GB2259610A - Transformer having a plurality of parallel primaries - Google Patents

Abstract

A transformer 10 comprises a plurality of parallel primary windings 11, 12, 13, and one or more secondaries 16. The current in each primary 11, 12, 13 is a fraction of the current in a single equivalent primary, enabling the use of lower-rated proprietary components, e.g. anti-surge NTC elements 23 for protecting lighting supplies. A thermal cutout 26, which may be within the windings, and MCB 20 can be in series with the transformer. <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO TRANSFORMERS The present invention relates to transformers and in particular, a transformer which is well suited for use in powering lighting systems.

Conventional lighting systems in domestic premises are typically powered directly from the mains but certain types of specialist lighting used in commercial and domestic premises are designed to operate from a lower voltage supply, for example 12 volts Hitherto, power to such systems has been provided from the mains via a transformer which transforms the 240 volts mains voltage down to 12 volts. A transformer of this type is illustrated schematically in Figure 1. The transformer may be for example a 600 VA transformer having a single primary winding 1 and a single secondary winding 2, with a turns ratio of 20:1. During normal operation in the 600 VA transformer the primary winding carries a current of 2.5 Amps which is transformed to 50 Amps on the secondary side (assuming no losses).

It is necessary to fuse a lighting circuit in order to prevent surges in the input current from damaging the components in the circuit and a suitable fuse is usually located in front of the input to the primary. When a transformer of this type is switched on, there is usually a surge in the input current before it settles to its 2.5A steady state. Therefore, the rating of the fuse has to be chosen so that it does not activate during the initial turnon surge. With a transformer of the type described above it has been standard practice to use a so-called "anti-surge" fuse rated at 6.3 Amps. Such a fuse will carry a current of up to 6.3 Amps but will "blow" if surges above that amount occur.

It would be advantageous to reduce the initial turn-on current surge of the transformer so that a lower rated fuse could be used, i.e. a fuse having a rating closer to the steady-state current, for better protection of the components of the lighting circuit. It would also be advantageous to reduce the current flowing in the primary winding so that commonly available proprietary components could be used for protection against short circuit faults and current surges. Such components are not designed to operate at such high currents as 2.5 Amps.

The present invention provides a transformer having a plurality of primary windings connected in parallel and one or more secondary windings. Thus, the input current is distributed between the primary windings and is less than the current which would be drawn by a transformer having a single primary winding. Thus, with a 600 VA transformer constructed according to the invention, proprietary components for reducing the turn-on surge may be used and the transformer may be fused to a current which is closer to its normal operating current.

The transformer preferably has a toroidal core and the primary windings are preferably wound around the core, one after the other, separated by layers of insulation. The or each secondary winding is then preferably wound on the outside of the primary windings.

The present invention also provides a power supply unit including a transformer of the type described above.

An embodiment of the invention will now be described in detail with reference to the accompanying drawings in which: FIGURE 1 is a schematic diagram of the type of transformer which has been used in conventional low-voltage lighting circuits; FIGURE 2 is a circuit diagram of a power supply for a lighting circuit including a transformer according to the present invention; and FIGURES 3(a) to (f) illustrate the steps in constructing a transformer according to the invention.

Referring to Figure 2, the illustrated power supply includes a transformer, generally indicated at 10 including three primary windings, 11,12,13 and a single secondary winding 16. The primary windings are connected in parallel to the live and neutral terminals 17,18 respectively of the mains supply.

A mini circuit breaker (MCB) or trip switch 20 is positioned in conductor 19 extending between the live terminal 17 and the circuit node 21 where the primary windings are coupled together. Connected in series with each of the primary windings 11,12,13 is a negative temperature coefficient device (NTC) 25. A thermal cut-out 26 is connected in series with the MCB 20 between the MCB 20 and the node 21.

The transformer 10 is a 600 VA transformer and thus the steady state operating current passing through the MCB 20 is 2.5 Amps. The three primary windings 11,12 and 13 are identical, and thus the current flowing through them is equal, i.e. 0.83A approx. (one third of 2.5A).

The NTC's 25 operate to reduce the current surge which normally occurs when power is initially applied to the transformer. An NTC has a resistance which decreases with increasing temperature. Thus, on initial start up, each NTC is relatively cold and has a relatively high resistance and so passes a relatively low current. The effect of the current flowing through the NTC is to warm it up so that its resistance lowers and the current increases and so on, until the resistance of the NTC becomes negligible. Thus the effect of the NTC's is to gradually apply the start-up current to the transformer 10, thereby reducing the initial surge. Commonly available NTC's are suitable for operating at steady state currents of 0.83A but they are not suitable for higher currents such as 2.5A, more especially they are not suitable for carrying the higher currents which occur in fault situations.

Having thus reduced the start-up surge current, the power supply can be fused at a rating which is closer to the desired steady state current. The MCB 20 is rated at 4A and would "trip" at 14-20A. With the conventional transformers described above there has been a tendency not to use MCB's because their rating has to be sufficient to carry the surge current which would occur on initial start up. Since the tripping current of an MCB is 3.5-4 times the rating current, this would mean a tripping current which is far higher than ideal. The ability to use an MCB in the arrangement shown in Figure 1 is an advantage because MCB's are simple to operate and do not require replacement after "tripping".

The purpose of an MCB or other type of fuse in a circuit of this kind is to avoid sudden large currents through the other components which might occur, for example, in a short circuit condition. The purpose of the thermal cut-out 26 is to avoid excessive heat in the event of a progressive (or gradual) overload.

A progressive overload will cause overheating which will operate the thermal cut-out. The MCB 20 is used in addition to the thermal cut-out 26 because thermal cutouts act too slowly to satisfactorily overcome short circuit faults.

It should be noted that the number of turns in each primary winding 11,12,13 must be the same as the number of turns in a 600 VA transformer having a single primary winding since the turns ratio determines the transformation ratio. However, the three primary windings of Figure 2 carry less current than a transformer with a single primary and hence thinner wire could be used for the turns.

An example of the physical construction of a transformer having several primary windings will now be described with reference to Figure 3. This example is a toroidal transformer which is based on a steel core 30, shown in Figure 3(a). This core is a spiral winding comprising a continuous strip of sheet steel. The core 30 is encased in an insulating casing shown in Figure 3(b).

The casing is formed in two similar pre-moulded parts 31a and 31b. Each part 31a,31b is shaped to fit closely around the core 30. Part 31a is shown in position around the core 30. Part 31b is shown spaced from the core 30.

Once the insulating casing is in place, a first primary winding 32 is wound around the toroidal core 30 as shown in Figure 3(c). When this procedure is complete a layer of insulation 33 is placed over the primary winding as shown in Figure 3(d). The insulation 33 is in the form of a relatively wide strip which is wrapped around the core over the primary winding as shown. When the wrapping of the insulation 33 is complete only the ends of the wire forming the primary winding protrude.

The steps described in the preceding paragraph as shown in Figure 3(c) and 3(d) are repeated until the desired number of primary windings has been wound around the core 30. Thus, the primary windings are separated from each other by a layer of insulating material such as insulating layer 33.

It should be noted here that the thermal cut-out 26 may be incorporated in the primary windings rather than being located outside the transformer as shown in Figure 2.

Figure 3(e) shows the core having three primary windings wrapped around it, 32,34 and 35. The final layer of insulation around the primary windings is indicated by numeral 36.

The secondary winding 37 is wound around the torus over the insulation 36 as shown in Figure 3(e). A further layer of insulation 38 is wrapped around the secondary winding as shown in Figure 3(f). Thus, in the finished product, the ends of the wires of the primary and secondary windings protrude from the final layer of insulation 38.

For certain applications it may be desirable to provide several secondary windings, in which case the steps shown in Figures 3(e) and (f) are repeated as often as necessary.

It will be appreciated that the present invention is not limited to 600 VA transformers, nor indeed to transformers for powering lighting equipment. However, the preferred transformer according to the invention is for powering lighting equipment and would typically have a rating between 20 and 750 VA, and preferably 200-750 VA.

Claims (12)

CLAIMS:

1. A transformer having a plurality of primary windings connected in parallel and one or more secondary windings.

2. A transformer as claimed in claim 1 having a toroidal core.

3. A transformer as claimed in claim 1 or 2 in which the primary windings are wound around a core and are separated from each other by a layer of insulation.

4. A transformer as claimed in claim 1,2 or 3 in which the or each secondary winding is wound on the outside of the primary windings.

5. A power supply unit comprising a transformer having a plurality of primary windings connected in parallel and one or more secondary windings.

6. A power supply unit as claimed in claim 5 in which a negative thermal coefficient resistive device is connected in series with each of the primary windings.

7. A power supply unit as claimed in claim 5 or 6 in which a thermal cut-out device is connected with the primary windings.

8. A power supply unit as claimed in claim 5 or 6 in which a thermal cut-out device is included in the primary windings.

9. A power supply unit as claimed in any of claims 5 to 8 in which power is input to the transformer via a mini circuit breaker.

10. A power supply unit as claimed in any of claims 5 to 9 in which the rating of the transformer is suitable for transforming mains power for supply to lighting equipment.

11. A transformer substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

12. A power supply unit substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.