ESAB Precision Plasmarc EPP-600, Instruction Manual, Page: 65 / 90

65
sectIon 6 trouBleshootIng
6.7 Main contactor (K1a, K1B and K1c) activation circuit
A power-up sequence takes place before the Main Contactor (K1) activates. K1 is actually three separate contactors – one
for each primary input phase. Thus, K1A, K1B, and K1C switch phases A, B, and C respectively to the Main Transformer, T1.
The power-up sequence begins with a remote Contactor Signal activating K3. Refer to the description entitled, “Auxiliary
Main Contactor (K3) & Solid State Contactor Circuits” for more information. K3 activates K2 closing the three contacts of
K2. K2 bypasses K1 contacts providing primary input power to the Main Transformer, T1. This current is limited by three
one Ohm resistors, R1, R2, and R3. The resistors eliminate the high surge currents typical of the turn-on inrush transients
associated with large transformers. The high current surge of charging the Bus Capacitor Bank is also eliminated by initially
powering the Main Transformer through K2 and the resistors.
The discharged Bus Capacitor Bank initially prevents the output of the Main transformer from reaching its normal value.
As the Bus Capacitor Bank charges, the Main Transformer output voltage rises and becomes high enough for K1A, K1B, and
K1C to close. Once the K1’s are closed, the contacts of the Starting Contactor, K2, are bypassed, and full primary line power
is supplied to the Main Transformer.
Because the starting sequence takes time, it is important at least 300 mS lapse between applying the Contactor Signal and
applying load to the power source. Applying load too soon will prevent K1 from closing, and fuses F1 and F2 will open.