Figure 1-32.-Saturable potential transform.

The use of the SCPT is relatively new to

motor-generator application. Since the basic operation

of each core in the SCPT is identical, only one core will

be explained.

The basic operation of the SCPT is explained with

the aid of figure 1-31. It consists of two voltage primary

windings

and

two current primary windings

and

two secondary windings

and

a dc control winding

In figure 1-31, these windings are arranged on a

three-legged E-type lamination. For simplicity, consider

and

the leg of the transformer with windings

The

winding and the secondary winding (Vs1)

function like a normal power transformer, and the

current primary winding

and secondary winding

function like a normal current transformer. When either

or ) induce a voltage into

of the primary windings

the secondary winding

(the secondary winding is

connected to a load), a current will flow in the secondary

winding

The SCPT is constructed in such a

manner that the current flow in the secondary is the sum

of the current that would be caused to flow by the

Figure 1-32.-Saturable potential transform.

and

As can be seen in fig-

separate windings

ure 1-31, there is a voltage and current primary winding

and a secondary winding on each of the cores of the

core, no voltage is induced into the control winding.

SCPT The function of each is as described in the

When voltage is applied to the primary windings,

previous paragraphs.

current flows in these windings, which are labeled in

figure 1-32. If the primary voltage is instantaneously

To understand the principle of operation of the dc

positive (+) at the start of winding

then the current

control winding, refer to figure 1-32. The action that

flowing through the turns of

and

should create

takes place between the primary winding (either current

the flux; 01 and 02, following the left-hand rule, which

primary or voltage primary), the secondary winding,

defines winding polarity.

and dc control winding is the same. Therefore, only a

voltage primary winding

is shown in figure 1-32.

The flux caused by

is in an upward direction,

The outer legs of the core are each wound with a primary

and the flux caused by

is in a downward direction.

and a secondary winding. The control winding is wound

These fluxes will close their loop through the center leg

on the center leg. The primary and secondary windings

of the laminated core because of the shorter path it

are connected so their flux oppose each other in the

presents; but because the fluxes are of equal magnitude,

center core. Thus, with the net flux of zero in the center

they cancel each other in the center leg and thus induce

no voltage in the control winding

Because the

fluxes, 01 and 02, link the secondary turns,

and

a voltage is induced in each of these with a sum of V

sec. The relationship that exists between the primary and

secondary windings when the core is not saturated is

identical to any voltage transformer with a core that is

not saturated. When a direct current flows through the

control winding

in the direction shown by a dc flux

is created, according to the left-hand rule, which is

in an upward direction opposing 01 and aiding 02. When

the magnitude of the dc flux becomes great enough,

Figure 1-31.-Saturable current-potential transformer.

it begins to force the core material into saturation.

1-27