66
3. Element Breakdown in Thyristor-Leonard Systems
In this system of DC motor control, if power outage or
commutation failure due to a thyristor control-circuit
fault occurs during inversion (while motor regenera-
tive power is being returned to the AC supply), the
DC motor, acting as a generator while coasting, will
be connected to a short-circuit path, as in Fig. 7.18.
For thyristor protection, MCCBs must be placed in the
DC side, as shown.
A Mag-Only MCCB with a tripping current of about
3 times the rated current is employed, either 3- or 4-
pole, series-connected as shown in Fig. 7.20. Since
the element short-circuit current is the same as the
MCCB current, circuit protection is effected provided
that the
e
i
2
dt limit for the element is larger than that
for the MCCB interruption duration. This must be es-
tablished by test.
Fig. 7.19 High-Speed Fuses for Thyristor-Circuit Protection
Fig. 7.20 Series Connection of MCCB Poles
Fig. 7.18 Ward-Leonard Thyristor Protection
3-Phase Fullwave Rectification
MCCB: Mag-Only
Tripping time
h
min
s
100
2
1
30
20
14
10
8
6
4
2
1
30
20
10
5
2
1
0.5
0.2
0.1
0.05
0.02
0.01
125
200
300 400 500 600 700 1000
1500 2000
3000 4000
Current (% of rating)
Region 1
Region 2
Region 3
Thyristor current-surge withstand
MCCB tripping
High-speed current-limiting fuse
Overcurrent-relay
Fig. 7.17 Thyristor and Protector Operating Curves
M
Short-circuit path in a commutation element failure
Short-circuit path in a power outage
Commutation element failure
AC
supply
High-speed fuses
M
M
M
b) 4-pole MCCB
a) 3-pole MCCB