GB2107139A

GB2107139A - Circuit arrangement for preventing re-establishment of the starting condition in an electric single-phase motor

Info

Publication number: GB2107139A
Application number: GB08226553A
Authority: GB
Inventors: Harald Mann Sorensen
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1981-09-25
Filing date: 1982-09-17
Publication date: 1983-04-20
Also published as: AT378634B; DE3234461A1; AU8864782A; AU558964B2; ATA353582A; DK425481A; ES8306937A1; ES515923A0; MX153626A

Abstract

In an electric single-phase motor having a starting circuit, which in connection with an auxiliary winding (2) produces a switching from a starting condition to a running condition when the number of revolutions per minute of the motor during the starting procedure has reached a predetermined value, the starting circuit (5-9) is so designed that after the switching from the starting condition to the running condition it cannot re-establish the starting condition until the feed voltage of the motor has been switched off. Hereby, damages, e.g. destroying of component parts, are avoided, which could otherwise occur if the starting condition is re- established during running, e.g. by an overloading of the motor. <IMAGE>

Description

SPECIFICATION Circuit arrangement for preventing reestablishment of the starting condition in an electric single-phase motor The invention relates to a circuit arrangement for preventing re-establishment of the starting condition in an electric single-phase motor having a main winding and an auxiliary winding, together with a starting circuit acting in connection with the auxiliary winding and switching the motor from the starting condition to a running condition, when during the starting operation the number of revolutions has reached a predetermined value.

In a known single-phase motor of this type, the starting operation is initiated by connecting the main winding to the mains voltage, and by connecting also the auxiliary winding to the mains voltage by means of the starting circuit. When the motor has reached the predetermined number of revolutions, which may, for instance, be about 75% of the synchronous number of revolutions, the auxiliary winding is switched off. The two windings are differently shaped, so that the phase displacement is smaller in the auxiliary winding that in the main winding. Hereby is obtained that when the auxiliary winding is switched on, the motor has a starting torque, and the running-up torque is increased.

If there is a need for a greater starting torque, a capacitor is inserted in series with the auxiliary winding, and this series connection is provided with the mains voltage by means of the starting circuit.

Finally, it is possible in connection with the auxiliary winding to use two series-connected capacitors, of which only one, the starting capacitor, is switched on and off by the starting circuit, depending on the number of revolutions of the motor, whereas the other capacitor, the running capacitor, is constantly connected to the mains voltage in series with the auxiliary winding.

Hereby, a better utilization of the motor during running and a greater starting torque are obtained.

Activation of the starting circuit, so that it switches from the starting condition to the running condition, can be accomplished by means of a centrifugal switch, in which the centrifugal forces on a number of rotating blocks are used to open an electric switch at the predetermined number of revolutions.

It is also known to use an electromagnetic relay, a so-called starting relay, the coil of which is connected in series with the main winding, in which the current is great during starting and running-up, so that the relay is energized and, thereby, inserts the auxiliary winding. When the motor has reached the predetermined number of revolutions, the current in the main winding has decreased so much that the armature of the relay is released, and the relay contact disconnects the auxiliary winding circuit.

In the said examples on known motor circuit arrangements, mechanical contacts are used to obtain the switching function. However, motor circuit arrangements are also known, in which semi-conductors, preferably triacs, are used in accomplishing the switching function.

A disadvantage of the known motor circuit arrangements is that the starting circuit will reestablish the starting condition by an overloading of the motor, whereby the auxiliary winding and the starting capacitor, if present, can be overloaded and destroyed, and furthermore the reinsertion of the starting capacitor in the case where a running capacitor is also present may lead to very great currents in certain circuit elements, because the starting capacitor is connected in parallel with the running capacitor, which has previously been loaded to a high voltage, whereby so great currents may occur that one of the circuit elements is overloaded.

It is the object of the invention to remove the said disadvantage, and this object is attained by means to prevent that the starting circuit after having switched the motor from the starting condition to the running condition re-establishes the starting condition, as long as the feed current of the motor is supplied. In this case, the above mentioned detrimental re-establishment of the starting condition during the running of the motor, e.g., by an overload, and the possible destroying of component parts caused thereby is avoided.

According to the invention, the idea of the invention can be realized by the fact that the starting circuit comprises a switching element, which during the starting of the motor is in a first inactive condition, which when the number of revolutions of the motor has reached the predetermined value shifts to a second active condition, in which it prevents the reestablishment of the starting condition, and which is maintained in this condition by the feed voltage of the motor.

According to the invention, the switching element may be an electromagnetic relay, which is energized by the transition from the starting condition to the running condition, to which the feed current of the motor is supplied via a holding contact, and which by another contact prevents re-establishment of the starting condition. In this case, the object of the invention is attained by very simple means.

In another expedient embodiment of the circuit arrangement according to the invention, the starting circuit can comprise a relay, which produces the starting condition by being supplied with energizing voltage as a result of the switching-on of the feed voltage of the motor, which produces the running condition by the removal of the energizing voltage when the predetermined value of the number of revolutions has been attained, said removal of the energizing voltage being obtained by the transition of the switching element to the second condition.

The invention is explained in further details in the following with reference to the diagrammatic drawing, in which: Fig. 1 shows a first embodiment of the circuit arrangement according to the invention, in which the number of revolutions of the motor is detected by a relay in series with the main winding, Fig. 2 a second embodiment, in which the number of revolutions of the motor is determined by measuring the voltage across the auxiliary winding, and Fig. 3 a third embodiment corresponding to that shown in Fig. 2, but having an electronic relay to switch over from the starting condition to the running condition.

The single-phase motor shown in Fig. 1 comprises a main winding 1 and an auxiliary winding 2 together with a starting capacitor 3 connected in series with the auxiliary winding 2.

The feed current of the motor can be switched on by means of a double-pole switch 4. In series with the main winding 1 is inserted the coil 5 of a relay, the contact 6 of which is connected in series with the auxiliary winding 2 and the capacitor 3. A second relay having a coil 7 has a break contact 8 connected in series with the auxiliary winding 2 and the capacitor 3 and a make contact 9 connected as a holding contact.

The circuit arrangement shown functions as follows.

When the switch 4 is closed, the mains voltage is supplied to the main winding 1 and the coil 5 connected in series therewith, whereby the contact 6 is switched. Since the relay having the coil 7 has a small delay in relation to the relay having the coil 5, there is not time enough for the former relay to be activated, before the contact 6 has been switched. By this switching, the connection to the relay coil 7 is interrupted, and the mains voltage is then supplied to the series connection of the auxiliary winding 2, the starting capacitor 3 and the relay contact 8, whereby the motor starts. When the number of revolutions of the motor has reached a predetermined value, e.g.

about 75% of the synchronous number of revolutions, the current through the relay coil 5 has decreased to such an extent that the relay contact 6 is again switched, whereby the current through the auxiliary winding 2 is interrupted.

Mains voltage is now supplied to the relay coil 7, whereby the contact 8 is opened, and the contact 9 is closed. The contact 9 hereby acts as a holding contact and ensures that the relay having the coil 7 is maintained in the activated condition, as long as the mains voltage is supplied. In this activated condition, the auxiliary winding 2 and the starting capacitor 3 cannot be reinserted because of the open relay contact 8. This also applies even if the current through the main winding 1, e.g, because of an overloading of the motor, should again rise to such a value that the relay having the coil 5 is again energized. Not until the mains voltage has been switched off, and the relay having the coil 5 is returned to its normal position, can the auxiliary winding be switched on again.

For parts shown in Figs. 2 and 3 and corresponding to parts shown in Fig. 1, the same references have been used as in Fig. 1.

In the embodiment shown in Fig. 2, the change from the starting condition to the running condition is realized by means of an electromagnetic relay having a coil 10 and a contact 1 The coil 10 is supplied with current when the mains voltage is connected by means of the switch 4. The current passes a resistor 12, a full-wave rectifier 1 3 and a resistor 14. Hereby the relay contact 11 is closed, and the motor starts.

As the number of revolutions of the motor gradually increases, the voltage across the auxiliary winding 2 also increases. This voltage is supplied to a measuring circuit consisting of a resistor 15, a capacitor 1 6 and a triggering diode (diac) 1 7. As the voltage across the winding 2 rises, also the voltage across the capacitor 1 6 will increase, and when the latter voltage reaches the triggering level of the triggering diode 1 7, a discharge through this diode and, dependent on the polarity, one of two diodes 1 8 and 19 of which the latter is included in an optically coupled thyristor 20 having a gate resistor 21 will take place.As soon as current flows through the diode 19, it will ignite the triac 20 which short-circuits the coil 10, whereby the relay contact 11 is opened. The current from the rectifier circuit 13 is a rectified direct current, the instantaneous value of which is zero at the zero crossings of the alternating current. However, the circuit arrangement comprising a diode 22, a capacitor 23 and the resistor 24, which has a much greater value than the resistor 14, has the effect that a certain current is maintained through the thyristor 20, even when the current from the rectifier circuit 1 3 is zero.The thyristor 20, therefore, remains conducting when it has been ignited by the light from the diode 19, which has the effect that even if the motor is overloaded, so that the voltage across the winding 2 decreases and there will be no igniting pulses from the diode 19, the relay coil 10 will remain short-circuited, as long as there is a voltage on the terminais of the motor. Hereby is obtained that the auxiliary winding 2 and the starting capacitor 3 are not reconnected to the supply voltage by an overloading of the motor.

In the motor shown in Fig. 3, the electromagnetic relay of Fig. 2 with the coil 10 and the contact 11 is substituted by an electronic relay in the shape of a triac 25 having a gate leak resistor 26. Otherwise, the starting circuit is constructed in the same way as shown in Fig. 2.

The current to the full-wave rectifier 1 3 is, however, in this case taken from a current transformer 27 through the primary winding of which the total motor current flows.

The circuit arrangement shown in Fig. 3 functions in the following way.

When the switch 4 is closed, a current flows through the main winding 1. Hereby, a current will be produced through the secondary winding of the current transformer 27, the fuil-wave rectifier 13, the resistor 14 and the resistor 26. Hereby, the triac 25 will be ignited, and it will be re-ignited after each zero crossing of the current passing through the auxiliary winding 2 and the capacitor 3. The motor will then start, whereby the voltage across the auxiliary winding 2 increases. When this voltage has reached a predetermined value, the thyristor 20 will ignite, as explained with reference to Fig. 2, whereby the gate of the triac 25 no longer receives an igniting voltage, so that the current through the triac will cease at the next zero crossing of the current.The thyristor 20 will remain ignited, as long as there is a current from the current transformer 27, even if the motor is being overloaded, and the voltage across the winding 2 is, therefore, reduced to a value below the limiting value for igniting the thyristor 20.

The invention is not limited to the embodiments shown. Thus, a centrifugal switch can immediately replace the relay having the coil 5 and the contact 6 in Fig. 1, or the voltage measuring circuit 15, 16, 17 in Figs. 2 and 3.

When electronic means are used, there are other possibilities for the elements for detecting whether the feed voltage of the motor is supplied.

Thus, instead of the thyristor 20 shown in Figs. 2 and 3, it is, for instance, possible to use a flip-flop with transistors.

The current supplies to the starting circuits in Figs. 2 and 3 can also be constructed in another way. Instead of the current supplies shown, it is, for instance, possible to use a mains voltage transformer or a winding inserted in one of the slots of the motor containing the main winding.

The invention is not limited either to the motor circuit arrangements shown. It can be used, for instance, in connection with single-phase motors without a starting capacitor, or single-phase motors having a starting capacitor as well as a running capacitor.

Furthermore, the invention can be varied in different ways within the scope of the patent

Claims

claims. CLAIMS

1. Circuit arrangements for preventing reestablishment of the starting condition in an electric single-phase motor having a main winding (1) and an auxiliary winding (2) together with a starting circuit (5, 6; 10-14; 13, 14, 25-27) acting in connection with the auxiliary winding and switching the motor from the starting condition to a running condition, when the number of revolutions of the motor during starting operation has reached a predetermined value, characterized by means (7-9; 1 5-24) for preventing that the starting circuit after having switched the motor from the starting condition to the running condition re-establishes the starting condition as long as the feed voltage is supplied to the motor.

2. Circuit arrangement according to claim 1, characterized in that the starting circuit comprises a circuit element (7-9; 20) which during the starting operation of the motor is in a first inactive condition, which when the number of revolutions of the motor has reached the predetermined value changes to a second active condition, in which it prevents re-establishing of the starting condition, and which is maintained in this condition by the feed voltage of the motor.

3. Circuit arrangement according to claim 2, characterized in that the circuit element is an electromagnetic relay (7-9) which is energized by the transition from the starting condition to the running condition, which is supplied with the feed voltage of the motor via a holding contact (9), and which prevents establishment of the starting condition by means of another contact (8).

4. Circuit arrangement according to claim 3, characterized in that the relay is energized via a contact (6), the state of which depends on the number of revolutions of the motor.

5. Circuit arrangement according to claim 2, characterized in that the starting circuit comprises a relay (10, 1 25) producing the starting condition by being supplied with an activating voltage as a result of the switching-on of the feed voltage of the motor, which produces the running condition by the removal of the activating voltage when the predetermined value of the number of revolutions has been reached, the said removal of the activating voltage being produced by the change of the circuit element (20) to the second condition.

6. Circuit arrangement according to claim 5, characterized in that the circuit element (20) in the second condition short-circuits the feed current of the relay (10, 11;25).

7. Circuit arrangement according to claim 6, characterized in that the circuit element is a thyristor (20) which is activated when the number of revolutions reaches the predetermined value, and the feed current of which is maintained as long as the feed current of the motor is supplied.

8. Circuit arrangement according to one or more of claims 5 to 7, characterized in that the relay is an electronic relay, e.g. a triac (25).

9. Circuit arrangement according to one or more of the preceding claims, characterized in that the attainment of the predetermined number of revolutions is detected by measuring the voltage across the auxiliary winding (2).

10. Circuit arrangement for preventing reestablishment of the starting condition in an electric single-phase motor, mainly as described with reference to the drawing.