Dry Type Transformers for Motor Starting

Dry Type Transformers

TMC produces dry type motor starting auto-transformers to suit all motor capacities.

Motor Starting Transformer Characteristics

Motor starting transformers are auto transformers with tappings in the range of 50% to 70% of rated voltage (customer specified). The motor to be started is switched through the auto transformer in such a way as to start the motor at a lower voltage to minimise starting load and the associated supply voltage drop. This method is often referred to as the “Korndorfer Starter” method.


The transformers can be supplied in enclosures to various degrees of protection, or open (un-enclosed) if preferred. Enclosure ratings of IP21 to IP33 are common and protection up to IP66 is also available for extreme conditions and flameproof applications. Enclosures are fitted with sealed marshalling boxes for temperature and performance monitoring equipment. Enclosures can be fitted with interlocked doors and limit switches to provide additional safety.

Fan Cooling

TMC dry type transformers can be supplied with fans to increase the rated capacity (kVA). Forced directed air circulation permits a continuous power increase of up to 35% above the rated capacity. Fans can be used to economically provide extra capacity for overload conditions such as cyclic load peaks and traction loads.

Information Required with Enquiry:

  • Voltages (at no-load)
  • Voltage tapping range.
  • Number of phases (normally 3)
  • Operating frequency
  • Motor rating
  • Motor locked-rotor current
  • Motor start duration (seconds)
  • Number of starts per hour
  • Ambient conditions
  • Design standard
  • Enclosure protection rating
  • Temperature sensing equipment
  • Current sensing equipment (CT’s).