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Basics You Need to Know about the AC Frequency Inverter

The AC frequency inverter (VFD) is a motor drive, intended for electromechanical drive systems, that regulates the speed and torque of AC motors by varying the motor input frequency and, depending on the topology, controlling the associated voltage or current changes. An alternative common name for AC frequency inverter is VFD (variable frequency drive), which in addition can also be called “AFD” (adjustable frequency drive), “ASD” (adjustable speed drive), “VSD” (variable speed drive), “AC drive,” “AC motor drive,” “inverter,” “micro drive,” “inverter drive” or simply, “drive.”

As with any instrument or control device, the selection of the right motor control system requires care and knowledge. This article will provide you with a general understanding of AC drives.

AC Drive Control Technology

Based on the control method, AC frequency inverters fall into 4 types: variable frequency variable voltage (U/F) control, sensor-less vector control (SV), vector control (VC), and direct torque control.

-U/F control

U/F control deals with the control of the ratio of frequency to frequency. A fundamental feature is that it controls the voltage and frequency of the inverter output and obtains the required torque characteristics by keeping U/F constant. Such an approach to control is low-cost and used for less precision universal drives.

-Vector control

Vector control, otherwise known as field-oriented control (FOC), allows the speed and torque of rotation on the motor shaft to be independently adjusted in a similar but more complex way to DC control methods. The principal idea behind VC is to control more than just the magnitude and frequency of the supply voltage but also the phase. Put differently, the magnitude and angle of the space vector are controlled.

VC makes high-performance induction motors possible. Vector frequency inverters are not only comparable to DC motors in terms of the speed range, but they can also directly control the torque variation of motors and have found a wide range of applications in many areas where precise or fast control is required.

-Sensor-less vector control

Sensor-less vector control can be implemented as constant volt/Hertz or flux vector control. As a move to reduce capital investment in adjustable speed drives, SV eliminates sensors from the drive system. As sensor-less vector control offers comparable performance to drives with sensors, it is often applied in projects involving independent speed and torque control.

-Direct torque control

Direct torque control, similar to vector control, decouples torque and flux for independent control. DTC controls the instantaneous speed of a motor’s stator flux by controlling the instantaneous input voltage to the motor, changing the instantaneous slip of its rotor, thus achieving direct control of the motor output as DTC controls the motor torque directly without a modulator. Hence, the torque response is much faster.

Design Specifications

In the search for an AC motor drive, a broad range of design specifications need to be considered. Highlights of these include:
1) Continuous current: The current exerted on the motor during continuous operation
2) Supply voltage: The voltage supplied to the drive
3) Output voltage: Output voltage range of the drive
4) Input frequency: AC input frequency received by the equipment
5) Output frequency: Frequency accepted by the motor after conversion by the AC inverter
6) Power: The rated output power of the drive motor system
7) Operating temperature: Operating temperature of the power supply
Additional important features include the installation configuration and the included functions.

Mounting Configuration

The mounting configuration of the frequency inverter is important for compatibility with the motor system. Depending on the design, drives can be mounted in various ways, including on PC boards, panels, DIN rails, or racks.


An AC frequency inverter is an electronic device that controls an AC induction or synchronous motor. The VFD controls the motor’s torque, speed, and direction, starting and accelerating the motor smoothly to the required speed at a controlled acceleration rate. Deceleration is also controlled, and braking is available as an option. In brief, AC frequency inverter features involve:

  • Soft start
  • Motor speed control
  • Motor output torque control
  • Contracting brake control
  • Motor protection
  • Proportional-integral-derivative (PID) control, etc.

The bottom line

The AC drives are very discreet; nevertheless, they fulfill an important and necessary role in everyday life, making the world and our way of life more accessible. With our high-performance and quality frequency inverters, GTAKE brings convenience to people’s lives. Different series of GTAKE AC drives are equipped with different control technology separately, including all the four types of control methods referred to above, variable frequency variable voltage (U/F) control, sensor-less vector control (SV), vector control (VC), and direct torque control, vastly fulfilling the demand of different applications.

Should you have any questions about VFDs or drive components, please feel free to contact GTAKE, and our experts will draw up the optimum solution for you on a case-by-case basis.

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