Variable frequency drive sizing
Select a variable frequency drive for a three-phase induction motor based on the actual required current: motor rated current from the nameplate or calculated from power, power factor and efficiency, switching-frequency and temperature derating, a duty-dependent overload reserve (VT, CT, heavy duty), plus guidance on motor cable length.
Calculation
Results
- Motor rated current I_N
- 14.15 A
- Switching-frequency derating factor k_freq
- 0.9
- Temperature derating factor k_temp
- 1
- Margin factor
- 1.1
- Required drive rated current I_VFD,req
- 17.3 A
- Overload factor
- 1.5
- Overload current I_overload (short-time)
- 21.23 A
- Recommended drive rated power
- 15 kW
- Rated current of the recommended size
- 28.3 A
Guideline values based on a generic standard power series. Motor protection, EMC filter selection and line reactor must be checked against the manufacturer; the drive manufacturer's datasheet is authoritative.
Sketch: mains supply, drive and motor with current path
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Formulas and fundamentals
Motor Rated Current from Power
If the motor rated current is not known from the nameplate, it follows from the rated power, power factor and efficiency of the motor:
Here U is the line-to-line supply voltage, cos φ the power factor (guideline value 0.85) and η the efficiency (guideline value 0.90). The drive is fundamentally sized on this current, not on the kW rating: two motors with the same power can have a different rated current depending on pole count, efficiency and voltage.
Switching-Frequency and Temperature Derating
The continuously deliverable drive current drops as the switching frequency rises (a higher switching frequency reduces motor noise but increases switching losses in the power semiconductors) and as the ambient temperature rises:
Guideline values for the switching-frequency factor: 4 kHz corresponds to 1.0, 8 kHz to 0.9, 12 kHz to 0.8 and 16 kHz to 0.7. For the temperature factor: up to 40 degrees C is 1.0, up to 50 degrees C is 0.9. The margin factor is 1.0 for normal duty (VT); for constant torque (CT) and heavy duty an addition is applied so that sufficient reserve remains on the rated current even under recurring overload peaks.
Overload Duty Classes VT, CT and Heavy Duty
In parallel with the continuous current, the drive's short-time overload reserve is checked. Depending on the duty class, a multiple of the motor rated current is required for a limited time:
Normal duty (Variable Torque, VT) requires a 110% overload factor, typical for pumps and fans with a load torque that rises with the square of speed. Constant torque (CT) requires 150% for 60 seconds, typical for conveyors, extruders or compressors with a load torque that stays roughly constant across the speed range. Heavy duty requires 200%, for applications with high starting or acceleration torque such as cranes or mills.
Recommended Drive Size
The recommended size is the next standard drive size whose rated current covers both the motor power P_N and the required rated current I_VFD,req. For constant torque or heavy duty, one size larger is additionally selected to keep margin for recurring overload peaks. This makes the key point clear: sizing follows the current, a power-only view in kW is not sufficient.
Motor Cable Length and EMC
A long, shielded motor cable acts as a capacitance to earth: the drive has to supply an additional capacitive leakage current on top of the motor current, steep switching edges (du/dt) stress the motor insulation, and bearing currents increase. Up to roughly 50 m of shielded cable, no additional measures are usually needed. Above that, an output reactor or a du/dt filter at the drive output is recommended, and a sine-wave filter for very long cables. The exact limit depends on the cable type and the drive manufacturer; the manufacturer's datasheet is authoritative.
Worked example
Reference example: an induction motor with P_N = 7.5 kW at U = 400 V, cos φ = 0.85 and η = 0.90 has a motor rated current of I_N = 7500/(sqrt(3)*400*0.85*0.90) = 7500/530.01 = 14.15 A. At an 8 kHz switching frequency, k_freq = 0.9, and at an ambient temperature up to 40 degrees C, k_temp remains 1.0. For constant torque duty (CT), the margin factor is 1.1, giving a required drive rated current of I_VFD,req = 14.15/(0.9*1.0)*1.1 = 17.30 A.
The short-time overload requirement under CT sizing is 150% of the motor rated current: I_overload = 14.15*1.5 = 21.23 A; this value only checks the drive's overload reserve and does not enter the continuous-current sizing. From the standard power series, an 11 kW drive (calculated rated current around 20.75 A) already covers the required 17.30 A; because the duty is CT, the calculator additionally recommends one size larger, i.e. a 15 kW drive with a rated current of around 28.30 A.
If an actually chosen drive with I_VFD = 20.75 A (11 kW size) is also entered, the resulting utilisation is 17.30/20.75 = 84% (green traffic light, pass). With a shielded motor cable of 30 m, no additional measures are needed; only above roughly 50 m is an output reactor or a du/dt filter recommended.
Frequently asked questions
How big does the frequency drive need to be?
The decisive value is the required drive rated current I_VFD,req, not the motor power in kW: it follows from the motor rated current I_N, derated for switching frequency and temperature and increased by a duty-dependent margin factor. The selected drive must be able to deliver this current continuously; the calculator's recommended standard size is the smallest size that covers both this current and the motor power P_N.
Is a drive sized by power or by current?
By current. Two motors with the same kW rating can have a different rated current depending on pole count, efficiency, power factor and supply voltage, and it is exactly this current that stresses the power semiconductors inside the drive. Most manufacturers' kW rating is only a rough matching figure for a standard motor under rated conditions; when in doubt, the current stated in the datasheet always governs.
What does switching-frequency derating mean?
Higher switching frequencies (PWM carrier frequency) reduce motor noise and current ripple but increase switching losses in the drive's power semiconductors. To avoid excessive heating, manufacturers therefore reduce the continuously permissible output current at higher switching frequency; the calculator models this with the derating factor k_freq (guideline values from 1.0 at 4 kHz down to 0.7 at 16 kHz). The exact derating behaviour is stated in the specific drive's datasheet.
What role does the motor cable length play?
A long, shielded motor cable acts as a capacitance to earth and additionally loads the drive with a capacitive leakage current; the steep switching edges (du/dt) also increase the voltage peaks at the motor windings and the bearing currents. Up to roughly 50 m, no additional measures are usually needed; above that, an output reactor or a du/dt filter is recommended, and a sine-wave filter for very long cables. The actual limit is manufacturer-specific.
What is the difference between VT and CT?
VT (Variable Torque, normal duty) describes applications where the load torque rises with the square of speed, such as pumps and fans; an overload reserve of around 110% is sufficient here. CT (Constant Torque) describes applications with a load torque that stays roughly constant across the speed range, such as conveyors or extruders, requiring a higher overload reserve of around 150% for 60 seconds. Heavy-duty applications with high starting torque, for example cranes, need additional margin and are set at 200% here.
How do I choose cos φ and efficiency if the motor rated current is not known from the nameplate?
For standard induction motors in the range of a few kW to a few tens of kW, a cos φ around 0.85 and an efficiency around 0.90 are common guideline values; smaller motors tend to sit lower and larger motors higher in efficiency. If the nameplate or datasheet is available, the values stated there, or the rated current given there, should be used directly, since the guideline values are only a rough approximation.
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