A variable-frequency drive (VFD or inverter) controls the speed of an induction motor by changing supply frequency and voltage. Used correctly, besides process control it cuts energy use on pumps and fans dramatically and eliminates starting current and mechanical shock.
Energy saving on variable-torque loads
For centrifugal pumps and fans, the affinity laws say flow is proportional to speed, head (pressure) to the square of speed and power to the cube of speed:
Example: reducing speed from 100 % to 80 % brings power down to about 0.8³ ≈ 51 %, a saving of nearly 49 %. By comparison, controlling flow with an outlet valve or damper saves almost no energy.
Drive selection criteria
Load type
| Load type | Example | Note |
|---|---|---|
| Variable torque (VT) | Centrifugal pump and fan | High saving; 110 % overload capacity is enough |
| Constant torque (CT) | Conveyor, reciprocating compressor, mixer | Needs 150 % overload and high starting torque |
| Constant power | Winch, machine-tool spindle | Field-weakening range |
| High-inertia load | Centrifuge, large fan | Acceleration time and braking resistor |
Electrical specification
- Rated current: select the drive from motor rated current, not kW alone.
- Overload capacity: choose normal duty (ND) or heavy duty (HD) to suit the load.
- Supply voltage and frequency and permitted voltage drop.
- Altitude and temperature: above 1000 m and above 40 °C ambient the drive must be derated.
- Ingress protection (IP) to suit the environment: IP20 in a panel, IP54/IP55 for field mounting.
Harmonics and power quality
An unfiltered six-pulse drive can cause 30 % to 40 % current THD. Remedies: AC line reactor or DC choke, 12- or 18-pulse drives, passive/active filters, or an Active Front End (AFE) drive. The goal is IEEE 519 compliance at the point of common coupling (PCC).
Motor cable and output voltage effects
- Shielded cable with proper earthing of both shield ends
- For long cables (over about 50 m depending on the model), an output reactor, dV/dt filter or sine filter
- Inverter-duty motors and, where needed, insulated bearings or shaft grounding rings to prevent bearing currents
Braking and safety
- Braking resistor for fast deceleration
- STO (Safe Torque Off) for safety applications
- Bypass contactor for critical processes
Drive or soft starter?
| Feature | Drive (VFD) | Soft starter |
|---|---|---|
| Speed control | Yes | No |
| Starting-current reduction | Very good | Good |
| Energy saving | High (variable load) | None |
| Cost | Higher | Lower |
| Harmonics | Yes (filter needed) | Low |
Commissioning steps
- Installation review: earthing, ventilation, power and control wiring, shielding, lugs and bolt torque.
- Enter motor nameplate parameters: power, voltage, current, frequency, speed and power factor.
- Run autotune (static or rotating) to identify motor parameters.
- Set acceleration/deceleration ramps and speed, current and torque limits.
- Set the speed reference (analogue, fieldbus or HMI) and digital I/O.
- Motor protection: thermal overload, PTC/PT100 and phase-loss functions.
- PLC communication via Modbus RTU/TCP, Profibus, Profinet or EtherNet/IP.
- No-load test, on-load test and tests of protective functions and emergency stop.
- Save parameters and prepare documentation.
Common troubleshooting
- Overcurrent fault during acceleration: lengthen the ramp or check boost settings
- DC overvoltage during deceleration: lengthen the ramp or add a braking resistor
- Earth fault: check cable and motor insulation
- Drive overheating: check the fan, air filter and panel temperature
ENP supports industry in selecting, supplying, commissioning and training on drives and soft starters; a drive training course is also available in the courses section.