Why can't power transformers change frequency?

Power transformers are indispensable for adjusting voltage to align with industrial production and daily life needs. A common question many users ask is: Can power transformers also modify electrical frequency? The straightforward answer is no—and below, we’ll break down why, while clarifying how frequency adjustment is actually achieved.

How Power Transformers Work (And Why Frequency Remains Unchanged)

Transformers operate based on the principle of electromagnetic induction. For induction to generate voltage, magnetic flux must change within a closed loop. In a transformer, this magnetic flux is transmitted through a closed iron core, so the rate of flux change—which directly corresponds to electrical frequency—remains identical on both the primary and secondary sides.
 
Voltage transformation happens because the primary and secondary windings have different numbers of turns. Since the magnetic flux change rate (frequency) is consistent across the core, the frequency doesn’t shift. Only the voltage is adjusted proportionally to the turns ratio—this is the core of a transformer’s function, and it doesn’t involve frequency modification.

What Is a Frequency Converter, and How Does It Adjust Frequency?

Frequency adjustment requires a specialized device called a frequency converter (or inverter)—a piece of equipment with a completely different structure and operating principle from transformers:
 
  1. Rectification Stage: First, an uncontrollable rectifier circuit converts alternating current (AC) into direct current (DC), which is then stored in a capacitor for stabilization.
  2. Inversion Stage: Power electronic devices switch rapidly to invert the stabilized DC back into AC. This switching process is what actively modifies the frequency.
 
While frequency converters are technically classified as switching power supplies, their output voltage waveform isn’t a perfect sine wave—instead, it’s a Pulse-Width Modulated (PWM) wave. However, the resulting current waveform closely approximates a sine wave, thanks to advanced control technologies like Space Vector Pulse-Width Modulation (SVPWM), which is widely used in modern converters.
 
Notably, frequency converters also adjust voltage when changing frequency—this is critical to keeping the magnetic flux in connected equipment (like motors) constant. A typical control strategy is:
 
  • For frequencies below 50Hz: Voltage is reduced proportionally to maintain constant torque.
  • For frequencies above 50Hz: Voltage is held at the motor’s rated value (no further increase) to achieve constant power control.

Key Differences Between Transformers and Frequency Converters

The fundamental reason transformers can’t change frequency lies in their design:
 
  • Transformers: Depend on a closed iron core to transmit magnetic flux, ensuring the flux change rate (frequency) stays consistent. Their only function is voltage transformation, via differing winding turns.
  • Frequency Converters: Use rectification, filtering, and inversion to break the fixed flux change rate. Their power electronic switching mechanism enables active frequency adjustment, paired with voltage regulation to protect equipment.

Conclusion

Transformers and frequency converters serve distinct, non-overlapping roles: Transformers adjust voltage without altering frequency, while frequency converters are specialized for frequency (and accompanying voltage) modification—ideal for applications like motor speed control. Understanding this difference helps select the right equipment for specific electrical needs, ensuring efficient and reliable operation.
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