The number that matters is the drive’s published single-phase input rating, not a blanket multiplication by sqrt(3). For the stated installation, the relevant values are 220 V single-phase supply, a 5.6 A three-phase motor, 7.1 A maximum drive output, 16.0 A single-phase drive input, and 9.3 A three-phase drive input. The 9.3 A value is an input rating; treating it as the output-current denominator produces the wrong 9.63 A estimate.
Wrong fixes and why they fail
Multiplying every three-phase current by sqrt(3) is the first common wrong fix. The factor relates phase topology when comparing apparent power at the same voltage, but it does not account for motor power factor, drive efficiency, rectifier harmonics, or different input and output operating conditions. It is not a universal VFD derating factor.
A second wrong fix is to apply a 50% derating rule to a drive that carries an explicit single-phase input rating. Derating is necessary when a three-phase-input-only drive is supplied from single phase because two rectifier paths and the DC-bus components carry more current. The required derating is manufacturer-specific; reported rules range from 20% to 50%. If the exact model is rated for single-phase input at the intended voltage, its single-phase table already defines the permitted loading.
A third wrong fix is to assume the difference between input amperes and output amperes becomes heat. Input and output currents are on different circuit topologies and cannot be subtracted as energy. Heat is determined from real power:
Drive loss (W) = input real power (W) - output real power (W)
Calculating efficiency from (20 A / 24 A) x 100 = 83.3%, for example, is invalid without voltage, phase arrangement, power factor, waveform, and simultaneous real-power measurements.
Current, apparent power, and thermal load
A VFD first rectifies the AC supply onto a DC bus, then synthesizes variable-frequency three-phase output. On single-phase input, all input power passes through two supply conductors. On three-phase input, three conductors share that transfer. The rectifier and DC bus therefore see different RMS and ripple-current stresses even when the motor receives the same shaft power. This is heat, not logic.
For sinusoidal systems, the apparent-power relationships are:
Single-phase kVA = V x I / 1000Three-phase kVA = sqrt(3) x V_LL x I_line / 1000
Using 220 V as the calculation voltage gives the following comparison. These are apparent-power calculations, not efficiency calculations.
| Quantity | Calculation | Result | Meaning |
|---|---|---|---|
| Motor at 5.6 A | sqrt(3) x 220 x 5.6 / 1000 |
2.13 kVA | Three-phase output apparent power at that current |
| Drive at 7.1 A output | sqrt(3) x 220 x 7.1 / 1000 |
2.71 kVA | Output apparent power at the stated maximum current |
| Drive at 16.0 A single-phase input | 220 x 16.0 / 1000 |
3.52 kVA | Input apparent power at the stated rating |
| Drive at 9.3 A three-phase input | sqrt(3) x 220 x 9.3 / 1000 |
3.54 kVA | Three-phase input apparent power at the stated rating |
The 16.0 A and 9.3 A ratings represent nearly equal input kVA at 220 V. Their ratio, 16.0 / 9.3 = 1.72, is close to sqrt(3). That relationship explains the two input values; it does not make 9.3 A an output rating.
Interpretation of the L100-15NFU ratings
The proposed drive has a stated maximum output of 7.1 A, while the motor nameplate current is 5.6 A. On current alone, the motor uses about 5.6 / 7.1 = 79% of the drive’s output rating, leaving 1.5 A, or about 27% above the motor nameplate current.
The single-phase rating must apply to the exact Hitachi L100-15NFU, supply-voltage class, and required output operating condition. A single-phase input-current entry strongly indicates that the manufacturer evaluated that input mode, but the installation table or manual must also show that full 7.1 A output remains available on single-phase power. If the table separates input ratings from derated output ratings, use the single-phase output value.
| Value | Correct classification | Engineering use |
|---|---|---|
| 5.6 A | Motor three-phase nameplate current | Minimum continuous drive-output current, subject to duty and overload requirements |
| 7.1 A | Stated maximum VFD output current | Drive-to-motor current comparison |
| 16.0 A | Stated single-phase VFD input current | Input conductor, disconnect, and protection selection per the manual and applicable code |
| 9.3 A | Stated three-phase VFD input current | Three-phase supply sizing; not an output-current denominator |
A 5 hp drive is not inherently required. Oversizing may be the correct path when a candidate is rated only for three-phase input and its manufacturer requires enough single-phase derating to reduce usable output below 5.6 A. It adds no technical value when the selected model already permits 7.1 A output from the specified single-phase source.
Input-current estimation limits
The proposed calculation, 16.0 x (5.6 / 9.3) = 9.63 A, mixes single-phase input current, motor output current, and three-phase input current. It therefore has no valid power basis.
If current scaled linearly with rated output current, the internally consistent estimate would be:
16.0 A x (5.6 A / 7.1 A) = 12.62 A
Treat 12.62 A only as a rough load estimate. Motor current is not a direct measure of shaft power because motor power factor and efficiency change with load. The VFD input uses a rectifier-capacitor waveform, so input true power, apparent power, RMS current, and peak current do not scale identically.
A power-based relationship is:
I_input = [sqrt(3) x V_LL_output x I_output x PF_motor] / [V_input x efficiency_drive x PF_input]
The motor power factor, drive efficiency, and input true power factor are not provided. Read them from the applicable motor and drive data, or measure real input power with an instrument suitable for distorted VFD input waveforms. For branch-circuit design, use the published 16.0 A input rating and the manufacturer’s specified protective-device and conductor requirements rather than a proportional estimate.
Drive-selection and supply procedure
- Read the motor nameplate and record rated voltage, rated current, frequency, connection, and duty. Confirm that the motor can be connected for the VFD’s three-phase output voltage.
- Read the exact drive manual entry for
L100-15NFU. Verify that 220 V single-phase input is an approved source and identify the output-current rating that applies in that mode. - Compare the applicable continuous output rating with the motor’s 5.6 A nameplate current. The stated 7.1 A value passes this current check if it remains valid on single-phase input.
- Compare the load’s acceleration and overload demand with the drive’s published overload curve. A mill that starts with material engaged or uses aggressive acceleration can require more short-duration torque than a current-only continuous-duty comparison reveals.
- Select feeder conductors, disconnecting means, and branch protection from the drive manual’s input-side table and the applicable electrical code. Use the 16.0 A rated input value where the table assigns it to this model and supply configuration.
- Program the motor’s nameplate data and set acceleration, deceleration, current-limit, and motor-protection functions for the driven machine. Use the parameter names in the installed manual; no parameter identifiers are established here.
- Run the mill unloaded, then increase to the highest normal mechanical load while observing VFD output current, input current, DC-bus or supply diagnostics, and temperature status.
Peak current and protection behavior
There are three different peak-current questions. At energization, the input rectifier charges the DC-bus capacitors; the drive’s charging circuit and the specified upstream protective device address that event. During motor acceleration, the inverter controls output frequency and current rather than applying across-the-line locked-rotor current. During a short circuit or ground fault, electronic protection and the required upstream protective device act according to their own limits.
Rated input current is not a prediction of the instantaneous current during an internal DC-bus fault or an output short. It is also not selected by imagining a dead short with every trip setting at maximum. Fault-current and short-circuit protection must follow the drive manual, device markings, and the available-fault-current requirements for the installation.
No waveform, off-state current, or repetition period is supplied for the charging or switching pulses. Converting those peaks to an equivalent RMS or continuous current would therefore be invalid. Use a true-RMS instrument with adequate crest-factor capability when checking input current, and use a drive-compatible method for output-current measurements.
Symptoms, causes, and verification
| Observed symptom | Likely decision point | Verification |
|---|---|---|
| Protection opens when the VFD is energized | Charging current, unsuitable protection characteristic, wiring fault, or inadequate supply circuit | Compare the installed device with the exact input-protection table; inspect input wiring and record voltage during energization |
| Trip occurs only during acceleration | Acceleration time, load torque, current limit, or insufficient overload capacity | Record output current during the ramp; test with the mechanical load removed and compare with the overload curve |
| Trip occurs at steady heavy load | Mechanical overload, motor current above 5.6 A, single-phase operation not approved, or supply-voltage drop | Measure loaded output current and input voltage; confirm the manual’s single-phase output rating |
| Drive overheats without an overcurrent trip | Input rectifier/DC-bus thermal stress, cooling restriction, high ambient temperature, or incorrect derating | Read the drive temperature diagnostic, inspect airflow, and compare the installation with the manual’s environmental limits |
| Motor lacks torque while current remains limited | Incorrect motor data, voltage/frequency setup, connection, or acceleration demand | Compare programmed motor data with the nameplate and verify motor lead configuration |
The acceptance test is operation at the highest repeatable normal load without exceeding 5.6 A continuously at the motor, the applicable drive output limit, or any published thermal and overload limit. Record input voltage and current at the same operating point. A lightly loaded demonstration that merely runs without tripping does not validate feeder sizing or full-load capacity.
Frequently asked questions
Why does a single-phase VFD input draw more current?
Two supply conductors must deliver the power that three conductors share on three-phase input. At 220 V, the stated 16.0 A single-phase and 9.3 A three-phase ratings correspond to about 3.52 kVA and 3.54 kVA respectively.
Why does multiplying 5.6 A by sqrt(3) give the wrong input current?
The calculation omits motor power factor, drive efficiency, input power factor, and rectifier waveform distortion. Use the published 16.0 A input rating for circuit design; 16.0 x 5.6 / 7.1 = 12.62 A is only a rough operating estimate.
Why does a three-phase-only VFD need derating on single phase?
Single-phase supply increases current and ripple stress in the input rectifier and DC bus. Apply the exact manufacturer derating; a 50% rule is a conservative selection shortcut, not a substitute for the model’s manual.
When should I stop testing and contact Hitachi support?
Stop if the manual does not explicitly connect the L100-15NFU single-phase input rating to the required 7.1 A output, or if the drive trips, overheats, or exceeds published limits at normal load. Record the nameplate data, supply voltage, input and output currents, load condition, and displayed diagnostic, then contact Hitachi through an official support channel before changing protection or increasing drive size.