Eurotherm 7000 Series, Руководство пользователя

Страница: 36 / 69

7300A User Manual
3-3
Firing modes
The ‘DLY’ potentiometer is a 3/4 turn type, and is used to set the delay angle for the first
firing:
• from 0° (turned anticlockwise to end stop)
• to 90° (turned clockwise to end stop).
The factory setting for the first firing delay with the XMFR option is 70° (typical value
suitable for starting most applications).
The optimum firing angle can be adjusted with the ‘DLY’ potentiometer to match the
cos
ϕ
of the load to obtain a minimal transient over-current (using an oscilloscope).
Figure 3-2 Typical switching with inductive load, at zero crossing (a) and with delay (b)
FIRING DELAY (XFMR option)
In ‘Burst mode’ firing with pure resistive loads, the thyristors are fired at zero voltage
crossing to avoid sharp current rises.
For an inductive load (e.g. transformer primary), switching the thyristors at zero crossing
generates transient over-currents (see figure 3-3a).
This transient could cause the high speed thyristor protection fuse to blow in certain cases.
To avoid the over-current, the first thyristor
firing must be delayed relative to the
corresponding zero for each phase.
The delay before thyristor firing starts may be
adjusted with the ‘DLY’ potentiometer available
with the XFMR option (C16 or C64 ‘Burst mode’).
Transient
Over-Current
a)
0
Vload
i
load
Without Transient
0
Vload
i
load
b)
Delay Angle
(0° à 90°)
Delay Angle
Setting
70
°
DLY
0
°
90
°
Figure 3-3 First firing delay adjustment potentiometer (XMFR option)