The crossed-E transformer assembly consists of five legs (poles). Each leg is encased by a coil. The
coil around the center leg is the primary, which is excited by an alternating voltage. The remaining four
coils are the secondaries. From this view, you can see how it gets the name, crossed-E.
When the reluctance dome (armature) is moved to the left of center, more flux links the left leg with
the primary coil, and the voltage induced in the left secondary increases. The right leg has fewer flux
linkages with the center coil; therefore, the voltage induced in the right coil will be less than that in the
left coil. Thus there will now be a net voltage out of the pickoff. The phase of the output will be that of
the larger voltage. If the dome were moved to the right, the opposite condition would exist. From this
brief description, you can see that the crossed-E transformer works on the same fundamental principle as
the basic type described earlier. The major difference between the two is in shape and the number of
secondaries, and in the fact that the armature has universal movement.
A commonly used magnetic error detector is the synchro-control transformer, which is used as a
control device in servo systems. Recall that we covered the CTs operation in depth in chapter 1 of this
module, and discussed its application to the servo system earlier in this chapter.
As an error detector, the CT compares the input signal impressed upon its stator with the angular
position of its rotor, which is the actual position of the load. The output is an electrical (error) signal taken
from the rotor, which is the difference between the ordered position and the actual position of the load.
A primary advantage of the CT over other types of error detectors is its unlimited rotation angle; that
is, both the input and the output to the synchro control transformer may rotate through unlimited angles.
A disadvantage is that the output supplied to the servo amplifier is always an ac error signal, and must be
demodulated if it is to be used in a dc servo system.
Q-19. In the output of an ac error detector, what indicates the (a) direction and (b) amount of control
necessary for correspondence?
Q-20. What two basic types of magnetic devices are used as error detectors?
RATE GENERATOR (TACHOMETER)
As we mentioned earlier, the tachometer in the velocity servo system is the heart of the feedback
loop. It is used to sense the speed (velocity) of the load. The tachometer is sometimes referred to as a
RATE GENERATOR. Whatever the name, it is a small ac or dc generator that develops an output voltage
(proportional to its rpm) whose phase or polarity depends on the rotor's direction of rotation. The dc rate
generator usually has permanent magnetic field excitation. The ac rate generator field is excited by a
constant ac supply. In either case, the rotor of the tachometer is mechanically connected, directly or
indirectly, to the load.
The AC Rate Generator
One type of ac rate generator used widely in the past is the drag-cup type.
The tachometer generator shown in figure 2-16 has two stator windings 90º apart, and an aluminum
or copper cup rotor. The rotor rotates around a stationary, soft-iron, magnetic core. One stator winding is
energized by a reference ac source. The other stator winding is the generator output, or secondary winding
the voltage applied to the primary winding produces a magnetic field at right angles to the secondary
winding when the rotor is stationary, as shown in view A. When the rotor is turned by mechanical linkage
from the load, it distorts the magnetic field so that it is no longer 90 electrical degrees from the secondary