secondary voltage of $120\,\text{V}$.

Multiple-Tapped Windings

Windings used for flexibility in connecting the primary to different supply voltages or providing different secondary voltages by altering the number of turns.

FCAN

Full capacity above nominal; a type of tap for variations in supply voltage.

FCBN

Full capacity below nominal; a type of tap for variations in supply voltage.

Transformer Polarity

Indicates connections that are in phase; windings connected dot-to-non-dot add voltages, while dot-to-dot connections are $180^{\circ}$ out of phase and subtract voltages.

Series Connections (Secondary)

Increases the voltage output and decreases the full load amps ($FLA$) by increasing the turns ratio.

Parallel Connections (Secondary)

Increases the current capacity and reducing the voltage output by decreasing the turns ratio.

Transformer Banks

The connection of individual, identical transformers together in series or parallel circuits to increase current capacity or use single phase transformers on a three phase system.

Current Transformer

A transformer where the voltage will rise to the point of burning out if the secondary circuit is opened while the primary is energized.

Step down transformer

The default type of power transformer that reduces applied voltage; it can be changed to a step up type by reversing the source and load connections.