A drive chosen only by the kW printed on the motor often trips, overheats or fails early in a hot Indian plant room. Sizing it by current, duty and site conditions gives a drive that starts the load every time and lasts for years.
VFD sizing means choosing a drive whose continuous current and overload capacity match the motor and load under real site conditions. This guide explains current based selection, duty ratings, derating and a worked calculator.
What Is VFD Sizing?
VFD sizing is the process of selecting a variable frequency drive whose continuous output current, overload capacity and environmental rating suit the motor and the driven load at the actual site. A variable frequency drive converts fixed mains supply into variable voltage and frequency, and its power devices have strict current and temperature limits.
The kW value in a drive catalogue is only a guide for a typical 4 pole motor at nominal voltage. Real motors differ in efficiency, poles and power factor, so their current can be higher than the catalogue assumes.
Good VFD sizing therefore starts from the motor nameplate current, not the kW, and then adds the load duty and site conditions. Read the nameplate carefully, as explained in motor full load current.
Why Current Matters More Than kW in VFD Sizing
ABB Technical Guide No 7, Dimensioning of a drive system, tells engineers to choose the drive according to the dimensioning current, calculated from the load torque profile. The drive must supply that current continuously and also cover short overloads.
A 6 pole or 8 pole motor of the same kW draws more current than a 4 pole motor, because its power factor is lower. Older or rewound motors and low efficiency designs, compared in IEC 60034 efficiency classes, can also draw more current than the catalogue figure.
In ABB guide examples, a 6 pole 55 kW motor needed 19 A less current than a 4 pole 75 kW motor for the same extruder duty. A sensible motor choice can shrink the drive as well.
Normal Duty vs Heavy Duty in VFD Sizing
Most drive makers publish two current ratings for each frame. ABB defines light duty current for the ACS580 as allowing 110 percent overload for 1 minute every 5 minutes, and heavy duty current as allowing 150 percent overload for 1 minute every 5 minutes.
| Duty Class | Overload Example | Typical Loads | Effect on VFD Sizing |
|---|---|---|---|
| Normal or light duty | 110 percent for 1 minute | Centrifugal pumps, fans, blowers | Use the higher continuous current rating |
| Heavy duty | 150 percent for 1 minute | Conveyors, crushers, mixers, hoists, extruders | Use the lower heavy duty rating, often one frame larger |
| High starting torque | Above 150 percent briefly | Loaded conveyors, positive displacement pumps | Check peak current and starting time with the maker |
Variable torque loads such as fans need little torque at low speed, so normal duty is usually enough. Constant torque loads need full torque from standstill, so their VFD sizing must use the heavy duty current, which is why such drives often trip as described in VFD overload trip causes.
When in doubt about the load type, ask the mechanical team for the starting torque and acceleration time. A crusher restarting full of material needs far more current than a free running pump.
7 Proven Steps for Correct VFD Sizing
Step one should use the motor current at the actual supply voltage. In India, motors are often rated at 415 V, so check the nameplate column for that voltage, and follow the drive setup advice in VFD parameter configuration.
Steps four to six are where most VFD sizing mistakes happen in the field. A drive that is correctly sized on paper can still overheat in a hot rooftop panel or trip on a long cable run.
VFD Sizing Derating for Heat and Altitude
Drive ratings are normally given at 40 °C ambient and up to 1000 m altitude. ABB specifies a 1 percent derate per 100 m above 1000 m for the ACS580, up to 4000 m.
VFDs.com cites the Galt G500 manual, which asks for a 1 percent derate for every degree above 40 °C and does not allow operation above 50 °C. Air at high altitude is thinner and cools less, an effect also seen in altitude effects on instrumentation.
VFDs.com calculates that a drive at 2500 m needs a 15 percent derate, so a 180 A unit can deliver only 153 A. A mine site near 4000 m would face about 30 percent.
VFD Sizing Formula
Altitude factor kA = 1 minus 0.01 × (H minus 1000) ÷ 100, when H is above 1000 m
Required drive current = motor FLC ÷ (kT × kA)
Example: FLC = 28 A, T = 45 °C, H = 1500 m
kT = 1 minus 0.05 = 0.95
kA = 1 minus 0.05 = 0.95
kT × kA = 0.9025
Required drive current = 28 ÷ 0.9025 = 31.0 A
In this VFD sizing example, choose a drive whose rated current at the chosen duty is at least 31.0 A in this example. Always confirm the actual derating curves in the manual of the drive you buy, because each maker publishes its own.
VFD Sizing Calculator
Second Worked Example: Single Phase Supply
A farm workshop has only single phase supply and wants to run a 3 phase motor with a nameplate current of 9 A. VFDs.com gives the safe rule of doubling the motor current when a 3 phase drive is fed from single phase, so the drive needs an 18 A three phase rating.
The input rectifier and DC bus capacitors carry much higher ripple on single phase, which is why the derate is so large. The difference in supply types is covered in single phase vs three phase power.
Switching Frequency and Motor Cable Length
A higher switching frequency makes the motor quieter but raises switching losses in the IGBTs, so most makers derate output current at higher carrier settings. ABB lists 1, 4, 8 and 12 kHz options for smaller ACS580 frames, so check the manual for the current allowed at each setting.
Long motor cables add capacitance, so the drive supplies extra charging current and the motor sees voltage spikes from reflected waves. Check the maximum cable length in the manual and add an output reactor or a dV/dt filter when the run is long.
Cable size also matters for voltage drop and heating, using the same method as in cable sizing for motor feeders. Use symmetrical shielded cable for good EMC behaviour.
If a drive must sit far from the motor, place it so the motor cable stays within the limit and run the longer section on the supply side. Supply cables carry no high frequency switching edges.
Load Types and Their VFD Sizing Approach
Torque rises with speed squared, little starting torque.
Full torque at all speeds, high breakaway.
Overhauling loads that regenerate energy.
One drive feeding several motors in parallel.
For hoists and quick stopping machines, regenerated energy must go somewhere, so VFD sizing must include the braking chopper and resistor, described in VFD braking resistor sizing. Group drives need individual motor overload protection because the drive cannot protect each motor separately.
- Reliable starting under full load.
- Fewer overload and overtemperature trips.
- Longer IGBT and capacitor life.
- Lower capital cost than heavy oversizing.
- Correct motor protection settings.
- Undersized drives trip during acceleration.
- Hot panels shorten drive life.
- Oversized drives waste money and panel space.
- Long cables damage motor insulation.
- Harmonics overload the supply transformer.
Supply Side Checks for VFD Sizing
A 6 pulse drive draws non sinusoidal current, so many drives on one transformer raise harmonic distortion. Use input reactors or DC chokes and consider an active harmonic filter when the drive load is a large share of the transformer rating.
The upstream breaker or fuse must suit the drive input current and the maker recommendation, as shown in fuse selection guide. Harmonic effects on other plant equipment are explained in power quality issues with VFDs and PLCs.
VFD Sizing Checklist Before Ordering
- Motor nameplate current at site voltage recorded.
- Load type and starting torque confirmed.
- Normal or heavy duty rating selected.
- Ambient temperature and altitude derating applied.
- Switching frequency and its derating checked.
- Motor cable length within limit or filter added.
- Braking need reviewed for stopping and lowering loads.
- Enclosure IP rating and panel ventilation planned.
Motor insulation should suit inverter duty, especially on older motors, as discussed in motor insulation classes. A careful VFD sizing review at this stage avoids costly replacements after commissioning.
ABB Technical Guide on Drive Dimensioning
How to Size and Select a Drive, Video Guide
VFD Sizing FAQ
It is the selection of a drive whose current, overload capacity and environmental rating suit the motor and the load. It always starts from the motor nameplate current and the type of load.
Site conditions such as heat, altitude and cable length then adjust the choice. A correctly sized drive starts the load reliably and runs cool for many years.
Size it by current, because motors of the same kW can draw quite different currents. Pole count, efficiency and power factor all change the current drawn from the drive.
ABB Technical Guide No 7 advises choosing the drive according to the dimensioning current. The catalogue kW value is only a convenient guide for typical 4 pole motors.
Normal duty allows a small overload, such as 110 percent for 1 minute, and suits pumps and fans. Heavy duty allows a larger overload, such as 150 percent for 1 minute.
Heavy duty suits conveyors, crushers and other constant torque loads. For the same frame, the heavy duty current rating is lower than the normal duty rating.
Many makers derate about 1 percent for every 100 m above 1000 m altitude. ABB states this rule for the ACS580 drive range up to an altitude of 4000 m.
At 2500 m the derate is about 15 percent, so a larger frame may be needed. Always confirm the exact curve published in the manual for your drive.
Most drives are rated for an ambient of 40 °C and lose capacity above that temperature. The Galt G500 manual cited by VFDs.com asks for 1 percent derating per degree above that limit.
Indian panel rooms in summer can easily exceed that temperature limit. Provide good ventilation or air conditioning, or choose a larger drive with enough margin.
Yes, many drives accept single phase input with derating, or special single phase input models exist. VFDs.com suggests doubling the motor current as a safe rule.
So a 9 A motor would need a drive rated for 18 A on 3 phase input. Check that the maker permits single phase input for the chosen model.
Long motor cables add capacitance, so the drive must supply extra charging current on every switching edge. Reflected waves can also nearly double the voltage seen at the motor terminals.
Check the maximum cable length given in the manual for each frame size. Use an output reactor or a dV/dt filter whenever the cable run exceeds that limit.
Related Articles
- VFD Working Principle Explained
- VFD Overload Trip Causes
- VFD Braking Resistor Sizing
- dV/dt Filter for VFD Motor Protection
- Motor Full Load Current FLC
External References
- Technical Guide No 7, Dimensioning of a Drive System, ABB
- How and When to Derate Variable Frequency Drives, VFDs.com
- Variable Frequency Drive, Wikipedia
What We Learn Today
- VFD sizing starts from the motor nameplate current, not the kW, because poles, efficiency and power factor change the current a motor draws.
- Choose normal duty for pumps and fans and heavy duty, often 150 percent for 1 minute, for conveyors, crushers and other constant torque loads.
- Derate for ambient above 40 °C, altitude above 1000 m, higher switching frequency, long motor cables and single phase supply before ordering the drive.