Variable switching frequency power converter
A drive system for an electrical load such as an electric motor or generator comprises a PWM converter and a controller therefore. A signal indicative of a condition of the converter outpost is used in a controller to drive an optimal value of PWM switching frequency based on a load characteristic that is used to control the pulse switching frequency. The system takes account of the overall system in deriving an efficient PWM switching frequency.
1. A power converter system, comprising:
an electrical load,
a power converter having a variable frequency output connected to the load, the variable frequency output being generated by pulse width modulation at a switching frequency,
a controller operably connected to control the output of the power converter,
means for producing a signal indicative of a load current,
means for deriving a value of switching frequency of the power converter based on an expression of a characteristic of the load and the signal, and
means for setting the switching frequency of the power converter according to the value.
2. A system as claimed in claim 1 in which the means for deriving include a look-up table.
3. A system as claimed in claim 1 in which the means for deriving include means for running an algorithm.
4. A system as claimed in claim 1 in which the means for deriving include means for running a mathematical expression.
5. A system as claimed in claim 1 in which the means for producing a signal include a current transducer arranged in relation to the output of the power converter.
6. A system as claimed in claim 1 in which the means for producing a signal include a current transducer arranged in relation to a supply to the power converter.
7. A system as claimed in claim 1 in which the power converter produces an a.c. power output from a d.c. power input.
8. A system as claimed in claim 1 in which the load is an electrical machine producing a mechanical output from an electrical power input.
9. A system as claimed in claim 1 in which the load is an electrical power network and the power converter is supplied by a renewable energy source.
10. A system as claimed in claim 1 in which means for setting are operable to set the switching frequency to optimise the efficiency of the system.
11. A method of power conversion from a power converter having a variable frequency output connected to an electrical load, the variable frequency output being generated by pulse width modulation at a switching frequency, the method comprising:
producing a signal indicative of a load current,
deriving a value of switching frequency of the power converter based on an expression of a characteristic of the load and the signal, and
setting the switching frequency of the power converter according to the value.
12. A system as claimed in claim 4 wherein the mathematical expression is a polynomial expression.
13. A system as claimed in claim 8 wherein the electrical machine is an electric motor.
14. A system as claimed in claim 13 wherein the electric motor is an a.c. induction motor.
15. A system as claimed in claim 9 , wherein the renewable energy source is a photovoltaic cell or a wind power generator.