Method for thermal protection of frequency converter and a frequency converter

1. A method for thermal protection of a frequency converter, which frequency converter comprises means for controlling the output current of the frequency converter, the method comprises the steps of

determining beforehand two or more data points (I sw1 , Tsink1 , I sw1 , Tsink2 , I sw1 , Tsink3 ) which define a thermal current limit for a semiconductor component of the frequency converter at specific temperatures (T sink1 , T sink2 , T sink3 ) at two or more switching frequencies (f sw1 , f sw2 , f sw3 ),

determining beforehand two or more data points (I fout , 0 , Tsink1 , I fout , 0 , Tsink2 , I fout , 0 , Tsink3 ) which define a thermal current limit for the semiconductor component at specific temperatures (T sink1 , T sink2 , T sink3 ) at a zero converter output frequency,

measuring the temperature (T sink ) of the semiconductor component or relating to the semiconductor component,

determining the switching frequency (f sw ) of the frequency converter,

determining the output frequency (f out ) of the frequency converter,

determining the highest allowable thermal current (I th ) as a function of the measured temperature (T sink ), determined switching frequency (f sw ) and determined output frequency (f out ) on the basis of the defined data points, and

limiting the output current of the frequency converter to the determined highest allowable thermal current (I th ), wherein

the step for determining the highest allowable thermal current (I th ) comprises the steps of

calculating allowable thermal currents (I sw1 , I sw2 , I sw3 ) at the two or more switching frequencies (f sw1 , f sw2 , f sw3 ) from the data points (I sw1 ,t sink1 , I sw1, Tsink2 , I sw1, Tsink3 ) on the basis of the measured temperature (T sink ),

defining a limit frequency (f out,lim ), the output frequencies higher than the limit being considered high output frequencies and the output frequencies lower or equal to the limit being considered low output frequencies,

calculating an allowable high-output frequency thermal current (I fout,hi ) for high output frequencies from the calculated allowable thermal currents (I sw1 , I sw2 , I sw3 ) and from the two or more switching frequencies (f sw1 , f sw2 , f sw3 ) on the basis of the determined switching frequency (f sw ),calculating an allowable zero converter output frequency thermal current (I fout, 0 ) for the zero converter output frequency from the data points (I fout , 0 , Tsink1 , I fout , 0 , Tsink2 , I, 0 , Tsink3 ) on the basis of the measured temperature (T sink ),

using the calculated allowable high-output frequency thermal current (I fout,hi ) for the high output frequencies as the highest allowable thermal current (I th ), if the determined output frequency is high,

calculating the highest allowable thermal current (I th ) from the calculated allowable high-output frequency thermal current (I fout,hi ) for the high output frequencies and from the calculated allowable zero converter output frequency thermal current (I fout,0 ) for the zero converter output frequency on the basis of the determined converter output frequency (f out ) if the output frequency is low.

2. A method according to claim 1 , wherein the step for determining beforehand two or more data points (I fout , 0 , Tsink1 , I fout , 0 , Tsink2 , I fout , 0 , Tsink3 ) at a zero converter output frequency comprises a step in which two or more data points are determined at specific temperatures by using two or more switching frequencies (f sw1b , f sw2b , f sw3b ).

3. A method according to claim 2 , wherein the method comprises the further steps of

calculating the highest allowable thermal currents (I fout , 0 , sw1b , I fout , 0 , sw2b , I fout, 0 , sw3b ) at a zero output frequency at two or more switching frequencies (f sw1b , f sw2b , f sw3b ), and

calculating the highest allowable thermal current for a zero output frequency from the highest allowable thermal currents (I fout , 0 , sw1b , I fout , 0 , sw2b , I fout , 0 , sw3b ) at the zero output frequency and from the two or more switching frequencies (f sw1b , f sw2b , f sw3b ) on the basis of the determined switching frequency (f sw ).

4. A method according to claim 3 , wherein the step of calculating allowable thermal currents (I sw1 , I sw2 , I sw3 ) on the basis of the measured temperature (T sink ) comprises a step of interpolating the allowable thermal currents (I sw1 , I sw2 , I sw3 ), which correspond to the measured temperature (T sink ) and switching frequencies (f sw1 , f sw2 , f sw3 ).

5. The method according to claim 4 , wherein the step of calculating the allowable high-output frequency thermal current (I fout, hi ) for high output frequencies on the basis of the determined switching frequency (f sw ) comprises a step of interpolating the allowable high-output frequency thermal current (I fout, hi ) for high output frequencies, which thermal current corresponds to the determined switching frequency (f sw ).

6. A method according to claim 3 , wherein the step of calculating the allowable high-output frequency thermal current (I fout, hi ) for high output frequencies on the basis of the determined switching frequency (f sw ) comprises a step of interpolating the allowable high-output frequency thermal current (I fout, hi ) for high output frequencies, which thermal current corresponds to the determined switching frequency (f sw ).

7. The method according to claim 6 , wherein the step of calculating the allowable zero converter output frequency thermal current (I fout, 0 ) for the zero converter output frequency on the basis of the measured temperature (T sink ) comprises a step of interpolating the allowable zero converter output frequency thermal current (I fout, 0 ) for the zero converter output frequency, which thermal current corresponds to measured temperature (T sink ).

8. A method according to claim 3 , wherein the step of calculating the allowable zero output frequency thermal current (I fout, 0 ) for the zero converter output frequency on the basis of the measured temperature (T sink ) comprises a step of interpolating the allowable zero output frequency thermal current (I fout, 0 ) for the zero converter output frequency, which thermal current corresponds to measured temperature (T sink ).

9. The method according to claim 8 , wherein the step of calculating the highest allowable thermal current (I th ) on the basis of the determined converter output frequency if the output frequency is low comprises a step of interpolating the highest allowable thermal current (I th ), which corresponds to the determined converter output frequency (f out ).

10. A method according to claim 3 , wherein the step of calculating the highest allowable thermal current (I th ) on the basis of the determined converter output frequency if the output frequency is low comprises a step of interpolating the highest allowable thermal current (I th ), which corresponds to the determined converter output frequency (f out ).

11. The method according to claim 3 , wherein the output frequency is a low-pass filtered value of the actual output frequency (f out ).

12. A method according to claim 1 , wherein the output frequency is a low-pass filtered value of the actual output frequency (f out ).

13. The method according to claim 12 , wherein the method comprises the further steps of

determining if the frequency converter is in the regenerative mode or in the motoring mode,

multiplying the determined highest allowable thermal current (I th ) with a factor (k), the value of which depends on the determined mode of the frequency converter, and

using the product (kI th ) as the highest allowable thermal current.

14. A method according to claim 1 , wherein the method comprises the further steps of

determining if the frequency converter is in the regenerative mode or in the motoring mode,

multiplying the determined highest allowable thermal current (I th ) with a factor (k), the value of which depends on the determined mode of the frequency converter, and

using the product (kI th ) as the highest allowable thermal current.

15. The method according to claim 14 , wherein the step of calculating allowable thermal currents (I sw1 , I sw2 , I sw3 ) on the basis of the measured temperature (T sink ) comprises a step of interpolating the allowable thermal currents (I sw1 , I sw2 , I sw3 ), which correspond to the measured temperature (T sink ) and switching frequencies (f sw1 , f sw2 , f sw3 ).