Method and arrangement for controlling permanent magnet synchronous machines

1. A method of controlling a sensorless permanent magnet synchronous machine using a frequency converter connected to feed the machine with electrical power, the method comprising:

producing a current vector having a magnitude to a stator of the machine with the frequency converter; and

rotating the current vector for rotating a rotor of the machine,

wherein the method further comprises:

prior to starting a drive of the machine, defining a value for the magnitude of the current vector and a frequency limit;

using the defined value as the magnitude of the current vector when the rotor of the machine is rotated with a frequency that is lower than the defined frequency limit; and

using a vector control method for controlling the machine when the frequency of the machine is higher than or equal, or below to the defined frequency limit, wherein the vector control method includes:

providing a signal proportional to a torque of the machine;

amplifying the signal proportional to the torque and feeding the amplified signal to a controller for obtaining a speed compensation term;

subtracting the speed compensation term from an estimated speed obtained from a vector control algorithm for obtaining a corrected speed estimate;

using the corrected speed estimate in a motor model for obtaining a stator current estimate;

calculating a stator current error as a difference between a measured stator current and the estimated stator current; and

estimating a speed of the machine from the stator current error for correcting an orientation of an estimate of a rotor angular position.

2. A method according to claim 1 , comprising:

using the vector control method in the frequencies below the defined frequency limit.

3. A method according to claim 2 , comprising:

producing a current reference for the rotating current vector;

producing current references with the vector control method;

summing the current reference for the rotating current vector to the current references obtained with the vector control; and

controlling the machine with the summed current reference.

4. A method according to claim 3 , comprising:

stopping the summing of the current references when the frequency of the machine exceeds the defined frequency limit.

5. A method according to claim 1 , comprising:

limiting the value of the speed compensation term when the frequency of the machine exceeds the defined frequency limit.

6. An arrangement for controlling a sensorless permanent magnet synchronous machine using a frequency converter connected to feed the machine with electrical power, the arrangement comprising:

means for producing a current vector having a magnitude to a stator of the machine with the frequency converter;

means for rotating the current vector for rotating a rotor of the machine;

means for setting a value for the magnitude of the current vector and a frequency limit;

means for taking into use the defined value as the magnitude of the current vector when the rotor of the machine is rotated with a frequency that is lower than the defined frequency limit; and

means for taking into use a vector control method for controlling the machine when the frequency of the machine is higher than or equal, or below to the defined frequency limit, wherein the means for taking into use the vector control method include:

means for providing a signal proportional to a torque of the machine;

means for amplifying the signal proportional to the torque and feeding the amplified signal to a controller for obtaining a speed compensation term;

means for subtracting the speed compensation term from an estimated speed obtained from a vector control algorithm for obtaining a corrected speed estimate;

means for using the corrected speed estimate in a motor model for obtaining a stator current estimate;

means for calculating a stator current error as a difference between a measured stator current and the estimated stator current; and

means for estimating a speed of the machine from the stator current error for correcting an orientation of an estimate of a rotor angular position.