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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
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Transformer Sizing
• Chart from page 1.4
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Jefferson Electric