A novel method of starting a 3-phase induction motor with one phase out from the source of supply

International Journal of Electrical Engineering Education, Jan 1999 by Basu, K P, Yusuf, Sheeba

Abstract Connection of supply neutral to the open terminal of a 3-phase induction motor under single phasing condition produces the required phase difference between winding voltages and starting torque is developed. Neutral connection is used only for starting and not for running. Otherwise the current exceeds the rated value even at no-load.

1 INTRODUCTION

The open conductor condition in any one phase of a Beta-phase supply may occur due to the burning of any one fuse/jumper connection, a loose contact in one conductor or any other reason. A Beta-phase induction motor, drawing power from a Beta-phase supply, continues to run under such a single phasing condition and delivers reduced output power at a lower speed and high losses. But a Beta-phase induction motor fails to start under single phasing condition as the starting torque becomes zero. However, a Beta-phase induction motor may produce a starting torque and continue to run satisfactorily from a 1-phase supply with the help of a static phase converter using a capacitive-inductive network 4.

This paper suggests a novel method of starting a Beta-phase induction motor under single phasing condition by using the supply neutral only and without having any phase converter network connected.

The inherent phase difference between line to neutral voltages of the remaining two conductors produces the necessary rotating magnetic field. This point may be emphasised to students in the classroom by performing a simple experiment as suggested below.

2 NEUTRAL WIRE CONNECTION FOR STARTING

Figs. 1(a) and (b) show the connection diagram of a Beta-phase induction motor with delta and star connected stator windings respectively, having the c-phase of the supply disconnected. The motor cannot start as no rotating magnetic field is produced due to the single phasing condition with the sole supply voltage Vab

The neutral of the supply may be connected to the open terminal c of the motor by closing the switch S. Phase difference between supply voltages Van, Vbn and Vab across the motor windings produces rotating magnetic field to develop the starting torque and the motor starts.

In most low and medium voltage distribution systems, the supply neutral terminal is solidly grounded. If the neutral wire is not available, the open terminal c of the motor may, therefore, be grounded to get the starting torque in such a situation.

4 CONCLUSION

A new and simple method of starting a Beta-phase induction motor under single phasing condition is proposed by connecting the supply neutral to the open terminal of the motor. The expression for the starting torque has also been derived for the proposed method. No additional network of static phase shifter is necessary for starting under the single phasing condition. Though the neutral connection provides low starting torque under the single phasing condition, the current exceeds the rated value even at no-load if the motor is allowed to run continuously at rated voltage. The new method is, therefore, proposed to be used to obtain the starting torque only. The motor should be allowed to run with two healthy conductor supply and the neutral connection removed just after starting.

ABSTRACTS - FRENCH, GERMAN, SPANISH

Une nouvelle methode de demarrage d'un moteur asynchrone triphase avec une phase couple de l'alimentation

La connexion du neutre de I'alimentation a la borne ouverte d'un moteur triphase sous alimentation monophasee produit la difference de phase requise entre enroulements et done le couple de demarrage. La connexion au neutre est uti]isee pour demarrer et non en permanence, autrement le courant excede la valeur nominate meme ii vide.

Eine neue Methode, einen Beta-phasigen Induktionsmotor bei Ausfall einer der Stromversorgungsphasen zu starten

Verbindung des Vorsorgungsnulleiters mit der offenen Klemme eines Drehstrominduktionsmotors unter einphasigen Bedingungen erzeugt den erforderlichen Phasenunterschied zwischen Wicklungsspannungen, und es entsteht Anlaufmoment. Verbindung des Nulleiters wird nur zum Starten und nicht zum Laufen verwendet. Andererseits ubersteigt der Strom den Nennwert selbst ohne Belastung.

Metodo novedoso para el arranque de un motor de induccion trifisico con una fase fuera de la fuente de alimentacion

La conexion del neutro al terminal abierto de un motor de induccion trifasico bajo la condicion de fase simple produce la diferencia de fase necesaria entre los devanados del motor y el par de arranque inicial. La conexi6n del neutro es utilizada solo para el arranque inicial y no para el regimen permanente. De lo contrario la corriente excederia los valores nominales incluso sin carga.

REFERENCES

[1] Habermann, R., `The single-phase operation of a three-phase motor with a simple static phase converter', Trans. American Institute of Electrical Engineers, 73, pt. 3 (1954)

[2] Brown, J. E. and Jha, C. S., `Starting of a three phase induction motor connected to a single phase supply system', Proc. IEE, 106A (1959)

[3] Jha, C. S., `The starting of single phase induction motor having assymetrical stator windings not in quadrature', Proc. IEE,109A, pp. 47-58 (Feb. 1962)