Control Techniques Unidrive Installation Manual page 36

Use the minimum tripping curves for the chosen manufacturer of thermal overload relay
in order to find the overload factor (F) that will cause the relay to trip after 5 seconds.
The tripping curve below is for a class 10A thermal overload relay.
Figure 5-1 Class 10A thermal overload relay tripping characteristics
10000
1000
100
10
1
0.1
From the thermal overload relay-tripping curve above it can be seen that at 5 seconds
the overload factor (F) will be approximately 3.5.
From the above information the thermal overload relay setting (I
I
=
set
I
=
set
Select a model of thermal overload relay that can be set at 1.26A. (e.g. Telemacanique
LRD-06).
2. Calculate the maximum fault current under fault conditions [I
Calculate the maximum current that could flow through the resistor (e.g. due to the
braking transistor becoming short circuit ).
(
I peak
r
Check that the maximum current under fault conditions is less than 17 times the current
setting of the thermal overload relay.
Overload factor (F) with short circuit =
( peak )
I
r
--------------------------
I
set
This is less than 17 times so this thermal overload relay would be suitable.
If the overload factor (F) is more than 17 times the current setting of the thermal overload
NOTE
relay, then an in-line fuse would need to be considered. In this case refer to the thermal
overload relay manufacturers recommendations.
36
1
overload factor, F
( )
I peak
rms
--------------------------------------------------- -
Overload factor(F)
4.4
1.26A
------- - =
3.5
LVDC brake turn on voltage (V)
)
= --------------------------------------------------------------------------------------- -
)
9.2A
7.3
= --------------- - =
1.26A
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10
=
R
Unidrive SP Low Voltage DC Installation Guide
thermal overload
min time cold (s)
thermal overload
average operating
time cold (s)
thermal overload
max time cold (s)
100
) will be:
set
(peak)]
r
63.6V
--------------- - = 9.2A
6.9Ω
Issue Number: 1