One-phase built-in variable transformers are the simplest and most straightforward type of variable transformers. They are operated using a slip-protected knob, which causes the voltage to increase steadily as the knob is turned clockwise.
Three-phase built-in variable transformers are designed to work with delta- or star-connected circuit types. These circuits have multiple wires that carry different types of electricity. The variable transformer itself is always star connected. The same controller operates each of the different voltage output sliders.
Motor-driven variable transformers get their name from the manner in which their control shaft, which is connected to their dial, is rotated. This variable transformer uses a synchronous motor to rotate its slider, as opposed to a knob or a dial. Limit switches are installed in the transformers to prevent the slider from turning too far. A manual switch is also included in this kind of transformer, allowing the operator to increase or decrease the voltage independently.
Desktop variable transformers are so-named because they are fully housed, so that the operator cannot come into contact with the inner parts of the transformer. They are useful for adjusting a.c. voltage. They are controlled with a slip-protected knob. The greatest advantage of this kind of variable transformer is its portability---it can be taken anywhere in a laboratory where adjustable voltage is required.
Toroidal variable transformers are so-named for their donut-like shape. They can come in single-phase or three-phase designs, and they are often used in lighting installations, electrical heat resistors, laboratory installations and testing equipment as well as in measuring and regulating circuits.
Variable column transformers are a different design of variable transformer. They are stand-up units that take up a bit more space than toroidal variable transformers. They also come in single-phase or triple-phase models.