How can you adjust a transformer's voltage range from just a few volts to over 100% input voltage?

Variable-voltage transformers are efficient means of changing voltage, especially if you want a greater degree of flexibility in changing the ratio between the primary and secondary coils. They are simple to operate, and controls are available for automatic adjustment to maintain "constant" (regulated) voltage output. The amount of voltage change obtained depends upon the type of variable transformer used.

One main reason for changing the secondary voltage is for compensation when the incoming line voltage changes. By using automatic equipment, the secondary voltage that serves the load will remain basically constant, or regulated, should the line voltage fluctuate. To avoid a continual voltage hunting condition, a voltage tolerance limit is maintained, usually from about a half volt to a few volts.

Methods of achieving variable voltage

There are two main methods you can use to achieve variable output voltage (other than using a rheostat). Both use autotransformers, but different types.

As shown in Fig. 1, auto transformers have at least two windings having a cam-man section. A regular autotransformer has fixed terminals. The step-down version has primary terminals at both ends of the coil; its secondary uses the same coil, but it will have one tap between the two primary terminals along with a terminal at one end of the coil. The voltage ratio is based upon the number of turns between the primary terminals and those between the secondary terminal and the tap.

Method 1. The first approach to achieving variable voltage is a configuration where one tap is fixed and the other tap is connected to a brush that slides along an uninsulated section of the coil. [ILLUSTRATION FOR FIGURE 2 OMITTED]. This is in lieu of having the secondary voltage based upon fixed output taps. One way of doing this is to have the coil wrapped around a toroidal-shaped core.

The voltage ratio relates to the location of the brush as it rides against the coil and depends on how much of the coil the brush is allowed to make contact with.

Limited ratio variable transformers are made as well as full range units, which can change the voltage from 0% to about 120% of the incoming line voltage. When the output voltage exceeds the input voltage, there are extra turns on the coil extending beyond the windings and lying between the incoming power terminals. In effect, the unit becomes a step-up transformer.

Ratings start at less than 1kVA for a 120V, single-phase variable transformer. Basic units are wired in parallel and/or in series to obtain higher power capacity. Two units in parallel have double the current and kVA rating. The individual units are stacked on top of each other, bolted together, and operated from a common shaft that rotates the brushes. Operating in a configuration that combines parallel and series connections, with several units stacked together, this type at 480V can have a rating exceeding 200kVA.

You can specify a control mechanism to achieve "constant" (regulated) voltage output automatically; it's attached to the rotor that turns the brushes.

An important design characteristic of this type of transformer relates to brush contact and the amount of current flowing through the carbon brush. A rating based solely on output kVA may cause serious problems because, for a given kVA load, the current drawn depends on the output voltage. Since the output voltage is variable, a load of a given kVA value may draw a safe current at 100% voltage, while at 25% voltage the current required to serve a load of the same kVA value would require four times as much current. This situation may cause the brush to overheat, so you should not exceed the maximum current rating.

Method 2. Another form of variable voltage transformer is the induction type, as shown in Fig. 3, which does not use brushes. The usual voltage change for this type of unit is [+ or -] 10% but can be greater. This unit is a variable-ratio autotransformer that uses two separate windings - a primary and a secondary. There's a laminated steel stator on which is wound a winding that serves as a secondary coil. This winding is connected in series with the load. The primary coil, or shunt winding, is connected across the supply line and is wound around a rotor. Construction is similar to that of a motor except that, in this case, the rotor can only turn 180 mechanical and electrical degrees.

As the primary core is rotated, the amount of primary flux passing through the secondary winding is decreased until the core reaches a position at right angles to the secondary winding. In this position, no primary flux passes through the secondary windings and, as a result, the induced voltage in this coil is zero.

The continued rotation of the core in the same direction again increases the amount of flux passing through the secondary, but it's now in the opposite direction, and so reverses the direction of the induced voltage. Thus, the output voltage can be varied by adding to or subtracting from it the voltage induced in the secondary winding.

The primary and secondary cores are circular, and the coils are assembled in recesses or slots similar to an induction motor.

Both single- and 3-phase transformers are available. The rotor is connected, via a worm gear assembly, to a motor that allows the rotor to turn up to 180 degrees. Limit switches prevent the rotor from turning further. Automatic controls can be installed so that if the line voltage varies, the output voltage remains constant (regulated). Ratings of these types of transformers vary from 8kVA, 120V, single-phase to 1500 kVA, 480V, 3-phase.

Robert B. Morgan, Senior Editor