Estimation of parameters of an induction machine

1. A method of identifying a parameter of an induction machine when the induction machine is connected to output phases of a voltage source inverter and the induction machine is in standstill state, the method comprising:

a first phase which includes:

magnetizing the induction machine by providing a first DC magnetization current (i m ) to the induction machine via the inverter until the induction machine reaches steady state, a current reference of the inverter having a first value (i dc — ref1 ),

controlling power semiconductors of the inverter to an off-state for a predetermined period of time (Δt),

controlling all output phases of the inverter to a same potential for providing a zero voltage vector, and

measuring a first maximum value (i mz — max ) of a stator current (i sd ) during the zero voltage vector; and

in a second phase:

magnetizing the induction machine by providing a second DC magnetization current to the induction machine for a magnetization time period (t 1 −t 0 ) via the inverter, the current reference having a second value (i dc — ref2 ) which is higher than the first value of the current reference (i dc — ref1 ),

controlling the power semiconductors of the inverter to an off-state for the predetermined period of time (Δt),

controlling all the output phases of the inverter to a same potential for providing a zero voltage vector,

measuring a second maximum value (i sdz — max ) of the stator current during the zero voltage vector,

estimating a parameter of the induction machine when the measured first and second maximum values of the stator current (i mz — max , i sdz — max ) are equal, and

repeating the second phase with different magnetization of the induction machine when the first and second measured maximum values of the stator current are not equal.

2. A method according to claim 1 , wherein the identified parameter is a main inductance of the induction machine, and wherein the second phase comprises:

estimating rotor flux (ψ R (t 1 )) of the induction machine produced during the magnetization time period, and

estimating the main inductance of the machine from an estimated rotor flux of the machine and from the first DC magnetization current.

3. A method according to claim 2 , wherein the estimating of the main inductance comprises:

using the first value of the current reference of the inverter (i dc — ref1 ) as the first DC magnetization current.

4. A method according to claim 2 , comprising:

estimating the main inductance as a quotient between estimated rotor flux (ψ R (t 1 )) and the first value of the current reference used in the first magnetization (i dc — ref1 ).

5. A method according to claim 2 , wherein the estimating of the rotor flux comprises:

estimating stator flux of the induction machine produced during the magnetization time period until the stator current has dropped to zero; and

using the estimated stator flux as the rotor flux.

6. A method according to claim 1 , wherein the identified parameter is the rotor time constant (τ r ) of the induction machine, and wherein the method comprises:

calculating the rotor time constant from the magnetization time period (t 1 −t 0 ) and from the first and second values of the current references used in the first phase (i dc — ref1 ) and in the second phase (i dc — ref2 ).

7. A method according to claim 6 , comprising:

calculating the rotor time constant as:

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8. A method according to claim 1 , comprising:

when the measured first and second maximum values of the stator current are not equal, changing the magnetization of induction machine by changing the second value (i dc — ref2 ) of the current reference.

9. A method according to claim 1 , comprising:

when the measured first and second maximum values of the stator current are not equal, changing the magnetization of the induction machine by changing the magnetization time period (t 1 −t 0 ).

10. An arrangement for identifying parameters of an induction machine when the induction machine is connected to the output phases of a voltage source inverter and the induction machine is in standstill state, wherein the arrangement comprises:

means for controlling an induction machine during a first control phase by providing a first DC magnetization current (i m ) to magnetize the induction machine via an inverter until the induction machine reaches steady state, a current reference of the inverter having a first value (i dc — ref1 ), and for controlling power semiconductors of the inverter to an off-state for a predetermined period of time (Δt), and for controlling all output phases of the inverter to a same potential for providing a zero voltage vector; and

means for measuring a first maximum value (i mz — max ) of stator current (i sd ) during the zero voltage vector, the means for controlling then magnetizing the machine during a second control phase by providing a second DC magnetization current to the induction machine for a magnetization time period (t 1 −t 0 ) via the inverter, the current reference having a second value (i dc — ref2 ) which is higher than the first reference (i dc — ref1 ), the means for measuring also measuring a second maximum value (i sdz — max ) of the stator current during a zero voltage vector of the second control phase; and,

means for estimating a parameter of the induction machine when the measured first and second maximum values of the stator current (i mz — max , i sdz — max ).

11. An arrangement of claim 10 , in combination with an induction machine.