GB2223895A

GB2223895A - Three-phase motor operated from single phase supply

Info

Publication number: GB2223895A
Application number: GB8822578A
Authority: GB
Inventors: James Abbott
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-09-26
Filing date: 1988-09-26
Publication date: 1990-04-18
Also published as: GB8822578D0

Abstract

A three-phase motor 10 has a relatively large 'starting' capacitor 20 connected in parallel with a smaller 'running' capacitor 18 via a switch 22, which allows the 'starting' capacitor to be decoupled from windings 12a, 12b, 12c, of the motor, which are connected to a single-phase supply. The capacitor 20 provides an energy storage device whereby energy is stored from the single phase supply and is delivered to the windings 12a, 12b, 12c at the required instants. Further embodiments are described (Fig 2 & 3) where the windings are connected in star connection. To stop the motor switch 15 is opened and start switch 22 is closed so that capacitor 20 and resistor 24 are connected across the motor to enhance braking. <IMAGE>

Description

SINGLE TO THREE-PHASE ELECTRICAL CONVERTOR The present invention relates to a single to three-phase electrical converter and particularly, but not exclusively, to a single to three-phase electrical convertor for use with a three-phase motor.

Three-phase motors have several advantages over single-phase motors such as greater mechanical power per unit volume and efficiency. The torque developed by the motor of a single-phase motor is pulsating whereas the torque developed by the motor of a three-phase motor is relatively steady. However, a three phase supply, which is typically 415 volts, is often difficult to obtain particularly in remote locations eg. farms where 240 volt l-phase a.c. supply is normal and thus it is desirable that a three-phase motor can be operated from a single phase supply.

A three-phase motor can be run in a unbalanced form from a single phase supply, however, such a motor cannot be started from a single-phase supply as the single phase supply cannot develop a torque to start the motor. A three-phase motor requires a single to three-phase convertor if the motor is to be started and run efficiently from such a single-phase supply. Existing convertors tend to be relatively expensive and difficult to use in certain applicatons. It is desirable to provide a single to three-phase converter which allows a three-phase motor to be both started and run from a single-phase supply in such a way that it behaves as if it were connected to a three-phase supply. It is also desirable to provide a single to three-phase phase convertor which can generate a braking force to help the three-phase motor to be rapidly stopped when desired.

It is an object of the present invention to provide a single to three-phase electrical convertor which obviates, or mitigates, at least one of the aforementioned problems.

This is achieved by providing an energy storage device coupled to the windings of a three-phase motor to store energy from a single-phase supply and to deliver the stored energy to the motor windings at the required instants.

In one embodiment a relatively large 'starting' capacitor is connected in parallel with the 'delta' connected windings of a three-phase motor. The terminals of the motor windings are connected to the single-phase supply and the capacitor provides an energy storage device and links 1 terminal of the supply to the 3rd terminal of the motor windings. A second, smaller 'running' capacitor is connected in parallel with the starting capacitor through a switch which allows the starting capacitor to be decoupled from the windings. The switch is closed when the three-phase motor is to be started to provide a relatively large capacitance in parallel with the motor windings and thus provide greater energy storage to produce greater starting torque in the three-phase motor.

In an alternative embodiment starting and running capacitors are connected in parallel with the 'star' connected windings of a 3 phase motor, the motor winding being connected to the single-phase supply.

According to the present invention there is provided single to three-phase electrical convertor apparatus for permitting a single-phase supply to drive a three-phase motor said apparatus comprising; a three phase motor having motor windings adapted to be coupled to said single-phase supply; electrical energy storage means coupled to said motor windings for storing electrical energy from said single-phase supply means and for releasing said stored energy to said motor windings to provide a three-phase electrical supply to said motor windings.

Preferably said electrical energy storage means has first and second electrical storage means coupled in parallel to said windings, said first and second storage means being separated by first switch means.

Conveniently said first storage means is provided by a relatively large 'starting' capacitor coupled to said motor windings and said second storage means are provided by a smaller 'running' capacitor.

Preferably said 'starting' and 'running' capacitors are connected in parallel through said first switch means which allows said 'starting' capacitor to be decoupled from said motor windings during the normal running of the motor.

Preferably also said motor windings are connected in a delta configuration. Alternatively said motor windings are connected in a star configuration.

Preferably also said single-phase supply means are provided by an a.c. electrical source.

Preferably also said switch means is a 'start' switch, which is closed when the three-phase motor is being started so that said 'starting' capacitor is coupled to said motor windings.

Preferably also a second switch means is provided by a 'stop' switch coupled between said single-phase supply means and said motor windings to allow the single-phase supply means to be disconnected from the motor windings Conveniently when said 'stop' switch is open said 'start' switch is closed so that said 'starting' capacitor provides an extra load on the motor windings to accelerate the stopping of the motor.

These and other aspects of the present invention will become apparent from the following description when taken in combination with the accompanying drawings in which: Fig. 1 is a circuit diagram of a first embodiment of a single to three-phase electrical convertor according to the present invention; Fig. 2 is a circuit diagram of a second embodiment of a single to three-phase electrical convertor according to the present invention, and Fig. 3 is a circuit diagram of a third embodiment of a single to three-phase electrical convertor according to the present invention.

Reference is firstly made to Fig. 1 of the drawings which shows a single to three-phase electrical convertor generally indicated by reference numeral 10. The convertor comprises 'delta' connected motor windings 12a, 12b and 12c, of a three-phase motor having a rotatable rotor (not shown in the interest of clarity), said windings 12a, 12b, 12c, connected to a single-phase electrical supply (not shown) through isolator switchgear 14, isolator switches 16 and a'stop' switch 15 whose purpose will be described.

A first running capacitor 18 is connected in parallel with motor winding 12b. The capacitor 18 provides an energy storage device for the motor windings 12, 12a and 12b. Electrical energy supplied from the single-phase supply is stored in the capacitor 18 when switch 15 and switches 16 are closed and the stored energy is released to the motor windings 12a, 12b and 12c at the required instants ie the capacitor 18 is discharged so that the total power generated in by the windings 12a, 12b and 12c of the three-phase motor is substantially uniform.

The value of capacitor 18 is chosen to make the three-phase motor behave as if connected to a three-phase supply. The capacitor 18 introduces an impedence resonance effect when the motor windings 12a, 12b and 12c are driven by a single-phase supply. The impedence resonance effect causes the magnitude of the total current drawn from the single phase supply to be reduced: For example, tests with a 250 Watt, three-phase motor with Delta connected windings, running from a 240 volt single-phase supply showed that the current from the supply dropped from 2 Amps at a rated load for a true single-phase motor to 1.3 Amps with an optimum phase convertor.

The conditions in a three-phase motor vary significantly between the starting condition and the normal running condition. A greater amount of torque is required to be developed to start a three-phase motor than is required to keep such a motor running. Therefore more energy must be supplied to the motor windings 12a, 12b, and 12c to start the motor than is required to run the motor, thus a greater value of capacitance is necessary.

Therefore, larger 'starting' capacitor 20 is connected in parallel with the capacitor 18 through a 'start' switch 22. A resistor 24 is connected in parallel with capacitor 20.

Prior to starting, isolator switch 16 is closed and contactor switches 15 and 22 are closed to allow the single phase supply to supply electrical energy to capacitor 20 as well as capacitor 18 via windings 12a, b and c. The capacitance of the combined value of capacitors 18 and 20 provides a sufficient impedance resonance effect to allow sufficient energy to be supplied to the motor windings 12a, 12b and 12c to permit starting. When the three-phase motor reaches full speed, 'start' switch 22 is opened by a time delay mechanism (not shown) to disconnect capacitor 20 from capacitor 18. As hereinbefore described the impedance resonance effect produced by capacitor 18 is sufficient to provide the three-phase motor operating at full speed with the illusion of running from a three-phase supply.

When the three-phase motor requires to be stopped, the 'stop' switch 15 is opened to disconnect windings 12a, 12b and 12c from the single-phase supply. When the 'stop' switch 15 is opened, 'start' switch 22 is simultaneously closed. With capacitors 18 and 20 and resistor 24 connected in parallel with the windings 12a, 12b and 12c, the windings 12a, 12b and 12c act as a generator supplying energy to the 'load' of capacitors 18 and 20 and resistor 24. This enhances the braking action of the three-phase motor, which will stop almost immediately.

Reference is now made to Fig. 2 of the drawings which is a circuit diagram of a second embodiment of a single to three-phase electrical convertor, where like numerals generally refer to like parts with the suffix 'a' added.

The three-phase motor windings 12c, 12d and 12e are connected in a 'star' configuration. It is understood that capacitor 18a is connected across windings 12c and 12e and that the function of each of the circuit components is substantially as described with reference to the previous embodiment.

Reference is now made to Fig. 3 of the drawings which is a circuit diagram of a third embodiment of a single to three-phase electrical convertor. The convertor 30 comprises a three-phase motor with a rotatable rotor, and which has 'star' connected motor windings 32a, 32b and 32c, one of which, winding 32a, is connected to a single-phase electrical supply (not shown) through isolator switchgear 34, isolator switch 36 and a 'stop' switch 38. Capacitor 40 is connected across windings 32b and 32c and capacitor 42 is connected across windings 32b and 32c. When switches 36 and 38 are closed the capacitors 40 and 42 introduce an impedance resonance effect with windings 32a, 32b and 32c and the three-phase motor behaves as if connected to a three-phase supply.

Capacitor 44 and resistor 46 are connected in parallel with capacitor 40 through 'start' switch 48. To start the three-phase motor, isolator switch 36, 'stop' switch 38 and 'start' switch 48 are closed. The combined value of capacitors 40 and 44 provide a sufficient impedance resonance effect to allow the three-phase motor to start and run as if connected to a three-phase supply. After the three-phase motor attains the desired running speed, a time delay mechanism (not shown) opens 'start' switch 48 to isolate capacitor 44 from capacitor 40.

To stop the three-phase motor, 'stop'switch 38 is opened and 'start' switch 48 is simultaneously closed to reconnect capacitor 44 to the motor windings to provide rapid braking of the motor in a manner similar to that hereinbefore described.

Various modifications may be made to the embodiments hereinbefore described without departing from the scope of the invention. For example, any suitable number of capacitors may be coupled to the motor windings. The isolator switchgear and contactor start switch may be replaced by any form of suitable switching apparatus. The 'running' capacitor may be switched out of connection with the motor windings during the normal running of the motor as the motor may be run 'single-phase' from a single-phase supply. The 'start' switch need not be closed when stopping the motor, however, if the 'extra' capacitance load is not coupled to the windings during a 'stop' the three-phase motor will probably take longer to stop.

Advantages associated with the invention are that the single to three-phase electrical convertor allows a three-phase motor to be both started and run from a single-phase electrical supply as if it were connected to a three-phase supply. The three-phase motor has a greater power factor and efficiency than it would if supplied from a single-phase supply, for example the three-phase motor shows a 12% increase in efficiency across a rated load. The motor behaves as if connected to a three-phase supply. In addition, the convertor also helps the motor to be 'stopped' in a relatively short period of time i.e.

1 second. The convertor is made from a minimal number of parts each of which are readily available. In addition, the convertor complies with the standards of the British Approval Service for Electrical Equipment in Flammable Atmospheres.

Claims

1. Single to three-phase electrical convertor apparatus for permitting a single-phase supply to drive a three-phase motor said apparatus comprising; a three phase motor having motor windings adapted to be coupled to said single-phase supply; electrical energy storage means coupled to said motor windings for storing electrical energy from said single-phase supply means and for releasing said stored energy to said motor windings to provide a three-phase electrical supply to said motor windings.

2. Apparatus as claimed in claim 1 wherein said electrical energy storage means has first and second electrical storage means coupled in parallel to said windings, said first and second storage means being separated by first switch means.

3. Apparatus as claimed in claim 2 wherein said first storage means is provided by a relatively large 'starting' capacitor coupled to said motor windings and said second storage means are provided by a smaller 'running' capacitor.

4. Apparatus as claimed in claim 3 wherein said 'starting' and 'running' capacitors are connected in parallel through said first switch means which allows said 'starting' capacitor to be decoupled from said motor windings during the normal running of the motor.

5. Apparatus as claimed in any one of claims 1 to 4 wherein said motor windings are connected in a delta configuration.

6. Apparatus as claimed in any preceding claim wherein said motor windings are connected in a star configuration.

7. Apparatus as claimed in any preceding claim wherein said single phase supply means are provided by an a.c.

electrical source.

8. Apparatus as claimed in any one of claims 2 to 7 wherein said switch means is a 'start' switch, which is closed when the three-phase motor is being started so that said 'starting' capacitor is coupled to said motor windings.

9. Apparatus as claimed in any one of claims 2 to 8 wherein a second switch means is provided by a 'stop' switch coupled between said single-phase supply means and said motor windings to allow the single-phase supply means to be disconnected from the motor windings

10. Apparatus as claimed in claim 9 wherein when said 'stop' switch is open said 'start' switch is closed sothat said 'starting' capacitor provides an extra load on the motor windings to accelerate the stopping of the motor.

11. Single to three-phase electrical convertor apparatus substantially as hereinbefore desribed with reference to Fig. 1, or to Fig. 2, or to Fig. 3 of the accompanying drawings.