Method and arrangement in connection with a brake chopper

1. A method of controlling a brake chopper, the brake chopper being connected between the positive and the negative rails of an intermediate voltage circuit feeding an inverter bridge, wherein current measurement means are arranged between a DC voltage source and the brake chopper and adapted to measure DC current flowing in the intermediate voltage circuit, which method comprises steps of:

forming, with the inverter bridge, an output voltage to a load connected to the output of the inverter bridge,

determining a time instant when the DC current of the intermediate circuit is sampled, and

preventing the use of the brake chopper at the determined time instant.

2. A method according to claim 1 , wherein the step of

forming of the output voltage to the load with the inverter bridge comprises a step of

forming an output voltage vector in each control period by using only one zero vector and one or two active vectors; and

determining the time instant comprises the steps of

choosing the time instant to be in the middle of the control period if the formed output voltage vector sequence comprises a zero vector ‘000’, and

choosing the time instant to be at the beginning or at the end of the control period if the formed output voltage vector sequence comprises a zero vector ‘111’.

3. A method according to claim 2 , wherein the step of determining the time instant comprises steps of

synchronizing the brake chopper with control periods of the inverter control, and

receiving information from the used zero vector from a modulator.

4. A method according to claim 1 , wherein the step of

forming the output voltage to the load with the inverter bridge comprises a step of

forming an output voltage vector in each control period by using only one zero vector and one or two active vectors; and

determining the time instant comprises the steps of

choosing the time instant to be in the middle of the control period if the zero vector is at the beginning or at the end of the control period, and

choosing the time instant to be at the beginning or at the end of the control period if the zero vector is in the middle of the control period.

5. A method according to claim 4 , wherein the step of determining the time instant comprises steps of

synchronizing the brake chopper with control periods of the inverter control, and

receiving information from the used zero vector from a modulator.

6. A method according to claim 1 , wherein the step of determining the time instant comprises steps of

synchronizing the brake chopper with control periods of the inverter control, and

receiving information from the used zero vector from a modulator.

7. A brake chopper control arrangement, the brake chopper being connected between the positive and the negative rails of an intermediate voltage circuit feeding an inverter bridge, wherein current measurement means are arranged between a DC voltage source and the brake chopper and adapted to measure DC current flowing in the intermediate voltage circuit, wherein the arrangement comprises:

means for determining a time instant when the DC current of the intermediate circuit is sampled, and

means for preventing the use of the brake chopper at the determined time instant.

8. A brake chopper control arrangement according to claim 7 , wherein in the arrangement an output voltage vector in each control period is formed by using only one zero vector and one or two active vectors and the means for determining a time instant comprises means for choosing the time instant on the basis of the used zero vector such that the time instant is in the middle of the control period if the formed output voltage vector comprises a zero vector ‘000’ and at the beginning or at the end of the control period if the formed output voltage vector comprises a zero vector ‘111’.