DigitAx User Guide
Issue code: dgxu4
5-12
5.9 Example calculations for
a braking resistor
The required value and power rating of the braking
resistor are calculated from the following:
• Amount of energy to be absorbed
• Rate at which the energy is to be absorbed
(power)
• Time lapse between successive
decelerations
Conditions
Drive model: DBE2200
Drive rating = 22kW
Peak output current of the Drive (I
pk
) = 72A
Maximum duration of the peak current =
4 seconds
Motor rating = 22kW
Full-load speed of the motor (n) = 3000RPM
Nominal torque rating of motor (T
nom
)= 130Nm
System inertia (J) = 0.5kg.m²
Resistor operating voltage (V
R
) = 800V
Motor K
T
= 2.5Nm/ARMS
Deceleration time from full-load speed to rest
(t
d
) = 2 seconds
Repeat cycle time = 30 seconds
Minimum permissible deceleration time
The minimum permissible deceleration time is
limited by the following:
• The peak current of the Drive (Ipk)
• The intermittent torque limit of the motor
(the value of torque that the motor can
deliver for a specified time – see the motor
manufacturer’s data)
1. Calculate the maximum torque that the motor
would produce when the Drive is delivering
peak current (72A), as follows:
MIK Nm
bDRIVE pk T
==××==××==72 2 5 180.
The Drive would cause this value of torque to
be produced for up to 4 seconds.
2. Refer to the the motor manufacturer’s data to
obtain the permissible overload for 4 seconds.
Then use this figure to calculate the
intermittent torque limit for the motor for a
4-second duration. For this example, 3 times
the nominal torque rating is assumed, as
follows:
MT Nm
bINT nom
==××==××==3 130 3 390
3. For calculating the minimum permissible
deceleration time (t
bMIN
), use the lower of the
two calculated values, as follows:
MNm
bMAX
== 180
4. The following equation is used as the basis for
the calculations:
M
Jn
t
Nm
b
b
==××
ππ
30
()
Use the following derivative of the equation to
calculate the minimum permissible deceleration
time (t
bMIN
) for stopping the motor from
full-load speed:
t
Jn
M
t onds
bMIN
bMAX
bMIN
==
==
××××
××
==
ππ
ππ
30
0 5 3000
30 180
087
.
. sec
Resulting torque
1. Calculate the torque that results from the
required deceleration time, as follows:
Power rating of the braking resistor
1. Calculate the power to be dissipated in the
resistor during braking as follows:
P
Mn
W
PkW
b
b
b
==
××
==
××××
××
==
ππ
ππ
30 10
78 5 3000
30 10
24 7
3
3
()
.
.
2. Since braking is planned to occur
intermittently, the resistor can be rated for
intermittent rather than continuous power
dissipation so that the overload factor of the
resistor can be used. This factor can be
obtained from cooling curves for the resistor,
as shown in Figure 5–9.
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