A 4 kW spindle replacing a 3.5 kW unit raises shaft-power demand by 0.5 kW, or about 14.3%. That change can push an existing VFD beyond its output-current or thermal limit even when the motor turns normally without load. The number that matters is the new motor nameplate current compared with the VFD continuous output-current rating; kilowatts alone do not settle compatibility.
Failed substitution fixes
Several quick fixes can produce rotation without establishing a safe installation. This is heat, not logic: an incorrect link pattern, frequency setting, or current limit can overheat the windings before the mechanical symptom becomes obvious.
| Attempted fix | Why it fails | Required check |
|---|---|---|
| Reuse the old 3.5 kW VFD settings | The replacement is rated 4 kW and may have different voltage, current, frequency, speed, and acceleration requirements. | Read every available motor rating and compare it with the VFD output specification. |
Connect the spindle to R and S because those terminals were used previously |
On common VFD layouts, R/S/T are supply-input terminals, while U/V/W are motor-output terminals. Labels vary, so function must come from the drive diagram. |
Identify terminals from the VFD label or manual before applying power. |
| Copy the visible links without resolving the voltage marking | The enclosure information reportedly shows the same pairs for 220 V and 330 V, while the observed alternative was interpreted as 380 V star wiring. Those markings do not form a dependable selection instruction. | Obtain the motor connection diagram or winding data. |
| Run the spindle because it rotates | Rotation proves phase sequence and torque production, not correct volts-per-hertz ratio or acceptable current. | Measure output current during a controlled unloaded test and under normal load. |
Electrical load behind the symptom
A VFD synthesizes a three-phase motor supply. Motor heating follows winding current and cooling conditions, while magnetic flux depends primarily on voltage relative to frequency. Excess current, an incorrect connection, or excessive low-speed load can therefore damage a spindle even when the drive does not trip immediately.
For a three-phase motor, shaft power does not equal electrical apparent power. A current estimate requires efficiency and power factor:
I = Pshaft / (sqrt(3) × VLL × efficiency × power factor)
The spindle data supplied here contain no rated current, efficiency, power factor, or operating frequency, so an exact current must not be calculated from 4 kW alone. Read rated current from the motor nameplate or obtain it from the supplier.
| Quantity | Acceptance limit | Where to read it |
|---|---|---|
| Motor voltage | Must match the selected winding connection and commanded VFD voltage | Motor plate and terminal-lid diagram |
| Motor current | Must not exceed the VFD continuous output-current rating | Motor plate and VFD rating label |
| Motor frequency | Must match the motor base-frequency setting | Motor plate and VFD motor-data parameters |
| Motor power | 4 kW replacement versus 3.5 kW previous spindle | Motor plate and VFD selection data |
| Protective earth | Continuous from spindle frame to protective-earth system | Terminal enclosure, cable, and continuity measurement |
Terminal identification and link ambiguity
Three line terminals plus an earth terminal indicate the external connections expected for a three-phase motor: three VFD output conductors and protective earth. If the terminal housing actually contains six winding studs, each winding has two ends identified as U1/U2, V1/V2, and W1/W2. Movable plates then establish star or delta connection.
The reported 220 V pattern pairs W2-U1, U2-V1, and V2-W1. That is a delta link arrangement. With those exact markings, connect the three VFD output phases to the three linked junctions, commonly at U1, V1, and W1. The alternative pattern that joins W2, V2, and U2 together is star.
The unresolved issue is the lid showing 330 V while the interpreted star diagram indicates 380 V. Treat that discrepancy as a stop condition, not a typographical correction. A photograph, winding-resistance test, or assumption about ordinary motors cannot determine the intended rated voltage; obtain the connection diagram tied to this spindle.
Controlled wiring and setup procedure
- Isolate the VFD supply. Wait until its discharge indicator and documented DC-bus check show a safe state before touching input, output, or motor terminals.
- Record the VFD model, input rating, output voltage, continuous output current, power rating, and supported frequency range from its label or manual.
- Record the spindle voltage, current, frequency, speed, and connection diagram. If those ratings are absent, pause commissioning and request them from the supplier.
- Identify the drive input and output by function. On a drive using the common convention, supply enters
R/S/Tand the motor connects toU/V/W; verify the actual drive diagram rather than relying on the convention. - Confirm all six spindle stud markings before moving links. Use the 220 V delta pattern
W2-U1,U2-V1, andV2-W1only when the motor documentation specifies delta for the VFD output voltage. - Install four-core motor cable: three output phases plus protective earth. Terminate earth at the spindle earth stud, never as one of the three phase conductors.
- Enter the new motor data in the VFD. Replace the previous 3.5 kW motor values with the documented 4 kW spindle voltage, current, frequency, and speed data; review acceleration, deceleration, current limit, and motor-protection settings.
- Disconnect the VFD before any insulation-resistance test. Test the cable and motor using the motor maker's specified method and test voltage, then reconnect after the stored charge has dissipated.
Commissioning verification
Start with the spindle mechanically unloaded and command a low speed permitted by the spindle documentation. Observe direction, sound, vibration, VFD output current, and fault history. Stop immediately for abnormal bearing noise, rapid heating, erratic current, or a drive trip.
If rotation is reversed, isolate power and interchange any two of the three motor-output phases. Never change the protective-earth conductor. Repeat the unloaded test, increase speed within the documented range, and compare phase currents where suitable measurement equipment is available; a large imbalance directs attention to a loose terminal, cable defect, winding fault, or incorrect link.
Complete verification at normal operating load. Confirm that measured current remains within both the spindle rated current and the VFD continuous output-current rating, and check that the drive does not reach current limit during acceleration or cutting. Review motor temperature after a representative run using the spindle's specified measurement point or protection device.
Recurring failure modes and stop conditions
A drive can have a 4 kW label yet remain unsuitable because its output current, voltage, or frequency range does not match the spindle. Derating conditions and cooling requirements also come from the VFD documentation. A spindle intended for operation above ordinary supply frequency requires the correct base-frequency and speed relationship; entering an assumed value can produce excessive flux and current.
Loose link plates create localized resistance and heat. Missing protective earth leaves the spindle frame without a dependable fault-current path. Reusing unidentified conductors from the failed spindle can also preserve a wiring error, so verify each conductor end to end rather than relying on color alone.
FAQ
How do I identify the motor output terminals on my VFD?
Read the terminal diagram for the exact drive. U/V/W commonly identify motor output and R/S/T commonly identify supply input, but the drive documentation controls.
How do I wire the 220 V delta links on this spindle?
If the spindle documentation confirms 220 V delta operation, link W2-U1, U2-V1, and V2-W1. Connect one VFD output phase to each linked junction and connect protective earth to the separate earth stud.
How do I know whether the old VFD can run the 4 kW spindle?
Compare the new spindle's rated voltage, current, frequency, and power with the VFD output ratings. The decisive comparison is motor rated current against the drive's continuous output-current capacity under the installed conditions.
How do I reverse spindle rotation?
After isolating power and verifying the DC bus is discharged, exchange any two VFD output phases. Leave protective earth unchanged.
When do I stop wiring and contact official support?
Stop if the spindle has no readable rated current or frequency, if the 330 V and 380 V markings cannot be reconciled, or if the VFD terminals cannot be identified from its diagram. Contact the spindle supplier or the VFD manufacturer's official support channel with clear terminal photographs, both rating labels, and the exact link arrangement before applying power.