The 185 V measurement from T3 to ground does not identify the fault by itself. The number that matters for input-power diagnosis is each phase-to-phase voltage, especially while the VFD input capacitors charge at boot. Record the exact fault code, balance the rotary converter first at no load, and then capture voltage and current with the Delta VFD-B connected.
Symptom timing and measured quantities
The VFD faults during boot, before the spindle receives a run command. That timing moves motor acceleration, motor wiring, and mechanical load down the decision tree; the first checks are input voltage, source impedance, charging current, and the drive's stored fault record.
| Quantity or condition | Observed value | Diagnostic use | Where to read it |
|---|---|---|---|
| Rotary-converter rating | 15 hp |
Identifies the installed source, but does not prove adequate transient capacity | Converter or idler nameplate |
| Phase-to-ground voltage |
T1 = 118 V, T2 = 120 V, T3 = 185 V
|
Useful for grounding and insulation checks; insufficient for calculating phase balance | True-RMS meter at the VFD input terminals |
| Phase-to-phase voltage | Not recorded | Primary input-balance measurement |
T1-T2, T1-T3, and T2-T3 at the drive |
| Reported balance | Within 3% under load |
Static result did not clear the boot fault; it is not a universal acceptance limit | Calculated from simultaneous phase-to-phase readings |
| Operating-state effect | Another machine running allowed normal operation | Shows that converter operating point or source stiffness changes the result | Measurements before and after the second machine starts |
| Drive condition | Used; previously operated a lathe | Keeps an internal drive problem in the diagnostic path | Fault history and test on a compliant source |
Phase-to-phase electrical mechanism
A rotary phase converter synthesizes the third phase through the rotating idler and any balancing capacitors. Its generated conductor can have an unusual voltage to ground because the three phase vectors, grounding arrangement, parasitic capacitance, and connected loads establish that value. Three phase-to-ground magnitudes alone reveal neither the angles between those vectors nor the three phase-to-phase voltages.
The VFD rectifier transfers energy from the phase-to-phase input into its DC bus. At power-up, the charging interval can expose excessive source impedance, a collapsing phase, an excessive phase-to-phase voltage, or a charging-system defect. During sustained operation, unequal voltage drives unequal rectifier current and additional heating. This is heat, not logic.
Run capacitors change both phase magnitude and angle. Adding capacitance can raise the generated-leg voltage, as occurred here, but the direction and size of the useful correction depend on all three phase-to-phase measurements and the connected load. A ground-referenced value cannot select the capacitor bank.
Diagnostic measurement sequence
- Read and record the exact Delta VFD-B fault code before cycling power again. Distinguish an input-phase, undervoltage, overvoltage, or internal hardware indication using the drive documentation; “fault” alone is not a diagnosis.
- With the VFD isolated, run the rotary converter at no load and measure
T1-T2,T1-T3, andT2-T3. Also record current in each converter conductor. - Calculate a comparison value: average the three phase-to-phase voltages, find the largest absolute deviation from that average, divide by the average, and multiply by
100%. Compare the result with the input requirements stated for the installed drive, rather than treating the previously reported3%as a manufacturer limit. - Connect the VFD and capture all three phase-to-phase voltages directly at its input during boot. Use an instrument with min/max capture or a power recorder because a handheld display can miss the charging transient.
- Measure input current during the same event. A voltage that falls while current rises identifies source impedance or converter regulation as the active mechanism; excessive voltage points to the opposite correction path.
- Repeat the capture with the other machine stopped and running. If the second machine changes the VFD result, compare converter speed, all three voltages, and conductor currents between those states.
- If the input remains inside the drive's documented requirements throughout boot, test the used VFD on a known-compliant three-phase source. A repeated identical fault then directs attention to its input rectifier, precharge components, sensing circuit, or stored configuration.
Rotary-converter balancing procedure
- Establish the unloaded baseline before adjusting capacitors. Record all three phase-to-phase voltages and converter currents, not only the generated phase to ground.
- De-energize the system and allow stored energy to discharge before changing a capacitor connection. Apply the equipment's electrical-safety procedure because both the VFD DC bus and capacitor bank can retain hazardous voltage.
- Adjust the balancing branches between
T1-T3andT2-T3in controlled increments. Change one branch at a time so each measurement shows which phase vector moved. - Restart the converter and repeat the unloaded readings after every change. Reject any adjustment that improves one phase-to-phase value by forcing another outside the VFD input range or by creating excessive converter current.
- Connect the VFD, repeat the boot capture, and then measure again while the motor runs. Voltage should remain within the drive's documented range, while conductor current must remain within the applicable equipment ratings.
A different installation used reported capacitor values of 80 MFD from T1 to T3 and 44.5 MFD from T2 to T3, producing approximately 250-251 V phase to phase from a 244 V input. Do not copy those values: they belonged to a 10 hp converter with a different idler and load, and the reported unit notation must be verified before selecting components.
Verification across operating states
Verify more than one steady operating point. Record unloaded converter operation, VFD boot, motor stopped with the drive energized, acceleration, and normal motor load. For every state, retain the three phase-to-phase voltages, input currents, DC-bus indication if the drive exposes one, and the fault history.
A valid correction produces repeatable cold boots without relying on another machine, keeps the input inside the drive's published limits during charging and running, and does not overload the idler, conductors, or capacitor bank. If starting another machine remains necessary, the installation still depends on an uncontrolled load-dependent balance.
Recurring diagnostic pitfalls
Phase-to-ground readings are often mistaken for phase balance. Balancing only under load can also hide an unacceptable no-load condition, while balancing only at no load can miss voltage collapse during VFD charging. A static meter reading may look acceptable even when a short transient crosses the drive's trip boundary.
Capacitors are not a universal cure: excessive capacitance can elevate voltage and current. Likewise, a prior history of running another lathe shows that the used drive once operated, but it does not prove that its input hardware is healthy now or that both power sources behave alike.
FAQ
Can I diagnose the Delta VFD-B fault from 185 V to ground?
No. Measure T1-T2, T1-T3, and T2-T3 at the VFD during boot, then correlate those values with the exact stored fault code.
Does phase balance within 3% guarantee that the VFD will boot?
No. The reported 3% was measured under load, while the fault occurred during boot; capture the charging transient and compare it with the installed drive's documented input limits.
Can I balance the converter by copying 80 MFD and 44.5 MFD?
No. Those values came from a different 10 hp converter; select each T1-T3 and T2-T3 adjustment from measurements on the installed idler and load.
Does a successful boot with another machine running prove the drive is healthy?
No; it proves that changing the converter's operating point changes the result. Stop testing if phase-to-phase voltage exceeds the drive rating, current exceeds an equipment rating, capacitor heating appears, or the fault persists on a known-compliant source. Give Delta Electronics official support the exact fault code, VFD-B nameplate data, wiring arrangement, and synchronized voltage/current captures.