Electric drive and method for controlling it

An electric drive comprising:

• a brushless motor having windings and a rotor;
• a power supply bridge for powering the motor, each phase winding of the motor having a resistance (RS) and a synchronous inductance (LS) a first circuit for detecting zero crossings of a counter electromotive force (ES) induced in the windings by rotation of the rotor;
• a second circuit having a detector stage for detecting a phase current (IS) flowing in the windings;
• and
• a third circuit for controlling the power supply bridge;
• the first circuit comprising an inductive and resistive element which is an analog representation of the phase winding electrical impedance, said inductive and resistive element being connected in series with a phase winding of the motor, the inductive and resistive element having an attenuated resistance and inductance, according to an attenuation coefficient “α”, as compared to the resistance and inductance of one of the phase windings of the motor, the first circuit also comprising a circuit having three wye connected first, second and third resistors respectively valued
• “
• R
• α
• =
• R
• ⁢
• α
• 3
• -
• 2
• ⁢
• α
• ”
• ,
• “R”, “R”, for measuring a motor supply voltage applied to said phase winding, an output of the first circuit being an attenuated c.e.m.f, signal:
• α·ES=α·VS−α·RSαIS−α·LS·dIS/dt;
• where 0≦α≦1;
• the third circuit comprising a controller in communication with the first circuit and the second circuit, with the first circuit providing the zero crossings of the counter electromotive force and with the second circuit providing the phase current flowing in the windings, said third circuit utilizing the communications from the first circuit and second circuit for applying an advance angle (δopt) between the motor supply voltage (VS) and the induced counter electromotive force (ES), said advance angle (δopt) being a predetermined linear function of a peak value of the phase current (IS) according to an equation:
• δ
• =
• π
• ·
• L
• S
• ·
• p
• 60
• ·
• K
• E
• ·
• I
• S
• where “LS” is the synchronous inductance of the motor measured in Henrys, “p” is a number of poles of the motor and “KE” is a counter electromotive force constant measured in V/rpm.