GB2081993A - Electric motor control circuitry for domestic appliances - Google Patents

Abstract

In an electric motor control circuit for domestic appliances in which rapid braking of an electric motor is a safety requirement, dual switching means is provided to disconnect electrical power to the motor and, after a time delay, to activate braking. An electric motor 13, in series with a switching means 12, is connected to a D.C. output of diode bridge 2. A mains supply through a pipolar switch 14, is coupled across the diode bridge connected in series with a control circuit 3; the control circuit including an RC network 5, 6 and 7 in parallel with a triac 4, the gate of which is connected through a diac 8 and resistor 9 to the RC network. A conventional snubber network 10 and 11 is coupled across the the triac. A further switching means 13, activates dynamic braking of the motor, switches 12 and 13 being arranged to operate with a time delay, in a linked and opposite switching manner. <IMAGE>

Description

SPECIFICATION Electric motor control circuitry for domestic appliances The present invention relates to electric motor control circuitry for domestic appliances and more particularly to control circuitry for use with permanent magnet electric motors.

It is frequently desired in the domestic appliance field to bring a motor rapidly to a halt under certain conditions. An example is in food processors, that is to say, motor-driven devices which have a chamber in which, there is a motor-driven element such as a cutter, shredder, etc. where it is desirable to provide in association with the access cover or lid, a switch to ensure that the rotating element is brought to a halt before it could be touched by the user. With some types of motor it is sufficient for the switch merely to interrupt the supply to the drive motor. However, with some other motors there is a tendency to run on after the supply is interrupted.

According to the present invention there is provided in combination, a permanent magnet electric motor and a device for braking the motor, the braking device comprising first and second switches, the first switch being in series with a supply to the motor and the second switch being connected across the motor, the braking device being arranged so that on activation, the first switch operates to interrupt the supply to the motor and, after a short delay, the second switch operates to apply dynamic braking to the motor. The delay should be sufficient to ensure that any continued conduction through the first switch, e.g. by arcing in the case of a mechanical switch, has substantially ceased before the second switch is operated.It has been found that where mechanical switches are used, if the first switch is opened at the same time as the second switch, arcing occurs across the first switch which results in the second switch effectively short sircuiting the mains supply.

As permanent magnet motors generally require D.C. energization, it is necessary for them to be energized from an alternating main supply via one or more rectifiers and this momentary shorting of the mains supplied by the second switch can lead to destruction of the rectifier(s). The permanent magnet of the motor may for example comprise ceramic magnetic material.

In a food processor or similar application, the braking device may be arranged to be actuated by removal of the access cover or lid, to ensure that the drive motor has been stopped before access to the interior of the appliance can be gained. The switches could be micro-switches fitted to the body of the appliance and engaged by two fingers with the appropriate formations on the cover or lid, the end surfaces of the fingers being staggered in the direction of removal of the cover or lid so that the first switch is opened before the second switch is closed.

The switches need not, of course, be mechanical switches: they could be any suitable type of switch, for example reed-relays or controlled-conduction semiconductor elements.

The invention will be further described by way of example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of a food processor embodying the present invention; and Figure 2 shows somewhat schematically the operation of the switches of the safety device by removal of the access cover.

Fig. 1 shows the control circuitry associated with the drive motor 1 of a food processor.

The motor 1 is a permanent magnet motor and is connected across the D.C. terminals of a full-wave diode bridge 2 which is connected in series with a control circuit 3 for controlling the energisation of the motor 1.

The control circuit 3 comprises a triac 4 which controls the armature current of motor 1 and whose conduction angle is determined by an associated R.C. network comprising resistors 5 and 6 and capacitor 7. A diac 8 is connected to the junction of resistor 6 and capacitor 7 and, by a resistor 9, to the gate of triac 4.

Resistor 10 and capacitor 11 form a conventional snubber network for triac 4.

By changing the value of resistor 5 or 6 either continuously or by means of a potentiometer or by switching discrete resistance values into circuit, the set speed of the motor can be adjusted by the user.

The circuit provides closed pole control of the motor speed by varying the conduction angle of the triac in dependence upon the variation of the motor speed, tending to maintain the motor at a constant speed despite variations in the load. This is achieved by effectively using the back EMF of motor 1 as a signal representing the speed of the motor.

This is because the back EMF influences the armature current and hence the rate at which capacitor 7 charges.

If the motor speed falls from its equilibrium value, so does the back EMF, thus bringing about an increase in the armature current, which decreases the time taken for capacitor 7 to charge to the firing point of triac 4. This in turn advances the turn-on point of triac 4 and thus provides a larger conduction angle tending to increase the torque produced by the motor so the speed of the motor tends to be restored to its set value. Similar negative feedback occurs when the motor runs at a speed in excess of the equilibrium speed.

Connected in series between the positive terminal of the diode bridge 2 and the motor 1 is a switch 1 2 having contacts which, in normal operration, are closed, while connected across the motor 1 is a switch 1 3 which, in normal operation, is open. These switches are positioned so that when an access cover A of the processor is in its fitted position, respective tabs 1 5 and 1 6 holds the switches 1 2 and 1 3 in their normal operating positions.

When, as shown in Fig. 2, the access cover A is removed, because the end surfaces of the tabs 1 5 and 1 6 are staggered in the direction of removal of the cover A, the switch 1 2 opens a short time before the switch 1 3 closes. This time delay is chosen to provide sufficient time (at the normal speed of removal of the access cover) to ensure that any arcing across the contacts of switch 1 2 has ceased before the switch 1 3 is closed to apply dynamic braking to the motor 1. A low-value resistor may be connected in series with switch 13, if desired or neccessary, in order to limit the current which flows through the motor armature under dynamic braking conditions to a value sufficiently low as to ensure that demagnetisation of the motor permanent magnet does not occur.

It will be appreciated that the delay between the operation of switches 1 2 and 1 3 could be achieved electronically rather than mechanically, for example by having a switch or similar elements operatively associated with the cover A and arranged to operate two sets of relay contacts or other electro-mechanical or electronic switching elements in the correct timed relation when removal of the cover A is sensed.

A double-pole user-operable on/of switch 14 is the main on/off switch.

Claims (7)

1. An electric motor control circuit including a permanent magnet electric motor and means for braking said motor comprising first and second switching means, the first switching means being connected in series with an electrical supply to the motor and the second switching means being coupled across the motor, the braking device being arranged, on activation, to operate the first switch to interrupt the electrical supply to the motor and, after a time delay, to operate the second switch to apply dynamic braking to to the motor.

2. A circuit according to Claim 1 wherein the permanent magnet of said motor is formed of a ceramic magnetic material.

3. A circuit according to Claim 1 or Claim 2 wherein an electrical supply to the motor comprises a full-wave diode bridge having DC outputs connected to the motor and AC inputs connected in series with a control circuit to a source of AC power.

4. A circuit according to Claim 3 wherein the AC inputs of the diode bridge are connected to a control circuit comprising an adjustable RC network coupled in parallel with an electronic switching means.

5. A circuit according to any preceding claim wherein said first and second switching means comprise respective electronic switching means.

6. A circuit according to any at claims 1 to 4 wherein said first and second switching means comprise respective micro-switches.

7. An electric motor control substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.