Firing modes
7100A User Manual
3-6
3.6. SAFETY RAMP
The safety ramp involves progressively increasing the thyristor firing angle in order to apply the
voltage (and current) to the load smoothly and thus reduce the start-up current of loads which have a
low resistance when cold and inductive loads.
‘Phase angle’ mode allows the firing angle to be progressively varied on start-up, acting as a
safety
ramp.
3.6.1. Start-up ramp
The start-up ramp is
active
in the following firing modes:
• ‘Phase angle’ (codes
V2CL
and
VICL + PA
)
• ‘16-cycle Burst mode’ with current limit (codes
C16
+
V2CL
or
VICL
).
The start-up ramp (approx. 16 cycles) is applied on the first firing after the thyristor unit is powered
up and after the firing is cut for more than 5 seconds. The initial firing angle is approx. 6°.
After the ramp, the firing angle corresponds to the setpoint in ‘Phase angle’ mode;
in ‘Burst mode’ the thyristors fire fully once the ramp is complete.
t
Current voltage
Supply voltage
Initial
firing angle
Figure 3-6 Start-up ramp (resistive loads)
t
Load Voltage
Supply Voltage
Magnetisation gradiant
Delay for first firing
Load Voltage
Figure 3-7 Transformer primary power-up in ‘Burst mode’ (XFMR option)
3.6.2. Magnetisation ramp (XFMR option)
For inductive loads, the safety ramp prepares initial magnetisation.
To avoid saturating transformers on power up, the safety ramp acts as a magnetisation ramp. With the
XFMR option, after this ramp, the first ‘burst mode’ firing cycle starts with the first firing delay.
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