Variable Frequency Drives (VFDs), also known as inverters or variable speed drives, are one of the most important components in industrial automation. They control the speed and torque of AC motors by varying the frequency and voltage of the power supply. In the UK, where energy costs continue to rise, VFDs offer significant energy savings and process improvement opportunities.
How Does a VFD Work?
A VFD converts incoming fixed-frequency AC power through three stages:
- Rectifier stage: Converts AC to DC using a diode bridge
- DC bus: Smooths the DC voltage using capacitors
- Inverter stage: Converts DC back to variable-frequency AC using IGBT transistors with Pulse Width Modulation (PWM)
By controlling the output frequency and voltage, the VFD can precisely control motor speed from zero to above rated speed.
Benefits of Using VFDs
Energy savings are the primary driver for VFD adoption. In pump and fan applications, reducing motor speed by just 20% can save approximately 50% of energy consumption due to the affinity laws (power is proportional to the cube of speed).
Additional benefits include:
- Soft starting and stopping reduces mechanical stress on belts, gears, and couplings
- Process control enables precise speed regulation for quality improvement
- Reduced maintenance by eliminating mechanical speed control devices such as throttle valves and dampers
- Power factor improvement reduces reactive power demand from the utility
- Built-in motor protection with overload, overcurrent, and phase-loss detection
Common VFD Applications
- HVAC systems: Fan and pump speed control for building automation
- Conveyor systems: Variable speed for material handling and packaging lines
- Compressors: Energy-efficient compressed air generation
- Mixers and agitators: Precise speed control for batch processing
- Centrifuges: Controlled acceleration and deceleration profiles
- Winding and unwinding: Constant tension control for web handling
VFD Configuration Essentials
When commissioning a VFD, these parameters must be correctly set:
- Motor nameplate data: Rated voltage, current, frequency, speed, and power
- Acceleration and deceleration ramps: Time in seconds for speed changes
- Speed limits: Minimum and maximum frequency settings
- Control mode: V/f (voltage/frequency), sensorless vector, or closed-loop vector
- I/O configuration: Analogue speed reference, digital start/stop, and relay outputs
Integration with PLCs
Modern VFDs communicate with PLCs via industrial networks such as PROFINET, EtherNet/IP, or Modbus TCP. This enables the PLC to command speed setpoints, monitor drive status, read actual speed and current values, and handle fault diagnostics, all without hardwiring.
Training at EDWartens UK
Our motor drives and VFD training at EDWartens UK includes hands-on configuration of Siemens SINAMICS drives with real motors. Students learn parameterisation, PLC integration via PROFINET, and troubleshooting techniques that are directly applicable to the workplace.