# 100W autotransformer

In the above example, the basic data of an autotransformer of about 100-200W is defined. Next, we determine the data of individual sections of the step-up winding

(fig. 1).It is usually necessary to provide for the ability to regulate the voltage on the load when the mains voltage changes. If we accept sufficient voltage regulation after 10 V, then it is necessary to make taps in the step-up part of the winding every W0 * 100 = 5.6 * 10 = 56 turns. Since the increasing part of the winding will consist of Wпов = W2-W1 = 672 - 448 = 224 turns, the number of taps to it should be equal to 224: 56 = 4 taps. The cross-sectional area of the core is 15 cm2, which corresponds to the typical power of the transformer, as it is easy to verify from the formula, in about 160 W or an autotransformer with a power of about 500 W. Therefore, if we use the core plates of this transformer for our case, then we need to take only about half of its set. Then the area of the set taken will be about 8 cm2, which will correspond exactly to the power of the 44 W transformer. In the absence of a finished core, it can be made on its own. Only in this case, when calculating the autotransformer data, the magnitude of the magnetic induction must be taken depending on the type of plates available.

fig. 1.

Rice. 2.

Fig. 3.

For example, in the manufacture of a core from ordinary roofing steel, the value of the magnetic induction should be taken no more than 4000-5000 gauss. The thickness of the core plates should not exceed 0.35-0.5 mm.

The shape and dimensions of the plates are shown in Fig. 2. The plate (W-shaped) and the overlay (rectangular plate) each have two holes that serve for the bolts tightening the core. For an autotransformer with a power of 100 W, 75-90 such plates of each sample must be prepared. If the plates are prepared from roofing steel, then they must be calcined in a furnace - on hot coals, and then buried in hot ash for slow cooling. The cooled plates are cleaned of the iron scale formed on their surface. Each plate on one side is pasted over with tissue paper or covered with shellac varnish. The plates are assembled into an overlap core.

To wind the coils of the autotransformer, it is necessary to make a frame.

The frame is glued from press board or cardboard. The markings necessary for the manufacture of the base of the coil frame are given in Fig. 3, where the through cuts are shown by solid lines, and the cuts are shown by the dotted line. The depth of the notch is equal to half the thickness of the press-panel. Along the dotted lines, indicated in the figure with the letters a and b, cuts are made on the reverse side. Then, four cheeks are cut out of the press board, the dimensions of which are shown in Fig. 4, and the frame is glued in the following sequence. By bending the prepared press-and-panel plate along the notch lines, a tetrahedral prism with four sides at each end is obtained. A cheek is placed on each end of the prism, then the sides are bent and glued to it. The resulting empty corners between the sides are filled with pieces of press-panel and then a second press-panel cheek is glued to the outer side of the cheek worn on the frame - such strengthening of the cheeks provides sufficient strength of the frame. At the finished frame, it is necessary to clean the edges of the cheeks and seams with a small file and sandpaper, and then varnish the entire surface of the frame, after which you can start winding the coil.

Winding should be done in correct rows. A layer of insulation (made of paper or cambric) must be laid between the layers. It is necessary to ensure that individual turns of the layer do not fall through the edges of the layers (at the cheeks of the frame). It is necessary to pay special attention to the manufacture of leads from the autotransformer winding. It is very important that the possibility of accidental closure of individual taps with each other is excluded, for which the taps should be made on both sides of the frame. The outlet from the required number of turns is done by a tap, and the point of the tap on the tap must be carefully insulated. As a wire for the outlet, you need to take a soft wire of sufficient cross section. Taps, in order to save space for winding, must be placed at the outer ends of the winding rows. Bends can also be made in the form of loops.

One of the important elements of an autotransformer is a switch. The switch design can be selected depending on the available options. In its simplest form, it can be a socket made of insulating material with sockets arranged in a circle and a plug with shorted legs. The slide switch is more convenient. In this case, it is very important that, when switching, the slider does not close two adjacent outputs of the autotransformer with each other. For this purpose, idle contacts are installed between the contacts to which the leads are connected. The switch must provide reliable contact between the slider and the contacts, as otherwise it will heat up.

For autotransformers up to 50 W, one board from the switch used in the receivers can be used as a switch, but the stopper must be removed so that the slider can rotate 360 °. For more powerful autotransformers, these switches are unsuitable, since due to the insufficient density of the connection of the slider with the contacts, at high currents, strong heating is obtained, and the switch may burn out. As the material of the socket for mounting the switch, you can use ebonite, getinax or, in extreme cases, a wooden plank boiled in paraffin. fig. 4

To control the voltage supplied to the consumer by an autotransformer, it is desirable to have some kind of device - a voltmeter, a neon lamp, etc. The most suitable device for monitoring voltage is any voltmeter designed to measure AC voltage up to 150-250 V. output clamps. In the absence of a voltmeter, a neon lamp can be used to control the voltage, which will light up at normal voltage (127-220 V) at the output of the transformer. The neon lamp turns on in the same way as a voltmeter at the output terminals of the autotransformer (Fig. 5) voltage, resistance R is selected such that the lamp lights up at a voltage of 120 or 220 V.

fig. 5.

Source: N.V. Kazansky "Autotransformer"

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