A METHOD OF DETERMINING A RESIDUAL FLUX OF AN INVERTER-SUPPLIED SQUIRREL CAGE INDUCTION MACHINE

PCT No. PCT/FI93/00281 Sec. 371 Date Feb. 6, 1995 Sec. 102(e) Date Feb. 6, 1995 PCT Filed Jul. 1, 1993 PCT Pub. No. WO94/03965 PCT Pub. Date Feb. 17, 1994.The invention relates to a method of determining rotational speed, size and instantaneous direction of a residual flux of an inverter-supplied squirrel cage induction machine, to be used for starting the squirrel cage induction machine when there is a rotating residual flux ( PSI s) in the machine (1) and a total leakage inductance ( sigma Ls) of the machine is known. According to the invention measures are taken, comprising steps of short-circuiting stator windings; measuring a first stator current vector (+E,rar/i/ s1) and switching off the stator circuit at the end of a first time period (T1); short-circuiting the stator windings once more after a second time period (T2) from the end of the first time period (T1); measuring a second stator current vector (+E,rar/i/ s2) and switching off the stator circuit after a time identical with the first time period (T1) from the second short-circuiting; determining the frequency ( PSI s) of the residual flux ( omega s) on the basis of the directions of the first and the second stator current vector (+E,rar/i/ s1, +E,rar/i/ s2) and the lengths of the first and the second time period; determining the size of an absolute value ( PSI s ) of the residual flux on the basis of the total leakage inductance ( sigma Ls) of the squirrel cage induction machine and the first or the second stator current vector (+E,rar/i/ 1, +E,rar/i/ s2 and determining the instantaneous direction of the residual flux ( PSI s) on the basis of the direction of the first or the second stator current vector (+E,rar/i/ s1, +E,rar/i/ s2).