Products & Services

Ferroresonant Transformers

ferroresonant transformersFerroresonant transformers are a special type of laminated transformer which provides a regulated output. These are sometimes known simply as "ferros", or "CVTs" (constant voltage transformers). Using a special magnetic structure and a capacitor, ferroresonant transformers can supply a well regulated output voltage which remains constant despite changes in input voltage and load. In some specialized applications, these devices can be designed to supply a constant current output, rather than constant voltage. Current limiting is an inherent benefit of ferroresonant transformers. If an overload, or even a short circuit is applied, secondary current is limited to a safe level and primary current is actually lower than at normal full load. Another inherent benefit is that the ferro serves as a low-pass filter, supplying to load with power, but effectively removing transient and surge voltages. Energy stored in the transformer structure even allows the output to "ride through" brief input power interruptions.

For dc (rectified output) applications, ferros supply a quasi-square wave output which can reduce or even eliminate the need for filtering. For ac load applications, ferros can be designed to supply a high quality, low distortion sine wave output. Ferroresonant transformers are single phase devices, but combinations of two or three ferros can provide all of the ferros' advantages in three phase applications.

Ferros which rely upon only their rugged copper and steel structure for regulation can easily maintain a desired output voltage within 3% despite wide variation in input voltage and/or load. Another variant, known as the controlled ferro, adds electronic control and can provide an adjustable output with regulation to better than 1%.


  • ferroresonant transformerRegulated output voltage
  • Fixed output voltage (conventional ferros)
  • Adjustable output voltage (controlled ferros)
  • Inherent current limiting
  • Ride through of brief power outages
  • Excellent transient and surge suppression
  • Very high levels of isolation
  • Extremely robust, rugged, and reliable
  • Quasi-square wave output waveform reduces
    filtering requirements for d-c applications
  • Low distortion sine wave output available for a-c load applications

Typical Applications

  • Line conditioners to protect computers and other sensitive electronic devices from transients, noise, and surges on the power line
  • Power supplies
  • Battery chargers
  • Inverters
  • Pre-regulators for precise power supplies
  • Lighting

Ratings and Sizes

All of General’s products are custom designed for specific applications. Available ratings depend upon a number of factors including power requirements, maximum voltages, currents, and frequency. We can produce ferroresonant transformers with ratings from 100 va to 10 kva for 50/60 Hz operation. Please do not hesitate to inquire as to your requirements.

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