Electronic control and regulating system

Electricity: motive power systems – Alternating current commutating motors – Universal or a.c.-d.c. motors

Patent

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Details

318345D, 318345H, H02K 2364

Patent

active

043920934

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to an electronic control system defined by the category of the main claim, and more particularly to speed control of universal motors connected to a a-c power grid, for example fo A-C operated appliances.


BACKGROUND

Several phase control and/or regulating systems are known. In many cases these are merely control systems in which a particular speed is set in a trigger circuit by setting of a time constant.
Rotary speed control systems may use a tacho-generator coupled to the motor from which an electrical signal for the actual speed value is derived. Such a control system has the disadvantage of being relatively slow because as a rule the mechanical time constant of the motor is part of the control process or the stabilizing process. The control circuits of full wave control systems for universal motors without tacho-generators are still relatively expensive because either the entire control circuit must be a DC circuit or at least the feedback signal must be rectified and/or the feedback signal is fed to active semiconductor elements for amplification and for the processing.


THE INVENTION

It is an object to improve the control circuit for a speed control system, particularly for universal motors connected to an A-C network.
Briefly, the motor is serially connected with a semiconductor switch, typically a triac which, additionally, is serially connected with a current/voltage converter or transformer. The current/voltage converter or transformer has its secondary, forming the voltage winding, connected to a capacitor to form a resonance circuit. The resonance frequency of the capacitor/inductance circuit is so arranged that it is between 0.5 to 1 times the power network frequency, preferably in the vicinity of the power network frequency. The resonance circuit is connected to the trigger circuit for the triac. The triac trigger circuit includes a charge capacitor, the charge on which is modified by the voltage occurring within the resonance circuit, so that the phase angle control effected upon triggering or firing of the triac, is controlled not only by the charge on the trigger capacitor but, additionally, by a factor dependent on current flow in the resonance circuit.
The resonance circuit becomes more and more effective, that is, its impedance increases with increasing current angle of the load current, and directly affects the charging of the trigger capacitor, and hence triggering of the trigger circuit. In contrast with prior inductive converters in control systems, the inductivity of the secondary winding of the converter is used to form a resonant or oscillatory circuit together with the parallel capacitor, the frequency of the resonance circuit being in the vicinity of the network frequency with which the system is to be used. Speed control can be obtained in accordance with a feature of the invention by a potentiometer which is connected to simultaneously influence the firing of triggering of the trigger circuit and the damping of the resonance circuit. The number of primary and secondary turns can be suitably selected to adapt or match the system readily to the particular load expected, as well as to the characteristics of the components in the trigger circuit, for example the breakdown voltage of a triggering diode connected in the firing circuit of the triac. The speed controller according to the present invention has the advantage of being a symmetrical full wave A-C controller in which the feedback signal necessary for control is firstly derived as a pure AC-value and secondly is fed back to the trigger circuit with sufficient amplitude and correct phase. It increases in higher-than-proportional ratio, i.e. amplifies in accordance with the load of the appliance. This is particularly advantageous since then the control system of the invention has only small deviations from a controlled value.
Only a small number of elements is required, resulting in space and cost savings. The symmetrical, full-wave operation leads to a low level of spurious noise signal ge

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