GB2168556A

GB2168556A - Transformer with improved power factor in primary

Info

Publication number: GB2168556A
Application number: GB08529920A
Authority: GB
Inventors: Donald Keith Mitchell
Original assignee: MITCHELL TRANSFORMERS Ltd
Current assignee: MITCHELL TRANSFORMERS Ltd
Priority date: 1984-12-05
Filing date: 1985-12-04
Publication date: 1986-06-18
Also published as: GB8430769D0; GB8529920D0

Abstract

A transformer has a capacitor connected electrically in parallel with part of its primary winding (13), but not all of that winding (13, 14), so as to improve the power factor of the current in the winding. The winding has more turns than the secondary winding (12) and part of the primary winding is tapped. <IMAGE>

Description

SPECIFICATION Improvements in transformers This invention relates to transformers and has the object of increasing the KVA output of the transformer. The invention is particularly applicable to enable existing transformers supplying a lagging power factor load to be up-rated in order to increase their KVA output.

According to the present invention a transformer has a primary winding and a secondary winding with the primary winding having a greater number of turns than the secondary winding and with a capacitor connected electrically in parallel with some, but not all, of the turns of the primary winding.

In this way the power factor of the current in the primary winding is improved thereby reducing the primary current or for the same primary current enabling a higher KVA output to be obtained.

The invention is particularly, but not solely, applicable to transformers which are used to produce a large secondary current which can be used for heating applications with low lagging power factor loads. With such transformers it is usual to have many more turns on the primary winding than on the secondary winding and to enable the secondary winding to carry the large current it is usual for this winding to be of hollow construction and for provision to be made for cooling liquid to be circulated therethrough.

The power factor improvement is brought about by employing the capacitor in association with some, but not all, of the turns of the primary winding. It is necessary that, by connecting the capacitor electrically in parallel with some of the turns, the capacitive current is forced through the remaining turns of the winding. If the capacitor were connected across all the turns of the primary winding then the capacitive current is not forced through the turns of the winding.

Preferably the turns of the primary winding are arranged in two groups which constitute first and second parts of the winding respectively, the first part of the winding has tap contacts thereon whereby some or all of the turns of the first part of the winding are connectible electrically in series with the turns of the second part of the winding and the capacitor is connected electrically in parallel with at least some of the turns of the first part of the winding.

Furthermore the first group of turns are arranged in first and second sub-groups, one end of the first sub-group being connected to a power terminal and the other end of the first sub-group being permanently connected to one end of the turns of the second subgroup and it is the second sub-group which has the tap contacts thereon.

The capacitor may be connected in parallel with all the turns of the first part of the primary winding or it may be connected in parallel with only the second sub-group of the first part of the winding. Furthermore, the capacitor may be connected at one end to the end of the second sub-group which is away from the first sub-group and at the other end to a movable contact which is movable into engagement with selected ones of the tap contacts.

It will be appreciated, however, that with this arrangement the capacitor could be shorted out completely if the movable contact were put into engagement with the same contact to which the other end of the capacitor were connected. If improvement is required on this lowest secondary voltage tapping then provision has to be taken to prevent this from happening and usually it is in the form of a means for always including a certain number of turns of the primary winding electrically in parallel with the capacitor at whichever of the taps the movable contact is in engagement.

In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings in which: Figures 1 to 3 show schematically three alternative embodiments of the present invention.

A transformer, such as one suitable for supplying power with a lagging power factor to an electric heater for metal billets, has a primary winding 11 and a secondary winding 12.

For such an application the voltage applied to the primary winding 11 may be 3.3 KV and the voltage on the secondary winding 12 may be of the order of, say, 100 volts. For the particular application the secondary current is very large and, consequently, the relatively small number of turns of the secondary winding may be directly water cooled, that is, the turns are made up of a hollow conductor and coolant water is circulated continuously through the conductor. On the other hand, the primary winding is conveniently indirectly cooled, that is, heat generated in the primary winding flows through the surrounding insulation to the secondary winding where it is absorbed by the cooling water.

In all the embodiments of the invention the turns of the primary winding are arranged in two groups 13 and 14, respectively, and each of the two groups may have approximately the same number of turns.

The turns constituting the group 13 comprise two sub-groups 13A, 133, respectively.

One end of subgroup 13A is connected to power terminal 15 and the other end is permanently connected electrically in series with sub-group 13B. Sub-group 13B of the winding has a plurality, say 9, tap contacts 1-9 connected to it. Tap contacts 1 and 9 are connected to respective opposite ends of subgroup 13B. In the arrangement shown in the figures, purely as an example, there may be approximately 50 turns between tap 1 and tap 9, and approximately 25 turns in sub-group 13A.

A power terminal 16 is connected to one end of the part winding 14 and the opposite end of this part winding is connected to the movable contact 17 of the tap changing switch. Thus, by connecting movable contact 17 to tap contact 9, the primary winding consists of the turns between power terminal 15 and tap contact 9 and the turns of the part winding 14. Similarly, when the movable contact is connected to tap contact 1, all of the turns of the part winding 13 are connected in series with the turns of the part winding 14.

A power factor improvement capacitor 18 is connected to a part, but not all, of the primary winding 11 of the transformer.

Referring now to Figure 1, the power factor capacitor 18 is permanently connected to the opposite ends of the part winding 13 so as to be permanently connected in parallel with it. In the embodiment illustrated in Figure 2, the power factor correction capacitor 1 8A is permanently connected between taps 1 and 9 of the part 13 of the winding so that the capacitor 18A is permanently connected in parallel with the turns of the sub-group 13B.

In the alternative embodiment, illustrated in Figure 3, the capacitor 1 8B is connected at one end to tap 1 and at the other end to the movable contact 17 of the tap changing switch. Thus, the capacitor is connected in parallel with the turns between tap 1 and the other tap to which the movable contact 17 is connected. Clearly, with this embodiment, an arrangement is built into the winding to prevent the movable contact 17 being brought into contact with tap 1 since this would short out the capacitor 1 8B completely.

In alternative embodiments of the invention which are not illustrated, the primary winding can be a one part winding which is tapped and has the capacitor connected across an untapped part of the winding or the winding is in two parts, one part being tapped as shown in Figures 1-3 and the capacitor connected in parallel with the other part.

Claims

1. A transformer having a primary winding and a secondary winding with the primary winding having a greater number of turns than the secondary winding and with a capacitor connected electrically in parallel with some, but not all, of the turns of the primary winding.

2. A transformer as claimed in claim 1, in which the turns of the primary winding are arranged in two groups which constitute first and second parts of the winding respectively, the first part of the winding having tap contacts thereon whereby some or all of the turns of the first part of the winding are connectible electrically in series with the turns of the second part of the winding and said capacitor is connected electrically in parallel with at least some of the turns of the first part of the winding.

3. A transformer as claimed in claim 1, in which the turns of the primary winding are arranged in two groups which constitute first and second parts of the winding respectively, the first part of the winding having tap contacts thereon whereby some or all of the turns of the first part of the winding are connectible electrically in series with the turns of the second part of the winding and said capacitor is connected electrically in parallel with all the turns of the second part of the winding.

4. A transformer as claimed in claim 2, in which the capacitor is connected electrically in parallel with all of the turns of the first part of the winding.

5. A transformer as claimed in claim 2, in which the first group of turns are arranged in first and second sub-groups, one end of the first sub-group being connected to a power terminal and the other end of the first subgroup being permanently connected to one end of the turns of the second sub-group, and said second sub-group having the tap contacts thereon.

6. A transformer as claimed in claim 5, in which the capacitor is permanently connected across the turns of the second sub-group.

7. A transformer as claimed in claim 5, in which the capacitor is connected at one end to the end of the second sub-group which is away from said first subgroup and at the other end to a contact movable into engagement with selected ones of the tap contacts.

8. A transformer as claimed in any preceding claim, in which the secondary winding is of hollow construction and provision is made for circulating cooling liquid therethrough.

9. A transformer substantially as herein before described with reference to the accompanying drawings.