MIL-DTL-81963C
4.9.10.1.2 The Marshall potentiometer. Figure 9 shows the circuit diagram of the potentiometer, while
figure 10 shows the current balancing network, together with the formula for determining servocomponent
current. It is evident from the figures that the potentiometer depends for its action on the provision of a
current, which is in phase-opposition to, and is an integral multiple of, the servocomponent current.
Under balanced conditions, the servocomponent current is given by the ammeter reading divided by the
current division ratio, while the applied voltage is given by the voltmeter reading multiplied by a ratio
dependent on the taps of the transformer T1, from which the voltmeter and servocomponent are supplied.
The servocomponent impedance may then be calculated from wattmeter, voltmeter and ammeter
readings.
a. The method has the advantages that the meter losses do not have to be taken into account and
servocomponent current is not a limiting factor, since the current divider should be designed to be
of very low resistance so that power dissipation is small. A suitable value for the first tap (divide by
one) is 0.5 ohm, as shown on figure 10.
b. Transformer T1 should be designed so that the maximum possible load does not significantly
affect the voltage ratio between the taps, and in use, the tapping points should be chosen to give
large deflections on the voltmeter and wattmeter.
4.9.10.1.3 Incremental induction bridge. Figure 11 shows the circuit diagram and the appropriate
equations for calculating servocomponent impedance using the incremental induction bridge. The bridge
consists of the unknown (winding under test) and resistors Ra, Rb and Rc. The value of Rb shall be small
compared to the unknown. The current through the unknown causes a voltage drop across Rb. Rh is in
the circuit to compensate for the voltage drop across Rb. The setting of the inductance (L) balance
control causes the L isolation amplifier to create a current, in phase with the generator, to flow in the
standard capacitor (C). The resistance (R) balance control, along with the RaRc voltage divider and the
isolation amplifiers, cause a current to flow in the standard resistor (G), that is proportional but opposite in
phase to the generator voltage. The combination of the L current and the R current is adjusted to create
a null on the detector. The resistance and inductance can then be read directly from the dials on the
balance control and used to calculate the impedance as shown on figure 11.
4.9.11 Temperature rise. The servocomponent, mounted on the appropriate standard test fixture, shall
be placed in a chamber, the volume of which shall allow between three and five cubic feet of free air
space per servocomponent. Servomotors and motor-generators shall have the shaft locked in such a
manner that it cannot rotate. The internal ambient temperature of the chamber shall be adjusted to a
value within the range 23 ± 2 degrees C. This value shall be recorded and shall be maintained
throughout the test. The dc resistance of that winding designated in the applicable general specification
shall be measured and recorded, when the servocomponent has attained the stabilized non-operating
condition of 4.2.2.1. All windings specified in the applicable general specification shall then be energized
at the standard rated voltage and frequency, as specified in the applicable general specification. And the
dc resistance of the designated winding shall be measured, when the servocomponent has attained the
stabilized operating condition of 4.2.2.2. The methods used for measuring the dc resistance of the
designated winding shall not entail disconnection of that winding from the energizing supply while the
measurements are taken. (Figure 12 shows a suitable circuit for this purpose.) The temperature rise
calculated from the expression which follows shall be in accordance with 3.6.11.
Temperature rise, (°C) = Rh-Rc x (234.5 + tc)
Rc
where:
Rh = resistance of the designated winding at the final stabilized temperature
Rc = resistance of the designated winding at the starting temperature
tc = starting temperature
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