Abstract: 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..sub.s) in the machine (1) and a total leakage inductance (.sigma.L.sub.s) 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 (i.sub.s1) and switching off the stator circuit at the end of a first time period (T.sub.1); short-circuiting the stator windings once more after a second time period (T.sub.2) from the end of the first time period (T.sub.1); measuring a second stator current vector (i.sub.s2) and switching off the stator circuit after a time identical with the first time period (T.sub.1) from the second short-circuiting; determining the frequency (.PSI..sub.s) of the residual flux (.omega..sub.

Abstract: The invention concerns a method and apparatus for real-time measurement of refiner mechanical pulp quality index such as freeness. According to the method, refiner mechanical pulp transported in a steam-phase carrier is illuminated with electromagnetic radiation, and the change in the radiation spectrum caused by refiner mechanical pulp is detected. According to the method, the change in the radiation spectrum caused by refiner mechanical pulp is measured at a minimum number of four wavelength bands and the measured intensities are computationally processed to obtain the desired index of quality.

Abstract: The invention relates to a method for determining the stator flux of an asynchronous machine when a stator current (i.sub.2) and stator voltage (u.sub.s) of the asynchronous machine are measured and a short-circuit inductance (.sigma.L.sub.s), stator inductance (L.sub.s) and rotor time constant (.tau..sub.r) of the machine are assumed to be known, wherein the product of the stator current (i.sub.s) and a stator resistance estimate (R.sub.se) is determined, the obtained product is subtracted from the stator voltage (u.sub.s), and the obtained voltage difference is integrated with respect to time to obtain a first stator flux estimate (.psi..sub.se). In the invention, the method further comprises the steps of determining a second stator flux estimate (.psi..sub.se,id) on the basis of the stator voltage (u.sub.s), the stator current (i.sub.s), the short-circuit inductance (.sigma.L.sub.s), the stator inductance (L.sub.s), the rotor time constant (.tau..sub.r) and the stator resistance estimate (R.sub.

Abstract: The invention relates to a current measuring transducer operating on the compensation principle, comprising two magnetic cores (C1, C2) with windows; windings wound thereon for both a current (Ip) to be measured and for a compensation current (Is); and means (Um, Nm, R1, D, IA) for generating the compensation current and adjusting the magnitude thereof. To compensate for induced interference voltages, a further winding (Nc1, Nc2) is arranged about both magnetic cores of the current transducer. To widen the frequency band, the windings (Nc1, Nc2) are series-connected in a closed loop through a frequency-dependent impedance (Z).

Abstract: The invention relates to a semiconductor switch control circuit comprising a timing logic unit (1), a first and a second driver (2, 3) controlled by the timing logic unit, a first and a second transformer (T1, T2), to which the first and the second driver (2, 3), respectively, apply an unmodulated ac voltage signal (A, B) for bringing the semiconductor switch into a conducting and non-conducting state, respectively, and the secondary windings of which are connected at the opposite ends to a reference potential (RP); a first and a second rectifier (4, 5) for rectifying dc voltages (V+, V-) of opposite polarity from a voltage (A, B) acting across the secondary windings of the transformers; a first and a second driver (10, 11) for maintaining the semiconductor switch (12) in a conducting and non-conducting state, respectively; and capacitor units (8, 9) for storing the turn-on and turn-off energy of the semiconductor switch, respectively.

Abstract: The invention relates to a method of preventing the stalling of an asynchronous machine controlled by a controller (6) affecting the slip and having a response time less than a rotor time constant (.tau..sub.r) of the machine. To minimize the safety margin between the allowable maximum torque and the stalling torque, the method comprises the steps of measuring rotational vectors (i.sub.s, u.sub.s) for the stator current and stator voltage of the machine; determining, on the basis of the measured rotational vectors (i.sub.s, u.sub.s) of the stator current and stator voltage and a short-circuit inductance (.sigma.L.sub.s) of the machine, two vectors one of which (.phi..sub.s) is parallel to the direction of the stator flux (.psi..sub.s) and the other (.phi..sub.r) to the rotor flux (.psi..sub.r); determining an angle (.gamma.) between said vectors parallel to the stator and rotor fluxes or a value (.mu.

Abstract: The invention relates to a method of determining the estimate of the stator flux of an electric machine, comprising the determination of a stator flux estimate (.PHI..sub.sest) of the electric machine as a time integral of the difference between the voltage (u.sub.s) supplied to the stator and the product of the stator current (i.sub.s) and the stator resistance estimate (R.sub.sest), whereby the stator resistance estimate (R.sub.sest) of the electric machine is determined by correcting the initial value or another earlier value of the stator resistance on the basis of the stator current component (i.sub.d.sup.s, i.sub.d.sup..delta., i.sub.d.sup.r) parallel to the stator flux estimate (.PHI..sub.sest), the air gap flux estimate (.PHI..sub..delta.est) or the rotor flux estimate (.PHI..sub.rest). To be able to follow changes in the stator resistance of the electric machine in different operational conditions of the machine, said current component (i.sub.d.sup.s, i.sub.d.sup..delta., i.sub.d.sup.

Abstract: The invention relates to a fan for an electrically operated machine, comprising a propeller (1) arranged axially about the machine and including a hub (8) and several blades attached to it and projecting substantially radially from it; and a cover (2) surrounding the propeller (1) in the front and on the sides and comprising a suction opening (3) on the suction side of the propeller (1) and an opening (7) on the pressure side for passing ventilation air into the machine parts to be cooled. To achieve a fan of simple construction, good efficiency and low noise level the blades (9) have a radius of curvature (R) and they are attached to the hub (8) at a blade angle (.alpha.) between 25.degree. and 70.degree.; a gap (S) defined between the suction opening (3) or its cylindrical extension (5) and the blades (9) of the propeller (1) in the axial direction is no more than 20% of the outer diameter of the propeller (1); the inner diameter (D.sub.A ) of the suction opening (3) before the propeller (1) is 0.8 to 1.

Abstract: This invention relates to a method for controlling the torque of a synchronous machine supplied by a frequency converter, wherein starting from measured variables (i.sub.R, i.sub.S, i.sub.T, i.sub.f) representing the state of the machine the direct axis flux component (.psi..sub.d), the quadrature axis flux component (.psi..sub.q) and the pole angle (.delta.) for the machine are determined, and set values are produced for the flux (.psi. set value), excitation current (i.sub.fo set value) and current building the torque (i.sub.2 set value). To prevent the "pull out" of the drive in the method of the invention, from the determined pole angle (.delta.) there is formed, by means of a limiting block for the pole angle, a limited pole angle (.delta..sub.r) that equals the pole angle (.delta.) if the pole angle has an absolute value smaller than a separately adjustable limit (.delta..sub.max), but equals the limit (.delta..sub.max) if the pole angle has an absolute value equal to or greater than the limit (.delta..

Abstract: The present invention relates to a method for controlling an ac motor supplied by an inverter having several switches. In order that the torque can be adjusted to be as desired as rapidly as possible both with maximum voltage and in the vicinity thereof, one proceeds in accordance with the invention in such a way that a value (.DELTA..gamma.) representing the difference between the reference value and actual value of the angle between the stator flux and rotor flux of the motor is determined on the basis of a value representing the requirement for torque change of the motor, e.g. by means of the difference (.DELTA.T) between the reference valve (T*) and actual valve (T) of the torque, and a quantity representing the magnetic state of the motor, the next switching moment of the switches of the inverter, i.e. the so called limit between flux sectors (.DELTA..phi.), is shifted with a first quantity (k.sub.1 .DELTA..gamma.) proportional to the determined differential value to change the angle (.gamma.