Hypertherm MAX200 Service Manual - Manual (Page 47)

3-6 MAX200 Service Manual
MAINTENANCE
Theory of Operation: High Frequency
Introduction
The MAX200 power supply uses a high frequency transformer to initiate a pilot arc between the nozzle and the
electrode to start the plasma torch. When the torch is sufficiently close to the workpiece, the arc will “transfer” to
the workpiece. Once the arc has been transferred, relays open to disable the high frequency transformer and the
pilot arc.
Circuit Description
The following is a description of the operation of the high frequency section of the MAX200 power supply. See
wiring diagram 013-4-179 as well as component placement figures in the Parts List section for reference.
High Frequency Generation Circuit
A high voltage is necessary to activate the high frequency generation. 120 VAC is sent through REC26 to a high
voltage transformer T5. T5 steps up the 120 VAC to 5000VAC. This voltage charges C7 and C8. Once voltage
across the caps gets sufficiently high, the space between the sparkgaps (SG1) breaks down, causing a virtual short
circuit which creates a damped oscillating LC circuit between C7, C8 and the high frequency coil T6. The frequency
of this oscillating circuit is determined by the equation F = 1/(2
π
x
√
(LC)).
As C7 and C8 discharge, the decaying voltage across the spark gap causes the gap to once again become “open”.
The capacitors charge again, and the process repeats.
It is important that the proper spark gap be maintained. The MAX200 requires a spark gap setting of .020". If the
gaps are too wide, they will not break down at all because there will not be enough voltage available to jump the air
space between the gaps. If the gaps are too close together, an almost continuous series of pulses is produced, but
the amplitude and duration of each pulse may not be large enough to initiate the pilot arc.
Once there is ionized plasma flow between nozzle and electrode, the current present at the electrode will jump the
gap to the nozzle, completing the circuit through the parallel pilot arc resistors R6. The pilot arc is now initialized.
Pilot Arc Circuit
The current level for the pilot arc is determined by the parallel resistors R6. When the torch is sufficiently close to
the workpiece, the arc is transferred to the workpiece. Once the transferred arc is established, relay CR1 opens,
stopping the pilot arc. The controls for shutting off the pilot arc and the high frequency generator come from Control
PCB7.