NF800-REP datasheet(89/105 Pages) MITSUBISHI

89 / 105 page

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 i2dt 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

Hrs

Min

Sec

100

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

Short-circuit path in a commutation element failure

Short-circuit path in a power outage

Commutation element failure

supply

High-speed fuses

b) 4-pole MCCB

a) 3-pole MCCB

NF800-REP Datasheet(PDF) 89 Page - Mitsubishi Electric Semiconductor

NF800-REP Datasheet(HTML) 89 Page - Mitsubishi Electric Semiconductor

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