What size of a frequency converter do I need? (4 methods to find out)

Frequency converter (FC) rating

When determining the rating of a frequency converter at a given load the first step is to consider the load characteristics. There are four different methods for calculating the required output performance and the choice of method depends on the motor data. However, first let’s start with the basics of motor load characteristics.

Load characteristics

Before the frequency converter size can be determined, a distinction must be made between the two most widely used load characteristics. Figure 1 shows the four most typical motor loads and their characteristics.

Group 1 – Machines for winding material under tension (veneer cutting machines)
Group 2 – Conveyor belts, cranes, positive displacement pumps as well as machine tools
Group 3 – Machines such as rollers, smoothing machines and other processing machines
Group 4 – Machines operating by centrifugal force (centrifuges, centrifugal pumps and fans)

The reasons for distinguishing between load characteristics are the following //

When the speed of centrifugal pumps and fans increases, the power requirement increases by the speed cubed (P = n³). The normal working range of centrifugal pumps and fans is the speed range of 50 to 90%. The load ratio increases by the speed squared, i.e. approx. 30 to 80%.

These two factors are shown in the torque characteristics of a frequency converter controlled motor. Figure 3 and Figure 4 show torque characteristics for two different FC sizes – one of them (Figure 4) is one power range lower than the other.

For both torque characteristics the same load characteristics were entered for a centrifugal pump.

In Figure 3, the total working range of the pump (0-100%) lies within the rated values of the motor. Since the normal working range of the pump is 30-80%, a frequency converter with a lower output can be chosen.

If the load torque is constant, the motor must be able to generate more than the load torque as the excess torque is used for acceleration.

The overload torque also guarantees that the system is able to cope with sudden increases of load. A frequency converter that does not allow any overload torque has to be selected such that the acceleration torque (TB) lies within the rated torque.

4 methods with different sets of motor data //

When the load characteristics have been determined, there are four different sets of motor data for deciding the power size of the frequency converter (FC).

Method #1 (current Im)

The frequency converter can be determined quickly and precisely on the basis of the current I_M which the motor takes up. If the motor is not fully loaded, the motor current could be measured on a similar system in full operation.

Example // A 7.5 kW, 3 x 400 V motor draws 14.73 A.

Referring to the technical data of the frequency converter, a frequency converter is selected that has a maximum continuous output current higher than or equal to 14.73 A at constant or square torque characteristics.

Method #2 (apparent power Sm)

The frequency converter can be selected on the basis of the apparent power S_M taken up by the motor and the apparent power delivered by the frequency converter.

Example // A 7.5 kW, 3 × 400 V motor draws 14.73 A

Referring to the technical data of the frequency converter, it is selected whose maximum continuous output is higher than or equal to 10.2 kVA at constant or square torque characteristics.

Method #3 (power Pm)

A frequency converter can also be selected in accordance with the power P_M generated by the motor. However, since cos φ and efficiency η change with the load, this method is imprecise.

Example // A 3 kW motor with an efficiency and cos φ of 0.80 or 0.81 draws as follows //

A frequency converter is selected – referring to the technical data of the frequency converter – that has a maximum continuous output higher than or equal to 4.6 kVA at constant or square torque characteristics.

Method #4 (standard series of motors)

For practical reasons, the power rating of most frequency converters follows the standard series of the asynchronous motors. Consequently, frequency converters are often selected on this basis but this can lead to imprecise sizing, in particular if the motor is not subjected to a full load.

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Reference // Facts Worth Knowing About Frequency Converters by Danfoss