GB1566878A - Motor overload protective device - Google Patents

Description

(54) MOTOR OVERLOAD PROTECTIVE DEVICE (71) We, AKTIESELSKABET LAUR.

KNUDSEN, NORDISK ELEKTRI CITETS SELSKAB, a company incorporated under the laws of Denmark, of Haraldsgade 53, DK-2100 Co enhagen(2), Denmark, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a motor overload protective device designed to protect electric motors against thermal overload.

A known motor overload protective device consists of an electromagnetic contactor controlled by a relay which continuously supervises the thermal conditions in the motor windings by direct or indirect measurement and at the occurrence of an overload breaks the control current to the contactor with the result that the motor is disconnected from the supply mains.

Prior art motor overload protective devices use relay types in which the winding temperature is measured directly by means of a thermally responsive element fitted into the winding, and indirectly operating types in which the principles of current measurement and integration of the square of current multiplied by time function characterizing the thermal load of the motor winding are based on the thermal or electrodynamic effects of the current or on electronic methods. Relay types of the indirectly operating kind are characterized by the fact that the current range within which each individual relay is operative is limited. A series of motors of various sizes which are operable by one and the same contactor therefore requires a plurality of sizes of the relay. To facilitate exchange ability the relays and contactors are often manufactured as separate units, and the relays may have specially shaped connecting elements adapted to the terminals of the contactors to facilitate their mounting. In exchanging a mounted relay, however, both the external cables connected to the relay and other connections between the relay and contactor are removed and reconnected to the replacement relay, which involves a risk of interchanging the cables and faulty operation.

The problem in exchanging the relay is reduced in one known motor overload protective device, which is described in Patent Specification No 971292, in which the contactor is divided into two halves and the relay is built into one half thereof. The separation of the two halves takes place in the breaking point between the fixed contact in the contactor half housing the relay, and the movable contact. This solution results in that the external cables connected to the last mentioned contactor half are to be reconnected upon exchange of the relay.

Another known solution involves a relay provided with a contact system of the pin and socket type so that the relay is exchangeable by removing it from its socket and inserting a replacement relay therein.

Disadvantages of this solution is both a high price and the fact that the pin and socket system is only suitable for lower currents due to the contact resistance conditions between pin and socket contacts.

According to the present invention there is provided a motor overload protective device having at least two terminals and comprising a contactor and a replaceable relay connected in series with one another between the said two terminals, the contactor having a movable contact arranged to electrically interconnect a fixed contact thereof with a fixed terminal of the relay, wherein another terminal of the relay is electrically connected by a separable connection to one of the said two terminals of the device, the said other terminal of the relay being electrically coupled to the said fixed terminal thereof through the relay.

Thereby the relay is exchangeable in a very simple manner and without the risk of interchanging the connecting leads of the device.

Exchanging the relay is particularly simple if the separable electrical connection also mechanically secures the relay relative to the contactor.

In a preferred embodiment of the invention the separable connection is a screw joint. Hereby both a good electrical and mechanical connection is easily provided.

The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawing, which is a schematic perspective view of part of an embodiment of the invention.

In the drawing there is illustrated a three-phase motor overload protective device consisting of a contactor and a relay.

The device has two sets of three fixed terminals 7,7',7" and 8 (one shown), respectively and the contactor comprises in a well-known manner a coil (not shown) and an armature (not shown) actuating three movable contacts 9 (one shown) in the direction of an arrow 10 for engaging two sets of three fixed contacts 11 and 12 (one shown).

The relay is an exchangeable thermal relay 1 made up of the elements within the dot-and-dash line in the figure. The relay is inserted in the electrical circuit at the breaking point 2 of the movable contacts 9 of the contactor, the fixed contacts 11 forming part of the thermal relay. Screw connections 3,3',3" electrically connecect and mechanically secure the relay to the fixed terminals 7,7',7".

Three bimetallic strips 4,4' and 4", one in each phase, are provided with heating coils which pass the respective phase currents, each heating coil being connected in series between the respective contact 11 and the respective screw connection 3,3' or 3". The bending of the bimetallic strips is representative of the thermal load of the protected motor.

The current to the contactor coil (not shown) is controlled by a switch 6 in its connection line. A release slide 5 in the thermal relay conveys in a known manner the bending of the bimetallic strips to the relay switch 6. Thus, when the bimetallic strips 4,4' and 4" are sufficiently heated by their coils, the slide 5, driven by the bending of the strips 4,4' and 4", opens the switch 6 and so disconnects the contactor coil (not shown). Thereupon the movable contacts 9 (one shown) disengage from the fixed contacts 11 and 12. As a result, the motor is disconnected.

Although the embodiment described hereinbefore with reference to the drawing is a thermal protective device, other embodiments can be constructed which may be, for example, electromagnetic or electronic protective devices, Preferably, in each embodiment, the separable connections mechanically secure the relay relative to the contactor, as do the screw connections 3,3' and 3" in the example described hereinbefore.

WHAT WE CLAIM IS: 1. A motor overload protective device having at least two terminals and comprising a contactor and a replaceable relay connected in series with one another between the said two terminals, the contactor having a movable contact arranged to electrically interconnect a fixed contact thereof with a fixed terminal of the relay, wherein another terminal of the relay is electrically connected by a separable connection to one of the said two terminals of the device, the said other terminal of the relay being electrically coupled to the said fixed terminal thereof through the relay.

2. A device according to claim 1, wherein the said separable connection mechanically secures the relay relative to the contactor.

3. A device according to claim 1 or 2, wherein the separable connection is a screw joint.

4. A motor overload protection device substantially as described hereinbefore with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. relay being electrically coupled to the said fixed terminal thereof through the relay. Thereby the relay is exchangeable in a very simple manner and without the risk of interchanging the connecting leads of the device. Exchanging the relay is particularly simple if the separable electrical connection also mechanically secures the relay relative to the contactor. In a preferred embodiment of the invention the separable connection is a screw joint. Hereby both a good electrical and mechanical connection is easily provided. The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawing, which is a schematic perspective view of part of an embodiment of the invention. In the drawing there is illustrated a three-phase motor overload protective device consisting of a contactor and a relay. The device has two sets of three fixed terminals 7,7',7" and 8 (one shown), respectively and the contactor comprises in a well-known manner a coil (not shown) and an armature (not shown) actuating three movable contacts 9 (one shown) in the direction of an arrow 10 for engaging two sets of three fixed contacts 11 and 12 (one shown). The relay is an exchangeable thermal relay 1 made up of the elements within the dot-and-dash line in the figure. The relay is inserted in the electrical circuit at the breaking point 2 of the movable contacts 9 of the contactor, the fixed contacts 11 forming part of the thermal relay. Screw connections 3,3',3" electrically connecect and mechanically secure the relay to the fixed terminals 7,7',7". Three bimetallic strips 4,4' and 4", one in each phase, are provided with heating coils which pass the respective phase currents, each heating coil being connected in series between the respective contact 11 and the respective screw connection 3,3' or 3". The bending of the bimetallic strips is representative of the thermal load of the protected motor. The current to the contactor coil (not shown) is controlled by a switch 6 in its connection line. A release slide 5 in the thermal relay conveys in a known manner the bending of the bimetallic strips to the relay switch 6. Thus, when the bimetallic strips 4,4' and 4" are sufficiently heated by their coils, the slide 5, driven by the bending of the strips 4,4' and 4", opens the switch 6 and so disconnects the contactor coil (not shown). Thereupon the movable contacts 9 (one shown) disengage from the fixed contacts 11 and 12. As a result, the motor is disconnected. Although the embodiment described hereinbefore with reference to the drawing is a thermal protective device, other embodiments can be constructed which may be, for example, electromagnetic or electronic protective devices, Preferably, in each embodiment, the separable connections mechanically secure the relay relative to the contactor, as do the screw connections 3,3' and 3" in the example described hereinbefore. WHAT WE CLAIM IS:

1. A motor overload protective device having at least two terminals and comprising a contactor and a replaceable relay connected in series with one another between the said two terminals, the contactor having a movable contact arranged to electrically interconnect a fixed contact thereof with a fixed terminal of the relay, wherein another terminal of the relay is electrically connected by a separable connection to one of the said two terminals of the device, the said other terminal of the relay being electrically coupled to the said fixed terminal thereof through the relay.

2. A device according to claim 1, wherein the said separable connection mechanically secures the relay relative to the contactor.

3. A device according to claim 1 or 2, wherein the separable connection is a screw joint.

4. A motor overload protection device substantially as described hereinbefore with reference to the accompanying drawing.