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cmek2de1
5 Motor driving / braking
cmek2e12
DRIVING
The motor is powered from the smoothed +60 V supply. Motor speed is controlled
by pulse width modulation of transistor Q1. The pulse frequency is 12 kHz, and the
maximum on--time is 97% of the pulse cycle time. During the pulse spaces, the
motor current freewheels through diode D8.
At 24 V motor supply voltage, the wire feed rollers’ speed is 160 r/min. A wire feed
speed of 20 m/min. requires a roller speed of 212 r/min.
Resistor R19 produces a voltage drop proportional to the motor current. If the
current exceeds 7 A, IC5:2 turns off the gate pulses to Q1. When the current falls,
Q1 conducts again at the next gate pulse.
BRAKING
When the motor star ts, capacitor C43 charges up via diode D28: the voltage is
limited to 15 V by zener diode D21. When the motor is to brake, the LED in
optocoupler IC8 lights, causing the transistor in IC8 to conduct and discharge
capacitor C43 (15 V) to the gate of transistor Q3. The transistor conducts and
short--cir cuits the motor voltage through resistors R51 and R64, which limit the
current to about 20 A.
