Transformer Coils
The input coil of a transformer is called the primary, and the output coil is called the secondary.
Step-Down Transformer
A transformer that has a lower output (secondary) voltage than input (primary) voltage.
Step-Up Transformer
A transformer that has a higher output (secondary) voltage than input (primary) voltage.
Isolation Transformer
A transformer that has an equal output (secondary) and input (primary) voltages. It’s also called a 1-To-1 transformer.
Turns Ratio
The ratio of primary turns to secondary turns.
For example, the transformer below has 320 turns in its primary and 80 turns in its secondary coil. Using these values, the turns ratio would be:
Step-Down Transformer
Turns ratio is generally represented by the letter “a”.
a > 1 (NP>NS): the transformer is a step-down transformer
a < 1 (NP<NS): the transformer is a step-up transformer
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,
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
Step-Down Transformer
A transformer that has a lower output (secondary) voltage than input (primary) voltage.
Core Losses
Power losses in a transformer due to excitation of the magnetic core.
Copper Losses
A power that is dissipated when current passes through the primary and secondary coils of a transformer.
Efficiency
The transformer efficiency is the ratio of output power to input power, given as a per cent. The efficiency of a transformer is found as
PS: transformer secondary power
PP: transformer primary power
The transformer efficiency is the ratio of output power to input power, given as a per cent. The efficiency of a transformer is found as
We can make TWO IMPORTANT statements from the above formula:
- A transformer that steps the voltage down also steps the current up.
- A transformer that steps the voltage up also steps the current down.
Step-Down Transformer
Transformer Type
Step-Down
Step-Up
1-to-1
Turns Ratio
NP > NS
NP < NS
NP = NS
Voltage Relationship
EP > ES
EP < ES
EP = ES
Current Relationship
IP < IS
IP > IS
IP = IS
Power Relationship
PP = PS
PP = PS
PP = PS
Autotransformer
A transformer comprises a single coil that typically has three terminal connections. Because of its construction, it can be wired in either a step-up or a step-down configuration.
NEC Transformer Nameplate Requirements
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.
Y-Y Transformer Connection
When both primary and secondary windings are wired in Y-Y configuration, the transformer primary and secondary current and voltage relationships are as follows:
Where L represents line-to-line (or phase-to-phase), and P represents Phase-to-Neutral.
Δ-Δ Transformer Connection
When both primary and secondary windings are wired in Δ-Δ configuration, the transformer primary and secondary current and voltage relationships are as follows:
Where L represents line-to-line (or phase-to-phase), and P represents Phase-to-Neutral.
Distribution Transformer
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/7620 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 7620 V.
- The secondary is Y-connected with a line voltage of 208V and a phase voltage of 120V.
- This is a Step-Down transformer.
Transformer High and Low Voltage Leads Identification
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.
High and Low Voltage Windings Transformer KVA
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.
Sizing a Transformer
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.