Reviewed by: Göran Stenmark – Product Manager at CG Drives & Automation
Introduction: Why knowing your VFD type matters
Choosing the right type of VFD is not just about running a motor. It’s about saving energy, controlling torque accurately, and making sure your system works reliably for the job. Different variable frequency drive types offer unique benefits in terms of energy savings, torque response, implementation complexity, and cost. This practical overview is designed to help you select the most suitable VFD drives for your applications.
Main categories
1. Voltage-source inverter (VSI) drives
VSI drives use a DC voltage input are the most common type of VFDs, widely used in general industrial and commercial applications.
Key advantages:
- Most common and cost-effective type
- Simple and cheap topology with high switching frequency
- Compact design, easy to implement
- Supports most general-purpose motor applications
Best fit for:
Fans, pumps, conveyors, and general machinery where cost and ease of installation are key.
2. Current-source inverter (CSI) drives
CSI drives are an older topology, but are still used for very special applications.
Best used when:
- Regenerative braking is needed
- In a harsh environment
- System requires high power and high inertia applications
Best fit for:
It can be found in large industrial drives, where old steel and cement plants require a simple design for high reliability.
3. Direct torque control (DTC) drives
DTC drives provide high-performance torque response without relying on traditional modulation techniques.
Pros:
- Extremely fast torque response
- Excellent low-speed and zero-speed performance
- Highest control accuracy
- Perfect for dynamic load changes
Best fit for:
Cranes, hoists, robotics, mills, test benches and machinery that experience frequent or abrupt torque changes.
Control strategies
Scalar (V/Hz) control
Scalar control maintains a constant ratio between voltage and frequency. It is simple and cost-effective but lacks precision under varying load conditions.
Best for:
Simple applications without strict performance requirements, such as fans and pumps.
Vector control (field-oriented control)
Vector control separates torque and flux components of the motor current, enabling precise control even under load variations. It supports both open-loop and closed-loop systems.
Best for:
Applications needing consistent performance under dynamic conditions, such as conveyors and extruders.
Direct torque control (DTC)
DTC adjusts torque in real time using hysteresis-based switching, allowing extremely fast response without the need for modulation or feedback devices.
Best for:
High-performance systems requiring fast torque changes and high accuracy.
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Types that are not VFD drives
Commonly confused drive types include:
- Mechanical gear reducers
- Hydraulic drive systems
- Eddy-current drives
Choosing the right type for your application
The most common AC drive types for different applications
People also ask – quick answers
What are the types of variable frequency drives?
VSI, CSI, and DTC are the main types, each with unique applications and control characteristics. From a power architecture view, VSI is the most commonly used type, while CSI is more rare.
What is the difference between VFD and VSD?
VFD is a specific type of VSD. A VSD, or variable speed drive, is a term for technology that controls how fast a motor runs. This includes both mechanical and hydraulic methods.
What are the control methods of a VFD?
The three major control methods are V/Hz, Vector control, and Direct Torque Control. All three can be used with a VSI but only V/Hz is used with a CSI.
What is the difference between an inverter and VFD?
An inverter is the part of a VFD that converts DC into a controlled output AC voltage. A VFD converter includes additional stages, such as creating the DC voltage needed from an AC source. Both require a control system for motor speed and torque.
What is IGBT in a VFD?
IGBT stands for Insulated Gate Bipolar Transistor. It is a high-efficiency electronic switch. IGBTs are used in modern VFDs to enable quick and efficient power control.
What are the modes of VFD?
Common modes include V/Hz control, vector control, and direct torque control. Each mode has different levels of complexity and performance.
What is the difference between VFD and VVVF?
They are often used interchangeably. VVVF (variable voltage variable frequency) is a legacy term referring to the same function as modern VFDs. Other terms for a VFD include AC drive, motor drive, and frequency converter.
Conclusion: Know your drive, control your process
Understanding the differences between VFD types allows you to select the right solution for your system. This choice directly affects energy efficiency, motor performance, and long-term reliability. Whether you’re managing a small HVAC system or a large industrial compressor, choosing the right VFD and knowing how to troubleshoot them can significantly improve performance and cost-efficiency.
Contact us for a consultation or system review