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6KBU300
—————— DIMENSIONING... AND CORRESPONDING... ——————
4
1
4. DIMENSIONING OF THE BRAKING UNIT AND
CORRESPONDINGRESISTOR
What below indicated should be meant in general, because point 6 reports a list of the normalised resistor which
must be used with the braking units of the series 6KBU300-.. for the supposed conditions.
Taking into account that:
P
PBR
[W] Peak power while braking
P
NBR
[W] Rated power of the resistor
E
BR
[J] Braking energy
V
BR
[V] Braking voltage threshold
I
PBR
[A] Peak braking current
I
AVBR
[A] Average braking current
I
PBU
[A] Peak current of the braking unit
n
1
, n
2
[RPM] Initial and final speed
t
BR
, T [S] Braking time and cycle time
J
TOT
[Kg* m2] Total moment of inertia (referred to the motor shaft )
We will have:
P=J*n*
PBR TOT 1
n-n
t
12
BR
2Π
60
f001
*
I=
PBR
P
PBR
V
BR
f003
E=
BR
(n -n )
12
22
J
TOT
2Π
2
2
60
**()
Ohmic value of the resistor:
f004
R=
BR
≤
V
BR
I
PBR
Rated continuous power of the resistor:
P=
NBR
=
P*t
PBR BR
E
BR
2T T
f005
Attention! The value calculated here has to be considered carefully:
the formula calculates an
average power value which may be considerably different
from the istantaneous power in case of very low duty-cycles.
Normally, resistors are not able to sustain a peak power greater than 5 to 10 times their rated
power. For this reason if the duty-cycles are less than 10%, the value calculated here can not
be used as rated power of the resistor and considerations made at 4.1 and 6 have to be taken
into account. Consult your resistor manufacturer for overload capability of resistors.