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How to dimension a variable-speed drive

03 February, 2020

ABB’s Liam Blackshaw offers advice on how to dimension a variable-speed drive correctly to suit the application.

When assessing choosing a VSD for an application, it is important to avoid over-dimensioning that could lead to unnecessary costs, impaired performance and reduced efficiency.


Check the initial conditions  

Before choosing a VSD and motor, check the mains supply voltage and frequency. The frequency does not limit the speed range of the application. 


Check the process requirements  

Do you need a particular starting torque? What is the speed range? What type of load will there be? 


Speed range: It is essential to know the real operating speed range. While this is not critical with a centrifugal load, for a constant torque load it is possible to oversize the motor to cope with the high loads at lower speeds. A critical factor is the ventilation capability of the shaft-mounted fan. At low speeds, the cooling capability is reduced, but this reduction is not linear. “Loadability” is important for shaft-mounted fans. If this is too restrictive, separate forced ventilation can be used.


Load types: The motor is part of an AC drive system and cannot be considered in isolation from the converter. It is important to know the type of load to be controlled before making any selections.

There are three different load groups:

•  quadratic load: this is the most common and covers centrifugal pumps and fans; 

•  constant torque: this occurs when fixed volumes are being handled, such as in screw compressors, extruders, or straight-line web handling;

•   constant power: this is typically used when a material is being rolled up and the diameter changes.

There may also be combinations of these load types. However, selecting the wrong load characteristic could easily lead to oversizing.


Select the motor  Any AC motor is a source of torque, and yet it is the power rating that is most often used when looking at catalogue ratings. These ratings are based on the fixed-speed output when connected to the network. Torque is related to both power and speed, so that T (Nm) = P (kW) x (9550)/ n (r/min). When dimensioning a VSD it is important to think in terms of torque. The motor must withstand process overloads and be able to produce a specified amount of torque. The motor’s thermal overloadability should not be exceeded. It is also necessary to leave a margin of around 30% for the motor’s maximum torque when considering the maximum available torque in the dimensioning phase.


Select the VSD  The VSD is chosen according to the initial conditions and the selected motor. The drive’s capability to produce the required current and power should be checked. Advantage should be taken of the drive’s potential overloadability in the event of a short-term cyclical load. Even with simple applications such as pumps or fans there will be a duty cycle. Initial start-up may require a high torque to achieve a specific acceleration. The thermal time constant for a converter is relatively short – possibly 4–6 minutes – and it will heat up and cool down quickly, while the motor thermal time constant can range from 15 minutes for a small motor to a couple of  hours for a large motor. This means that the overload requirements are generally far more critical for converter sizing than for motor sizing.

Finally, it is often possible to improve performance by fine-tuning the settings in the drive software. Several motor control adjustments are available, such as slip compensation, acceleration and deceleration times, and maximum current. Taking these into consideration will help ensure trouble-free performance. Also bear in mind that it is never a good idea to select a drive based on cost alone – choosing a product simply because it is the cheapest may save a bit at the outset, but will usually end up costing much more through incorrect operation and poor reliability.


Download Technical Guide No.7: Dimensioning of a drive system from

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