The present invention refers to a control device for a universal electric motor.
As is already known, a universal motor is a motor with serial excitation whose speed of rotation can be regulated, for example, by modulating the supply voltage of the motor itself by means of a control circuit of the phase control type or the pulse width modulation type.
Normally, the rotor of the universal motor is associated with a speed transducer (for example, a tachometer generator) capable of providing the control circuit with a signal proportional to the effective speed of rotation of the motor. The control circuit compares that signal with a corresponding reference signal and controls the power supplied to the motor in proportion to the error signal resulting from that comparison, in order to maintain the desired speed of operation at a substantially constant value. Universal motors and their respective control devices are commonly utilized, for example, for driving the rotating basket of washing machines, in which the basket must be driven alternatively in opposite directions of rotation during predetermined operational phases.
For this purpose, the control device also comprises a reversing device for reversing the rotation of the motor which is substantially comprised of a switch and two contacts which are controlled (for example, by a control device associated with a programmer of the washing machine) to simultaneously switch between a first and a second position.
These contacts are connected to the field winding terminals (or to the armature winding) in such a way that in the first and second positions, the field winding is always connected in series with the armature winding and the power supply, but in respectively opposite directions of the windings.
Consequently, the motor rotates in one direction or the other according to whether the contacts of the reversing device are in the first or the second position.
Because the speed control circuit (or at least its power supply portion) is substantially connected in series with the motor and the electrical power supply source, the full voltage is applied to the motor in the case of a possible short circuit of the power supply portion of the control circuit.
The same thing can happen in the case of a failure of the speed transducer, which, for example, may erroneously signal a stopped condition of the motor, or a lower motor speed than the actual speed.
In both of these cases, the motor tends to turn at an extremely high speed which is likely to damage the housing in which it is mounted; this phenomenon is particularly serious in a case where the reversing device is activated in the direction of rotation of the motor.
To eliminate such an eventuality it was proposed to connect the motor in series with a normally closed electrical safety switch which is capable of being opened by turning off the excitation of the motor itself whenever the full voltage is applied to it.
The safety switch can be a centrifugal switch capable of being opened when the motor exceeds a predetermined rotational speed; in such a case, however, the switch is relatively complex and not very reliable, as well as imprecise in its functioning.
Alternatively, the safety switch may be activated by a relay, or similar device, capable of being excited by opening the switch when the signal generated by the speed transducer of the motor exceeds a predetermined figure. In particular, this signal controls a device which closes the relay when the signal has a value higher than that of a reference signal; this solution still becomes undesirably complex. Furthermore, in order to ensure a sufficient functioning safety, the relay must be associated with a self-sustaining electric circuit which further complicates the control device and absorbs electric power excessively when the relay is closed.
A simpler solution was also proposed, consisting of connecting a suitably calibrated fuse in series with the motor to open the electrical feed circuit when the current exceeds a predetermined safety factor. However, this solution is not satisfactory either, since the fuse easily deteriorates with the passage of time, even under the correct operating conditions, and hence tends to disable the feed circuit of the motor mistakenly.