Frequency converter and its control method

1. A method of controlling a voltage controlled PWM (Pulse Width Modulated) frequency converter comprising a single phase rectifier bridge ( 10 ) connectable to a sinusoidal single phase supply, a DC intermediate circuit ( 11 ) and a controlled inverter bridge ( 12 ) for generating an AC output voltage (U out ) with varying amplitude and frequency to a load,

said inverter bridge ( 12 ) having PWM controlled semiconductor switches (V 11 –V 16 ) and flywheel diodes (D 11 –D 16 ) connected in inverse-parallel with the semiconductor switches, the method comprising the steps of:

controlling the frequency converter so that the supply line current (I in ) is substantially sinusoidal and in phase with the supply line voltage (U in ),

controlling the inverter bridge so that a curve of filtered average current (I dc ) in the DC intermediate circuit substantially follows a curve of a rectified AC supply voltage (U dc ), and

directly connecting the rectifier bridge is ( 10 ) to the inverter bridge ( 12 ),

wherein a curve of the power fed to a load is formed substantially as sin 2 (2πft) (f=line frequency t=time).

2. A method as defined in claim 1 , further comprising the step of:

providing the DC intermediate circuit with a DC capacitor unit having a low capacitance value, the DC capacitor being for limiting voltage spikes produced in switching situations, but not acting as an energy source.

3. The method as defined in claim 1 , further comprising the step of:

connecting two phase switches of the inverter bridge to two windings of a single phase motor and

connecting a third phase switch of the inverter bridge to a third phase winding of the single phase motor for producing a start torque through the third phase winding in order to avoid a use of a separate start capacitor in the motor.

4. A voltage controlled PWM (Pulse Width Modulated) frequency converter comprising:

a single phase rectifier bridge ( 10 ) connectable to a sinusoidal single phase supply,

a DC intermediate circuit ( 11 ),

a controlled inverter bridge ( 12 ) for generating an AC output voltage with varying frequency to a load and a PWM controller unit ( 14 ),

said inverter bridge ( 12 ) having PWM controlled semiconductor switches (V 11 –V 16 ) and flywheel diodes (D 11 –D 16 ) connected in inverse-parallel with the semiconductor switches,

wherein the PWM controller unit ( 14 ) controls the frequency converter so that the supply line current (I in ) is substantially sinusoidal and in phase with a supply line voltage (U in ),

wherein the PWM controller unit ( 14 ) controls the inverter bridge so that the curve of filtered average current (I dc ) in the DC intermediate circuit substantially follows a curve of a rectified AC supply voltage (U dc ),

wherein the rectifier bridge is ( 10 ) directly connected to the inverter bridge, and

wherein a curve of a power fed to a load is formed substantially as sin 2 (2πft) (f=line frequency t=time).

5. The voltage controlled PWM (Pulse Width Modulated) frequency converter as defined in claim 4 , wherein the DC intermediate circuit is provided with a DC capacitor unit having a low capacitance value, the DC capacitor being for limiting voltage spikes produced in switching situations.

6. The voltage controlled PWM (Pulse Width Modulated) frequency converter as defined in claim 4 , further comprising:

a filter unit consisting of inductors and capacitors on the supply line side in order to filter off high-frequency harmonics from a supply current.

7. The voltage controlled PWM (Pulse Width Modulated) frequency converter as defined in claim 4 , wherein two phase switches of the inverter bridge are connected to windings of a single phase motor and third phase switch of the inverter bridge produces a start torque through a third phase winding of the single phase motor in order to avoid a use of a separate start capacitor in the motor.