Control of the mains bridge of a frequency converter to regenerate energy from the motor to the supply

1. A method for controlling the mains bridge of a four-quadrant PWM frequency converter provided with a DC intermediate circuit when the power is flowing in the direction towards the supply network, comprising:

providing said frequency converter with an AC inductor to be connected to an alternating voltage source, a controlled mains bridge, a DC intermediate circuit and a controlled load bridge for feeding a variable-frequency alternating voltage into a load, and providing said mains bridge with controlled semiconductor switches and shunt diodes; and

controlling the mains bridge so that the controlled semiconductor switch in the upper branch of the phrase having the highest supply voltage instantaneous value and the controlled semiconductor switch in the lower branch of the phase having the lowest supply voltage instantaneous value are conducting;

advancing the control of the mains bridge semiconductor switch to be conducting next by a time sufficient to cause the current of the conducting phase turned from negative to positive before commutation, the size of the time being determined by the size of the current in said mains bridge.

2. A method according to claim 1 in a frequency converter wherein the mains bridge semiconductor switches are gate turn-off components, such as IGBTs,

and the control of the switch to be conducting next is advanced by at least a time step T 1 , where

T1

=

1

2

⁢

f

×

{

1

-

1

π

×

[

cos

-

1

(

2

×

π

×

f

×

L

×

i

D

⁢

⁢

C

U

-

1

]

}

3. A method according to claim 1 in a frequency converter wherein the semiconductor switches in the mains bridge are thyristors having a recovery time of (t Q ),

and the control of the thyristor to be conducting next is advanced by at least a time step T 2 +0.5*t Q , where

T2

=

⁢

1

2

⁢

f

×

{

1

-

f

×

t

Q

-

1

π

×

cos

-

1

[

2

×

π

×

f

×

L

×

i

D

⁢

⁢

C

U

+

⁢

cos

⁡

(

π

×

(

1

-

f

×

t

Q

)

)

]

}

.

4. A four-quadrant PWM frequency converter, comprising:

a mains bridge, said mains bridge being controlled by means of a control unit so the power flows in the direction towards the supply network;

an AC inductor connected to an alternating voltage source, a DC intermediate circuit and a controlled load bridge for feeding a variable-frequency alternating voltage into a load, said mains bridge being provided with controlled semiconductor switches and shunt diodes;

the mains bridge being controlled so that the controlled semiconductor switch in the upper branch of the phase having the highest supply voltage instantaneous value and the controlled semiconductor switch in the lower branch of the phase having the lowest supply voltage instantaneous value are conducting;

the control unit advancing the control of the semiconductor switch to be conducting next by a time sufficient to cause the current of the conducting phase to turn from negative to positive just before commutation, the size of the time being determined by the size of the current in said mains bridge.

5. A frequency converter according to claim 4 , wherein the mains bridge semiconductor switches are gate turn-off components, such as IGBTs,

and the control unit advances the control of the mains bridge semiconductor switch to be conducting next by at least time T 1 , where

T1

=

1

2

⁢

f

×

{

1

-

1

π

×

[

cos

-

1

(

2

×

π

×

f

×

L

×

i

DC

U

-

1

]

}

.

6. A frequency converter according to claim 4 , wherein the mains bridge semiconductor switches are thyristors having a recovery time (t Q ),

and the control unit advances the control of the thyristor to be conducting next by at least time T 2 +0.5*t Q , where

T2

=

⁢

1

2

⁢

f

×

{

1

-

f

×

t

Q

-

1

π

×

⁢

cos

-

1

[

2

×

π

×

f

×

L

×

i

DC

U

+

⁢

cos

⁢

(

π

×

(

1

-

f

×

t

Q

)

)

]

}

.