Digitax DBE1100S, Руководство пользователя, Страница: 41 / 96

DigitAx User Guide
Issue code: dgxu4
5-13
0.5 1 2 5 10 20 50 100 200 500
1
2
3
4
5
6
7
8
9
10
Repeat cycle times
15s
30s
1min 5min
30min
Overload
factor
Deceleration time
Figure 5–9 Example cooling curves for
power resistors (in practice, refer
to the cooling curves for the
resistor to be used)
3.
The cooling curves indicate that for a braking
time of 2 seconds and repeat cycle time of 30
seconds, the overload factor (
F
) is 6.5.
4.
Calculate the required power rating of the
resistor, as follows:
P
P
F
kW
R
b
== == ==
24 7
6 5
3 8
.
.
.
Value of the braking resistor
1.
Calculate the maximum suitable value for the
braking resistor, as follows:
(( ))
R
V
P
MAX
R
b
== ==
××
==
2
2
3
800
24 7 10
25 9
.
.
Ω Ω
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 = 22
Ω Ω
3.
Refer to the following table to find out if the
internal braking resistor (when fitted) is
suitable for the application. If it is not, an
external resistor must be used. In this case,
ensure the resistor value is not less than the
minimum permissible value.
Model Internal resistor External
resistor
Value Power ratings Minimum
permissible
value
DBE140
DBE220 80
Ω
150W continuous
1.5kW for 10 secs
braking time with
90 secs minimum
cooling time.
65
Ω
DBE420
DBE600
DBE750
DBE1100S 40
Ω
300W continuous
3.0kW for 10 secs
braking time with
90 secs minimum
cooling time.
27
Ω
DBE1500
DBE2200 11
Ω
Current setting for the
thermal overload relay
1. Calculate the maximum permissible continuous
current through the braking resistor, as follows:
I
P
R
A
R
R
max
.
.
== ==
××
==
3 8 10
22
13 1
3
where:
P
R
is the power rating of the resistor to be used
(not the calculated power dissipation)
R
is the actual value of the braking resistor
(not the calculated value)
2. Use the tripping curves for the thermal
overload relay in order to find the overload
factor (
F
) that will cause the relay to trip after
10 seconds. See Figure 5–10.