   HOME | Site map | Contact us

Transformer Fundamentals
To order a transformer you will need to know -
• Input voltage, single or three phase
• Output voltage
• Output current (Amps) or power required (VA or KVA)
ie. 240V / 110V 2Amps
The above basically means 240V input, 110V output at 2 amps.

• Transformer enclosure
• Output socket
• Fuses (Input or output)
• Handles

Calculating transformer rating (VA or KVA)

To calculate the rating of a single phase transformer -

VA = Output Volts x Amps
ie. 240V / 110V 2Amps = 220VA

Sometimes VA is shown as KVA, basically 1000 VA = 1 KVA

To calculate the rating of a three phase transformer -

VA = (Output Volts x Amps) x 1.732
ie. 415V / 200V @ 14A = 4850VA or 4.85KVA

Is Watts (W) the same as VA ?

Not exactly, if your output / load is inductive (ie. a motor) it needs to be converted to VA.

VA = Watts (W) / 0.85
ie. 240V / 110V 400W = 470VA

If the load is a large electric motor, then further upsizing of the VA may be required.

Basic principals

A simple transformer consists of two electrical conductors called the primary winding and secondary winding, and a steel core that magnetically links them together

These two windings can be considered as a pair of mutually coupled coils. Energy is coupled between the windings by the magnetic field that links both primary and secondary windings. Classifications

Transformers are adapted to numerous engineering applications and may be classified in many ways:

• By power level (from fraction of a VA to over a thousand MVA),
• By application (power supply, impedance matching, circuit isolation),
• By frequency range (power, audio, RF)
• By voltage class (a few volts to about 750 kilovolts)
• By cooling type (air cooled, oil filled, fan cooled, water cooled, etc.)
• By purpose (rectifier, arc furnace, amplifier output, etc.)
• By ratio of the number of turns in the coils -
• Step up
• Step down
• Isolating
• Variable

Circuit symbols Two windings and an iron core, step-up or step-down as windings are different ratios. Transformer with two windings and an iron core. Transformer with three windings, two secondary windings. Transformer with an earth screen.

Three phase circuit symbols

The three primary windings are connected together and the three secondary windings are connected together. This is also sometimes referred to as a polyphase transformer. The most common connections are Y-A, A-Y, A-A and Y-Y.
There are many possible configurations that may involve more or fewer than six windings and various tap connections.