Characteristics: Triac - nptel

Since V g = 10 – i g R is tangent to V g i g = 0.5 at V go , i go .
Slope of the tangent of V g i g = 0.5 at (V go , i go ) = -R
∴
∴
∴
dvg
- vg
v
- R = = = -
di ( v go,igo) i ( v go,igo
) i
g
v v i 0.5
∴ R = = =
i i i
go go go
2 2
go go go
g
0.5
i × - 10i + 0.5 = 0 or 10i = 1 or i = 0.1
2
go 2 go go go
igo
0.5 0.5
R = = = 50 Ω
i .01
2
go
go
go
Back emf.
V a
t
i a
(normal)
t
i a
(with field loss)
3. Figure shows the armature voltage (firm line) and armature current of the motor under normal
operating condition at rated speed. If there is a sudden loss of field excitation back emf will
become zero and armature current will be limited solely by the armature resistance.
The peak magnitude of the fault current will be 220 2 = 1556(Amps) .
.2
It the thyristors have to survive this fault at least for 1 2 cycle (after which the fuse blows) I sM >
1556 Amps.
t
The fuse blows within 1 2
cycle of the fault occurring. Therefore the thyristors must withstand
the fault for at least 1 2 cycle.
Therefore, the i 2 t rating of the thyristor should be
Version 2 EE IIT, Kharagpur 37