Drive belt slip detection

1. A method for detecting slippage between an electric machine and a machine drivingly connected to the electric machine by a drive belt, the method comprising:

determining at least two operational parameters selected from a rotational speed of a rotor of the electric machine, a current supplied to or generated by the electric machine and a voltage supplied to or generated by the electric machine; and

determining whether belt slip is occurring based only upon said at least two determined operational parameters.

2. The method as claimed in claim 1 wherein the electric machine is an electric motor.

3. The method as claimed in claim 2 wherein the method further comprises reducing at least one of the current and the voltage supplied to the motor in order to reduce the torque produced by the motor when belt slip is determined to be occurring.

4. A method for detecting slippage between an electric machine and a machine drivingly connected to the electric machine by a drive belt, comprising:

adjusting at least one of the current and the voltage supplied to the electric machine in order to reduce the torque produced by the electric machine based on at least two operational parameters selected from a rotational speed of a rotor of the electric machine, a current supplied to or generated by the electric machine and a voltage supplied to or generated by the electric machine.

5. The method as claimed in claim 3 further comprising determining first and second rotor speeds (RPM a ) and (RPM b ) separated by a short time delay, subtracting the first rotor speed (RPM a ) from the second rotor speed (RPM b ) to produce a speed difference value and combining the speed difference value with the determined values of voltage (V) and current (A) supplied to the motor to produce a belt slip value.

6. The method as claimed in claim 5 wherein the belt slip value (BSV) is calculated in accordance with the equation BSV=((RPM b −RPM a ) 2 *A)/V.

7. The method as claimed in claim 6 wherein belt slip is determined to be occurring when the belt slip value is greater than a predetermined threshold.

8. The method as claimed in claim 3 further comprising:

determining a first internal resistance of the motor (R a );

determining whether the first internal resistance (R a ) is between predetermined resistance limits;

determining whether a first motor rotor speed (RPM a ) is between predetermined speed limits; and

determining that slip is occurring only if the first internal resistance (R a ) is between its predetermined resistance limits and the first rotor speed (RPM a ) is between its predetermined speed limits.

9. The method as claimed in claim 1 wherein the electric machine is an electric generator.

10. The method as claimed in claim 9 further comprising:

reducing at least one of the current and the voltage generated by the generator in order to reduce the torque required to rotate the rotor of the generator when belt slip is occurring.

11. The method as claimed in claim 10 wherein the machine to which the electric machine is drivingly connected is an internal combustion engine.

12. A belt slip control system for controlling slip between an electric machine and a machine drivingly connected to the electric machine by a drive belt, comprising:

a controller adapted to determine at least two operational parameters selected from a rotational speed of a rotor of the electric machine, a current supplied to or generated by the electric machine and a voltage supplied to or generated by the electric machine said controller using only the determined operational parameters to determine whether belt slip is occurring.

13. The control system as claimed in claim 12 wherein the electric machine is an electric motor and the controller is further adapted to reduce the torque generated by the motor when belt slip is determined to be occurring.

14. The control system as claimed in claim 13 wherein the electric machine is an electric generator and the controller is further adapted to reduce the torque required to drive the rotor of the generator when belt slip is determined to be occurring.

15. The control system as claimed in claim 14 wherein the electric machine is a combined motor/generator and, when belt slip is determined to be occurring, the controller is operable to reduce the torque generated by the combined motor/generator when the combined motor/generator is operating as a motor and to reduce the torque required to drive the rotor of the motor/generator when the combined motor/generator is operating as a generator.

16. A control system as claimed in claim 12 wherein the controller determines a belt slip value using the determined rotational speed, voltage and current and compares the determined belt slip value with a predetermined threshold to determine whether belt slip is occurring.

17. The control system as claimed in claim 12 wherein the controller determines a first internal resistance of the electric machine from the voltage and current, determines whether the first internal resistance is within predetermined resistance limits, determines whether a first rotor speed is within predetermined speed limits and determines that belt slip is occurring only if the first internal resistance and the first rotational speed are both within their respective limits.