GB2251740A

GB2251740A - Induction motor control arrangement

Info

Publication number: GB2251740A
Application number: GB9021985A
Authority: GB
Inventors: Kevin Ray
Original assignee: GREENFIELDS EUROP Ltd
Current assignee: GREENFIELDS EUROP Ltd
Priority date: 1990-10-10
Filing date: 1990-10-10
Publication date: 1992-07-15
Also published as: GB9021985D0

Abstract

Although an induction motor under full load acts essentially as a resistive load, under no load the motor tends to act predominantly as an inductive load. Accordingly, although no output work is done by the motor, significant currents flow in the motor. These significant currents cause substantial I<2>R losses in the motor and the electrical supply system. In order to reduce these losses, a standard form of universal motor speed control circuit/light dimmer circuit is included in series with the motor and the speed control circuit has a fixed or preset resistor 22. The circuit is set so that, at full load, the circuit chops the first 36 DEG of each half cycle. This produces a small reduction in full load power. However, under no load, the circuit causes a substantial reduction in the RMS current through the motor, and thus the I<2>R losses are reduced. A capacitor 20 and a diac 24 can be an integrated circuit (26, Fig 2) and a triac 14 may also be integrated with the capacitor and diac (28, Fig 3). <IMAGE>

Description

INDUCTION MOTORS This invention relates to induction motors and in particular to improving the performance of such motors under par-t- or no-load conditions.

As is well-known, in an induction motor, a rotating magnetic field is set up by the stator, and the field rotates at a frequency dependent on the supply current frequency. The rotating stator-field induces a current in the rotor, and the rotor current depends upon the relative speed of rotation of the stator-field and the rotor, or slip.

The rotor induces a back EMF in the stator which opposes the applied voltage. The mechanical power of the motor is equal to the product of back EMF and the stator current.

Under full load conditions, an induction motor has a power- Factor approaching unity, that is to say the motor acts almost as if it is a purely resistive load. However, under no load conditions, the inductance of the motor windings becomes significant and the power factor reduces towards zero, for example to 0.1 or 0.2. Accordingly, relatively high currents still flow in the motor even though no external work is performed by the motor. These high currej0!s cause resistive (I2R) power losses in the supply system to the motor and in the motor itself.As the loading of the motor is changed from Eull load to no load, the power factor chal)ges progressively.

Many induction motors run most of the time under mid- or low-lvatl conditions, and may, for example, experience full load only upon start- up. Accordingly, there is a significant waste of energy and it is desirable to increase the power factor. To do this it iniglit I)e considered appropriate to arrange one or more capacitors in palailel with the motor and to switch the capacitors in the ( i rc'uit- in dependence upon the loading of the motor.However, such an arrangement would be large and expensive, and although it would reduce the power losses in the supply system to the motor, it would not affect the power losses in the motor itself under low- or no-load.

The present invention seeks to provide a solution to the above problem which is effective and which can be provided cheaply altdd using only a small amount of space.

In accordance with the present invention, a cotitrol circuit is provided in series with the motor. The control circuit comprises a triggerable switch, such as a triac, for switching the motor current, and a trigger circuit. In operation, ininediately after the motor current has fallen to zero with each AC half cycle, the motor current is switched off, and it is then switched on again in a preset manner, for example after a certain time, or when the voltage has risen to a certain value, or some combination of these features.Accordingly, when the motor is under full load and tending to act purely resistively, the beginning of each half cycle will not be supplied to the motor, but when the switch is turned Ol), the motor current waveform will follow the supply voltage waveform substantially proportionately.

Provided that the off-time is small, for example for a phase angle of 360, the decrease in RMS voltage applied to the motor and the conimensurate decrease in motor power will not be significant. In contrast, when the motor is under no load and is acting predominantly inductively, the motor current will lag the supply voltage by a phase angle approaching 900. Accordingly, when the current falls to zero and is switched off, the supply voltage will be near a peak. When the current is subsequently turned on, the current will not immediately rise to a value proportionate to the voltage, but instead will rise gradually from zero, because of the inductive nature of the motor, and will therefore be substantially less than if the switch-off period had not been provided.Accordingly, tile RMS value of the current for the complete half cycle will be substantially less than if no switch-off period had been provided, and so the IR losses in the motor and the supply system will be even more substantially reduced because of the square-law nature of these losses.

Conveniently, the trigger cii-cuit and switch may be provided by a standard for-m of universal motor speed controller circuit or a standard fort of light dinner circuit without any modification except thaL the variable resistor of such circuits may be replaced by a fixed or preset resistor. For minial.ur-isation and reliability, the trigger circuit may be provided by an integrated circuit, such as type 656-754 obtainable from RS Components, or indeed both the trigger circuit and switch may be provided by a single integrated circuit, such as United Automation type CSR1504A.

Specific embodiments of the preseiit invention will now- be described by way of example with reference to the accompanying drawings, in which: Figures 1 to 3 show circuit diagrams of three different motor circuits according to the invention; and Figures 4A to 4D are waveform diagrams to illustrate the operation of the circuits.

Referring to Figure 1, a motor arrangement comprises an AC power supply 10 (for example, a single-phase mains supply), an induction motor 12 and a triac 14 (for example, type TiC246D available from RS components) connected in series, with a suppressor 16 being connected across the main terminals Mt 1, Mt 2 of the triac. A standard form of universal motor speed control circuit or light dimmer circuit 18 comprises a capacitor 20 of, for example, 0.1uF and a 10k resistor 22 connected in series across the triac main terminals Mt 1, Mt 2, with a diac 24 connected between the junction of the capacitor 20 and resistor 22, and the gate G of the triac 14.However, unlike the standard speed control or light dimmer circuit, the resistor 22 is fixed (or possibly is a 'preset' type resistor). With a resistive load and a mains supply, the precise values of the capacitor 20 and resistor 22 are chosen to provide triggering of the triac 36 after the start of a mains half-cycle, and as is well known, the triac will switch off when the current therethrough falls to zero.

This circuit of Figure 2 is similar to t that of i;'igure 1, except that the capacitor 24 and diac 24 are provided by an integrated circuit 26 type 656-754 available from RS components.

The circuit of Figure 3 is also similar, except that the triac 14 is also included in the integrated circuit 28 of type CSR1504A available from United Automation.

Referring to Figure 4A, if an induction motor under full loa(l acts essentially resistively, the motor current 1R!N will be in phase with and proportionate to the supply voltage Vb, both beiiig shown in Figure 4A by a single line. With the inclusion of the circuits shown in Figures 1 to 3, the mains voltage V5 will not be supplied to tlie motor during the first 360 of each half cycle as shown in Figure 4B, and thus the motor voltage VH will be zero for 360 and subsequently will equal Vs. Again, the current I will be propol-tionate to the motor voltage V. The switch-off period of 360 is sufficiently small that the loss of power caused thereby is not substantial.

Referring to Figure 4C, if the motor is under no load and is acting essentially inductively, without the circuits of Figures 1 to 3, the motor current IHi' would lag the supply voltage V5 by 900. With any of the circuits of Figures 1 to 3 included, the motor voltage VW will be chopped to zero at about the middle of each half cycle, at phase angle P1. Then, at phase angle P2, when the triac is switched on again, the motor current IM will not suddenly step up to the value which would have arisen without the control circuit, but instead, due to the inductive nature of the motor, will progressively rise from zero, being smaller than the current IKI' by a constant amount.Accordingly, the current Ii will fall back to zero earlier than the current Ij', at phase angle P1', and the triac will then switch off, the current Thi will remain zero until phase angle P2' when the triac is triggered again.

By a comparison of the waveforms Thi and IN,' in Figure 4C, it can be seen that the RMS value IRHS of the switched current Thi is about 65% of the RMS value IRMS' of the unswitched current Thi' Accordingly the resistive losses which are proportional to IRMS will be, with the control circuit about 42% of the losses without the control circuit.

Figure 4D illustrates the current and voltage waveforms when the motor is under part-ioad. It will be appreciated that as the loading increases and the motor becomes more predominantly resistive, the switch-off phase angle P, will approach a zero phase angle, and the motor current Th? will, during the switch-on period, tend more closely toward the motor current Igp' which would arise without the control circuit.

The following table sets out the currents lKys and IHD' and speeds S and S' with and without the control circuits for a typical single phase induction motor at various loadings: Load IRMS S IRMS' S No load 1.9 1400 3.0 1400 Quarter load 2.9 1380 3.8 1385 Half load 5.5 1315 6.1 1330 Full load 10.0 1230 10.2 1250 It can be seen that at no load, the control circuit produces a substantial current reduction with no noticeable affect on speed. At full load, there is a smaller reduction in speed and current, and accordingly a small reduction in maximum work output from the motor.

However, for motors which are used primarily on part-load this loss of work output can be disregarded.

It will be appreciated that the invention is also applicable to three-phase motors, requiring a circuit as described above in each phase.

Claims

1. An AC motor arrangement comprising an induction motor and a control circuit in series therewith, the control circuit comprising a triggerable switch for switching the motor current and a trigger circuit for the switch, the switch and trigger circuit being arranged such that immediately after the motor current has fallen to zero with each AC half cycle, the motor current is switched off arid is then switched on again in a preset manner.

2. An arrangement as claimed in claim 1, wherein the switch is operable to produce said switching-off of the motor current, and in response thereto the trigger circuit is operable to trigger the switch in said preset manner to produce said switching-on of the motor current.

3. An arrangement as claimed in claim 1 or 2, wherein the switch comprises a triac arranged to be connected via first and second main terminals in series with the motor and an AC supply, wherein the trigger circuit is connected to the main terminals in parallel with the triac so as to sense voltage across the triac upon switching-off of the triac, and wherein the trigger circuit has a trigger' output connected to a gate terminal of the triac and operable to switch on the triac.

4. An arrangement as claimed in claim 3, wherein the voltage across the triac is sensed by tire trigger circuit via a fixed resistor.

5. An arrangement as claimed in claim 3, wherein the voltage across the triac is sensed by the trigger circuit via a preset resistor.

6. An arrangement as claimed in any of claims 3 to 5, wherein the trigger circuit comprises a standard universal motor speed control circuit or a standard light dimmer circuit in which the sensing resistor is provided by a fixed or preset resistor.

7. An arrangement as claimed in any preceding claim, wherein the trigger circuit is provided by a single integrated circuit.

8. An arrangement as claimed in any preceding claim, wherein the trigger circuit and triggerable switch are provided by a single integrated circuit.

9. An arrangement as claimed in any preceding claim, wherein upon full load of the motor, the motor circuit is switched off for a phase angle of about 360 in each half cycle.

10. An AC motor arrangement substantially as described with reference to the drawings.

11. A method of controi} ing an AC induction motor, in which immediately after the motor current has fallen to zero with each AC half-cycle, the motor current is switched off and is then switched oii again in a preset manner.

12. A method of controlling an AC induction motor, substantially as described with reference to the drawings.