Synchronization of machine and load characteristics
An electrical machine is coupled to a load which has a torque profile which varies with angle. The machine is coupled so that the torque ripple of the machine is synchronized with the torque ripple of the load in such a manner as to provide an efficient drive system which can start at any angular position. This allows the size of the electrical machine to be reduced to a point where its average torque may be below the peak requirement of the load.
1. A method of starting a load having a cyclically variable input requirement and a repeatable rest position, the method comprising:
driving the load by an electric motor having a cyclically variable output, including a low output in the output cycle of the motor which is less than a peak in the variable input requirement of the load; and
coupling the motor with respect to the load so that the peak input requirement of the load coincides with a region in the output cycle of the motor in which the peak input requirement is exceeded by instantaneous output from the motor and such that the cyclically variable output of the motor is rising at start-up to boost the acceleration of the load from the rest position;
wherein the input requirement of the load and the output of the electric motor are torque; further wherein the cyclical variability of the output of the motor is related to the changing angular position of a rotor of the motor with respect to a stator of the motor.
2. A method as claimed in claim 1 in which the period of the output cycle of the motor is equal to or is an integral dividend of the period of the input requirement cycle of the load.
3. A method as claimed in claim 1 , in which the peak input requirement of the load substantially coincides with a peak output of the motor.
4. A method as claimed in claim 1 , in which the load is an internal combustion engine and the electric motor is coupled as a starter motor to the engine.
5. A method as claimed claim 1 , in which the motor is a switched reluctance motor.
6. A drive system for starting a load having a cyclically variable input requirement and a repeatable rest position, the system comprising:
an electric motor having a cyclically variable output, including a low output in the output cycle of the motor which is less than a peak in the variable input requirement of the load; and
a connection between the load and the motor arranged so that the peak input requirement of the load substantially coincides with a region in the output cycle of the motor in which the peak input requirement of the load is exceeded by instantaneous output from the motor and such that the cyclically variable output of the motor is rising at start-up to boost the acceleration of the load from the rest positions;
wherein the input requirement of the load and the output of the electric motor are torque; further wherein the cyclical variability of the output of the motor is related to the changing angular position of a rotor of the motor with respect to a stator of the motor.
7. A system as claimed in claim 6 , in which the period of the output cycle of the motor is equal to or is an integral dividend of the period of the input requirement cycle of the load.
8. A system as claimed in claim 6 , in which the peak input requirement of the load coincides with a peak output of the motor.
9. A system as claimed in claim 6 , in which the load is an internal combustion engine and the electric motor is coupled as a starter motor to the engine.
10. A system as claimed in claim 6 , in which the motor is a switched reluctance motor.
11. A method of starting an internal combustion engine having a cyclically variable starting torque requirement and a repeatable rest position, the method comprising:
coupling the engine to a switched reluctance machine having a torque output with a cyclically variable ripple, having a period equal to or an integral dividend of the period of the starting torque requirement of the engine, and which includes a low torque output which is less than a peak in the starting torque requirement of the engine; and
arranging the relative angular orientation of the machine and the engine so that a peak in the starting torque requirement of the engine coincides with a region in the torque output of the machine in which the starting torque requirement is exceeded by instantaneous torque output of the machine and such that the cyclically variable output of the machine is rising at start-up to boost the acceleration of the load from the rest positions;
wherein the cyclical variability of the output of the machine is related to the changing angular position of a rotor of the machine with respect to a stator of the machine.
12. A starting system for an internal combustion engine having a cyclically variable starting torque requirement and a repeatable rest position, the system comprising:
a switched reluctance machine, having a torque output with a cyclically variable ripple, having a period equal to or an integral dividend of the period of the starting torque requirement of the engine, and including a low torque output which is less than a peak in the starting torque requirement of the engine, the machine being coupled to the engine such that a peak in the starting torque requirement of the engine coincides with a region in the torque output of the machine in which the starting torque requirement is exceeded by instantaneous torque output of the machine and such that the cyclically variable output of the machine is rising at start-up to boost the acceleration of the load from the rest position;
wherein the cyclical variability of the output of the machine is related to the changing angular position of a rotor of the machine with respect to a stator of the machine.