Inverter circuit

Abstract

The invention relates to an inverter circuit for operating an asynchronous motor having an adjustable speed. The inverter has its output frequency regulatable by a frequency regulator and the D.C. supply has its voltage regulatable by a voltage regulator. There is a desired value frequency regulator having a signal (f) and voltage and current measuring devices having output signals (E) and (I). A control circuit processes these signals (f, E, I) to form input values for the referred to voltage and frequency regulators. The control circuit includes a computer circuit which, from at least two output values and at least one constant, calculates a computed value of the third output value which, together with the first-mentioned output values, given by way of computation a substantially constant slip frequency for a given desired value, and this computed value is compared with a value substantially corresponding to the actual third output value. The three output values are interlinked in the computer circuit in such a way that a constant slip frequency is obtained for the desired frequency value that is fed in. The D.C. voltage supplied to the inverter is regulated so that this condition is maintained independently of the load. The result of this is that the nominal slip for a nominal load also occurs on partial load and this leads to the desired constant speed. This slip frequency can be kept constant even at low loads in that the air gap magnetization is made substantially proportional to the rotor current, i.e. there is under-magnetization.

Dependencies are inferred from the claim text.