What Is a Frequency Converter?
A frequency converter is a device designed to convert an input frequency to an arbitrary output frequency.
Frequency, also referred to as the frequency of oscillation, represents the number of times an electric current or radio wave completes a full cycle per second. Electric currents can be either direct current (DC) or alternating current (AC). Typically, power sources in homes and factories supply AC at a fixed frequency.
However, there are situations where specific machines or equipment require a different frequency than what the power supply provides. In such cases, frequency converters are employed to transform the incoming current to the desired frequency.
Applications of Frequency Converters
Frequency converters find applications when there is a need to modify frequencies. For instance, this is relevant in home appliances and industrial settings.
Regions around the world may have different standard frequencies for their power supply. For example, Eastern Japan uses a 50 Hz frequency, while Western Japan uses 60 Hz. To ensure compatibility, frequency converters are installed to enable the use of electrical devices in regions with different frequency standards.
Furthermore, certain machine tools may require power supplies with lower or higher frequencies than the standard 50 Hz or 60 Hz, necessitating the use of frequency converters.
Principle of Frequency Converters
Frequency converters achieve their function by utilizing both inverters and converters.
An inverter is a device that transforms DC into AC, while a converter is a device that changes AC into DC.
1. Converter Circuit
The converter circuit initially converts the incoming AC into DC. Since AC is a sinusoidal wave with positive and negative values, the converter circuit uses diodes to perform full-wave rectification, ensuring that both positive values are extracted.
Next, the extracted current passes through a capacitor. The capacitor’s role is to smooth the current by repeatedly charging and discharging, resulting in a continuous and stable DC.
2. Inverter Circuit
Subsequently, the current proceeds through an inverter circuit, which generates an AC with the desired frequency. This is achieved through the use of transistors within the inverter circuit, allowing for the adjustment of the ON/OFF switching interval, which, in turn, varies the output frequency.
Pulse Width Modulation (PWM) is a common technique employed in inverter circuits. PWM controls the output by managing the ON/OFF ratio of switches, offering advantages such as reduced power consumption and compactness. In an inverter circuit, the direction of current flow through the load is reversed when the switch is toggled ON/OFF. This technology, known as switching technology, is essential for generating alternating current.
3. PWM (Pulse Width Modulation)
One of the PWM control methods involves triangular wave comparison. In this method, a sine wave of the desired frequency (AC) is compared with a high/low triangle-shaped wave known as a carrier, using an operational amplifier. When the sine wave exceeds the carrier, the switching control signal is set to ON. Conversely, when the sine wave falls below the carrier, it switches to OFF. This comparison is repeated to produce a switching control signal, resulting in a pulse wave output.
AC voltage is supplied by switching DC using semiconductor switching devices like thyristor.
Other Information on frequency converters
1. Thyristor
The thyristor, also utilized in frequency converters, consists of a diode with a gate terminal. The diode permits the flow of electricity in one direction only.
The gate terminal’s role is to control the opening and closing of the electrical circuit. When combined with a diode, it enables control over the direction and output of the current.
2. Thyristor valve
A device capable of withstanding high voltages and possessing the required electrical capacity is known as a valve or rectifier. It is created by connecting several thyristor elements in series or parallel.
While earlier thyristor valves used air-insulated wind-cooling or oil-insulated oil-cooling systems, modern systems often employ air-insulated water-cooled systems, which use large-capacity thyristor elements to efficiently handle high-power applications.