GB1568507A - Electric motor control units - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRIC MOTOR CONTROL UNITS (71) We, COLAIR ELECTRONICS LI MITED, a British Company, of 11 South Square, Gray's Inn, London WC1 and DAVID JOHN MILLWARD, a British subject, of 20, Conway Court, Stephenson Road, Clacton-on-Sea, Essex (formerly of 7 Birch Avenue, Great Bentley, Colchester, Essex), do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention relates to control units for electric motors, and in particular concerns certain improvements in the construction and operation of alternating current control units operating on a phase-control basis, by which term is meant the regulation of power from an a.c. supply by variation of the firing angle of an electronic switch (included in the control unit) arranged between the supply and the motor to be controlled.

When it is desired to control the rotational speed of an electric motor running on alternating current, it is often convenient to use a system which regulates the power supplied to the electric motor on the phasecontrol basis. This manner of control relies upon switching electronically the power supplied to the motor at least once - but usually twice - for each cycle of alternating current.The control is conveniently effected by means of a timing unit synchronized with the frequency of the alternating current supply and which operates to switch on the power at a predetermined time interval after the start of one cycle and to switch the power off at the half-cycle point (i.e. 1800 after the start of the cycle); the power is then switched on again at the same predetermined time interval after the 1800 point and switched off at the completion of the cycle (i.e. 360 ). Clearly, when on full power, power is switched on at 0" and 1800, or at a very small time interval thereafter, but for any proportion of power less than 100% the power is switched on electronically some finite time after 0 and 1800 - for example, when operating at 50% power, the current is switched on at 90 , off at 1800, on at 270 and off at 360".

Such phase-control for regulating the power supplied, and hence the speed of operation of, AC motors is well known and widely used both domestically and industrially. One particular common application is the control and operation of cooling, heating or ventilation fans, for it is relatively easy to provide a compact and reliable form of control unit which is easy to operate. The fan motors themselves are usually single or three phase induction motors, such as capacitor-run motors, or shaded pole motors, though sometimes universal motors are employed. Whenever an induction motor is employed it is necessary for the motor to have a high rotor resistance, if satisfactory results are to be achieved.

There are however certain disadvantages with the known forms of speed control unit for fan motors, and this invention aims at reducing at least in part some of those disadvantages.

According to this invention there is provided a control unit for an alternating current electric motor, which control unit is arranged to allow the selection of a particular rotational speed for the motor and to regulate the power supplied to the motor on a phase-control basis (as defined above) dependent upon the selected speed, the control unit including means arranged to increase the power supplied to the motor to a starting power value for a predetermined time following initial switch-on of the control unit, which starting power is substantially greater than that power required to allow the motor to overcome its initial starting friction and commence rotating, should the regulated power supplied for the selected rotational speed be less than a pre-set amount smaller than the starting power.

It will be appreciated that the control unit of the invention aims at eliminating the problem of starting a motor where the power supplied is sufficient to maintain very slow rotation of a motor but is insufficient to overcome the initial starting friction of the motor and its load (for example a fan). For instance, with the known form of control unit it is possible for the selected speed to require the supply of, say 5% power, but the supply of 5% power to the motor when stationary would be insufficient to run the motor up to the slow speed from stationary.

The motor would thus remain stalled. This invention overcomes this difficulty, by providing sufficient power to allow the motor to start rotating, and once the motor is rotating, the power is reduced to the required value.

The precise level of the starting power supplied for the predetermined time following initial switch-on of the control unit will depend upon the type of motor which is to be controlled. Some motors will start with only 30% power being supplied thereto, but a more typical figure is 40%. On the other hand, some motors unusually may require as much as 75% to 80% power to start them.

Thus, the control unit is preferably arranged to supply a starting power of from 30% to 80% following initial switch-on for the predetermined time, and for many applications, substantially 50% power will suffice.

The starting power is supplied for the predetermined time if the regulated power is less than a pre-set amount. Conveniently, this pre-set amount is the same as the starting power, so that if the regulated power is greater than the starting power the regulated power is supplied instead.

As mentioned above, in a control unit employing phase-control, the power is switched electronically and usually- by means of a semi-conductor device such as a thyristor or triac. The precise operation of such a semi-conductor device is with convenience controlled by the charging of a capacitor to a particular voltage allowing a comparator to trigger, and thus to fire the semi-conductor device. The precise period for which the power is supplied to the motor can be varied in a simple way by changing the time-constant of the capacitor charging circuit or the triggering level of the comparator, and advantageously this can be effected by means of a variable resistor in the capacitor charging circuit.

For such a design of control unit, it is preferred for there to be two inputs to the comparator circuit serving to control the operation of the thyristor or triac which regulates the power supplied to the electric motor, one of the inputs being the usual speed-selectable input coupled to the capacitor charging circuit, and the second input being coupled to a circuit to provide the starting power and which is actuated to over-ride the usual input only when the control unit is initially switched on.Preferably, this second input is coupled to a capacitor charging circuit forming a starting power timing circuit which is similar to that circuit for the speed selection, but having a time constant appropriate for the required starting power by allowing the semiconductor device to be switched on for a sufficiently long period, there being a shortcircuiting device for the capacitor charging circuit which device is arranged to become operative after the predetermined time following initial switch-on. Conveniently, this short-circuiting device comprises a semiconductor switching element (such as a transistor) arranged in parallel with the capacitor of the starting power timing circuit, which semi-conductor element is switched on to conduct (and hence to effect a short-circuit) after said predetermined time.Most conveniently, the control of the predetermined time is effected by means of a further capacitor charging circuit, which gradually charges up a capacitor following initial switch-on and once this capacitor has been charged to a particular value, an output is provided to the said semiconductor element to render the element conductive and thus to short-circuit the starting power timing circuit capacitor, thereby rendering inactive from that point onwards the starting power timing circuit.

In a preferred aspect of this invention, the control unit is arranged to supply substantially 80% power to the motor following initial switch-on, and advantageously this is supplied for not more than 5 seconds following the initial switch-on whereafter the control unit reverts to the normal phasecontrol arrangement.

The means whereby the starting of the motor is ensured following initial switch-on will hereinafter be referred to as a "guaranteed start" circuit.

When the control unit of this invention is employed with an electric motor driving a fan, it is often convenient to couple the control unit to a temperature sensor such that the speed of operation of the fan is dependent upon the detected temperature.

Such a control unit is often arranged so as to cause the motor to rotate relatively quickly should the ambient temperature rise above a Predetermined value, and once the ambient temperature has fallen to the predetermined value to revert to operation at a minimum, pre-set speed to give ventilation only. For this arrangement, it is necessary for there to be a temperature sensor for the ambient temperature, the sensor providing either a continuous, analogue output or a discrete output whenever the temperature rises above a required minimum value.

Although such control units function generally satisfactorily, there are certain disadvantages in the construction employed; for example apart from their complexity it usually happens that when high voltage motors are controlled, there are relatively high voltages present in the temperature sensor element and this can be a considerable source of danger.

To overcome this difficulty, it is preferred for the control unit of this invention to have an over-ride for the selected rotational speed dependent upon a parameter of the ambient environment which over-ride is rendered operative to increase the rotational speed should the ambient parameter change from a predetermined value, the over-ride including a parameter sensor providing an output if the parameter changes from said predetermined value, and said output being used to illuminate the light emitter of an optical coupling assembly, the emitted light being received by the light detector of the assembly which detector serves to increase the power supplied to the motor by the control unit.

Above, reference is made to the control of an ambient parameter, and by this is meant any parameter of the environment which the motor is arranged to control. For example, the parameter could be humidity, with the motor controlling a pump injecting water vapour in to the environment, or the parameter could be pressure with the motor driving a blower. Normally, however, it is envisaged that the controlled parameter will be temperature, with the motor driving a fan to introduce cooler air into the environment should the temperature rise above a pre-set value. In the following, references will be made solely to temperature, though it will be understood that other parameters could equally well be controlled in a similar manner. For other parameters, the temperature sensor must of course be replaced by a sensor suited to the parameter to be controlled.

It will be appreciated that in this preferred arrangement of the invention, the optical coupling serves to isolate electrically the temperature sensor from the remainder of the control circuit, thereby allowing the temperature sensor to operate on low voltages so as to eliminate risk of electric shock.

In a preferred embodiment, the optical coupling assembly comprises a lightemitting diode optically aligned with a light sensitive diode or photo-transistor, the light sensitive diode or photo-transistor when receiving light serving to cause an over-ride capacitor charging circuit to be rendered operative, the output from this over-ride capacitor charging circuit also being coupled to the comparator causing triggering of the semi-conductor device switching power to the electric motor. Provided the time constant of the over-ride charging circuit is smaller than that of the selected rotational speed, more power will be supplied to the motor whenever optical coupling occurs.

Various forms of temperature-sensor may be used, but the preferred arrangement is for this to be a thermistor biassed to operate on a substantially linear part of its characteristics. In this way, the setting of the desired temperature can easily be effected by means of a linear variable resistor, and with the correct biassing a calibration scale for the variable resistor can be linear.

If required, the transfer of the selected rotational speed of the motor to the phase control device switching current to the motor can also be via an optical coupling assembly, and preferably the same optical coupling assembly as transfers the parameter control. This has the advantage that the manual control used for selecting the rotational speed is isolated from the current driving the motor. In a similar way, the "guaranteed start" circuit may also be optically coupled. Advantageously, the components effecting all three modes (that is, manual speed selection, external parameter control and "guaranteed start") are driven by the same, relatively low voltage source and their outputs appear in parallel across the light emitter of the optical coupling assembly.Then, whichever circuit has for the time being the shortest time constant will cause the emitter to be illuminated, thereby indirectly turning on the power to the motor in each half-cycle. The control unit must of course then be arranged to re-set each charging circuit each time one of the circuits triggers operation.

To control a three-phase motor, all three phases have separately to be triggered at the appropriate times - usually by three separate semi-conductor switching devices. To operate such control, it is advantageous for three separate optical coupling assemblies to be arranged one associated with each triggering circuit for each phase respectively but with their emitters in parallel across the same source. If such an arrangement is used with all three modes driving the light emitters, with the mode having the shortest time constant for the time being driving the emitters, then a simple three-phase control can be obtained without the need for providing three similar sets of control equipment, one for each phase.

By way of example only, one specific embodiment of this invention will now be described, reference being made to the accompanying drawings, in which: Figure 1 is a block diagram of a control unit constructed in accordance with the invention; Figure 2 is a circuit diagram of the control unit shown in Figure 1; and Figure 3 is a circuit diagram of a second embodiment of control unit, two operating circuits being omitted therefrom.

Referring to Figure 1, there is shown a block diagram of a control unit constructed in accordance with this invention, and including a "guaranteed start" circuit and an isolated temperature sensor. In the Figure, a load 1 represents an electric induction motor driving a fan, having power fed to it both from a single phase alternating current power supply and through a radio interference suppression network 4 from a phasecontrol basis power supply 3. The operation of the power supply 3 is governed by a trigger circuit 5, which is itself controlled by one of three sources of information, namely an isolated temperature sensor 6, a manual speed selector 7 and a "guaranteed start" source 8. Power is supplied as shown to the temperature sensor 6 and the "guaranteed start" source 8.

The manual speed selector 7 serves two purposes: in the first, manual mode of operation this selector allows any particular operating speed for the motor to be selected at will, but in a second automatic mode serves solely to set the minimum running speed of the motor. The temperature sensor 6 operates only when the unit is in the automatic mode, and serves to detect a rise in temperature above a predetermined level; when this is detected a signal is transferred across an optical isolator to the trigger circuit 5, to over-ride the minimum speed selected on the manual speed selector 7 and to cause high power to be fed to the motor so that it runs at a high speed.On restoration of the temperature to the required value, the signal from the temperature sensor is removed, and the trigger circuit 5 is thereafter controlled by the manual speed selector 7 such that the fan rotates at the predetermined minimum rate set on the selector 7 giving a small degree of ventilation.

The "guaranteed start" source 8 serves to apply approximately 80% power to the motor for about five seconds after initial switch-on of the control unit, unless either the manual speed selector 7 or the temperature sensor 6 controls the trigger circuit 5 to supply even more power to the motor.

The "guaranteed start" source 8 is of particular value because although the speed of an induction motor can be governed to be at a very low value, the motor will not in general start from rest under such low power conditions, for insufficient power is provided to overcome the starting inertia.

However, different motors have different starting characteristics requiring various starting powers, and the proportion of power as well as the time for which it is supplied by the "guaranteed start" source 8 can be programmed into the control unit so as to meet widely varying circumstances.

Referring now to the circuit diagram of Figure 2, switch 1 serves to connect power from a mains supply through a high speed fuse 2 to the "U" phase winding 4 and "Z" phase winding 5 of a motor represented by the broken lines 3. The "Z" phase winding together with a motor start/mn capacitor 6 are not used in the case of a shaded-pole or series-universal motor.

Inductor 7 together with capacitors 8 and 9 and resistor 10 suppress radio frequency interference symmetrical voltages, whilst capacitors 11, 12 and 13 suppress radio frequency interference asymmetrical voltages so that these interferences do not exceed acceptable limits. Triac 14 controls the conduction of current through the "U" phase winding of the motor 3 by acting as an electronic switch, the period (or the phase angle) of conduction being under the control of voltages applied to its gate through resistors 15, 16, and derived from bridge rectifier 17, diode 30 and thyristor 33. The bridge rectifier 17 also provides a full wave rectified power supply via resistor 18 to a trigger circuit for the triac 14, the supply being chopped to a fixed value rectangular wave by zener diode 19. Resistors 20, 21 and 22 form a potential divider network to set the operating point of programmable unijunction transistor 23, resistor 20 being shorted by link 24 for 50 Hz. operation but being left in circuit by cutting the link for 60 Hz. operation. The effect of the link is to alter the time-base of the trigger circuit from 10 milliseconds to 8.3 milliseconds.

Variable resistor 25 constitutes the manual control for selecting a particular speed, and together with capacitor 27 forms a timing network for the trigger circuit. This network is connected to the uni-junction transistor 23 through diode 28 and operates such that when the voltage across capacitor 27 reaches the peak point of the unijunction transistor 23, the uni-junction transistor will conduct and discharge capacitor 27 through the diode 28, into the gate of thyristor 33, thus turning on the thyristor.

Resistor 29 limits the peak current into the gate of thyristor 33, and also ensures that the gate-to-cathode impedance of the thyristor is kept to a value below the maximum specified for the device.

Resistor 31 together with capacitor 32 serve the dual purpose of (a) limiting the rate of rise of voltage across thyristor 33 which otherwise might turn on the thyristor and produce false triggering in the tnac 14, and (b) ensuring that when the thyristor is triggered, current flow from the capacitor 32 through the resistor 31 will hold the thyristor in its conducting state - that is the thyristor will latch - should the comparatively slow rate of current rise in the inductive load ("U" phase winding 4) not supply sufficient latching current during the duration of the thyristor gate pulse.

Diode 34 and resistor 35, serve to (a) provide temperature compensation for the uni-junction transistor 23, and (b) to present a high impedance network to the unijunction transistor, thus keeping the peak point current, i.e. the current needed to trigger the device through diode 28 (or diodes 38 or 39 described below), at a relatively low value.

Should the inductive load be of such a value as to resonate with the radio frequency interference suppression network, triac 14 when triggered might well turn off again, when current flow due to this resonance approaches zero. Diode 30 is therefore included to ensure that the capacitor 32 discharges only through thyristor 33, and not through resistor 18 when the triac 14 is triggered. The time constant formed by the capacitor 32 and the resistor 31 is longer than the period of any resonant period in the load.

The timing network formed by the variable resistor 25 and capacitor 27 is adjustable to compensate for manufacturing tolerances, and to pre-set the minimum load voltage, by the series network of resistor 25A and the pre-set resistor 25B, this network being in parallel with the variable resistor 25. Resistor 26 provides some source impedance to capacitor 27 when the variable resistor 25 is set to its minimum value.

When the output of the uni-junction transistor 23 fires thyristor 33, the output of the bridge rectifier 17 is short-circuited and a high current flow then occurs through the resistors 15 and 16. The resultant potential developed across resistor 15, connected between the main terminal (terminal 1) and the gate of the triac 14, renders the triac conductive, whilst resistor 16 limits the instantaneous current to a safe value for the relatively low current thyristor 33. Firing of the thyristor 33 and triac 14 reduces the rectangular waveform to zero, synchronising the timing network to the beginning of the next half cycle.The advantages of using the thyristor 33 to fire the triac 14 via the bridge rectifier 17 are twofold, viz: (a) the thyristor requires a very much smaller gate current than the triac to fire, and hence the power required from the timing circuit is less, resulting in a simpler timing circuit power supply; and (b) rectification through the bridge rectifier 17 enables the triac to fire in phase quadrants I and III, which are the most sensitive and closely matched quadrants of the triac, hence resulting in much greater switching symmetry.

Resistor 36 and capacitor 37 form another timing network which constitutes part of the "guaranteed start" circuit. If this network has a faster timing sequence (i.e. causes the triac 14 to be turned on more quickly following the start of a cycle of operation) than that of the manual control timing network or that formed by the automatic control (to be described later), this "guaranteed start" network will predominate on initial switching on of the control unit. The "guaranteed start" timing network is connected by diode 38 to the uni-junction transistor 23, and will conduct to reverse bias the diodes 28 and 39 if this timing network is faster than the other networks, until the uni-junction transistor 23 discharges all the timing network capacitors.

Resistor 36 and capacitor 37 are conveniently set to cause about 80% full power to be supplied to the motor, but either or both of these components can be varied to cause other power levels to be supplied.

Operation of the "guaranteed start" network is for a pre-set period only after initial switch-on, whereafter it is inhibited. Diode 40 connected to switch 1 allows current to flow through resistor 41 to charge an electrolytic capacitor 43 on alternate positive half cycles as soon as switch 1 is closed, turning on the control unit. A small amount of energy from the capacitor 43 discharges through resistor 42 on each negative half cycle but nevertheless the result is a generally steady increase in the capacitor voltage.

When this voltage reaches the Zener voltage of Zener diode 44, the diode conducts and provides base current for N-P-N transistor 45, to render this transistor 45 conductive.

This shorts out the capacitor 37, and thus switches off the timing network for the "guaranteed start" circuit, and hence reverse biasses the diode 38 for all conditions.

The electrolytic capacitor 43 is sufficiently large to provide a holding current for the zener diode 44 during the negative half cycles when diode 40 does not conduct.

The timing network formed by resistors 41 and 42 and capacitor 43 is such that the "guaranteed start" circuit is operational for about 5 seconds, but this can be varied by changing the values of any of the resistors 41 and 42, capacitor 43 and zener diode 44.

When the control unit is switched off, the capacitor 43 discharges quickly through resistor 42 to be re-set for a further operation when the unit is next switched on.

The network formed by resistor 46 capacitor 47 and light-sensitive photo-transistor 50, comprising half of an optical isolator, constitute a timing network for operation in the automatic mode. Light falling on the transistor 50 reduces its resistance enabling capacitor 47 to charge more quickly.

On fast charge the voltage across capacitor 47 will rise faster than the voltage across the manual control timing network capacitor 27, and thus diode 39 will conduct and reverse bias diode 28. On slow charge the voltage on capacitor 27 will rise faster than the voltage on capacitor 47, so that diode 28 will conduct and reverse bias diode 39. In both cases the voltage on capacitor 47 is reset by the firing of uni-junction transistor 23.

An isolating transformer 51 together with diodes 52 and 53 provide a full wave rectified low potential supply which is fully isolated from the mains supply for the temperature sensor. This low potential supply is smoothed by an electrolytic capacitor 54. Resistor 55 and Zener diode 56 stabilise this power supply by well-known principles. The power unit can be switched on by switch 57' which, when actuated, changes the control unit over from the simple manual control mode to the automatic temperature-dependent mode.

The non-inverting input of a monolithic operational amplifier 57 is maintained at a fixed reference voltage decided by resistive potential dividers 58 and 59. The potential at the inverting input of the amplifier 57 is set by the combination of variable resistor 60 (which serves as the desired temperature control), resistor 61 in parallel with a remote thermistor 62, pre-set resistor 63 in series with fixed resistor 64, and resitor 69.

The remote thermistor 62 detects the ambient temperature, and can be positioned anywhere it is desired to sense the ambient temperature. The thermistor is conveniently provided in a suitable housing equipped with wires connecting it to the main part of the control unit. Capacitor 70 in the control unit is in parallel with the thermistor 62 and serves to remove any interference pulses, which may have been picked up on the thermistor remote connecting wires.

The characteristic curve of the temperature measuring thermistor 62 is non-linear but its combination with the parallel resistor 61 produces a curve which is substantially linear. Pre-set resistor 63 compensates for the manufacturing tolerances in the variable resistor 60, thermistor 62 and other fixed resistors associated with this chain.

An increase in the ambient temperature will cause a decrease in the resistance of the thermistor 62, and this decrease relative to a set value of resistance on the temperature control 60 will turn on the amplifier 57 if the two resistances have the right relationship, allowing current to flow through the light emitting diode 65, which is the other half of the optical isolator comprising also phototransistor 50. The current flow through the light emitting diode 65 is limited to a safe value by variable resistor 67, which also serves to set the operating point, and to compensate for different transfer ratios of optical isolators. The light is received by the photo-transistor 50 and this causes the speed of the motor to be increased as explained above.If it were necessary for the speed of the motor to decrease with an increase in temperature, then it would be necessary to connect the temperature sensing network, comprising the remote thermistor 62, the resistors associated with this chain, together with variable resistor 60 to the non-inverting input of the operational amplifier 57. The fixed resistors 58 and 59 would then be connected to the inverting input.

Resistor 66, connected to the noninverting input of the amplifier 57, provides a substantially constant source impedance for different conditions of temperature settings. Thus the gain of the amplifier 57 can accurately be set by the negative feedback resistor 68. The temperature bandwidth, i.e.

the necessary temperature increase to change the speed of the motor from a slow speed to full on, can thus accurately be set.

A convenient bandwidth could be 4"C.

Capacitor 71 in parallel with resistor 68 is of such a value to ensure that the gain of the amplifier 57 is very substantially reduced to all alternating frequencies, so that the amplifier is sensitive to changes in D.C. levels only. Any pick-up of mains frequency is thus prevented from producing asymmetrical triggering in the automatic mode.

It will be appreciated that the optical isolator comprising the photo-transistor 50 and light-emitting diode 65 serve to isolate the temperature sensor itself from the high voltages associated with the motor 3, and thus with the main part of the power control circuitry. Since the temperature sensor circuitry has its own isolated power supply, the unit can operate in a particularly safe manner.

Switch 68' is a simple over-riding switch for the whole unit, allowing full power to be applied to the motor irrespective of the operation of the control unit.

Figure 3 shows another embodiment of this invention, in which all three modes of operation (namely, automatic temperature sensing, manual control, and "guaranteed start") are arranged to be connected through the optical isolator to the trigger circuit. The part of the circuit up to the bridge rectifier 17 of Figure 2 is the same here, and the circuit of Figure 3 is to be connected to points A and B of Figure 2 in place of the circuit shown there. Parts common to both circuits are given like reference characters, provided their function is essentially the same in both cases.

In Figure 3, uni-junction transistor 23 is biassed by the potential divider network comprising resistors 20, 21, and 22, the output of the uni-junction transistor 23 firing the thyristor 33 as previously explained with reference to Figure 2. The main difference of this circuit is that the timing network is formed directly by phototransistor 50, fixed resistor 46 and capacitor 47, interconnecting diodes being omitted.

The operational amplifier 57 is connected in a similar manner to that shown in Figure 2, with the remote temperature sensing thermistor 62, together with its associated resistor chain, 63, 64, 69 and 61 are connected to the inverting input through resistor 66, whilst fixed resistors 58 and 59, together with a variable resistor 72 are connected to the non-inverting input. This illustrates an alternative connection for the temperature sensing variable control, corresponding to variable resistor 60 of Figure 2, it being connected to the non-inverting input here. Resistors 66 and 68 provide the negative feedback necessary to set the gain of the amplifier and thus the temperature bandwidth, as before, and capacitor 71 renders the gain very low for stray pick-up interference voltages.

Resistors 73 and 74, together with variable resistor 75 (which serves as the manual control) form a potential divider which is variable by adjusting the manual control 75.

This potential divider thus forms a bias network for the base of N-P-N transistor 76 through a coupling diode 77. The emitter of the transistor 76 is directly connected to the light emitting diode 65 constituting a half of the optical isolator including phototransistor 50, and thus current flow through transistor 76 and the light emitting diode 65 can be controlled by the setting of the variable resistor 75. In this way, the timing network comprising photo-transistor 50, resistor 46 and capacitor 47 in the trigger circuit is varied by altering the control setting of resistor 75. Diode 78 prevents the operational amplifier 57 from interfering with this setting should the automatic mode dictate a low speed setting, or the automatic mode is switched off.

Resistors 79 and 80 form a potential divider chain also to bias transistor 76, though a diode 81. These resistors comprise part of the "guaranteed start" circuit and can be changed to give different characteristics thereto. Transistor 82 is connected to the junction of resistors 79 and 80, and when this transistor is conducting the potential at the junction of resistors 79 and 80 becomes low, and reverse biasses diode 81. Resistor 83 and electrolytic capacitor 84 form a timing network, such that when the control unit is switched on, capacitor 84 commences to charge. When the voltage on capacitor 84 reaches the Zener voltage of Zener diode 85, this Zener diode will conduct, causing the transistor 82 to conduct, thus turning off the "guaranteed start" circuit. Diode 86 rapidly discharges capacitor 46 when the unit is switched off.A convenient charge time for capacitor 84 would be 5 seconds but this time can be altered by changing the values of any of capacitor 84, resistor 83 or Zener diode 86.

Switch 87 serves to switch on the operational amplifier, and thus operate the automatic mode. When switched on, the conduction of current through the light-emitting diode 65 is controlled by the setting of the temperature control 72 and the ambient temperature controlling the thermistor 62.

Resistor 88, in series with the light-emitting diode 65, is variable and is used both to limit the current through the light-emitting diode and to compensate for different transfer ratios of optical isolators.

From the above, it will be appreciated that all three modes of operation are coupled through the optical isolator. One advantage of this is that a three-phase motor can be controlled by adding in parallel with light-emitting diode 65 the light-emitting diodes of further optical isolators, such as those shown in dotted lines at 65a and 65b.

The other half of each of these optical isolators is connected to trigger circuits identical to those shown in the upper half of Figure 3. Each trigger circuit is connected to an associated, independent triac for controlling one of the phases of a three-phase supply, by being connected in series with a winding between the neutral and a phase wire. Variable resistors 88a and 88b are used to balance all three optical isolators, and thus each phase of the three-phase supply.

WHAT WE CLAIM IS: 1. A control unit for an alternating current electric motor, which control unit is arranged to allow the selection of a particular rotational speed for the motor and to regulate the power supplied to the motor on a phase-control basis (as defined herein) dependent upon the selected speed, the control unit including means arranged to increase the power supplied to the motor to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. sensing, manual control, and "guaranteed start") are arranged to be connected through the optical isolator to the trigger circuit. The part of the circuit up to the bridge rectifier 17 of Figure 2 is the same here, and the circuit of Figure 3 is to be connected to points A and B of Figure 2 in place of the circuit shown there. Parts common to both circuits are given like reference characters, provided their function is essentially the same in both cases. In Figure 3, uni-junction transistor 23 is biassed by the potential divider network comprising resistors 20, 21, and 22, the output of the uni-junction transistor 23 firing the thyristor 33 as previously explained with reference to Figure 2. The main difference of this circuit is that the timing network is formed directly by phototransistor 50, fixed resistor 46 and capacitor 47, interconnecting diodes being omitted. The operational amplifier 57 is connected in a similar manner to that shown in Figure 2, with the remote temperature sensing thermistor 62, together with its associated resistor chain, 63, 64, 69 and 61 are connected to the inverting input through resistor 66, whilst fixed resistors 58 and 59, together with a variable resistor 72 are connected to the non-inverting input. This illustrates an alternative connection for the temperature sensing variable control, corresponding to variable resistor 60 of Figure 2, it being connected to the non-inverting input here. Resistors 66 and 68 provide the negative feedback necessary to set the gain of the amplifier and thus the temperature bandwidth, as before, and capacitor 71 renders the gain very low for stray pick-up interference voltages. Resistors 73 and 74, together with variable resistor 75 (which serves as the manual control) form a potential divider which is variable by adjusting the manual control 75. This potential divider thus forms a bias network for the base of N-P-N transistor 76 through a coupling diode 77. The emitter of the transistor 76 is directly connected to the light emitting diode 65 constituting a half of the optical isolator including phototransistor 50, and thus current flow through transistor 76 and the light emitting diode 65 can be controlled by the setting of the variable resistor 75. In this way, the timing network comprising photo-transistor 50, resistor 46 and capacitor 47 in the trigger circuit is varied by altering the control setting of resistor 75. Diode 78 prevents the operational amplifier 57 from interfering with this setting should the automatic mode dictate a low speed setting, or the automatic mode is switched off. Resistors 79 and 80 form a potential divider chain also to bias transistor 76, though a diode 81. These resistors comprise part of the "guaranteed start" circuit and can be changed to give different characteristics thereto. Transistor 82 is connected to the junction of resistors 79 and 80, and when this transistor is conducting the potential at the junction of resistors 79 and 80 becomes low, and reverse biasses diode 81. Resistor 83 and electrolytic capacitor 84 form a timing network, such that when the control unit is switched on, capacitor 84 commences to charge. When the voltage on capacitor 84 reaches the Zener voltage of Zener diode 85, this Zener diode will conduct, causing the transistor 82 to conduct, thus turning off the "guaranteed start" circuit. Diode 86 rapidly discharges capacitor 46 when the unit is switched off.A convenient charge time for capacitor 84 would be 5 seconds but this time can be altered by changing the values of any of capacitor 84, resistor 83 or Zener diode 86. Switch 87 serves to switch on the operational amplifier, and thus operate the automatic mode. When switched on, the conduction of current through the light-emitting diode 65 is controlled by the setting of the temperature control 72 and the ambient temperature controlling the thermistor 62. Resistor 88, in series with the light-emitting diode 65, is variable and is used both to limit the current through the light-emitting diode and to compensate for different transfer ratios of optical isolators. From the above, it will be appreciated that all three modes of operation are coupled through the optical isolator. One advantage of this is that a three-phase motor can be controlled by adding in parallel with light-emitting diode 65 the light-emitting diodes of further optical isolators, such as those shown in dotted lines at 65a and 65b. The other half of each of these optical isolators is connected to trigger circuits identical to those shown in the upper half of Figure 3. Each trigger circuit is connected to an associated, independent triac for controlling one of the phases of a three-phase supply, by being connected in series with a winding between the neutral and a phase wire. Variable resistors 88a and 88b are used to balance all three optical isolators, and thus each phase of the three-phase supply. WHAT WE CLAIM IS:

1. A control unit for an alternating current electric motor, which control unit is arranged to allow the selection of a particular rotational speed for the motor and to regulate the power supplied to the motor on a phase-control basis (as defined herein) dependent upon the selected speed, the control unit including means arranged to increase the power supplied to the motor to

a starting power value for a predetermined time following initial switch-on of the control unit, which starting power is substantially greater than that power required to allow the motor to overcome its initial starting friction and commence rotating, should the regulated power supplied for the selected rotational speed be less than a pre-set amount smaller than the starting power.

2. A control unit as claimed in claim 1, wherein the starting power supplied is from 30% to 805? of full power.

3. A control unit as claimed in claim 2 wherein the starting power supplied is substantially 50% of full power.

4. A control unit as claimed in any of the preceding claims, wherein there is a semi-conductor device to switch the power and a comparator the output of which fires the semi-conductor device, the comparator having two inputs each of which are compared with a standard voltage, one of the inputs being coupled to a capacitor-charging circuit the time constant of which is variable to regulate the power supplied to the motor and the second input being coupled to a capacitor-charging circuit forming a starting power timing circuit pre-set to have a time constant appropriate for the required starting power, the second capacitor-charging circuit being actuated only when the control unit is initially switched on.

5. A control unit as claimed in claim 4, wherein there is a short-circuiting device for the capacitor of the starting power timing circuit, which device is arranged to become operative after the predetermined time following initial switch-on.

6. A control unit as claimed in claim 5, wherein the short-circuiting device comprises a semi-conductor switching element arranged in parallel with the capacitor of the starting power timing circuit, which semiconductor element is switched on to conduct and hence to effect a short-circuit after said predetermined time.

7. A control unit as claimed in any of the preceding claims wherein the predetermined time for which the starting power is supplied is determined by a further capacitor charging circuit, which gradually charges up a capacitor following initial switch-on, and once the capacitor has been charged to a particular value an output is provided to inhibit the supply of the starting power.

8. A control unit as claimed in any of the preceding claims, wherein the starting power is supplied for not more than five seconds following initial switch-on.

9. A control unit as claimed in any of the preceding claims, wherein there is an over-ride for the selected rotational speed dependent upon a parameter of the ambient environment which over-ride is rendered operative to increase the power supplied to the motor should the ambient parameter exceed a predetermined value, the over-ride including a parameter sensor providing an output if the parameter changes from said predetermined value, and said output being used to illuminate the light emitter of an optical coupling assembly, the emitted light being received by the light detector of the assembly which detector serves to render more power to be supplied to the motor by the control unit.

10. A control unit as claimed in claim 9, wherein the parameter sensor is arranged to sense ambient temperature.

11. A control unit as claimed in claim 10, wherein the temperature sensor is a thermistor biassed to operate on a substantially linear part of its characteristics.

12. A control unit as claimed in any of claims 9 to 11, wherein the optical coupling assembly comprises a light-emitting diode optically aligned with a light-sensitive diode or photo-transistor, the light sensitive diode or photo-transistor when receiving light serving to cause an over-ride capacitor charging circuit to be rendered operative, the output from this over-ride capacitor charging circuit being arranged to control the power supplied to the motor.

13. A control unit as claimed in any of the preceding claims, wherein the transfer of the selected rotational speed of the motor to means within the control unit operating on the phase control basis to regulate the power supplied to the motor is effected via an optical coupling assembly.

14. A control unit as claimed in any of the preceding claims, wherein the operation of the means arranged to increase the power supplied on starting is effected via an optical coupling assembly.

15. A control unit as claimed in claim 13 and claim 14, wherein the components for controlling the manual speed selection, the external parameter control and the supply of starting power are driven by the same, relatively low voltage source with their outputs appearing in parallel across the light emitter of an optical coupling assembly, such that whichever circuit has for the time being the shortest time constant will cause the light emitter to be illuminated, thereby indirectly regulating the power supplied to the motor.

16. A control unit as claimed in any of claims 9 to 14, wherein three separate transistorised optical coupling assemblies are arranged one associated with a triggering circuit for each phase respectively of a three-phase supply but with their emitters in parallel across the same source.

17. A control unit substantially as hereinbefore described with reference to and as illustrated in Figure 1 or in Figure 2 or in Figure 3 in combination with Figure 2.