Tweco 15XC CE Merlin PAK - Manual (Page 89)

Manual 0-2529 4 - 17 SERVICE
4.10 TROUBLESHOOTING THEORY
The main transformer (T1) has buss bar connections to
select the 380 VAC input voltage (see page 2-6). Each main
transformer secondary has two taps. The higher voltage
secondary tap is automatically selected by the voltage
selection PC board when primary power is applied. A red
LED indicator (D18) on the voltage selection PC board will
be ON. This arrangement provides secondary voltages
close to the optimum levels.
The 115 VAC and lower voltage taps are selected directly
by relays on the voltage selection PC board. The fan and
pump motors are supplied by the motor control contactors
(MC1 and MC2). MC1 is for the high voltage tap and MC2
is for the low voltage tap. Three-phase primary power is
controlled by one of the main contactors (W1 for the high
voltage tap or W2 for the low voltage tap).
Primary three-phase power is rectified by diodes D1-D6.
The resulting negative DC voltage (approximately -320
VDC) is applied to the switching transistor (Q1). The
switching transistor controls the output current by pulse
width modulation (PWM). PWM varies the duty cycle (or
on-time versus off-time) of the switch. The greater the on-
time, the higher the output current will be. The switching
transistor output is a series of pulses which are filtered
back into pure DC voltage by the output network. The
output network consists primarily of the main inductor
(L2A and L2B), resistor R13, and capacitor C23.
The switching control PC board compares the shunt amp
output with the current control pot setting and generates
logic level PWM signals. The shunt and shunt amp PC
board are located between the positive bridge and work
lead to measure the output cutting current. The signals are
sent to the driver PC board, where they are converted into
the current and voltage levels needed to drive the switch-
ing transistor.
The switching control PC board senses if the remote
control panel is connected by a jumper in the remote cable.
If the remote cable is not used, a relay on the switching
control PC board shorts J15-35 to J15-36. The short by-
passes the E-Stop function of the remote. The switching
control PC board also routes the shunt amp output to the
digital ammeter in the remote. CSR (current greater than
25A) and over-current shutdown (greater than 175A)
signals are sent to the logic PC board.
Input Voltage Selection
Switch-Mode Power
Supply Operation
Switching Control Board