ESAB ESP-150 Manuel D'instruction page 248

SECTION 5
Voltages in plasma cutting equip-
ment are high enough to cause se-
rious injury or possibly death. Be
particularly careful around equip-
ment when the covers are re-
moved.
Be sure that all primary power to the
machine has been externally dis-
connected. Open wall disconnect
switch or circuit breaker before at-
tempting inspection or work inside
of the power supply.
Voltages in plasma cutting equip-
ment are high enough to cause se-
rious injury or possibly death. Be
particularly careful around equip-
ment when the covers are removed.
D. TESTING AND REPLACING BRIDGE ASSEMBLY COMPONENTS
The silicon diodes and SCR's used in the power supply are devices which allow
current to flow in only one direction. They block current in the other direction. The
diodes and SCR's are designed to provide long troublefree operation; however,
should a failure occur, they may require replacement.
1. Testing Diodes
a. Locate the main rectifier assembly containing the silicon diodes and
SCR's.
b. Electrically isolate main bridge rectifier assembly by disconnecting the
main transformer secondary fuse links F1 and F2.
c. With ohmmeter on RX1 scale, place negative lead on the diode heat sink
and touch positive lead to each pigtail terminal. Meter should read a low
resistance of approximately 3 to 15 ohms on each diode.
d. Reverse leads and check each diode. All readings should show high
resistance of 2 K (2000) ohms or higher. With most ohmmeters on the
RX1 scale, 2K ohms is the highest possible reading available.
e. Since diode resistance is non-linear with voltage (using any scale), di-
odes are good when they show low resistance in one direction and high
resistance in the opposite direction. They are bad when they show no or
very low resistance in both directions (shorted), or if they show very high
assistance in both directions (open).
2. Testing SCR's
a. Follow steps a. and b. under Testing Diodes. Disconnect the SCR pig-
tail.
b. With ohmmeter on RX1 scale place the negative lead on the anode (end
of SCR with screw threads) and positive lead on the cathode (pigtail
end). Meter should read a high resistance of 2 K (2000) ohms or higher.
c. Reverse leads and check each SCR. All readings again should show
high resistance. The SCR's are bad is they show low resistance in either
direction.
d. Now check the gate circuit on the SCR's by placing the negative lead to
the gate (wht wire off SCR) and the positive lead to the cathode. Meter
should read approximately 20 ohms and should vary only slightly (5 ohms)
when leads are reversed. If the meter reads zero or infinity in either direc-
tion the gate circuit is faulty and the SCR should be replaced.
IMPORTANT: When replacing Diodes or SCR's make sure mounting surfaces
are clean. Coat mounting surfaces with Alcoa No. 2 EJC Electrical
Joint Compound (no substitutes) available in 8 oz. bottles under
P/N 73585980. Use a torque wrench to tighten diodes and SCR's.
Recommended torques are 20-30 in.-lbs. for diodes; 125-150 in.-
lbs. for SCR's.
TROUBLESHOOTING
Check the problem against the symptoms in the following troubleshooting
guide. The remedy may be quite simple. If the cause cannot be quickly
located, shut off the input power, open up the unit, and perform a simple
visual inspection of all the components and wiring. Check for components,
bulged or leaking capacitors, or any other sign of damage or discoloration.
The cause of control malfunctions can also be found by referring to the se-
quence of operations and electrical schematic diagram (Fig. 3) and check
out the various components. A volt-ohmmeter will be necessary for some of
these checks.
242
MAINTENANCE