winding between B and C. In the part of the winding
Operating the reverse button duplicates the steps for
that is between A and B, the load current of 7 amperes
the forward button described in table 6-9 with the
is opposed by the line current of 2.22 amperes.
exception of the F1 and F2 contacts. The R1 and R2
Therefore, the current through this section is equal to
contacts are closed to reverse the direction of current
the difference between the load current and the line
through the armature and thus the direction of rotation.
current. If you subtract 2.22 amperes from 7
amperes, you will find the secondary current is 4.78
AUTOTRANSFORMER
amperes.
CONTROLLERS
Autotransformers are commonly used to start
A single-phase autotransformer has a tapped
three-phase induction and synchronous motors and to
winding on a laminated core. Normally, only one coil
furnish variable voltage for test panels. Figure 6-19
is used on a core, but it is possible to have two
shows an autotransformer motor starter, which
autotransformer coils on the same core. Figure 6-18
incorporates starting and running magnetic contractors,
shows the connections for a single-phase
an autotransformer, a thermal overload relay, and a
autotransformer being used to step down voltage. `he
mercury timer to control the duration of the starting
winding between A and B is common to both the primary
cycle.
and the secondary windings and carries a current that is
equal to the difference between the load current and the
ONE-STAGE ACCELERATION
supply current.
CONTROLLERS
Any voltage applied to terminals A and C will be
Figure 6-20 shows a typical dc controller. The
uniformly distributed across the winding in proportion
connections for this motor controller with one stage of
to the number of turns. Therefore, any voltage that is
less than the source voltage can be obtained by tapping
the proper point on the winding between terminals A and
c.
Some autotransformers are designed so that a
knob-controlled slider makes contact with wires of the
winding in order to vary the load voltage.
The directions for current flow through the line,
transformer winding, and load are shown by the arrows
in figure 6-18. Note that the line current is 2.22 amperes
and that this current also flows through the part of the
Figure 6-18.--Single-phase autotransformer.
Figure 6-19.--Autotransformer controller.
6-13
