Selecting a variable frequency drive (VFD) for a motor starts with its rated current, voltage and the characteristics of the driven machine. Matching the power rating in kilowatts is not enough: the drive must handle the required overloads, work with the motor across the entire speed range, and be compatible with the electrical installation and automation system. Only then can you safely assess whether speed control will improve process operation and energy efficiency.
How do you select a VFD for a motor? The key principle
The VFD output current available under actual operating conditions must meet the motor’s current requirements, while its overload capacity must accommodate starting and load changes. You must also confirm the voltage, motor type, speed range, cooling, cable length and control method. A drive selected solely because it is labelled “15 kW” may be unsuitable even for a motor also rated at 15 kW.
We recommend checking the following, in order:
- Define the process requirements: torque, speed range, operating time, and the frequency of starting and braking.
- Read the motor ratings and check whether its design permits operation from a variable frequency drive.
- Select the voltage class and verify the drive’s continuous output current after accounting for installation conditions.
- Compare the required peak current and overload duration with the characteristics of the selected drive.
- Check cooling, insulation, cables, protection, electromagnetic compatibility and automation.
- Plan commissioning, including electrical measurements and checks of machine functions.
This sequence – from the supply and load, through the motor, to the drive current rating – is also described in ABB’s guide to drive system dimensioning.
What information should you read from the motor nameplate?
The nameplate is the starting point, but final selection also requires data on the driven machine and the motor documentation. Particular care is needed when retrofitting older drives if full information on winding insulation, cooling or the permissible speed range is unavailable.
| Parameter | What to check | Why it matters for selection |
|---|---|---|
| Voltage and winding connection | E.g. 230/400 V Δ/Y or 400/690 V Δ/Y | These must match the drive’s output voltage and the motor’s winding connection. |
| Rated current Iₙ | The current in amperes for the relevant voltage and connection | This is the basis for checking the drive’s continuous output current capacity. |
| Rated power Pₙ and speed nₙ | Mechanical shaft power and rated speed | These help assess torque and the required drive characteristics. |
| Frequency and motor type | 50/60 Hz, induction, synchronous or another type | These determine control algorithm compatibility and parameter settings. |
| Duty and cooling method | Continuous/intermittent duty; shaft-mounted or independent fan | These determine the permissible thermal load at low speeds. |
| Sensors and technical condition | PTC/PT100, encoder, insulation and bearing condition | These affect protective functions and retrofit risk. |
Do not confuse the motor’s rated mechanical shaft power, expressed in kW, with the electrical power drawn from the supply. For a broader drive efficiency assessment, see our article on electric motor efficiency.
Output current, overload capacity and VFD operating conditions
Why is the kW rating alone insufficient?
VFD catalogues often associate drives with typical motor power ratings. For a detailed selection, however, you must compare the motor’s rated current with the drive’s permissible continuous output current, taking account of the selected voltage class, duty rating and any derating. A manufacturer may specify different current ratings depending on switching frequency, ambient temperature or mounting conditions. See Schneider Electric’s technical explanation of current-based drive selection.
It is also important to distinguish between Normal Duty and Heavy Duty modes where a model offers them. A higher permissible short-term overload may mean lower available power or current in continuous operation. In the Altivar Process range, for example, these ratings depend on the model and its Dual Rating setting; they are not universal values for all VFDs. See Schneider Electric’s description of Normal Duty and Heavy Duty.
What does the load characteristic mean?
| Load type | Typical application | Implication for VFD selection |
|---|---|---|
| Approximately constant torque | Conveyor, mixer, selected process machines | Check low-speed torque, starting conditions and overload capacity. |
| Torque increasing approximately with the square of speed | Some centrifugal pumps and fans | A different load characteristic; the operating point and process constraints still need to be checked. |
| Approximately constant power over part of the range | Winder, certain machining systems | Torque varies with speed; the full operating range must be checked. |
The “power proportional to the cube of speed” relationship must not be applied automatically to a conveyor, positive-displacement pump or any process machine. Even for centrifugal pumps, the result depends on the system curve, static head and the need to maintain the required process function. This article concerns electrical drive selection; detailed savings calculations for pumps and fans require a separate process analysis.
Starting, braking and low-speed operation
Assessing overload capacity requires more than peak torque and current: the duration and frequency of the load also matter. A loaded conveyor starting up and a system with high inertia may need a different margin than a fan started only occasionally. Rapid braking can return energy to the drive’s DC link; depending on the application, check whether resistor braking or regenerative braking is required and what braking duty cycle is permissible.
A standard motor with a fan mounted on its shaft generally cools less effectively at low speed. Sustained high torque at low speed calls for a review of the thermal loadability curve, temperature sensors and sometimes independent cooling. Above base speed, field weakening, reduced available torque and the mechanical limits of the motor and machine must be considered. These relationships are covered in ABB’s sections on motor loadability and drive dimensioning.
230/400 V supply, winding connections and motor compatibility
The term “three-phase VFD” may describe the output, not necessarily the input. Check the drive’s input supply requirements separately from the three-phase voltage it can provide to the motor.
For example, a typical motor nameplated 230/400 V Δ/Y may, subject to the manufacturer’s approval, operate in delta at 230 V line-to-line or in star at 400 V. A VFD with a 1 × 230 V input and a 3 × 230 V output may therefore be suitable for such a motor with the correct winding connection, but it does not replace a 3 × 400 V drive for a motor requiring 400 V in its selected configuration. Not every motor or drive supports this arrangement. The final decision must be based on the nameplate and both manufacturers’ instructions, not merely the number of phases in the product name.
Also verify that the selected VFD supports the specific motor type and the required control method. A conventional induction motor, a permanent-magnet synchronous motor and an encoder-equipped drive may have different requirements.
Motor cables, EMC and protection – an often-overlooked part of selection
The VFD is one component of the overall installation. On the input side, check supply characteristics, protective devices, supply currents, any line reactors and coordination with switchgear. On the output side, factors include the type and length of motor cable, cable capacitance, shielding, protective earthing and compliance with the equipment manufacturer’s recommendations.
Long cables and fast output voltage pulses can increase stress on winding insulation and the risk of bearing currents. Depending on the particular VFD–motor combination and cable length, the manufacturer may recommend a du/dt filter, sine-wave filter or other measures. There is no single safe cable-length threshold or mandatory filter for all models. Compare ABB’s grounding and cabling guide with Danfoss’s output filter guide.
Selection must also account for:
- electromagnetic interference immunity and emissions (EMC), including routing of power and signal cables;
- protection against electric shock and the types of protective devices specified in the VFD instructions;
- temperature, dust, cabinet ventilation, enclosure protection rating and required installation clearances;
- compatibility with motor temperature sensors and required protective functions;
- special environmental conditions, such as a potentially explosive atmosphere, where applicable.
Ecodesign requirements for certain motors and variable speed drives placed on the EU market arise, among other provisions, from Commission Regulation (EU) 2019/1781, as amended. These set efficiency requirements for equipment within their scope, not a table for sizing a VFD for a particular load. The installation must also be designed to meet applicable safety and electromagnetic compatibility requirements, taking the manufacturers’ documentation into account.
Control: V/f, vector control and automation interfaces
Scalar V/f control may be sufficient for a simple system with moderate dynamic requirements. If high torque at low speed, a fast response to changing loads or more precise drive control is needed, vector control – sometimes with an encoder – should be considered. The choice must reflect the required machine function and the motor’s capabilities.
Before ordering the VFD, it is worth listing the required signals and interfaces:
- start, stop and direction commands, plus ready-status feedback;
- speed or torque reference – from a panel, analogue input or PLC;
- operating status, alarm, frequency, current and temperature feedback;
- a suitable communications protocol, such as Modbus or PROFINET, if required by the existing automation system;
- process control (e.g. PID), process interlocks, behaviour after power loss and machine safety requirements.
The Safe Torque Off (STO) function, if available, does not automatically replace every emergency-stop function or isolation of the power supply for maintenance. The safety solution must be selected on the basis of a risk assessment of the specific machine.
Example: a 15 kW motor and two VFDs with seemingly identical power ratings
Consider an induction motor rated 15 kW, 400 V, 50 Hz, 28,5 A and 1470 rpm. Starting analysis for the machine indicates a demand of approximately 42 A for 5 seconds, with the starting cycle repeated every few minutes. These are assumed figures to illustrate the method, not measurements from a particular plant.
- VFD A is marketed as “15 kW”, but in the required duty mode and after accounting for installation conditions its continuous output current is 27 A. It fails to meet the basic 28,5 A requirement despite the matching power label.
- VFD B has a continuous current rating of 32 A under the same conditions, and its illustrative rating allows 150% of that current for a defined period. This gives 48 A, exceeding the assumed 42 A. It is a candidate for further evaluation, but the permissible duration, intervals between overloads, actual torque characteristic and motor cooling must still be checked.
Motor torque can also be estimated from rated power and speed: Tₙ ≈ 9550 × 15 / 1470 ≈ 97,4 Nm. If the machine requires 80 Nm at 1200 rpm, its mechanical shaft power is approximately P ≈ 80 × 1200 / 9550 ≈ 10,1 kW. This is not the power drawn from the supply: motor and VFD efficiency at this operating point and other process loads must also be considered. The calculation neither determines energy savings nor, on its own, settles VFD selection.
Commissioning: what should be checked after installation?
Even a correctly selected model needs proper configuration and testing. During commissioning:
- Check connections, earthing, protective devices, shielding, permissible cable length and cooling.
- Enter the motor’s rated data and configure the required control method, current limits and thermal protection.
- Carry out the motor identification procedure permitted by the manufacturer and confirm rotation direction.
- Set acceleration and braking times, and test responses to load steps, signal loss and faults.
- Check PLC communications, process sensors, interlocks and safety functions.
- Record current, frequency, temperatures, alarms and the key process parameter under representative operating conditions.
Measurements must use methods suited to distorted waveforms and the type of signal. A current reading from an arbitrary meter is no substitute for checking power and the energy balance of the entire system.
Will a correctly selected VFD automatically reduce energy consumption?
No. A VFD enables changes in speed and control method, but also introduces losses of its own. The potential energy benefit depends on whether control reduces the energy needed to perform the same useful process work. Compare the active power demand of the entire system before and after the retrofit under comparable conditions; with varying output, also compare a specific indicator such as kWh per tonne or batch of product.
From an energy auditor’s perspective, I would start with the load profile and process requirements. If the equipment operates at the same required speed most of the time, buying a VFD solely “to save energy” may not be justified. A system that genuinely needs flow or output control may be different. That, however, is a separate question from electrical VFD selection.
What should you prepare before deciding to buy?
An application description and the available data are enough for an initial assessment; there is no need to commission a full measurement campaign immediately. The most useful inputs are a photo of the motor nameplate, machine documentation, supply arrangement, approximate duty cycle, required speed and torque range, motor cable length, and details of control and automation.
If you are planning a drive retrofit, send the available information using the Energy Trend contact form. This provides a basis for identifying missing data and deciding which next steps make technical and energy-efficiency sense.
Technical and legal sources
- ABB – Technical Guide No. 7: Dimensioning of a Drive System – load characteristics, current, torque, speed range and cooling.
- Schneider Electric – VFD selection by voltage and current and Normal Duty and Heavy Duty modes – continuous current and overload capacity.
- ABB – Grounding and Cabling of Drive Systems – cabling, earthing and EMC.
- Danfoss – Output Filters Design Guide – limitations associated with pulsed output voltage and filters.
- Commission Regulation (EU) 2019/1781 – consolidated text – ecodesign requirements for equipment within its scope.
