The present invention relates in a general way to alternating current synchronous electric motors with permanent magnet rotors.

A first object of the invention is to provide a method for determining in a simple, reliable and economical way the mechanical load of a synchronous electric motor of this type.

This object is achieved according to the invention with a method characterized in that - the rotor of the motor is associated with an electrical position sensor, stationary in operation, which can supply an alternating electrical position signal, - the phase difference between the said electrical position signal and the alternating voltage supplied to the motor is then determined, and - the determined phase difference is compared with at least one predetermined reference.

A further object of the invention is to provide a device for determining the mechanical load of an alternating current synchronous electric motor with a permanent magnet rotor, and according to the invention this further object is achieved with a device whose essential characteristics are defined in Claim 5. Further characteristics and advantages of the invention will be made clear by the following detailed description, provided purely by way of example and without restrictive intent, with reference to the attached drawings, in which:

Figure 1 is a block diagram of a device according to the present invention associated with an alternating current synchronous electric motor with a permanent magnet rotor;

Figure 2 is a set of three diagrams which show, in a qualitative way, as a function of the time t shown on the horizontal axis, the variation of the motor supply voltage and of two voltages generated in a device according to the present invention; and

Figure 3 is a diagram which shows in a qualitative way the variation of the phase difference <p between the position signal and the supply voltage of a synchronous electric motor used for driving a hydraulic pump, as a function of the liquid flow rate Q shown on the horizontal axis.

In Figure 1, the letter M indicates a single-phase alternating current synchronous electric motor with a permanent magnet rotor, of a known type.

The motor M is associated with a control circuit D which is also of a known type.

The motor M is supplied for operation with an alternating voltage V, such as the ordinary mains voltage at 50 (60) Hz. The rotor of the motor M is associated with an electrical position sensor S, such as a Hall effect sensor. This sensor, which is mounted in a stationary way, supplies at its output in operation an alternating electrical position signal, indicated by H in Figures 1 and 2. When the rotor reaches synchronism, this signal is a square wave having the same frequency as the alternating supply voltage V.

In Figure 1, ZCD indicates a squaring circuit, to whose input the supply voltage V is applied during operation. A square-wave signal, indicated by Vsq in Figures 1 and 2, appears at the output of the squaring circuit ZCD during operation, this signal being isochronous and in phase with the voltage V.

In general, there is a phase difference, indicated by φ in Figure 2, between the square-wave signals H and Vsq.

The present invention is based on the finding that the phase difference φ varies significantly with a variation of the mechanical load (resistive torque) of the synchronous electric motor M.

With reference again to Figure 1, the squaring circuit ZCD (which, incidentally, can be what is known as a zero crossing detector) and the electrical position sensor S are connected to a phase detector PD, which supplies at its output a signal indicating the phase difference φ between the two signals H and Vsq.

The output of the phase detector PD is connected to a first input of a comparator TC, which compares the determined phase difference φ with at least one predetermined reference, supplied to this comparator by a (constant or variable) threshold generator circuit TG.

The comparator circuit TC can also be designed to compare the phase difference cp with a plurality of predetermined reference values .

A device according to the present invention can be used, for example, to determine the liquid flow rate Q of a hydraulic pump driven by an alternating current synchronous electric motor of the type defined above. For this purpose, a device according to the invention is used to determine the phase difference between the alternating supply voltage and the electrical signal indicating the position of the rotor of the motor, and this phase difference is compared with at least one predetermined reference indicating a corresponding value of the liquid flow rate Q.

Figure 3 shows a graph of the experimentally determined variation of the phase difference φ as a function of the variation of the liquid flow rate Q of an electric pump of the type in question.

The determination of the liquid flow rate of a hydraulic pump driven by a synchronous electric motor can be useful, for example, in a dishwasher, in which an electrical resistance heating device for heating the washing liquid is associated with the intake of the electric pump which supplies the washing liquid to the spray arms of the dishwasher. In a prior art application of this type, use is made of what is known as a thermal cut-out, which switches off the electrical resistance heater when, for any reason, the flow of washing liquid through the electric supply pump decreases or stops altogether. In such an application, a device according to the present invention could be used to supply an electrical signal indicating the instantaneous flow rate of the pump, and also to detect a reduction or interruption of the liquid flow through this electric pump, so that the electrical resistance heating device could then be switched off without the use of a thermal cut¬ out.

However, the application described above is mentioned purely by way of example.

Another useful advantage of a device according to the present invention is the possibility of detecting the jamming of the rotor, which may occur in conditions such that the sensor (S) is switched not by the rotor' s magnetic field, but by the magnetic field dispersed in the winding (a condition which may occur when, for example, the rotor is jammed in such a position that the strength of the magnet's magnetic field acting on the sensor is zero) . In the conditions described above, the angle φ which is present is unstable, but in any case is very different from the angle which would be present in correct operation (in normal operation, the angle can vary between 100 and 180 , while when the rotor is jammed it oscillates between 0° and 100°) . Clearly, provided that the principle of the invention is retained, the forms of application and the details of construction can be varied widely from what has been described and illustrated purely by way of example and without restrictive intent, without thereby departing from the scope of protection of the invention as defined in the attached claims.