Selection Guide for Power Transformers - DigiKey TechForum

a = 1 (NP=NS): 1-to-1 (Isolation) transformer has equal input and output voltages

The turns ratio of a transformer is important because it determines the ratio of primary voltage to secondary voltage. By formula,

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For the transformer above, this formula indicates that the voltage across the primary is four times the voltage across the secondary.

IMPORTANT!

Power on the primary and secondary sides of the transformer always remains the same. KVA on the Primary Side = KVA on the secondary Side (Under Ideal Conditions, Ignoring Losses) KVA on the Primary Side = 1.732 × Voltage across Primary × Current through Primary KVA on the Secondary Side = 1.732 × Voltage across Secondary × Current through Secondary

According to Section 450.11 of the NEC, each transformer must have a nameplate indicating the following:

• Name of the transformer manufacturer
• Transformer KVA rating
• Frequency
• Primary and Secondary Voltages
• Transformer Impedance (25 KVA and higher only)
• Amount and kind of insulating liquid (liquid-type transformer)
• Insulation temperature class (dry-type transformer)

Tap Changer

Taps on the primary winding compensate for variations in line voltage. Tap increments are usually 2.5 % or 5 %, both above or below the rated line voltage.

A transformer that is used to step down the voltages transmitted by power utilities to values that the customers require.

Distribution transformers are classified as being either Dry-Type or Wet-Type. These classifications indicate the means used to cool and/or insulate the components.

Dry-type transformers are generally air-cooled and may have louvres to aid in cooling.

Wet-type transformers are high-power transformers that generate too much heat and are typically cooled using mineral oil or high-temperature hydrocarbons.

EXAMPLE:

A 13,000Y/ to 208Y/120 transformer would have the following characteristics:

• The primary is Y-connected to a 13,000 V line voltage or a phase voltage of V.
• The secondary is Y-connected with a line voltage of 208V and a phase voltage of 120V.
• This is a Step-Down transformer.

ANSI standards require that the leads connected to high-voltage windings be marked with an H and leads connected to the low-voltage windings be marked with an X. If the transformer steps up the voltage, the primary winding (the winding to which voltage is applied) is the low-voltage, or X, winding. If the transformer steps down the voltage, the primary winding (the winding to which voltage is applied) is the high-voltage, or H, winding.

KVA is determined by the connected load of your equipment. The primary and the secondary KVA are exactly the same.

AS AN EXAMPLE:

Primary-Side: 100A @ 120V = 12,000VA or 12KVA [same as] Secondary-Side: 50A @ 240V = 12,000VA or 12KVA.

The following information is required to size any transformer:

• Is the system Single or Three-Phase?
• What is the frequency (Hz)?
• What is the voltage (line to line) of the “supply power” (i.e., primary)?
• What voltage is necessary for the load equipment (i.e., secondary)?
• What is the total load (in amperes) of your connected equipment (on the secondary side)?

With the above information, the transformer KVA can be calculated as:

Single-Phase Calculations:

Example:

Three-Phase Calculations:

Example:

After calculating the KVA rating, choose a Standard size transformer that is no less than the KVA needed.

IMPORTANT!

The Volts and Amps above can be either the Primary values OR the Secondary values, but NOT both. The KVA is the same on the Primary side and the Secondary side. The “Volts” are the Line to Line, NOT the Line to Neutral.