Method and device for starting an electric motor

1. A method for starting up an electric motor, which has a rotor, comprising the steps of:

driving the rotor in a first direction of rotation with a first torque, wherein a maximum value of the first torque is not greater than a maximum counter torque opposing the rotation of the rotor, causing the rotor to come to a standstill in a first stationary position;

starting from the first stationary position, driving the rotor in a second direction of rotation, which is opposite from the first direction of rotation, until the rotor comes to standstill in a predefined second stationary position; and

starting from the second stationary position, starting up the rotor in the first direction of rotation.

2. The method as claimed in claim 1 , wherein the electric motor comprises phase windings and the rotor is driven by applying excitation currents to the phase windings.

3. The method as claimed in claim 1 , wherein the rotor is driven by a pulse width modulated signal.

4. The method as claimed in claim 3 , wherein the pulse width modulated signal is a pulse width modulated current signal.

5. The method as claimed in claim 3 , wherein at least one of a duty cycle and a clock cycle of the pulse width modulated signal is selected such that the maximum value of the first torque is not greater than the maximum counter torque.

6. The method as claimed in claim 1 , wherein the electric motor is driven in a stepping mode before reaching the predefined first or second stationary position.

7. The method as claimed in claim 1 , wherein the second stationary position is located within an angular range from 0° to 90° behind the position which corresponds to the maximum counter torque.

8. The method as claimed in claim 1 , wherein the second stationary position is located within an angular range from 30° to 60° behind the position which corresponds to the maximum counter torque.

9. The method as claimed in claim 1 , wherein the second stationary position is located within an angular range from 35° to 45° behind the position which corresponds to the maximum counter torque.

10. The method as claimed in claim 1 , further comprising producing an increasing second torque when the motor is started from the predefined second stationary position.

11. The method as claimed in claim 10 , wherein the second torque increases linearly in steps.

12. The method as claimed in claim 10 , wherein the second torque increases over time with an increasing slope.

13. The method as claimed in claim 10 , wherein the second torque follows a start ramp and switches into an auto-commutation mode at an end of the start ramp, wherein at the end of the start ramp the counter torque is less than half of the maximum counter torque.

14. The method as claimed in claim 13 , wherein at the end of the start ramp the counter torque is less than one third of the maximum counter torque.

15. The method as claimed in claim 1 , wherein the electric motor is initially started up in current mode and switches over into voltage mode when reaching a predetermined rotational speed.

16. The method as claimed in claim 1 , further comprising holding the rotor in a rest position for a predetermined period of time before driving the rotor with the first torque.

17. The method as claimed in claim 1 , wherein the electric motor is employed to drive a compressor, in particular a compressor in a refrigerator, wherein the maximum counter torque corresponds to a counter torque at a compression point.

18. The method as claimed in claim 1 , further comprising:

applying control signals to the electric motor to drive the electric motor,

determining the control signals in a first start-up attempt by reference to parameters for a state with a first maximum counter torque, and

determining the control signals in a second start-up attempt, which follows the first run-up attempt, by reference to parameters for a state with a second maximum counter torque, wherein the second maximum counter torque is greater than the first maximum counter torque.

19. A method for starting up an electric motor, which has phase windings and a rotor, comprising the steps of:

producing a torque which moves the rotor in a first direction of rotation, in order to start up the electric motor;

when the start-up of the electric motor is unsuccessful

driving the rotor in a first direction of rotation with a first torque, wherein a maximum value of the first torque is not greater than a maximum counter torque opposing the rotation of the rotor, causing the rotor to come to a standstill in a first stationary position;

starting from the first stationary position, driving the rotor in a second direction of rotation, which is opposite from the first direction of rotation, until the rotor comes to standstill in a predefined second stationary position; and

starting from the second stationary position, starting up the rotor in the first direction of rotation.

20. A device for starting up an electric motor with a rotor designed as the drive for a compressor having a compression point, in particular a compressor in a refrigerator, wherein the device is configured

to drive the rotor in a first direction of rotation with a first torque, wherein a maximum value of the first torque is not greater than a maximum counter torque opposing the rotation of the rotor, causing the rotor to come to a standstill in a first stationary position;

starting from the first stationary position, to drive the rotor in a second direction of rotation, which is opposite from the first direction of rotation, until the rotor comes to standstill in a predefined second stationary position; and

starting from the second stationary position, to start up the rotor in the first direction of rotation;

wherein the maximum counter torque corresponds to a counter torque at the compression point.

21. An electric motor with a rotor designed as the drive for a compressor having a compression point, in particular a compressor in a refrigerator, the electric motor comprising a device for starting up the electric motor wherein the device is configured

to drive the rotor in a first direction of rotation with a first torque, wherein a maximum value of the first torque is not greater than a maximum counter torque opposing the rotation of the rotor, causing the rotor to come to a standstill in a first stationary position;

starting from the first stationary position, to drive the rotor in a second direction of rotation, which is opposite from the first direction of rotation, until the rotor comes to standstill in a predefined second stationary position; and

starting from the second stationary position, to start up the rotor in the first direction of rotation;

wherein the maximum counter torque corresponds to a counter torque at the compression point.