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M’Ax Installation Guide
11
Issue Number: 6
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6.
Calculate the minimum required power rating of the resistor, as
follows:
If the braking resistor is to be mounted inside the enclosure, make a
note of this value; you will need it when calculating the enclosure
size.
In practice, use a resistor having a power rating higher than the
calculated value. For this example: P
R
= 750W or 0.75kW
2.20
Value of the braking resistor
1.
Calculate the maximum suitable value for the braking resistor, as
follows:
2.
In practice, use a resistor having a preferred value close to and
lower than the calculated value. This is because the calculated value
would cause the braking transistor to be switched on almost
continuously during braking. In this case, the drive will not have full
control of the DC-bus voltage. A lower value of braking resistor will
cause the braking transistor to act as a chopper which will then allow
the drive to control the DC-bus voltage more accurately.
The reduction in value does not increase the power dissipation since
the average voltage across the resistor is reduced by the braking
transistor operating as a chopper.
For this example: R = 200
Ω
2.21
Disabling protection of the internal
braking resistor
The internal braking resistor is protected against I
2
t overload by
calculations performed in the drive software. When an external braking
resistor being used, this calculation must be disabled in order to remove
the possibility of it causing the drive to trip unnecessarily.
To disable the I
2
t protection for the internal braking resistor, make a note
to set parameter 10.55 at 1 when following Chapter 6 Setting Up the
drive for Basic Applications in the User Guide.
Do not disable the I
2
t protection when the internal
braking resistor is to be used.
2.22
Current setting for a thermal overload
protection relay
1.
Calculate the maximum permissible continuous current through
the braking resistor that is to be used, as follows:
where:
P
R
is the continuous power rating of the resistor to be used (not
the minimum required power rating)
R is the actual value of the braking resistor (not the calculated
value)
2.
Select a model of thermal overload relay that can be set at 1.9A
3.
Calculate the maximum current that could flow through a resistor
(e.g. due to the braking resistor becoming short circuit), as follows:
4.
Calculate the overload factor for this condition, as follows:
5.
Use the tripping curves to find the time that the thermal overload
relay will take to trip (e.g. 30 seconds approximately).
2.23
Enclosure layout
Refer to Figure 2-6 for minimum clearances above and below the drive.
The bookcase format allows drives to be mounted in rows with no need
for horizontal spacing.
Figure 2-6
Minimum clearances above and below the drive
Refer to Figure 2-7 for the arrangement of the associated equipment and
wiring in the enclosure. This diagram shows two drives, one having an
external braking resistor connected. When EMC emission standards are
to be met, an RFI filter will need to be included for each drive; see the
sections on EMC emission standards later in this chapter.
P
RMIN
P
P K
F
----------
2.4
10
3
×
3.5
------------------------
0.7k W
=
=
=
R
MAX
V
R
(
)
2
P
P K
---------------
780
2
2.4
10
3
×
------------------------
250
Ω
=
=
=
CAUTION
I
Rmax
P
R
R
-------
750
200
----------
1.9A
=
=
=
I
Rp k
V
R
R
-------
780
200
----------
3.9A
=
=
=
F
S\C
I
Rpk
I
S ET
---------------
3.9
1.9
--------
2
=
=
=
1
2
5
10
0.5
1
2
5
10
17
X current setting (F)
20
50
100
Time (s)
Balanced operation 3-phase,
from cold state
Balanced operation 2-phase,
from cold state
Balanced operation 3-phase,
after a long period of set current
flow (hot state)
