The 2.2 kW spindle was commanded to 400 Hz while the VFD displayed about 11,000 to 11,520 rpm. The number that matters first is the commanded electrical frequency, followed by measured shaft speed and output current. A displayed rpm value is often a calculated engineering-unit conversion; changing that conversion can alter the display without changing the waveform delivered to the motor.
In this installation, changing Parameter 144 from 1440 to 3000 changed the indication from 11,520 rpm to 24,000 rpm at the same 400 Hz command. The spindle sound did not change. That result identifies a display-scaling change, not proof of acceleration.
Frequency, current, and thermal limits
Frequency sets the rotating magnetic-field speed. Current produces torque and winding heat. Operating time determines how much of that heat accumulates. A speed investigation must therefore separate four quantities: commanded frequency, actual output frequency, measured shaft speed, and motor current.
For a motor whose speed follows induction-motor relationships, synchronous speed is:
Nsync = 120 × f / P
where Nsync is synchronous speed in rpm, f is electrical frequency in hertz, and P is the motor pole count. If the spindle is a two-pole motor, 400 Hz corresponds to 24,000 rpm synchronous speed:
Nsync = 120 × 400 / 2 = 24,000 rpm
An induction rotor runs below synchronous speed while producing torque because it needs slip. The exact loaded shaft speed must therefore come from a tachometer rather than the ideal equation. Read the motor nameplate or manufacturer data to establish the motor type, rated frequency, rated speed, voltage, and current before treating 400 Hz or 24,000 rpm as permissible operating points.
| Quantity | Known value | Where to verify | Decision it controls |
|---|---|---|---|
| VFD input supply | 220 V single phase in this installation | VFD input nameplate and manual | Whether the drive is correctly supplied and rated |
| Commanded frequency | 400 Hz | VFD frequency monitor | Requested electrical speed |
| Displayed speed | About 11,000 to 11,520 rpm before scaling correction | VFD speed display | Whether the engineering-unit conversion is credible |
| Calculated two-pole synchronous speed | 24,000 rpm at 400 Hz | Motor pole data and formula | Expected upper reference if the motor is two pole |
| Actual shaft speed | Not established by the display | Independent tachometer | Whether the spindle is physically slow |
| Output current | Read from the operating VFD | VFD current monitor and motor nameplate | Electrical loading and heating risk |
Displayed speed versus mechanical speed
A VFD can show frequency directly or convert frequency into an estimated rpm value. That conversion may use a base-speed constant, pole count, rated-speed entry, or another manufacturer-specific scaling value. It is not normally a closed-loop shaft-speed measurement unless the system has speed feedback and the drive is configured to use it.
The observed arithmetic is decisive. Multiplying 11,520 rpm by the ratio between the new and old settings gives:
11,520 × (3000 / 1440) = 24,000 rpm
The exact linear relationship shows that Parameter 144 scaled the displayed value in this setup. Because frequency stayed at 400 Hz and the audible sound stayed the same, the edit did not demonstrate that the spindle doubled its mechanical speed. It corrected—or at least rescaled—the indication.
| Symptom | Likely mechanism | Decisive check |
|---|---|---|
| 400 Hz with an 11,520 rpm indication | Incorrect frequency-to-speed display scaling | Measure the shaft with a tachometer |
Displayed rpm changes after editing Parameter 144, but sound does not |
Display conversion changed while motor frequency remained constant | Compare VFD frequency before and after the edit |
| Measured speed is genuinely low while frequency reaches 400 Hz | Wrong motor assumptions, excessive slip, load, current limiting, wiring trouble, or a motor-drive mismatch | Check nameplates, unloaded speed, output current, and drive diagnostics |
| Frequency cannot reach 400 Hz | Maximum-frequency limit, reference scaling, command-source configuration, or an active drive limit | Compare command, setpoint, and output-frequency monitors |
Control approaches and recommendation
The spindle can receive a speed reference from the VFD keypad, an external potentiometer, or a machine controller when the drive supports those sources. These methods select a reference; they do not redefine the motor’s safe maximum frequency or prove its actual speed.
| Approach | What it changes | Best use in this case | Main pitfall |
|---|---|---|---|
| VFD keypad | Frequency command through the local interface | Initial unloaded test because it removes external control scaling | Confusing the rpm display with measured shaft speed |
| External potentiometer | Analog speed reference | Manual remote adjustment after the analog input is configured | Incorrect input range or scaling can prevent full frequency |
| Machine controller | Automated analog or digital speed command | Normal machine operation after standalone testing | Controller scaling and VFD scaling can create two independent limits |
| Display-parameter edit | Engineering-unit indication | Correcting a known readout conversion | It can make the screen say 24,000 rpm without changing the shaft |
Use the keypad for the first diagnostic run, measure speed independently, and correct display scaling only after confirming motor data. Add the potentiometer or controller afterward. This sequence keeps reference-command faults separate from motor, drive, and display faults.
Nameplate and command-chain checks
The 220 V single-phase supply describes the VFD input in this installation, not necessarily the motor winding topology. A compatible VFD rectifies its accepted input supply and synthesizes its specified motor output. Whether this particular VFD accepts that supply and produces the output required by the spindle must be read from both nameplates.
- Record the VFD input rating and compare it with the 220 V single-phase source.
- Record the VFD output voltage, current, and frequency ranges.
- Record the spindle voltage, current, rated frequency, rated speed, and connection information.
- Verify that the drive output ratings cover the spindle nameplate requirements. Supply phase, motor output phase, and voltage class are separate fields.
- Check the configured run-command source and frequency-reference source. Select the keypad for the standalone test.
- Check the configured maximum frequency and any upper-frequency limit against the spindle nameplate. A controller or potentiometer cannot command beyond an active drive limit.
Applying an unverified three-phase supply can damage a VFD whose input is rated only for another source. Read the VFD input label before changing supply topology. A correctly rated single-phase input does not by itself limit a properly configured drive to half of the expected motor speed.
Independent speed-measurement procedure
A tachometer separates a false display from a real low-speed condition. Use an instrument and target suitable for the expected shaft speed, follow the instrument’s stand-off requirements, and keep hands and loose material away from the rotating spindle.
- Remove cutting load and secure the spindle installation for an unloaded test.
- Return the speed command to zero, start the drive from the keypad, and raise frequency gradually while watching output frequency and current.
- Record frequency, displayed rpm, measured rpm, and output current at several stable points up to the nameplate-approved maximum frequency.
- At 400 Hz, compare measured speed with the speed predicted from the documented motor pole count or rated-speed data.
- Stop if current exceeds the spindle or VFD rating, if the drive reports a fault, or if abnormal vibration, noise, or rapid heating develops.
If measured speed rises proportionally with frequency while the VFD rpm display has a different ratio, the fault is display scaling. If measured speed stays abnormally low and current rises, investigate mechanical load, wiring, current limiting, or motor-drive compatibility. If both frequency and measured speed stop below the command, inspect the reference source, frequency limits, and active drive status.
Display-scale correction
In the documented setup, changing Parameter 144 from 1440 to 3000 produced a 24,000 rpm indication at 400 Hz. Preserve the original value before editing, because parameter meanings vary between VFD models. Confirm the definition of Parameter 144 in the manual for the exact drive rather than transferring the number to a different VFD.
- Record the original
Parameter 144value of1440and the current values of related motor and display settings. - Hold the frequency at a safe, stable test point and record displayed rpm plus tachometer rpm.
- Enter the value justified by the exact VFD manual and spindle data. The observed value
3000belongs only to the documented setup. - Run the same frequency point again and compare display, tachometer, sound, and current.
- Repeat at multiple frequencies. A correct conversion should track proportionally across the tested range rather than matching at only one point.
This is a calibration task, not a torque or speed command. If changing the parameter alters the actual output frequency, stop and restore the recorded setting; the parameter has a different function on that drive or interacts with another configuration item.
Verification and operating decision
Accept the correction only when the command chain, motor behavior, and displayed value agree. At the 400 Hz test point, capture the VFD frequency monitor, tachometer speed, output current, and any diagnostic indication. Compare current with both nameplates and compare measured rpm with the motor’s rated data, allowing for the operating principle and load rather than forcing the display to an ideal number.
A potentiometer is optional for reaching the full configured frequency. When used, verify zero command, full-scale command, direction of adjustment, and loss-of-signal behavior from the exact drive documentation. The potentiometer cannot repair an incorrect maximum-frequency setting, wrong analog-input selection, current limit, or inaccurate rpm display.
If the independent measurement is near the expected rated speed and current remains within rating, retain the verified display scale and document it. If the shaft is truly slow, restore attention to current, load, wiring, motor data, and drive diagnostics; further display edits only conceal the symptom.
FAQ
How do I know whether my 2.2 kW spindle is really turning 24,000 rpm?
Measure the shaft with a tachometer while recording VFD output frequency and current. The VFD rpm screen may be a calculated value rather than feedback from the spindle.
How do I calculate spindle speed from 400 Hz?
For a two-pole motor, synchronous speed is 120 × 400 / 2 = 24,000 rpm. Verify the pole count and rated-speed data; an induction motor’s loaded shaft speed is below synchronous speed because torque requires slip.
How do I correct an 11,520 rpm display at 400 Hz?
On the documented VFD, changing Parameter 144 from 1440 to 3000 changed the display to 24,000 rpm. Confirm that parameter’s definition in the manual for the exact VFD, then verify the result with a tachometer.
How do I use a potentiometer to reach full spindle speed?
Configure the drive to use its analog input as the frequency reference, then verify that full-scale input commands the configured maximum frequency. A potentiometer changes the reference only; it does not override frequency limits or correct display scaling.
How do I know when to stop troubleshooting the spindle?
Stop testing if current exceeds a nameplate rating, the VFD faults, or the spindle develops abnormal vibration, noise, or rapid heating. Escalate to the VFD or spindle manufacturer’s official support channel when the exact parameter definition, supply compatibility, winding connection, or rated 400 Hz operating point cannot be verified from the product documentation.