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Unidrive SPM User Guide
71
Issue Number: 3 www.controltechniques.com
5. Calculate the start-up time.
Where:
t
charge
: Approximate time to charge the DC bus.
R
: Total resistance of the soft-start resistor network. (Ohms)
Start-up time should not be less than 0.5s and generally should not be
more than 4s although this upper limit can be chosen by the user. 1s
charge time is recommended.
6. Calculate the supply current at 0.1s, 0.2s, 0.4s, 0.7s and 1s.
Where:
I(t)
: Peak current at time = t seconds.
Note that these calculation times are based on a 1s charge time. If the
charge time not 1s, then the time steps can be calculated as follows.
7. 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.
8. Check that the MCB prevents the resistor from overheating. An
example will best illustrate the process.
Example:
SPMD1204 on a 10% line power supply.
Step 1
C = 13200µF
Step 2
W = 1.45 x 13200 x 10
-6
x 230
2
W = 1013J
Step 3
Select resistor CT part number 1270-2483
Number of resistors required = 1013 / 1700 = 0.6
One resistor is sufficient at 48
Ω
, but if a shorter start-up time is required,
then two resistors can be used in parallel giving 24
Ω
.
Step 4
Peak supply current is:
A 1.2A MCB is required.
Step 5
Calculate the start-up time:
A start-up time of 1.58s is acceptable.
Step 6
Calculate the supply current throughout the start-up time.
Step 7
Comparing the data from step 6 with the tripping characteristic of the
selected MCB, Figure 6-11, shows that the supply current is less than
the MCB trip curves for each time interval.
Step 8
To 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 2 x 1270-2483 which is 24
Ω
296W
10 x nominal power = 2960W
But the MCB current rating from step 4 was 1.2A
11.1A is 9 x the rated current
From Figure 6-11 the MCB will trip in approximately 3s
From the resistor manufacturer's data shown in Figure 6-10, 10 x rated
power can be withstood for 5s
The MCB will protect the resistor. Design complete.
6.5.2 Design data
Capacitance
DC bus capacitance values and peak allowable supply current for
Unidrive SPM drives are as follows.
Table 6-10 DC bus capacitance and peak supply current values
Softstart resistor
The following resistors can be configured in series and parallel
arrangements to meet the requirements.
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
t
ch
e
arg
5 R C
×
×
=
I t
( )
I
pk
e
t
–
R C
×
--------------
⎝
⎠
⎛
⎞
×
=
I
pk
1.56 230
×
24
----------------------------
14.95Apk
=
=
t
ch
e
arg
5 24 13200 10
6
–
×
×
×
1.58s
=
=
Time
s
Supply Current
Apk
0.1
10.9
0.2
8.0
0.4
4.2
0.7
1.6
1
0.6
Model
Total DC bus
capacitance
µF
Maximum allowable
peak supply current
A
SPMA14X1
4400
75
SPMA14X2
5500
SPMA16X1
2200
52
SPMA16X2
SPMD12X1
8800
75
SPMD12X2
11000
SPMD12X3
13200
SPMD12X4
SPMD14X1
4400
52
SPMD14X2
5500
70
SPMD14X3
6600
SPMD14X4
SPMD16X1
2200
91
SPMD16X2
SPMD16X3
2933
SPMD16X4
I
P
R
----
the current needed to give 10 x power is
=
I
P10
2960
24
-------------
11.1A
=
=