Method for estimating the sum of stator and rotor resistances

1. A method for estimating the sum of the stator and rotor resistances of an induction machine using a frequency converter when the frequency converter is connected to the induction machine, the method comprising the steps of

controlling the stator current of the induction machine with the frequency converter such that the rotor flux (ψ R ) changes substantially linearly while the stator current is substantially constant,

determining the magnitude of stator voltage and stator current while the rotor flux is changing substantially linearly, and

calculating the sum of the stator and rotor resistances from the determined magnitudes of stator voltage and stator current.

2. A method according to claim 1 , wherein the controlling of the stator current comprises the steps of

controlling the stator current to the first reference value (I ref1 ) for a first time period (t 1 ) so that the rotor flux starts to grow in the direction of the current,

controlling the stator current to the second reference value (I ref2 ) for a second time period (t 2 ) so that the rotor flux starts to grow in the direction of the current, whereby preferably I ref2 >0, i ref1 =−i ref2 and t 2 =2×t 1 .

3. A method according to claim 1 , wherein the magnitudes of stator voltage and stator current are determined during a period in which the average value of the rotor flux is substantially zero.

4. A method according to claim 1 , wherein the sum of the stator and rotor resistances is calculated by calculating an average of the stator voltage and an average of the stator current during the period in which the rotor flux is substantially zero and dividing the average stator voltage by the average stator current.

5. A method according to claim 1 , wherein the controlling of the stator current comprises the steps of

controlling a DC-current to the stator of the induction machine and,

inserting a square pulse current into the stator current of the induction machine using the frequency converter, the pulse current having an amplitude (i s2 -i s1 ).

6. A method according to claim 1 , wherein the magnitudes of stator voltage and stator current are determined during periods in which the average value of the rotor flux is substantially equal during consecutive positive and negative pulses.

7. A method according to claim 6 , wherein the determining of the magnitude of stator voltage comprises steps of:

determining the stator voltage (u s,ave1 ) of the induction machine during the square pulse when the stator current is at the first amplitude (i s1 ), and

determining the stator voltage (u s,ave2 ) of the induction machine during the square pulse when the stator current is at the second amplitude (i s2 ).

8. A method according to claim 5 , wherein the sum of the stator and rotor resistances is calculated by:

calculating a difference between stator voltages (u s,ave2 −u s,ave1 ) that are determined when the stator current is at the first amplitude and second amplitude, and

dividing said difference of the voltages by the amplitude of the pulsed current (i s2 −i s1 ).

9. A method according to claim 5 , wherein the determined stator voltages (u s,ave1 , u s,ave2 ) are average voltages of the measurement period.

10. A method according to claim 5 , wherein the step of inserting square pulse current into the stator current comprises a step in which multiple n consecutive square pulses are injected into the stator current, whereby the sum of the stator and rotor resistances is calculated by

calculating the sum of stator voltages

∑

i

=

1

n

⁢

u

s

,

ave

⁢

⁢

1

,

i

 determined when the stator current is at the first amplitude (i s1 ) during consecutive pulses,

calculating the sum of stator voltages

∑

i

=

1

n

⁢

u

s

,

ave

⁢

⁢

2

,

i

 determined when the stator current is at the second amplitude (i s2 ) during consecutive pulses,

subtracting the sum of the stator voltages

∑

i

=

1

n

⁢

u

s

,

ave

⁢

⁢

1

,

i

 determined at the first amplitude from the sum of the stator voltages

∑

i

=

1

n

⁢

u

s

,

ave

⁢

⁢

2

,

i

 determined at the second current amplitude, and

dividing the subtraction result by the number of consecutive pulses n and by the amplitude of the pulsed current (i s2 −i s1 ).

11. A method according to claim 5 , wherein the stator DC-current, into which the pulsed current is injected, has a magnitude that is substantially equal to the nominal magnetization current.