Changing Speed by Using Frequency Converters

The speed of an electric motor can be changed by altering the frequency of the electric current. This is because the ratio is the same as 60 or 50 ftp (f = the frequency of the current, p = the number of poles in the stator). Frequency converters are built of electronic components, frequently combined with mi­croprocessors. They provide good motor protection and are superior to the traditional bimetal protection. The characteristic curve for a pump and fan motor is also quadratic, making lower demands to the frequency converters When the frequency of the electrical current is changed in the frequency converter, the main AC supply is transformed into DC. The DC is then treated

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In the frequency converter’s components, before being transformed back into AC at the required frequency.

Frequency converters constructed for use with three-phase motors! ess than 4 kW can therefore be delivered with a single-phase supply from the elec­trical mains. For greater power requirements, it is most common to use a three-phase power supply. Through switches and potentiometers or the pro­gramming module of the frequency converter, the important parameters of electrical motors can be adjusted as needed.

The advantage of using frequency converters is that the possibility ex­ists to use a ramp function when starting and stopping the motor (soft start). By using this function, it is possible to avoid starting both fans at full speed with closed dampers; it also reduces stresses on the fan transmis­sion (belts) at the start.

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