GB2197762A - Motor control circuit - Google Patents

Abstract

An analogue to digital convertor applied to eg fan motor speed control provides for a continuously variable setting control 12, the output of which is applied to a plurality of comparators, each associated with a fixed resistance value RA, RB... and by which one or more fixed resistances are switched in or out of circuit with the electrical load. The threshold valve of comparator 13 (controlling resistor RA) is set by resistors R3 and R4, but influenced by the switching of comparators 14, 15 via R5 and R6. Thus, as the control 12 is adjusted, comparator 13 switches on RA, then comparator 14 switches on RB, but by changing the threshold of comparators 13 RA is switched off. As the control 12 is adjusted further both RA and RB may be switched on in parallel before RC is switched on, and RA and RB switched off. <IMAGE>

Description

Analogue to digital convertor This invention relates to analogue to digital convertors. It is for use when it is desired to represent an analogue control to the user and to carry out the control by digital techniques. It has application in the field of electric motors and has particular application to motors requiring a variable speed control, for example, a fan motor.

A typical electric motor requiring to be operated at various different speeds may have a multi-position switch with each discrete switch position connected through a different value resistance. The different resistances result in different voltages across the motor, and therefore different speeds, in respect of each switch setting. Triac controls may alternatively be used in which the triac gate is triggered at different positions in the mains cycle such that the triac conducts for different proportions of the mains cycle. Such triac controls may give rise to motor vibration which with a fan motor, for example, can result in undue noise from the fan.

It is an object of the invention to provide an improved electrical motor control.

According to one aspect of the invention we provide an analogue to digital convertor eg. for an electric motor, including a plurality of comparators, each associated with a solid-state switch and a resistance of fixed value, the comparators being arranged in parallel with one another, each having a first input from a continuously variable setting control which first input is compared with a second input which can be modified by the presence or absence of an output from at least one other comparator, such that when the first input exceeds the second input the switch is caused to conduct and cause its associated resistance of fixed value to be connected in series with the electric motor winding.

When a particular resistance of fixed value is connected in series with the electic motor winding, the threshold of at least one comparator not associated with that resistance may be pulled down or pushed up by the application of part of the output of the comparator associated with that resistance to the second input of the or each comparator not associated with that resistance. The variable setting control may be variable throughout a fixed range; it may for example include a variable potentiometer, variable between the voltage output values which effectively represent the highest speed setting and the lowest speed setting for the electric motor.

One embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a motor control system according to the invention applied to a fan motor, and Figure 2 is a schematic diagram of the electrical circuitforming part of the control system shown in Figure 1.

An electric motor 10 has windings which may be connected in series with each of, or any combination of three resistances of fixed value represented by RA, RB and RC. The resistance of RA is greater than RB; the resistance of RB is greater than RC. The connection is carried out by means of the devices represented schematically by switches within box 11, which devices are further described with reference to Figure 2.

It should be noted however that one of the "switches" is associated with a resistance of nominally zero value, such that when only this "switch" is closed, the motor windings have the maximum voltage applied across them.

Referring to Figure 2, a control panel 12 has a resistor R1 which is used as a variable potentiometer to allow the voltage applied to the control circuit to be continuously varied from 12 to Zero volts. A variable resistor R2 is preset within the panel 12 to give a particular minimum set voltage.

The output from the potentiometer is applied simultaneously to the positive inputs of four comparators identified as first comparator 13, second comparator 14, third comparator 15 and fourth comparator 16. There may be as many comparators as are required by any particular application. Each comparator is a high gain amplifier and the 12v power supplies for the comparators have for clarity been omitted from Figure 1.

Considering the operation of the first comparator 13, if its first (positive) input voltage level from the potentiometer is above the voltage of its second (negative) input then there will be a positive output signal from first comparator 13. The voltage level of the second input is fixed by the respective values of two resistors R3 and R4 which act as a potential divider across which 12v is applied. The voltage level of the second input is additionally modified by the resistors R5 and R6 respectively from the outputs of the second and third comparators 14 and 15. The output signal from the first comparator 13 is applied to a drive circuit for a triac TR1 shown within a dashed line box 17 in Figure 2.When the output from the first comparator 13 switches to zero volts, triac TR1 is triggered to conduct and switch resistance RA into series between the neutral mains voltage line and the windings of fan motor 10.

The circuitry associated with each of the second to fourth comparators 14 to 16 is similar to that associated with first comparator 13. The second input to the second comparator 14 is only modified by the output from the third comparator 15 through resistor R7, whilst the second input to each of the third and fourth comparators 15 and 16 has no connections to permit such modification of the second input voltage.

Also the ratio of the equivalent resistors to R3 and R4 will be different for each comparator. The outputs of each of the second to fourth comparators 14 to 16 are connected into a respective drive circuit for a triac for first comparator 13 and these triacs and drives are shown respectively in dashed box outline as TR2, TR3 and TR4.

The values of R3 and R4 (and resistors for each of the potential dividers at the second inputs to each of the second, third and fourth comparators 14, 15 and 16) and the values of R5, R6 and R7 are selected such that when the voltage determined by the potentiometer on control panel 12 is set at its lowest speed setting, the voltage at the first inputs of all the comparators is above the voltages at the respective second inputs. Hence, the outputs of all the comparators are at the 12v supply line and the triac drives are switched off. (It should be noted that the drive to each triac is switched on when each comparator output switches to zero volts).

If the speed setting is gradually increased at control panel 12, the resistance values of R3 to R7 are such that eventually the voltage at the first input to the first comparator 13 will drop below the voltage at its second input and triac TR1 associated with first comparator 13 will be switched on to bring in resistance RA and therefore switch on motor 10 at the minimum speed. If the speed setting is further increased, eventually the voltage at the first input to the second comparator 14 will drop below the voltage at its second input and the output from the second comparator 14 will switch on its associated triac TR2 and bring in RB to further increase the speed of motor 10.At the same instant, the output of second comparator 14 through R5 reduces the voltage at the second input of the first comparator 13 below the voltage at its first input, and the first comparator output switches to 12v, switches off TR1 and removes RA from circuit with motor 10.

If the speed setting is further increased, eventually the voltage at the first input to the third comparator 15 will drop below the voltage at its second input, and the output from the second comparator 15 will switch on its associated triac TR3 and bring in RC to further increase the speed of motor 10. At the same instant, the output from third comparator 15 through R6 and R7 will reduce the voltage at the second inputs to both the first and second comparators 13 and 14, such that the outputs of the first and second comparators 13 and 14 switch to 12v and triacs TR1 and TR2 are switched off, so that only RC is in circuit with the windings of motor 10.

As the speed setting is further increased (ie. the voltage at the wiper of potentiometer R1 is reduced), the voltage at the first input of comparator 13 drops below the modified voltage at its second imput and hence TR1 switches RA in series with the motor 10.

However, since RC is already switched in by comparator 15 via TR3, the parallel combination of RA and RC in series with the motor 10 increases the motor speed further. In a similar manner, as the speed setting is further increased, resistances RB and RC are brought into circuit, then RA, RB and RC and finally comparator 16 switches on TR4 which switches the motor directly across the mains, hence producing maximum speed.

The invention thus provides means for switching in discrete resistance values and parallel combinations of them in response to what is a continously variable adjustment ie. the variable potentiometer. The number of discrete stages can be varied to suit the application, but it becomes more expensive the more stages are used.

Claims (3)

CLAIMS:

1. An analogue to digital convertor eg. for an electric motor, including a plurality of comparators, each associated with a solid-state switch and a resistance of fixed value, the comparators being arranged in parallel with one another, each having a first input from a continuously variable setting control which first input is compared with a second input which can be modified by the presence or absence of an output from at least one other comparator, such that when the first input exceeds the second input the switch is caused to conduct and cause its associated resistance of fixed value to be connected in series with the electric motor winding.

2. An analogue to digital convertor as claimed in claim 1 in which the variable setting control is a variable potentiometer.

3. An electric motor control system as hereinbefore described with reference to and as shown in the accompanying drawings.