MB30-CS datasheet(83/105 Pages) MITSUBISHI

83 / 105 page

for 2 minutes. Thus:

1.2 x P x 103

IMCCB ≥

where 1.2: Allowance for random variations in

arc-welder current, and supply-volt-

age fluctuations

P: Welder rated capacity (kVA)

E: Supply voltage (V)

The switching transient in the arc welder is mea-

sured as 8~9 times the primary current. Consequently,

using 1.2 allowance, it is necessary to select instan-

taneous-trip characteristics such that the MCCB does

not trip with a current of 11 times the primary current.

7.5 MCCBs for Transformer-Primary Use

Transformer excitation surge current may possibly

exceed 10 times rated current, with a danger of nui-

sance tripping of the MCCB. The excitation surge

current will vary depending upon the supply phase

angle at the time of switching, and also on the level of

core residual magnetism. The maximum is as shown

for switching-point P in Fig. 7.7. During the half cycle

following switch-on the core flux will reach the sum of

the residual flux

fr, plus the switching-surge flux 2fm.

The total, 2

fm +fr, represents an excitation current in

excess of the saturation value. The decay-time con-

stant of this tends to be larger for larger transformer

capacities. Table 7.2 shows typical values of excita-

tion surge current, but as these do not take circuit

impedance into account, the actual values will be

larger. If both the primary leakage impedance and cir-

cuit impedance are known, the surge current may be

derived by considering the transformer as an air core

reactor; otherwise the values in Table 7.2 should be

used. This table gives maximum values, however, that

are based on the application of rated voltages to rated

taps; it should be noted that supply overvoltage will

result in even larger surges.

Since it is the instantaneous-trip function of the

MCCB that responds to the transient current, ther-

mal-magnetic MCCBs, which can more easily be

manufactured to handle high instantaneous-trip cur-

rents, are advantageous over completely electromag-

netic types, where the instantaneous-trip current is a

relatively small multiple of the rated current.

Table 7.2 Transformer Excitation Surge Currents

Capacity

(kVA)

100

150

200

300

500

Decay time constant

(Hz)

First 1/2-cycle peak

(multiple)1

Decay time constant

(Hz)

First 1/2-cycle peak

(multiple)1

1ph transformer

3ph transformer

Table 7.3 Transformer Capacities and Primary-Side MCCBs

Tran.

kVA

7.5

100

150

200

300

500

3 phase 400V

NF30-SP

(

30)

NF50-SP

(

40)

NF50-SP

(

50)

NF100-SP

(

50)

NF100-SP

(

60)

NF100-SP

( 100)

NF250-SP

( 150)

NF250-SP

( 175)

NF250-SP

( 225)

NF400-SP

( 300)

NF400-SP

( 350)

NF630-SP

( 600)

NF1000-SS ( 900)

MCCB Type (rated current (A))

1 phase 230V

NF100-SP

(

75)

NF100-SP

( 100)

NF250-SP

( 150)

NF250-SP

( 200)

NF400-SP

( 300)

NF400-SP

( 400)

NF630-SEP ( 600)

NF1000-SS ( 500)

NF1000-SS ( 800)

NFE2000-S (1200)

NFE2000-S (1500)

—

1 phase 400V

NF100-SP

(

40)

NF100-SP

(

60)

NF100-SP

(

75)

NF250-SP

( 125)

NF250-SP

( 150)

NF250-SP

( 225)

NF400-SP

( 400)

NF630-SP

( 500)

NF630-SP

( 630)

NF1000-SS ( 500)

NF1000-SS ( 600)

NF1000-SS ( 900)

NFE2000-S (1400)

3 phase 230V

NF50-SP

(

50)

NF100-SP

(

40)

NF100-SP

(

60)

NF100-SP

( 100)

NF250-SP

( 125)

NF250-SP

( 175)

NF400-SP

( 250)

NF400-SP

( 300)

NF400-SP

( 400)

NF630-SP

( 500)

NF630-SP

( 600)

NF1000-SS ( 900)

NF1600-SS (1400)

Note: 1 “Multiple” means the first 1/2-cycle peak as a multiple of the rated-current peak.

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