ESAB LKA 150, Service Manual

Page: 10 / 24

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RFI SUPPRESSION AND BASE LOAD RESISTORS
Capacitors C4 − C6 protect against RFI. Resistors R12 and R15 provide a minimum
base load for the rectifier bridge: in addition, they act as discharge resistors for the
capacitors.
4 MOTOR DRIVE CIRCUIT
The wire feed motor is powered by the rectified secondary voltage from main
transformer TM1.
Pressing the welding torch trigger switch SB1 provides a supply to thyristor TY1 via
contact B4 from the positive side of the main power rectifier VC1.
Resistors R10 and R14 limit the motor starting current: excessive starting current
would demagnetise the motor. D9 is a squelch diode protecting against back − emf
from the motor.
The ignition circuit for TY1 consists of R2, R4, R5, D5 and C2.
When Q1 is not conducting, operation of the circuit is as follows:
If the voltage at B4 exceeds the motor voltage, C2 charges via R2, R4 and R5.
When the voltage on C2 reaches the trigger voltage (0.5 − 1.5 V), thyristor TY1 fires.
This means that TY1 conducts each half − cycle when Q1 is not conducting.
D5 protects C2 against negative voltage when Q1 conducts.
MOTOR VOLTAGE CONTROL
C3, R11 and potentiometer R13 form a low − pass filter circuit for the motor supply.
R13 picks off a suitable fraction of the motor voltage for connection to the base of Q1
via diode D6. Q1 starts to conduct at a bases voltage of 0.7 V.
C2 charges more slowly, which means that thyristor TY1 fires later.
Motor voltage is lowest when R13 is in its upper position, the minimum position.
The base voltage is then high and Q1 conducts. The charging time for C2 becomes so
long that thyristor TY1 misses some half cycles, firing only (for example) on every
third cycle.
When R13 is in its lower (= maximum) position, the base voltage on Q1 is low and
Q1 does not conduct. In this state, zener diode D7 determines the maximum motor
voltage.
See on page 10 for the motor voltage and current waveforms.