A tap changer is a key device of transformers that allows for variable turn ratios to be selected in distinct steps. This is done by connecting to a number of access points known as taps along either the primary or secondary winding.

There are 2 primary types of tap changers such as no-load tap changers (NLTC) and on-load tap changers (OLTC). This article will show the concept of on- a load tap changer and a No-Load Tap changer.


  1. No-load tap changers
  2. On-load tap changers

1. No – load tap changers

No–load tap changers, called Off-circuit tap changers (OCTC) or De-energized tap changers (DETC), are frequently used in low power, low voltage transformers in which the tap point often may take the form of a transformer connection terminal, requiring the input line to be disconnected manually and connected to the new terminal. Furthermore, it is employed in situations in which a transformer's turn ratio does not require frequent changing and is allowed to switch off the transformer system.

Besides that, NLTC are also used in high voltage distribution-type transformers in which the system includes a no-load tap changer on the primary winding to accommodate transmission system variations within a narrow band around the nominal rating. In such systems, the tap changer will often be set just once, at the time of installation, although it may be changed later to accommodate a long-term change in the system voltage profile.

2. On-load tap changer

On-load tap changer (OLTC), also known as On-circuit tap changer (OCTC), is used in a place where there is a supply interruption due to unacceptable tap change, the transformer is often equipped with a more complex and expensive on-load tap changing mechanism.  The ratio of the number of turns can be changed without breaking the circuit.

These systems usually are fitted with 33 taps (one tap: center "Rated" tap and 16 taps: to increase and decrease the turn ratio) and allow for ±10% variation (each step providing 0.625% variation) from the nominal transformer rating which, in turn, allows for stepped voltage regulation of the output.


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