14.7 Delay Triggering with Burst Firing
The Delay Triggering firing is used
the control a primary of transformer
coupled with the normal
resistances on the secondary (N.B.
don’t connect cold resistances on
the secondary like: Superkanthal,
Molybdenum, Platinum, Tungsten,
Quartz Lamp).
For an inductive load (ex
transformer), switching the
thyristors at zero crossing can
generates transient over currents
that can blow the fuses, to avoid this
problem you must use the Delay
Triggering. This firing delay the first
half cycle of Burst for an angle from
0 to 100° relative to the zero.
For understand the Delay Triggering firing, we have represented the waves generate by vectors that rotates in
counterclockwise:
Without delay at zero crossing
when V1 is to zero (projected on
the X axis) the unit switch On.
In this case the instantaneous
value of the currents are i1, i2 and
i3 and this condition, for the curve
of magnetization, could generate
transient over currents that can
blow the fuses.
With Delay Triggering the firing of
the thyristor are triggered with a
delay until the instantaneous value
of the current i1=0, i2 positive and i3
negative like represented.
In this case the risk of transient
over currents is reduced and the fuses don’t blow.
The angle alpha is the delay to have i1=0 and this angle depends on the power factor.
The delay angle suggest for most applications is 80°.
Current
Voltage
Transient over-current
Without Delay Triggering
Zero
crossing
voltage
0
Current
Voltage
Delay angle
(0° to 100°)
With Delay Triggering
Zero
crossing
voltage
0
Without Delay Triggering
With Delay Triggering
V
2
V
1
V
3
l
2
i
2
i
3
i
1
l
1
l
3
y
X
V
2
V
1
a
V
3
l
2
i
2
i
3
l
1
Delay Angle
l
3
y
X
www.west-cs.co.uk
51
