PL114946B2 - Current system designed to protect ac motors against operation without complete number of supply power phases - Google Patents

Description

The object of the present invention is a current system which protects AC motors against non-phase operation.

Hitherto known current systems protecting three-phase motors against single-phase operation use transformers, secondary windings, which they feed directly or through symmetrical component filters of the relay windings. The disadvantage of these systems is the need to use very sensitive current transmitters, and in the case of using symmetrical component filters, the efficiency of operation with underloaded motors is low and the need to determine the phase sequence of the motor supply network.

The system for protecting AC electric motors against non-full-phase operation, known from the Polish patent specification No. 88191, is equipped with a measuring unit made of two current-voltage transformers, which compares the absolute value of the vector difference of the output voltages of the transformers with the absolute value of the output voltage of the transformer. The output voltage of the comparing element is fed through a full-wave rectifier and a retarder to the actuator, which influences the state of the automatic switch installed in the power supply circuit of the protected motor.

The system according to the patent specification No. 88191 having an advantage over systems with symmetrical components filters, however, has a rather complex structure.

The protection system according to the invention is equipped with three current-voltage transformers, the primary windings of which are connected in series with the stator windings of the AC motor. The secondary windings of any two of these transformers are connected in series, however, so that the output terminals are formed as single ended terminals and are connected, through a rectifier, to one winding of the relay. The secondary winding of the third transformer supplies, via a rectifier, the second winding of said relay. Each of the windings of this relay is bypassed by a capacitor.

The protection system according to the invention is characterized by a simple structure, operational reliability, is insensitive to distortions of the current waveforms flowing in the network supplying the protected motor, and this does not require high qualifications of the personnel involved in the installation and operation.

The object according to the invention is shown in an exemplary embodiment in the drawing which shows the system schematically.

The protection circuit according to the invention comprises three transformers T1, T2, T3, current-voltage rectifiers D1, D2, capacitors C1, C2 and a two-winding relay P. The primary windings of the transformers T1, T2, T3 are connected in series with the windings of the AC motor. The secondary windings of two of these transformers are associated with each other in series in such a way that one pair of homonymous terminals (ends or beginnings) is a galvanic connection, while the other pair of terminals, through a rectifier D1, is connected to one with one winding-3 114 946 4 yen of relay P. The secondary winding of the third transformer is connected in series, through a rectifier D2, to the second winding of relay P. The capacitors C1 and C2 are connected in parallel to the relay coils.

With a symmetrical three-phase system of currents consumed by the motor, the current values of the currents Lsr and Lsr flow in the relay windings. that the magnetic flows 0i and 02 they produce are equal and opposite to each other, as a result of which the relay P is in an inoperative state. When the circuit of any of the power supply phases is interrupted, the protected motor changes the values of the voltages induced in the secondary windings of transformers T1, T2, T3, as a result of which the ratio of currents flowing through the windings of relay P will change. no and disconnecting the motor from the mains.

The system according to the invention can find wide application in the protection of three-phase motors against single-phase operation, and therefore it can contribute to a significant reduction of the failure rate of drive systems.

Patent claims A current system protecting AC motors against half-phase operation, including rectifiers, capacitors, a two-winding DC electromagnetic relay, three current-voltage transformers, the primary windings of which are connected in series to the motor supply network, characterized by the fact that the secondary winding is any two transformers (Ti, T2, T3) constitute a series circuit having at the output identical terminals, which is connected in series with the input of the rectifier (D1), to the output of which is connected in parallel the first winding of the relay (P) in parallel to it the first capacitor (C1) is connected, while the secondary winding of the remaining third transformer is connected in series with the input of the rectifier (D2), to the output of which the second relay winding (P) is connected in parallel, with a second capacitor (C1) connected in parallel.

To the Prac engine. Typographer. UP PRL. Mintage 120 copies

Price: PLN 100