GE AMX-II Service Manual page 180

GENERAL ELECTRIC MEDICAL SYSTEMS AMX-II X-RAY UNIT
10-377 SM A0654E
SEC. 10 MOTOR CONTROL
The operator controls the motor speed by rotating a potentiometer (6R1 96,D) on the brake handle,
illustration 1-2, Chapter 1. The potentiometer (6R1) voltage is combined with a sawtooth voltage to
control the conduction of 8A109 and Q10(101,F)which regulate motor current and hence motor speed.
Power transistors 8A109 and 8A1Q10 are controlied by a pulse width modulated signal applied to the
base of 8A109. The period of the transistors remains constant but the on and off times are adjusted to
give a desired average current. As the on pulses become wider and the off pulses shorter more current
flows through the motor resulting in a higher speed. See W59.
When the brake handle is depressed 6LS1 (95,F) closes to apply power to the motor drive system and
start 8A102 (96,F) oscillating (W-51). The sawtooth output of 8A1Q3 (w-52) is combined with the DC
voltsge from 6R1 (Speed Control) and 8A1R15 (max. speed adjust) to provide pulse width modulation
bias for 8A105.
Pulse width modulation of 8A1Q5 is achieved in the following manner.
+V
SLOW SPEED
SHADED AREA — Q5 CONDUCTS
M
y мм GUT PUT
À \ N N / we
"Ll — - —
Q5 TURN OFF
BIAS VOLTAGE
DC BIAS
FROM 6R1
0 >
T
18790-010
ILLUSTRATION 6-17
The frequency and amplitude of the output of 8A 103 remains constant but the DC bias from 6R1 is
changed by the operator. At a slow speed 6R1 applies high DC bias causing 8A105 to conduct for a
short time (shaded area) and remain off for a long time; Illustration 6-17.
When the operator applies less DC bias, SA105 conducts for a longer period of time (shaded area, lilus-
tration 6-18). In this manner the operation controls the conduction time of 8A1Q5 resulting in a pulse
modulated output. Examples of the pulse modulated output of 8A105 are shown in W-53 and W-54.
6-21