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　　What is a variable frequency drive?

　　For electricity, frequency is the number of cycles per second in an alternating current. A variable-frequency drive (VFD) changes the number of cycles per second (in hertz). In the figure below, the output frequency is six times the input frequency. This has nothing to do with voltage or current; only the frequency is altered. Conversely, a higher frequency can be converted into a lower frequency.

　　How does a variable frequency drive work?

　　Initially, frequency converters were mechanical. An engine or electric motor would be connected to a pulley or gear train to drive a generator at a speed faster or slower than that of the motor. Since frequency is directly proportional to the generator’s speed, it would increase or decrease in proportion to the pulley or gear ratio. This approach imposed limitations on speed, available ratios, and stability. Moreover, if the motor or power supply experienced fluctuations, the generator’s output frequency would likewise vary. The electrical efficiency of such systems—between the motor and the generator—was typically quite low.

　　Today, frequency conversion is accomplished electronically. Any type of power source can be converted into direct current (DC). DC has no frequency, much like a battery. This DC can then be pulsed in tiny segments to generate any desired frequency.

　　This solid-state frequency converter features high efficiency, is quartz‑crystal‑controlled, and operates independently of input power fluctuations.

　　It requires minimal maintenance and eliminates the need for lubrication or mechanical wear.

　　Applications of Solid-State Frequency Converters

　　Variable-frequency converter

　　Since motor speed is directly proportional to frequency, changing the frequency will alter the speed. This is highly beneficial in many applications, such as printing and conveyor systems. In air-handling systems, energy efficiency is of paramount importance. Reducing fan speed by half decreases the fan’s pressure output by a factor of four (according to the square law), while the power consumption drops by a factor of nine (according to the cube law). Lowering the fan speed—rather than closing dampers to reduce airflow—can result in substantial energy savings.

　　Fixed-frequency converter

　　Although variable speed drives have many applications, they typically require a fixed frequency that differs from the utility grid. In many countries, the grid operates at 60 Hz, while in others it runs at 50 Hz. Due to global trade, manufacturers producing for foreign markets must test their products at the target country’s grid frequency. Variable-frequency drives enable factories in 60-Hz regions to test equipment on 50-Hz power supplies, and vice versa. Renewable energy systems, such as solar or wind, rely on inverters to convert their output to the utility‑grid frequency so that consumers can utilize the generated electricity. Frequency is a critical factor in the design of electrical equipment. A key relationship is that higher frequencies allow for smaller iron cores in motors and transformers. Consequently, operating on a utility grid above 60 or 50 Hz can reduce equipment size and weight—particularly important for devices that are frequently transported or flown. Aircraft and military systems take advantage of this by being designed to operate at 400 Hz; the lighter weight of such equipment permits greater payload capacity for fuel and cargo. Converting utility‑grid power to 400 Hz is a cornerstone of both commercial and military aviation worldwide. Other military platforms, including missile systems and computers, also employ 400 Hz. Despite differing national standards for grid frequency, aviation‑specific frequency requirements remain uniform across all regions.