21
Negative te rminal
Cutting m achine
Electrode
Nozzle
Work piec e
Positive t erminal
(2)
Non-transferred arc
Power source negative output terminal is connected with electrode; positive terminal
is connected with work piece. Contact nozzle with work piece firstly, then plasma
flame flow will be generated between electrode and nozzle after power on. Spray arc
out of nozzle by plasma airflow which goes through nozzle, then plasma arc is formed.
Fig. 7-3: Non-transferred arc
2.
Plasma cutting technological parameter
Parameters include cutting current, cutting voltage, cutting speed, gas flow and distance
between nozzle and work piece.
(1)
Cutting voltage: though we can increase cutting thickness and cutting speed by increasing
current, but arc will become thicker and cut become wider. So for thick plate cutting, better to
increase cutting voltage. High open circuit voltage is easy to start arc. Increase gas flow rate
and change gas composition can increase cutting voltage. But if cutting voltage is over 2/3 of
open circuit voltage, arc will be no stable, and blow out easily. In order to increase cutting
voltage, need to use power source with high open circuit voltage. So open circuit voltage of
plasma cutting machine is no less than 15V, and 2 times of cutting voltage.
(2)
Cutting current: current and voltage decide power of plasma cutting machine. As power
increases, cutting speed and cutting thickness will increase correspondingly. Normally choose
cutting current according to plate thickness and cutting speed. For same thick plate, the larger
the current is, the quicker of cutting speed is. But too large of cutting current may burn
electrode and nozzle, cause double arc easily. So proper current is needed for electrode and
nozzle. Cutting current also influences cutting speed and cut width. Larger cutting current may
make arc become thicker, and cut become wider, and form “V” type cut. Below table is
relationship about cutting current and cut width.
Cutting current/A
20
60
120
250
500
Cut width/mm
1.0
2.0
3.0
4.5
9.0
Table 7-1: Relationship about cutting current and cut width
