Electrical Transformer
Tap Changers

The purpose of a tap changer is to regulate the output voltage of a transformer by altering the number of turns in one winding and thereby changing the turns ratio of the transformer. There are two types of tap changers: a de-energized tap changer (DETC) and an on-load tap changer (LTC). Waukesha® transformer designs can accommodate both/either types of tap changers to meet customer specifications.

On Load Tap Changer
On Load Tap Changer


Prolec GE Waukesha’s de-energized tap changers are designed to last with no maintenance under normal operating conditions. The tap changer, a manually operated device, includes an external operating handle with a clearly marked position-indicating dial. Provisions are made for padlocking in each tap position and for the addition of key interlocks.

A Geneva drive is typically used to move the contacts to the correct position and also lock them in place for operation. Other benefits of the modular DETC include:

  • No coking on contacts
  • Allows for reduction in tank size
  • Ease of operation due to simple design and few moving parts
  • No need to exercise switch
  • Easy contact replacement
  • Moveable copper contacts fitted with multi-contact bands
  • Location flexibility within transformer
  • Ample ampacity and BIL
  • Meets IEEE PC57.131 and IEC 60214-1 test requirements


​Waukesha® transformers can be designed with either arcing-in-oil or vacuum tap changers, as specified by the customer.


Prolec GE Waukesha’s UZD® load tap changer is a field-proven design. When the minimally required maintenance is performed, UZD has a reliability rate of 99.95% for units built since 1997.

Designed for high speed, reliable operation and reduced maintenance, the UZD offers important features with benefits:

BUE Motor Drive Mechanism

Charges the spring battery operating the load tap changer; housed separately in an air-filled compartment. Located at a convenient height and not in the oil-filled switching compartment. Readily accessible at ground level for inspections and maintenance.

Spring Drive Mechanism

Tap changer can be operated manually while energized and carrying load. Stored energy in the spring mechanism results in high speed operation and minimum arcing time. Each tap change is consistent and unaffected by voltage interruptions or manual hand cranking speed.

Power-Class Series Transformer/Booster

Series transformers reduce current levels passing through the contacts, thereby limiting arcing intensity and duration and extending LTC life. Power-class design means the series transformer used by Prolec GE Waukesha employs circular core/coil construction with disc or helical type windings.

Single Arm Design

Single arm simultaneously operates both the tap selector and arcing contacts for a simple basic design with few parts that helps reduce operating and maintenance costs.

Variety of Other Features:

  • Designed to withstand full vacuum in tank without bypass piping to equalize pressure
  • Large opening to tap changer compartment helps make inspections quick and easy
  • Design utilizes just three openings into the main transformer tank protected with O-ring gaskets for maximum protection from leaks—fewer openings = less maintenance
  • Compact design requires only 110 gallons of oil to drain and replace
  • Full-featured control compartment equipped to meet customer requirements
  • Standard regenerating breathers maintain dry oil in a free-breathing tap changer
  • Meets all requirements of IEEE standard for Load Tap Changers (IEEE C57.131-1995)