What is a Frequency Converter?

A frequency converter is an electronic device that converts alternating current (AC) electricity from one frequency to another. The most common use of frequency converters is to allow equipment that requires a particular input frequency to be powered from a supply that operates at a different frequency.

The key components of a frequency converter are rectifiers, inverters, and an electronic control system. The process involves:

Rectifying the input AC to direct current (DC)

Inverting the DC back to AC at the desired output frequency

Controlling the output voltage and frequency

There are two main types of frequency converters: 

Rotary converters - These use electromechanical components like motors and generators to convert the frequency. They are bulky and less efficient but can handle high power applications.

Static converters - These use solid-state electronics with no moving parts. They are more compact, efficient, and suitable for lower power applications. Common static converter designs include matrix converters and PWM converters.

By converting the input frequency to a different output frequency, frequency converters allow equipment designed for one frequency to operate using a different supply frequency. This eliminates the need to replace existing motors, transformers or generators when power supply changes.

1. Why Use a Frequency Converter?

Frequency converters, also known as variable frequency drives (VFDs), offer several key benefits that make them a useful addition to many industrial and commercial applications. Here are some of the main reasons to use a frequency converter:

Allows speed control of AC motors - One of the primary functions of a frequency converter is to control the speed of AC induction motors. By adjusting the frequency of the power supplied to the motor, the speed can be increased or decreased smoothly over a wide range. This adjustable speed capability is useful for applications like conveyors, pumps, fans, and machine tools where variable speed is required.

Matches frequencies between power sources - Frequency converters can match different frequencies between two AC power sources. For example, they can connect a 50 Hz motor to a 60 Hz power supply, allowing flexible operation between mismatched sources. This makes it possible to run equipment designed for one frequency on a different grid frequency.

Reduces energy consumption - By enabling adjustable speed operation, frequency converters can save significant energy compared to running an AC motor at constant full speed. The converter adjusts voltage and frequency to meet the immediate demands of the load, rather than providing a continuous full voltage. This optimization reduces wasted energy and can lead to major efficiency improvements.

In summary, frequency converters bring the key benefits of motor speed control, frequency matching, and energy savings. Their flexibility and optimization capabilities make them an essential component for many industrial applications involving AC motors and power electronics. With the right converter, operations can be streamlined, efficiency improved, and costs reduced.

2. Frequency Converter Applications

Frequency converters are used in a wide range of industrial and commercial applications to control the speed and torque of electric motors. Some of the most common applications include:

Motor Speed Control

One of the primary uses of frequency converters is for precise speed control of three-phase AC induction motors. By varying the frequency of the power supply, the speed of the connected motor can be accurately controlled. This allows operations to match the motor speed to the specific requirements of the application. Frequency converters are used for motor speed control in conveyors, pumps, fans, compressors, mills, and more.

Connecting Power Systems

Frequency converters allow the connection of electric grids, generators, and motors that operate at different frequencies. For example, they can connect a 50 Hz grid to a 60 Hz motor. The converter creates a new output frequency that matches the required device. This prevents mismatches between power systems.

HVAC Systems

HVAC applications like fans, pumps and compressors often benefit from using frequency converters. By controlling the motor speed, airflow and cooling capacity can be finely regulated to match demand. This avoids energy waste from fixed speed operation. Frequency converters also allow soft starting of HVAC motors, reducing start-up surge currents.

Conveyors

Frequency converters enable variable speed control of conveyor motors. This allows the belt speed to be precisely matched to the flow rate of material. Conveyor speed can be automatically varied to prevent backups or keep product evenly spaced. Gradual speed changes enabled by converters also reduce shocks to the conveyor system. 

Pumps & Compressors

Pumps and compressors often utilize AC induction motors. Frequency conversion allows their motor speed to be adjusted for more efficient operation by matching speed to the flow or pressure requirements. Soft starting capability also minimizes surges to the connected piping or equipment. Overall, converters improve performance while reducing energy consumption.

3. How do Frequency Converters Work?

Frequency converters, also known as variable frequency drives (VFDs), work by converting the fixed frequency AC power from the electric utility to a variable frequency and variable voltage used to drive motors. There are three main stages in a basic VFD: 

Rectifier Stage

The rectifier converts the input AC power to DC power. It typically uses a full-wave diode bridge rectifier to convert the incoming AC into pulsating DC. The rectifier stage provides a steady DC voltage that can then be filtered and converted to the desired output waveform.

DC Link/Bus

After the rectifier stage, the DC link consists of capacitors that smooth out the pulsating DC. This creates a steady intermediate DC voltage bus that connects the rectifier and inverter stages. The DC bus voltage powers the inverter stage components.

Inverter Stage

The inverter takes the DC bus voltage and converts it to a variable frequency and voltage output waveform using power transistors like IGBTs or MOSFETs. Pulse Width Modulation (PWM) techniques are used to vary the width of the pulses applied to the inverter switching devices. This creates the desired AC output waveform to drive the motor at different speeds. 

The PWM output waveform is a chopped up sine wave rather than a clean sinusoid. But the inductance in the motor windings helps smooth out the pulses. Varying the PWM pulse width varies the frequency and voltage applied to the motor. This allows variable speed control of three phase AC motors with a VFD. 

Choosing the Right Frequency Converter

When selecting a frequency converter for your application, there are several key factors to consider:

Power Rating

The power rating, expressed in kW or HP, must be appropriately sized for your motor. Generally, the converter should have a power rating around 110-125% of the motor's rated power. Oversizing improves performance, while undersizing can damage the converter or motor. 

Consider both the input and output power. Input power depends on your supply voltage, while output power depends on the motor's voltage.

Keep in mind that the converter's output capacity diminishes as the output frequency increases above the motor's base frequency (usually 50-60 Hz). Consult power derating curves.

Input & Output Voltage

The input voltage range must match your power supply. Common input voltages are 480V, 600V and 690V for higher power applications.

The output voltage must match your motor's rated voltage. Standard options are 230V, 460V, 575V. Some converters support multiple output voltages.

Using a converter with an output..