VFD / Variable Frequency Drive
Drive
A variable frequency drive rectifies mains to DC then synthesises variable-frequency AC, giving a three-phase motor smooth speed control, gentle starts, and big energy savings on fans and pumps.
What it looks like
VFD / Variable Frequency Drive
A typical drive — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Power / current rating
kW / AMatch the motor's kW and full-load current — current is what really matters.
Input/output voltage
V230 V 1-ph in → 230 V 3-ph out is the small-shop staple.
Output frequency range
HzTypically 0–400 H z; motor cooling limits low-speed running.
Control I/O
Analog speed input, digital run/direction, relay outputs, Modbus.
Markings
Rating plate with input/output specs; terminals L/N (or R/S/T) in, U/V/W to motor, control strip labelled per manual.
Standard values
0.75–2.2 kW single-to-three-phase drives cover most workshop machines.
How to choose
Size by motor current with margin, confirm input phase availability, check low-speed torque needs (vector > V/f), and budget for a brake resistor on loads that overhaul (saws, hoists).
Pinout & package
Book-style heatsinked module. Motor cable short and screened; NEVER put a contactor between VFD and motor while running.
Example circuits
- Lathe spindle speed control from a 1-ph supply
- Pump with PID pressure control via the analog input
- Conveyor with preset speeds on digital inputs
Common failures
Blown input rectifier/DC bus caps from surges and age, IGBT failure from output shorts, overvoltage trips on decelerating high-inertia loads (needs brake resistor), cooked drives from clogged fans.
How to test
Check DC bus voltage (~1.41 × input) at the test points power-off-and-discharged FIRST — bus caps hold lethal charge for minutes. Fault codes in the manual diagnose most issues; diode-test the input bridge with power removed.
Substitutes
Any VFD of matching voltage/current with the control features you use; for single-speed needs, a plain starter is simpler and cheaper.