GB2086672A - Single phase inverter - Google Patents

Abstract

A single phase inverter for converting a D.C. voltage supply into a single phase A.C. supply, which comprises a pair of transistors (Q1, Q2) and a pair of capacitors (C1, C2). The emitter-collector paths of the transistors are connected in series across the D.C. supply voltage. The two capacitors are also connected in series across the D.C. supply voltage. The respective junctions between the transistors and capacitors are connected to the stator winding of the motor (M) to be supplied with single phase A.C. Square waveforms (W1, W2) are applied to respective base electrodes of the transistors (Q1, Q2), the waveforms being 180 DEG out of phase with respect to one another. <IMAGE>

Description

SPECIFICATION Single phase inverter The present invention relates to inverters capable of converting D.C. into single phase A.C., and in particular to single phase variable frequency square waveform inverters.

There are various types of inverters, including machines, choppers, feedback oscillators and relaxation oscillators. In some of these the inverter is used as an oscillator and this is generally the case where the frequency is above 100 KHz, and when the function is to provide A.C. power for some other circuit or equipment.

The present invention is more concerned with low frequency application and thus the true inverter, and is applicable to generally small single phase A.C. electric motors provided with capacitor start, and run as well as two or three phase motors which can be suitably adapted to operate on single phase A.C.

The design of an inverter which is used for A.C. machines depends very much upon the parameters, resistances and the reactances of the motor to be supplied, and also whether it is an induction motor or a synchronous motor.

It is an object of the present invention to provide an inverter in which the speed of the A.C. motor can be varied or controlled by varying the frequency of the output from the inverter.

According to the present invention there is provided a single phase inverter for converting a D.C. voltage supply into a single phase A.C.

supply, said inverter comprising a pair of electronic switches connected in series across the D.C. supply; a pair of electrical charge storge means connected in series across the D.C. supply; a first output terminal connected to the junction between the two electronic switches; a second output terminal connected to the junction between the two electrical charge storage means; means for supplying two variable frequency square waveform signals in phase opposition to respective control elements of said electronic switches to cause alternate conduction and non-conduction in phase opposition.

Preferably said electronic switches are transistors and said electrical charge storage means are capacitors.

Alternatively, said electronic switches are each formed by two transistors connected in Darlington configuration.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings wherein: Figure 1 is a circuit diagram of one preferred form of inverter; Figures 2a and 2b are waveforms of the base drives for the respective transistors shown in the circuit of Fig. 1; Figures 3a and 3b are waveforms of the voltage and current supplied to the motor shown in the circuit of Fig. 1; Figures 4a and 4b are equivalent circuits illustrating the circuit connections during respective alternate half-cycles of operation; Figure 5 is a circuit diagram of a Darlington pair which may be used for the transistors in the circuit of Fig. 1; Figure 6 is an oscillogram illustrating typical current and voltage waveforms obtained for an A.C. motor driven by the inverter circuit shown in Fig. 1; and Figures 7a, 7b and 7c show different examples of electrical connections for two phase and three phase wound motors which can be run from the single phase inverter using a capacitor.

Referring first to Fig. 1, the inverter circuit includes a pair of NPN transistors Q1 and Q2 and a pair of capacitors C1 and C2. The emitter-collector paths of the two transistors are connected in series across the D.C. supply voltage V. The two capacitors C1 and C2, which have equal values of capacitance, are also connected in series across the D.C. supply voltage V. The stator winding of the motor M to be supplied with single phase A.C. is connected to terminals A and B of the inverter circuit. The junction between the emitter electrode of the transistor Q1 and the collector electrode of the transistor Q2 is connected to the terminal A. The junction between the capacitors C1 and C2 is connected to the terminal B.The base electrodes of the two transistors Q1 and 02 are connected to respective terminals 1 and 2.

Referring to Figs. 2a and 2b, the square wave voltage waveforms W1 and W2 are effectively 180* out of phase and are applied to respective terminals 1 and 2 of the inverter circuit shown in Fig. 1. The duty ratio or mark space ratio of each waveform W1 and W2 is slightly less than unity. The waveforms W1 and W2 can can be generated and synchronized by any suitable type of square waveform generator of which the frequency can be readily adjustable either manually or automatically.

During the first half cycle when the transistor Q1 is rendered conductive by the waveform W1 being at V, whilst the transistor Q2 is non-conductive because the waveform W2 is at zero, the capacitor C2 is charged to a voltage V/2, by current flowing from the D.C.

supply V, whilst the capacitor C1 discharges through the motor M, the direction of current flow in the armature winding of the motor being from terminal A to terminal B.

The operation of the inverter during the first half cycle can be readily appreciated from the equivalent circuit shown in Fig. 4a. As will be seen, the motor M is effectively in parallel with the capacitor C1, and the parallel combination of motor and capacitor C1 is in series with the capacitor C2 across the D.C. voltage supply V, the terminal A being effectively connected to the positive supply rail through the conducting transistor Q1.

During the second half cycle when the transistor Q2 is rendered conductive by waveform W2 being at V, whilst the transistor Q1 is non-conductive because the waveform W1 is at zero, the capacitor C1 is charged to a voltage V/2, by current flowing from the D.C.

supply V, whilst the capacitor C2 discharges through the motor M, the direction of current flow in the stator winding of the motor being now from terminal B to terminal A.

The operation of the inverter during the second half cycle can be readily appreciated from the equivalent circuit shown in Fig. 4b.

As will be seen, the motor M is effectively in parallel with the capacitor C2, and the parallel combination of motor and capacitor C2 is in series with the capacitor C1 across the D.C.

voltage supply V, the terminal A being effectively connected to the negative supply rail through the conducting transistor Q2.

The resulting voltage and current waveforms are graphically shown in Figs. 3a and 3b. These are theoretical waveforms and do not take into account the characteristics of the transistors and also some inherent resistance in the circuit which together with the capacitor C1 and C2 introduce some unwanted time constants into the actual waveforms which are shown in Fig. 6, these oscillograms having been photographed from an oscilloscope.

In a preferred form, each transistor T1 and T2, may be replaced by a monolithic Darlington pair as shown in Fig. 5. The Darlington pair in integrated form is formed on a monolithic chip indicated by dashed lines having external terminals B, C and E for connection to the inverter. The Darlington pair comprises transistors Q3 and Q4 connected in conventional Darlington configuration having its commoned collector electrodes connected to the external terminal C, the base electrode of the transistor Q3 connected to the external terminal B, and the emitter electrode of the transistor Q4 connected to the external terminal E. A diode Do, which constitutes a protective element is connected between the emitter electrode of the transistor 04 and the commoned collector electrodes.A pair of biasing resistors R1 and R2 are connected across the base and emitter electrodes of respective transistors Q3 and 04.

The application of the inverter circuit shown in Figs. 1 and 5 to various types of A.C.

motors will now be illustrated with reference to Figs. 7ato 7c.

As shown in Fig. 7a a two phase motor M has its first phase winding L1 directly connected across the single phase supply produced by the inverter, whereas the second phase winding L2 is connected in opposite sense across the single phase supply and in series with a capacitor C3.

Fig. 7b shows the application of the inverter to a three phase motor having its windings L3, L4 and L5 connected in delta. A capacitor C4 is connected in parallel with the winding L5, whilst the single phase supply from the inverter is connected across the winding L3.

Fig. 7c shows the application of the inverter to a three phase motor having its windings L6, L7 and L8 connected in star. A capacitor C5 is connected across the series combination of the winding L6 and L7, whilst the single phase supply from the inverter is connected across the series combination of the windings L6 and L8.

The principal advantages of the above described inverter are as follows: (a) The speed of an alternating current rotating electric motor can be varied and controlled either manually or automatically by controlling the frequency of the square waveforms W1 and W2. This can be done very simply and the speed range which can be achieved is very wide.

(b) The inverter is constructionally very simple and uses few cheap devices which are easily obtainable. Because of its simplicity and low power dissipation it is cheap to manufacture and it occupies only a small volume. For some applications, the inverter can be made to be an integral part of the motor, which means it can be mounted on the non-drive end of the motor occupying a volume of the order of one third that of the motor itself.

(c) At any one instant only a half the current the motor draws comes directly from the D.C. supply. Additionally, since equal amounts of current drawn from the D.C. supply during both the positive half-cycle and the negative half-cycle, the D.C. source ripple current is very much reduced.

(d) For most applications there would be no need for extra circuitry to limit the current because its method of operation imposes limits to the current that is being drawn from the D.C. supply.

(e) The inverter is practical over a wide range of D.C. voltage.

(f) The motor associated with the above described inverter may additionally be either a two phase or a three phase wound motor, but start and run as a single phase motor with a capacitor.

Claims (5)

1. A single phase inverter for converting a D.C. voltage supply into a single phase A.C.

supply, said inverter including: a pair of electronic switches connected in series across the D.C. supply; a pair of electrical charge storage means connected in series across the D.C.

supply; a first output terminal connected to the junction between the two electronic switches; a second output terminal connected to the junction between the two electrical charge storage means; means for supplying two variable frequency square waveform signals in phase opposition to respective control elements of said electronic switches to cause alternate conduction and non-conduction in phase opposition.

2. An inverter according to Claim 1, wherein each electronic switch is a transistor.

3. An inverter according to Claim 1 or 2, wherein each electrical charge storage means is a capacitor.

4. An inverter according to Claim 1, wherein each electronic switch comprises a pair of transitors connected in Darlington configuration.

5. A single phase inverter for converting a D.C. voltage into a single phase A.C. supply constructed and arranged to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.