High voltage two-transformers frequency converters for asynchronous motors

General characteristics

The frequency converter (FC) converts an alternating current with a frequency of 50 Hz, a voltage of 6kV or 10 kV into a three-phase voltage variable in value and frequency.

Frequency conversion take place under low voltage conditions, the conversion scheme is displayed in next figure.

Functional diagram of two-transformers frequency converter

A frequency converter with a low-voltage conversion link is designed for control of asynchronous electric motors with power range from 250 to 1250 kW and a voltage of 6kV and 10 kV.
Depending on the converter’s power, the low-voltage link can be made to a voltage of 0.4 kV or 0.69 kV.

Advantages of FC made according to a two-transformers circuit:

  • simple maintenance of the low-voltage converter;
  • sinusoidal voltage at the FC output;
  • lower overall dimensions as compared to a high-voltage converter;
  • lower cost as compared to a high-voltage converter and, as a result, shorter payback period.


External appearance of frequency converter with rated current of 1260A

The frequency converter ensures:

  • Motor start and speed control;
  • Reverse;
  • Acceleration, deceleration, shutdown;
  • Motor and converter protection;
  • Dynamic braking during shutdown;
  • “Pick up” in motion;
  • Technological parameter PI-control;
  • Preset speed parameters;
  • Auto-reclosing;
  • Linear or S-shaped velocity controller.

The converter consists of

  • step-down transformer;
  • low-voltage frequency converter of PC5 series comprising of:
    • circuit breaker;
    • converter power unit;
    • control system;
  • sinus filter;
  • step-up transformer.

Protection system

  • short-circuit currents protection;
  • prohibited current overload protection;
  • dump or unacceptable decrease in the supply input voltage;
  • unacceptable increase of the supply input voltage;
  • overtemperature of semiconductor power devices;
  • phase failure of the input and output voltages;
  • short circuits to ground protection;
  • engine overheating protection.


  • support of network interfaces such as Profibus DP, Modbus, Ethernet, CAN;
  • flexible structure of the control and regulation system, open for monitoring and control from Automated Process Control System via network interfaces.

Main characteristics of the converter conform with characteristics of low voltage frequency converters of PC5 series.

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