Method and arrangement for determining rotation speed of a motor

1. A method for determining the rotation speed of a motor fed by an inverter, wherein a feed connection between the inverter and the motor has three phases, the method comprising:

determining the rotation speed of the motor in at least two alternative manners, wherein one of the at least two alternative manners comprises:

measuring the frequency of a voltage fed to the motor by an inverter; and

estimating the rotation speed of the motor on the basis of the measured frequency,

wherein the measurement of the frequency of the voltage fed to the motor by the inverter comprises:

measuring two phase to phase voltages of the feed connection between the inverter and the motor;

determining, on the basis of the measured phase to phase voltages, the rate of change of the angle of the voltage fed to the motor by the inverter; and

determining the frequency corresponding to the rate of change of the angle.

2. The method of claim 1 , comprising,

low-pass filtering the measured two phase to phase voltages of the feed connection between the inverter and the motor to remove the modulation frequency, whereby the rate of change of the angle of the voltage fed to the motor by the inverter is determined on the basis of the low-pass filtered phase to phase voltages.

3. The method of claim 1 , wherein the motor is an induction motor or a synchronous motor.

4. An arrangement for determining the rotation speed of a motor fed by an inverter, wherein a feed connection between the inverter and the motor has three phases, the arrangement comprising:

means for determining the rotation speed of the motor in at least two alternative manners, wherein the means for determining the rotation speed of the motor comprise for performing one of the at least two alternative manners:

means for measuring the frequency of the voltage fed to the motor by the inverter; and

means for estimating the rotation speed of the motor on the basis of the measured frequency,

wherein the means for measuring the frequency of the voltage fed to the motor by the inverter comprise:

means for measuring two phase to phase voltages of the feed connection between the inverter and the motor;

means for determining the rate of change of the angle of the voltage fed to the motor by the inverter on the basis of the measured phase to phase voltages; and

means for determining the frequency corresponding to the rate of change of the angle.

5. The arrangement of claim 4 , wherein the means for measuring two phase to phase voltages of the feed connection between the inverter and the motor comprise means for low-pass filtering the measured two phase to phase voltages of the feed connection between the inverter and the motor.

6. The arrangement of claim 4 , wherein the motor is an induction motor or a synchronous motor.

7. A device for determining the rotation speed of a motor fed by an inverter, wherein a feed connection between the inverter and the motor has three phases, the device being configured to:

determine the rotation speed of the motor in at least two alternative manners, wherein the device is, in order to perform one of the at least two alternative manners, configured to:

determine the frequency of the voltage fed to the motor by the inverter; and

estimate the rotation speed of the motor on the basis of the determined frequency, wherein the device is configured to determine the frequency of the voltage fed to the motor by the inverter by:

measuring two phase to phase voltages of the feed connection between the inverter and the motor;

determining, on the basis of the measured phase to phase voltages, the rate of change of the angle of the voltage fed to the motor by the inverter; and

determining the frequency corresponding to the rate of change of the angle.

8. An inverter, the inverter being configured to:

determine the rotation speed of a motor fed by the inverter in at least two alternative manners, wherein the inverter is, in order to perform one of the at least two alternative manners, configured to:

measure the frequency of the voltage fed to the motor by the inverter; and

estimate the rotation speed of the motor on the basis of the measured frequency, wherein the inverter is configured to measure the frequency of the voltage fed to the motor by the inverter by:

measuring two phase to phase voltages of a feed connection between the inverter and the motor;

determining, on the basis of the measured phase to phase voltages, the rate of change of the angle of the voltage fed to the motor by the inverter; and

determining the frequency corresponding to the rate of change of the angle.

9. A frequency converter, the frequency converter being configured to:

determine the rotation speed of a motor fed by the frequency converter in at least two alternative manners, wherein the frequency converter is, in order to perform one of the at least two alternative manners, configured to:

measure the frequency of the voltage fed to the motor by the frequency converter; and

estimate the rotation speed of the motor on the basis of the measured frequency, wherein the frequency converter is configured to measure the frequency of the voltage fed to the motor by the frequency converter by:

measuring two phase to phase voltages of a feed connection between the frequency converter and the motor;

determining, on the basis of the measured phase to phase voltages, the rate of change of the angle of the voltage fed to the motor by the frequency converter; and

determining the frequency corresponding to the rate of change of the angle.