VFD Volts-per-Hertz Calculator

A VFD volts-per-hertz calculator determines the reference voltage associated with a selected output frequency. The primary result is the motor’s Base volts per hertz, calculated from the motor nameplate voltage and motor nameplate base frequency.

For a constant V/Hz motor control reference, voltage normally changes in proportion to frequency below the motor’s rated base frequency. This preserves the intended voltage-to-frequency relationship used by the drive’s motor-control setup. The calculated voltage is useful when reviewing VFD commissioning values, checking a programmed V/Hz curve, documenting a replacement drive, or comparing a target speed command against the motor nameplate reference.

Use the commissioning reference screen with the exact motor nameplate and drive parameter manual. It does not prescribe a universal VFD setup.

Motor Nameplate Reference

The calculation begins with two motor nameplate values:

Input	Electrical purpose
Motor rated voltage (V)	The voltage identified on the motor nameplate for the selected operating connection and rating
Motor rated frequency (Hz)	The motor’s nameplate base frequency used to establish the base V/Hz relationship
Target frequency (Hz)	The output frequency being reviewed
Entered voltage boost (V)	A drive-specific voltage addition, entered only when supported by the drive manual or commissioning record
Maximum output voltage (V)	The voltage ceiling applied to the calculated reference voltage
Drive parameter note	A separate record for control mode, field weakening, boost, and motor insulation data

A motor rated at 480 V and 60 Hz has a base ratio of 8 V/Hz. At 30 Hz, the proportional reference voltage is 240 V before any voltage boost or output-voltage limit is applied.

The values must match the actual motor connection. A dual-voltage motor connected for 480 V operation requires the 480 V nameplate reference, not the lower-voltage connection rating.

Volts-Per-Hertz Calculation

The calculator uses the motor’s base voltage and base frequency to calculate Base volts per hertz:

\(\displaystyle \text{Base volts per hertz} = \frac{\text{Motor rated voltage}}{\text{Motor rated frequency}}\)

It then calculates the voltage reference at the selected Target frequency:

\(\displaystyle \text{Unlimited reference voltage} = (\text{Base volts per hertz} \times \text{Target frequency}\)

• text{Entered voltage boost} ]

The voltage ceiling is then applied:

\(\displaystyle \text{Limited reference voltage} = \min(\text{Unlimited reference voltage}, \text{Maximum output voltage}\)

\(\displaystyle \text{Voltage limited amount} = \text{Unlimited reference voltage} - \text{Limited reference voltage}\)

The Control region identifies whether the reviewed target frequency is within the motor’s base-frequency region or above the motor rated frequency, where the drive configuration and motor operating limits require separate review.

Calculation Example

Using the stated commissioning reference values:

Field	Entered value
Motor rated voltage	480 V
Motor rated frequency	60 Hz
Target frequency	30 Hz
Entered voltage boost	0 V
Maximum output voltage	480 V

\(\displaystyle \text{Base volts per hertz} = \frac{480\text{ V}}{60\text{ Hz}} = 8\text{ V/Hz}\)

\(\displaystyle \text{Unlimited reference voltage} = (8\text{ V/Hz} \times 30\text{ Hz}) + 0\text{ V} = 240\text{ V}\)

Because 240 V is below the 480 V maximum output voltage:

\(\displaystyle \text{Limited reference voltage} = 240\text{ V}\)

\(\displaystyle \text{Voltage limited amount} = 0\text{ V}\)

Worked result: At 30 Hz, the 480 V, 60 Hz motor reference is 240 V, with no voltage limitation. The reported control region is Base-frequency region.

Voltage Boost and Output Limiting

Entered voltage boost is not a universal motor value. It is a drive-specific setting that may be associated with low-speed torque compensation, manual V/Hz adjustment, or a documented commissioning requirement. Enter a boost value only when it is established by the drive parameter manual or commissioning record.

A positive boost increases the Unlimited reference voltage. If that value exceeds Maximum output voltage, the calculator limits the displayed voltage and reports the difference as Voltage limited amount.

For example, if a calculated reference voltage were 490 V while the maximum output voltage were 480 V, the limited reference voltage would be 480 V and the voltage limited amount would be 10 V. That limit is a mathematical ceiling in this calculation; it does not confirm that the actual drive, supply system, motor, or insulation system is suitable for the operating condition.

Drive and Field Verification

The calculated V/Hz reference does not replace drive setup or equipment verification. Confirm the following separately against the motor nameplate, VFD documentation, installation requirements, and the AHJ where applicable:

• Drive control mode, including whether the installation uses a basic V/Hz mode, vector control, sensorless vector control, or another manufacturer-specific method.
• Motor connection, motor rated voltage, rated frequency, rated current, speed range, service factor, and insulation information.
• Drive input supply voltage and the actual available output-voltage capability.
• Output reactor, dV/dt filter, sine-wave filter, motor lead length, and reflected-wave limitations where required by the drive or motor manufacturer.
• Branch-circuit and feeder conductor ampacity, terminal rating, overcurrent protection, grounding, disconnecting means, and equipment listing requirements.
• Voltage drop on the VFD line side and output-circuit installation conditions; a calculated motor V/Hz reference is not a voltage-drop calculation.
• Motor cooling and load characteristics at reduced speed, particularly where a shaft-mounted fan provides less cooling at low frequency.

A VFD can affect motor current, torque capability, heating, harmonics, and output waveform conditions in ways not represented by a simple proportional voltage calculation. Use the displayed Base volts per hertz, Unlimited reference voltage, Limited reference voltage, and Voltage limited amount as commissioning-reference values alongside the exact drive parameter manual and documented motor data.