What is a variable frequency drive?
A variable frequency drive (also called frequency converter, inverter or VFD) is a power-electronic device that controls the speed of an induction motor by changing the frequency and voltage of the current that feeds it. Since the speed of an induction motor is proportional to the line frequency (n = 60·f/p, where f is the frequency and p the number of pole pairs), changing the frequency changes the motor's speed.
Before drives, controlling motor speed required complex mechanical solutions (pulleys, mechanical variators) or inefficient electrical ones (resistors, voltage variation). The variable frequency drive does this electronically, with high efficiency and full precision.
How does a variable frequency drive work?
Internally, the drive performs three conversion stages:
- Rectification: mains AC current (50 Hz in Europe) is converted to DC by a diode rectifier bridge. This stage creates the "DC bus".
- Filtering: the DC current is filtered with capacitors to obtain a DC voltage that is as stable as possible.
- Inversion (PWM): using IGBT transistors controlled by pulse-width modulation (PWM), AC current is regenerated with the desired frequency and voltage. The output frequency can be set from practically 0 Hz up to 400 Hz or more, with 0.01 Hz resolution.
What is a variable frequency drive used for?
1. Speed control
The most obvious application: precisely controlling motor speed. A pump, fan, conveyor or compressor that previously ran at fixed speed can now run at exactly the speed the application needs at any given moment.
2. Energy savings
This is the most important benefit in many applications. Pumps and fans follow the fluid affinity laws: if you reduce speed to 80%, flow drops to 80% but power consumption drops to 51% (80³). In practice, a properly tuned drive can cut the electrical consumption of a pumping or ventilation system by 20% to 60%.
3. Soft starts
Direct motor starting produces an inrush current of 5 to 8 times the rated current, causing mechanical stress on the coupling, pump and pipework, and voltage spikes on the electrical network. The drive starts the motor by smoothly accelerating from 0 Hz, eliminating these spikes and extending equipment life.
4. Process control
Combined with a PLC and process sensors (pressure, flow, temperature), the drive forms part of a control loop that automatically regulates the process: a pump that holds constant pressure in a circuit, a fan that regulates the temperature of an oven, or a compressor that maintains compressed-air system pressure.
Types of variable frequency drives
- Scalar drives (V/f): the simplest and cheapest. They keep the voltage-to-frequency ratio constant. Suitable for pumps, fans and applications where torque precision at low speed is not required.
- Sensorless vector drives (SVC): they control the motor's flux vector through a mathematical model, with no need for a speed encoder. Better torque precision than scalar. For conveyors, compressors and applications with variable loads.
- Closed-loop vector drives (FOC/DTC): with motor encoder for speed feedback. The most precise. For positioning, cranes, extruders and applications where torque at low speed is critical.
- Servo drives: technically high-performance vector drives for servomotors with very high-resolution encoders. Used in Motion Control applications: machining centres, robots, high-speed packaging lines.
Main variable frequency drive manufacturers
- Siemens SINAMICS: G120 range (standard applications) and S120 (servo and Motion Control). Seamless integration with Siemens PLCs and TIA Portal.
- ABB ACS: ACS310 (pumps and fans), ACS580 (general purpose) and ACS880 (high-end). Recognised for reliability in process applications.
- Schneider Electric Altivar: ATV320 (machinery) and ATV630/930 (process). Good integration with Schneider Modicon PLCs.
- Danfoss VLT: particularly strong in pumps, HVAC and marine applications. Recognised for ruggedness in demanding environments.
- Yaskawa: A1000 and GA700 ranges. Very popular in machinery for their value-for-money ratio.
When does it make sense to install a variable frequency drive?
- Pumps and fans that don't always work at 100% load (typical payback: 6-18 months).
- Motors that start and stop frequently (eliminating current spikes prolongs motor life and reduces grid impact).
- Applications where process speed needs to be adjusted (production lines, extruders, agitators).
- When you want to implement process control (PID loop for pressure, flow, temperature or speed).
- To meet energy efficiency requirements or ISO 50001 certification.
At Bluemation we integrate drives from the leading brands into industrial automation projects: drive programming, communication setup with the PLC (Profinet, EtherNet/IP, Modbus) and control-loop tuning. Talk to our team.