Spindle wiring basics: what the VFD actually does
A variable frequency drive sits between your supply and your spindle and does two conversions in a row. Understanding those two conversions makes almost every wiring question answerable without looking anything up.
Rectifier, DC bus, inverter
Inside the drive:
- A rectifier turns the incoming AC into DC and charges a capacitor bank — the DC bus.
- An inverter chops that DC back into three AC outputs at whatever frequency the drive is commanded to.
Two consequences follow directly, and they explain most of what confuses people:
- The output voltage cannot exceed roughly the input voltage. A drive fed from 220 V produces about 220 V three-phase on its output, no matter what it is asked for. This is why a 380 V spindle cannot simply be plugged into a 220 V drive — see single-phase input.
- The DC bus is live whenever the supply is connected, even with the spindle stopped. Disconnecting the output does not make the inside of the drive safe.
Terminal names are not standard
| Function | Labels you will see | Notes |
|---|---|---|
| Supply, phase 1/2/3 | L1/L2/L3 · R/S/T · L/N | A single-phase drive uses two of the three |
| Motor output | U/V/W · T1/T2/T3 · A/B/C | Never connect supply here |
| Protective earth | PE · ⏚ · E · GND (chassis) | Not the same as the signal GND pin |
| Brake resistor | B1/B2 · PB/PC · (+)/BR · P/+ & BR | Polarity usually irrelevant; minimum Ω is not |
| Digital input common | COM · DCM · DGND | Defines whether inputs are active-high or active-low |
| Run / direction | FOR · FWD · REV · X1–X5 | Function is assigned to a terminal, not fixed |
| Analog speed reference | AI1 · AVI · AIN · VCI | Paired with an analog ground, not chassis earth |
| Reference supply | +10V · 10V · VREF | Feeds a potentiometer; do not use it as a general supply |
| Fault relay | FA/FB/FC · RA/RB/RC · TA/TB/TC | A dry contact, so it can switch either way round |
The same model number from the same maker has shipped with different labels across production runs. Read the silkscreen next to the terminals, not a diagram from a forum.
Phase order sets direction
Swap any two of U, V and W and the spindle turns the other way. That is the whole mechanism: the rotating field follows the order the three phases energise in.
Both routes are legitimate:
- Swap two output conductors — physical, permanent, works even if the drive is later replaced with a dumb contactor.
- Reverse via a control input — the drive swaps two phases internally. More flexible, and it survives moving the spindle between machines.
Star and delta on the spindle side
Many spindles wound for two voltages bring six leads out instead of three, and the link arrangement inside the terminal box decides which voltage they want:
- Delta (Δ) puts the windings across the line voltage — the low-voltage arrangement.
- Star (Y) puts them across line voltage divided by √3 — the high-voltage arrangement.
A nameplate reading 220 V Δ / 380 V Y means the same motor can run from a 220 V
three-phase output when linked in delta, or from 380 V when linked in star. That single line is
the difference between “my drive works” and “I need a different spindle”, and
it is worth opening the terminal box to check before you order a drive.
What never to do
- Never land the supply on U/V/W. The inverter stage is destroyed essentially instantaneously.
- Never switch the drive output with a contactor or a relay while running. Breaking current into an inductive load on the inverter side is what kills output transistors. Use a run command on the control terminals instead.
- Never insulation-test or megger the spindle while the drive is connected. The test voltage appears across the inverter.
- Never fit power-factor correction capacitors on the drive output. They form a resonant circuit with the motor inductance.
- Never lift the protective earth to “fix” noise. If noise is the problem, fix it properly.
A sane first power-up sequence
- Supply wired to the input terminals, correct voltage class, on a dedicated circuit you can isolate.
- Output wired to the spindle; earth to both chassis; screen clamped at the drive end.
- Spindle mechanically mounted, tool not yet fitted, cooling running.
- Motor parameters entered from the nameplate — see the list.
- Command a low speed first (10–20 Hz) with the spindle clear. Confirm direction. Stop.
- Raise speed in steps to the maximum, listening for bearing noise and watching the drive's output current readout.
- Only then fit a tool and cut.
Parts
Connectors and cable
Questions
What happens if I swap two phases?
The spindle runs backwards. Swap any two of the three phase conductors to reverse it — do it at the drive output or in the connector, never inside the spindle.
Can I put a contactor between the VFD and the spindle?
No. Switching the output of a running drive can destroy the output stage. Stop the drive first, or better, do not put a switch there at all.
Why do terminal labels differ between drives?
There is no standard for control terminal names. L1/L2/L3, R/S/T and U/V/W all appear, and the manual for your drive is the only authority — which is why the diagram page describes functions rather than labels.
Sources and method
Three kinds of statement appear on this page, and each is labelled where it occurs:
- Derived — conductor sizing follows from current and from your local wiring code — the page shows the reasoning, not a table of sizes.
- Rule of thumb — workshop practice, not a standard. Given with the range it is meant to hold over, and it loses to your manual wherever the two disagree.
- Maker-specific — parameter numbers, connector pin numbers, minimum brake resistance, maximum cable length, bearing speed limits. Not published here. They move between firmware revisions of the same model, and a wrong one is worse than none.
Standards and documents
- IEC 60204-1 — electrical equipment of machines
- IEC 60364-5-52 — low-voltage installations: selection and erection of wiring systems
- NEMA MG 1 — motors and generators (nameplate current, service factor)
- The manual for your own drive and spindle — the only valid source for maker-specific values