Nidec Unidrive M600 Series - 11.3 Thermal; magnetic overload protection for soft start circuit

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312 Unidrive M Regen Design Guide
Issue Number: 4
11.3 Thermal / magnetic overload protection for soft start circuit
Protection for the softstart circuit must be provided. The recommended protection is to use a miniature circuit breaker (MCB) having a thermal
magnetic trip. The thermal part of the tripping mechanism protects against a high impedance short circuit and the magnetic part of the trip protects the
resistor against a direct short circuit. The overload should be sized as following to provide thermal and magnetic protection:
11.3.1 Thermal / magnetic overload characteristics
Figure 11-2 Example of tripping characteristic
11.3.2 Sizing of magnetic overload
The magnetic overload should be selected to the peak current and charging time at power up with the trip being at for example 20 times the nominal
rated current of the overload. Therefore for a 20 A peak current an MCB with a 1 A nominal current rating could be used.
The charging of a system takes a total of 5 time constants with this having a decaying exponential current due to the RC network, therefore at 5 time
constants the system will have charged up with the current being at approximately zero.
The peak current and charge time during power up can be calculated using the following formula.
Example: Peak current
480 Vac supply +10 %, total softstart resistance of 16  (3 x 48  in parallel):
I
peak
= Vac (+10 %) x 1.414 / Resistance
softstart
= (480 + 48) x 1.414 / 16 = 46.7A I
peak
Example: Charging time
Total softstart resistance of 16  (3 x 48 ) in parallel, and a total DC bus capacitance of 14040 F
T
constant
= Resistance
softstart
x Total Capacitance
DC bus
0.225 = 16 x (14040 x 10
-6
)
T
charge =
T
constant
x 5
1.13 s = 0.225 x 5
0.01
0.02
0.04
0.06
0.1
0.2
0.4
0.6
1
2
4
6
10
20
40
1
2
4
6
10
20
40
60
120
1.5 2 3 41 56 8
10
15
20
30
14
21
10
Tripping time
Seconds Minutes
Multiple of rated current
Thermal Trip
Area
Hot
Cold
Magnetic Trip
Area