The invention relates to a method of starting an electric motor (10) powered by an alternating power supply voltage, said method consisting of supplying current to the motor (10) with a delay t.sub.ret, during each period of the power supply voltage. The starting method comprises a first sequence in which the following steps are carried out during each n.sup.th period of the power supply voltage, where n is an integer number greater than 0: a) determine the value of a variation of a resistance of the electric motor during the previous n periods of the power supply voltage; b) compare said value of the variation of the determined motor resistance with a first threshold resistance value; c) if the value of the variation of the motor resistance is less than the first threshold value, reduce t.sub.ret. The invention also relates to a starter device and a computer program.

Systems and techniques detecting a reverse current are disclosed. An apparatus comprises a switching circuit coupled to a load and a reference node. The switching circuit may be capable of conducting a reverse current from the reference node to the load when a voltage at the load is lower than a voltage at the reference node. A voltage source has a first terminal coupled to the load, a second terminal configured to follow a voltage at the load, and produces a voltage proportional to a voltage drop across the switching circuit. A comparator circuit is coupled to compare a voltage at the second terminal of the voltage source to the voltage at the reference node and configured to indicate when the reverse current has a magnitude greater than a predetermined threshold.

The present invention provides an acceleration method for V/f controlled induction motor in flux-weakening region, which comprises: acquiring no-load magnetizing current I.sub.m of the induction motor at current stator frequency; selecting a smaller one of 0.5.Math.I.sub.m(1/σ+1) and (I.sub.m.sup.2+σ)/(I.sub.m+σI.sub.m) as magnetizing current set point, in which σ is an estimated total leakage inductance coefficient; getting an error signal by subtracting the magnetizing current of the induction motor from the magnetizing current set point; determining the stator frequency for the next control period according to the error signal which is provided as a controlling variable of negative feedback. The acceleration method of the present invention can provide the maximum output torque in flux-weakening region and has a larger tolerance for the error of the estimated leakage inductance.

The present invention provides an acceleration method for V/f controlled induction motor in flux-weakening region, which comprises: acquiring no-load magnetizing current I.sub.m of the induction motor at current stator frequency; selecting a smaller one of 0.5.Math.I.sub.m(1/σ+1) and (I.sub.m.sup.2+σ)/(I.sub.m+σI.sub.m) as magnetizing current set point, in which σ is an estimated total leakage inductance coefficient; getting an error signal by subtracting the magnetizing current of the induction motor from the magnetizing current set point; determining the stator frequency for the next control period according to the error signal which is provided as a controlling variable of negative feedback. The acceleration method of the present invention can provide the maximum output torque in flux-weakening region and has a larger tolerance for the error of the estimated leakage inductance.

Examples include a method for controlling a motor soft-starter for starting an electric motor on a three-phases electric network in order to compensate a misbalance between the windings of the electric motor due to a misbalance between the phases of the electric network.

A motor starter and a method for starting an electric motor are disclosed. In an embodiment, the motor starter includes a phase gating unit and a control unit. The phase gating unit is configured to be deactivated via the control unit after an activity period of the phase gating unit has elapsed and is configured to be deactivated during operation after the activity period has elapsed.

Pairs of phases of an AC power source are connected to pairs of phases of a motor in a first sequence that repeats at a first frequency. The motor is braked by connecting pairs of phases of the AC power source to pairs of phases of the motor in a second sequence that is reversed with respect to the first sequence and that repeats at a second frequency less than the first frequency. In further aspects, pairs of phases of an AC power source are connected to pairs of phases of a motor in a first sequence. All of the phases of the motor are subsequently disconnected from the AC power source for a predetermined dwell interval having a duration greater than a time constant of the motor, and thereafter the motor is braked by connecting pairs of phases of the AC power source to pairs of phases of the motor in a second sequence that is reversed with respect to the first sequence. Motor starters implementing such operations are also disclosed.

A method is for operating a three-phase cage motor on a multiphase electrical grid via a soft starter, with which one or more grid phases of the grid being respectively switchable by firing thyristors. Apart from a grid-related firing criterion, a rotor-flux-related firing criterion is taken into account.

A motor starter apparatus includes an enclosure, a bypass contactor configured to be installed in the enclosure and configured to selectively couple a power source to a motor, and a reduced voltage soft starter (RVSS) circuit assembly (e.g. a wheeled truck) configured to be installed in the enclosure, to be coupled to the power source and to apply a variable voltage to the motor. The bypass contactor is configured to preserve a connection of the bypass contactor to the power source and the motor when the RVSS circuit assembly is removed from the enclosure.

A method is for operating a three-phase cage motor on a multiphase electrical grid via a soft starter, with which one or more grid phases of the grid being respectively switchable by firing thyristors. Apart from a grid-related firing criterion, a rotor-flux-related firing criterion is taken into account.