Figure 1-8.-Single-phase autotransformer.

coil remains energized after the push button is released,

than the source voltage can be obtained by tapping the

closing holding contacts HA. The coil, HM, also closes

proper point on the winding between terminals A and C.

and

applying full-

main line contacts

Some autotransformers are designed so a knob-

line voltage to the motor high-speed winding. The motor

controlled slider makes contact with wires of the

will run at high speed until coil HM is de-energized

winding to vary the load voltage.

either by opening the stop switch, a power failure, or an

overload.

The directions for current flow through the line,

transformer winding, and load are shown by the arrows

Pressing the low-speed push button closes the low-

in figure 1-8. Note that the line current is 2.22 amperes

speed contactor by energizing coil LM. The coil remains

and that this current also flows through the part of the

energized after the button is released, through the holdi-

winding between B and C. In the part of the winding that

ng coil contacts, LA. The coil, LM, also closes the

is between A and B, the load current of 7 amperes is

mainline contacts,

and

which apply the

opposed by the line current of 2.22 amperes. Therefore,

full-line voltage to the low-speed motor winding. The

the current through this section is equal to the difference

motor will run at low speed until coil LM is de-

between the load current and the line current. If you

energized. The LM and HM contractors are mechanically

subtract 2.22 amperes from 7 amperes, you will find the

interlocked to prevent both from closing at the same

secondary current is 4.78 amperes.

time.

Autotransformers are commonly used to start three-

phase induction and synchronous motors and to furnish

Autotransformer Controllers

variable voltage for test panels. Figure 1-9 shows an

autotransformer motor starter, which incorporates

A single-phase autotransformer has a tapped

winding on a laminated core. Normally, only one coil is

used on a core, but it is possible to have two auto-

transformer coils on the same core. Figure 1-8 shows

the connections for a single-phase autotransformer

being used to step down voltage. The winding between

A and B is common to both the primary and the second-

ary windings and carries a current that is equal to the

difference between the load current and the supply

current.

Any voltage applied to terminals A and C will be

uniformly distributed across the winding in proportion

to the number of turns. Therefore, any voltage that is less

Figure 1-8.-Single-phase autotransformer.

Figure 1-9.-Autotransformer controller.

1-8