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Unidrive M Regen Design Guide 313
Issue Number: 4
MCB Selection
From the above calculations for a peak charging current of 46.7 A with a charge time of 1.13 s a magnetic overload with the following characteristics
could be used:
3 A nominal rating (46.7/20 = 2.335)
The exponential charging current for the soft start circuit must be checked against the MCB trip characteristic curve for the overload to ensure no
spurious trips occur during charging time. Calculate the supply current throughout the start-up time (5 time constants):
Calculate the supply current at 0.1 s, 0.2 s, 0.4, 0.7 s and 1 s.
Where:
I(t): Peak current at time = t seconds.
t = Time in seconds
Example: supply current throughout the start-up time
Table 11-3 Supply current throughout the start-up time
Note that these calculation times are based on a 1 s charge time. If the charge time not 1 s, then the time steps can be calculated as follows.
Compare the supply currents at time t1 to t5 with the circuit breaker worst-case trip characteristic. Make sure that the current is less than the trip curve
for all the time intervals calculated.
Comparing the data from Table 11-3 with the tripping characteristic of the selected MCB, Figure 11-2 shows that the supply current is less than the
MCB trip curves for each time interval.
11.3.3 Sizing of thermal overload
The thermal overload should be sized to provide protection against a high impedance short circuit. Under this condition the current flowing would not
be high enough to result in the magnetic overload tripping, but the power dissipated would exceed the nominal power rating resulting in heating of the
resistor.
In order to size the thermal overload correctly, the power rating and overload characteristics of the resistor are required. The power characteristic
curve for the resistor should be converted from multiples of power to current in order to size the thermal overload correctly.
Check that the MCB prevents the resistor from overheating assume a system fault which results in a continuous power of 10 x the nominal power
being dissipated by the resistor.
Resistor selected earlier was 3 x 48 which is 16 1500 W
10 x nominal power = 15000 W
MCB rating from previous selection was 3 A.
30.6 A is 10.2 x rated current.
From Figure 11-2 the MCB will trip in 3 s.
From the resistor manufacturers data shown in Figure 11-3,10.2 x rated power can be withstood for 3 s.
It() I
pk
e
t–
RC×
--------------
×=
It() 46,7 e
t–
16 14040 10
6–
××
-------------------------------------------------
×=
Time
s
Supply Current
Apk
0.1 29.9
0.2 19.2
0.3 12.3
0.4 7.9
0.7 2.1
10.5
Time interval
t1 = 0.1 x t
charge
t2 = 0.2 x t
charge
t3 = 0.4 x t
charge
t4 = 0.7 x t
charge
t5 = t
charge
I
P10
15000
16
---------------- 30,6A==
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