Autotransformers

An autotransformer has a single winding which is tapped as shown in Fig. 195 to provide a fraction of the primary voltage across the secondary load.

Fig. 195. Autotransformer voltages and currents.

The connections may be reversed so that a step-up voltage is obtained. The regulation, leakage inductance, and size of an autotransformer for a given rating are all less than for a two-winding transformer handling the same power. Where the voltage difference is slight, the gain is large. Where the voltage difference is great, there is not much advantage in using an autotransformer, nor can it be used where isolation of the two circuits is required.

Autotransformers are used in electronic applications chiefly for the adjustment of line voltage, either to change it or to keep it constant. Examples are the reduction of plate voltage for tuning an amplifier and the maintenance of constant filament voltage. Taps may be chosen by means of a tap switch to adjust the load voltage. The load voltage may be adjusted to within half the voltage increment between taps.

If the voltage is adjusted while load remains connected, bad switching arcs occur, either from breaking the circuit or from short-circuiting turns. To provide for adjustment under load conditions, a resistor may be momentarily connected in the circuit as the tap switch bridges from one tap to the next, and current is limited to full-load value. In large power tap changers, a reactor replaces the resistor to avoid heating and losses.

The v-a rating of an autotransformer depends on the ratio of input to output voltage. In Fig. 195 the output current I₂ = I₁ + I₃. Let p = per cent tap/100 = E₂/E₁. Neglecting losses, I₂ = I₁/p and I₃ = (1/p - 1)I₁. Then

Volt-amperes (in the upper portion) = (1 - p)E₁I₁

Volt-amperes (in the lower portion) = pE₁I₃ = (1 - p)E₁I₁

which satisfies equality of volt-amperes in each section. For ratio p close to unity, the v-a rating and hence size for a given output can be made very small; for small values of p the size is not much less than that of a two-winding transformer, but the autotransformer has much less regulation. Its effective winding reactance and resistance decrease as (1 - p)²; that is, for a given unit,

[111]

Appreciably less regulation is obtained in an autotransformer, even when size is not reduced much, because the right-hand term in equation 111 is squared.

When the power for electronic equipment is supplied by a 230-volt line, but auxiliary items such as relays and small motors are used at 115 volts, a convenient way of obtaining the latter voltage is to center-tap the primary of a large plate transformer, and use it as a 2:1 step-down autotransformer. The larger primary winding copper requires little extra space, and an additional transformer is thereby saved.

To improve the closeness of voltage control, a variable autotransformer has been developed in which the moving tap is a carbon brush which slides over exposed turns of the winding. Brush resistance prevents excessive transition current and permits smooth voltage control; yet it offers little additional series resistance to the load. The same idea can be applied to two-winding transformers for secondary voltage adjustment. A typical unit of this kind is shown in Fig. 196.

Fig. 196. Adjustable primary anode transformer.

When autotransformers are used on three-phase supply lines, they may be connected the same as two-winding transformers in star, delta, open-delta, or Scott connections. The last two connections are less subject to objectionably high regulation in autotransformers and, if they supply three-phase anode transformers, cause no serious primary voltage unbalance for voltage ratio p close to unity.