Transformer Basics & Sizing

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Transformer Basics

• Standard transformers consist of:

– Coil (Conductor, windings)

– Core (Laminations)

– Insulation

– Enclosure

Com ponents

CORE

PR IM ARY

W IND ING

SECONDARY

W IND ING

IN SULAT ION

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Typical 3-Phase Ventilated Unit

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Typical Encapsulated Unit

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Basic Terms to Know

• Dry type transformer – A transformer that is cooled by means other than a liquid, usually

airflow

• KVA – Kilovolt ampere or thousand volt ampere

– This is how transformers are rated

• Line Voltage – Also known as the primary voltage

– This is the voltage from the source

• Load Voltage – Also know as the secondary voltage

– Voltage required to operate the load

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Basic Terms to Know

• Single Phase

– Two lines of AC power (Hot & Neutral)

• Three Phase

– Three lines of AC power where each

line is 120 degrees out of phase with the

other two (A, B & C)

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Basic Terms to Know

• Weather Shield – Otherwise known as a Drip-Shield

– These are metal “awnings” added on front and back of ventilated units to make the unit meet NEMA 3R requirements

– The Weather Shield Kit includes both front and back pieces

• Electrostatic Shield – Internal electrical shielding to minimize

electrical noise

– Copper foil placed between primary and secondary windings

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Configuration: Single Phase

• Many single phase transformers have dual primary

and secondary voltages

• For example 240x480 – 120/240

– Primary is 240 or 480

• Typically marked H1, H2, H3, H4

– Secondary is 120 or 240 or

120 and 240

• Typically marked X1, X2, X3, X4

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Configuration: Three Phase

• Primary

– Most three phase transformers have a single

primary voltage in a “Delta” (3-wire) configuration.

– Typically marked H1, H2, H3

• Example voltage configurations

– 480 – 208Y/120 (delta – wye)

– 600 – 240 (delta – delta)

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Configuration: Three Phase

• Secondary

– Three phase secondary voltage may be a “Delta”, 3-wire or

“Y” (Wye), 4-wire configuration

– 3-wire = X1, X2, X3

– 4-wire = X0 (neutral),X1, X2, X3

– The secondary can have a three phase voltage and a

single phase voltage

• Example voltage configurations

– 480 – 208Y/120 (delta – wye)

– 600 – 240 (delta – delta)

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Configuration: Three Phase

• Delta

• Wye

480 V 480 V

480 V

208 V

208 V

208 V 120 V

Neutral

120 V

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• Secondary

– Delta w/ center tap

240 V

208 V “high

leg”

120 V 120 V

240 V

240 V

Configuration: Three Phase

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Transformer Sizing

• Transformer size is determined by the kVA rating of the load – (secondary = equipment)

1. Determine the load voltage

2. Determine the load current (amps)

3. Determine if the load is 1 Ph or 3 Ph

4. Determine the line voltage

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Transformer Sizing

• Transformer size is determined by the kVA of the load!

• Formulas – Single Phase: VOLTS x AMPS = kVA

1000

– Three Phase: VOLTS x AMPS x 1.73 = kVA 1000

– See Catalog page 1.3 for reference

Transformer Sizing

• Chart from page 1.4

Jefferson Electric