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10
M’Ax Installation Guide
www.controltechniques.com
Issue Number: 6
2.18
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 (19A), as follows:
The drive would cause this value of torque to be produced for up to 2
seconds.
2.
Refer to the the motor manufacturer’s data to obtain the permissible
overload (continuous stall torque) for 2 seconds.
Then use this figure to calculate the intermittent torque limit for the
motor for a 2-second duration. For this example, 3 times the nominal
torque rating is assumed, as follows:
3.
For calculating the minimum permissible deceleration time (t
bMIN
),
use the lower of the two calculated values, as follows:
4.
The following equation is used as the basis for the calculations:
Use the following derivative of the equation to calculate the minimum
permissible deceleration time (t
bMIN
) for stopping the motor from full-
load speed:
Check that t
bMIN
is less than td; if not, system design must be
reconsidered.
Resulting torque
Calculate the torque that results from the required deceleration time,
as follows:
2.19
Power rating of the braking resistor
1.
Calculate the kinetic energy (E
K
) that will be dissipated in the
braking resistor, as follows:
2.
Calculate the average power over deceleration time (td):
3.
Calculate the average power (P
av
) that will be dissipated over the
whole cycle:
When the value of P
av
is less than 125W, the internal braking
resistor can be used. For this example (which shows marginal
conditions), an external braking resistor must be used in order to
reduce the risk of the drive tripping under braking; tripping would
remove control from the motor, allowing it to coast.
4.
Since braking is planned to occur intermittently, an external 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 2-5.
Figure 2-5
Example cooling curves for power resistors (in
practice, refer to the cooling curves for the resistor to be used)
5.
The cooling curves indicate that for a braking time of 0.5 second and
repeat cycle time of 7 seconds, the overload factor (F) is 3.5.
M
bDRIVE
I
p k
K
T
×
19
1.6
×
30.4N m
=
=
=
M
bINT
T
CS
3
×
12.2
3
×
36.6N m
=
=
=
M
bMAX
30.4Nm
=
M
b
J
T
n
t
b
----------
π
30
------
Nm
(
)
×
=
t
b MIN
J
T
π
n
30M
bMAX
--------------------------
=
t
bMIN
13.7
10
3
–
×
π
×
4000
×
30
30.4
×
------------------------------------------------------------
0.19
ond
sec
=
=
M
b
J
T
n
t
d
----------
π
30
----------
×
N m
(
)
=
M
b
13.7
10
3
–
×
π
×
4000
×
0.5
30
×
-----------------------------------------------------------
11.5Nm
=
=
E
K
0.5
J
×
n
π
×
30
-------------
è
ø
æ
ö
2
×
=
E
K
0.5
13.7
×
10
3
–
4000
π
×
30
----------------------
è
ø
æ
ö
2
×
×
=
E
K
1.2kJ
=
P
P K
E
K
td
-------
=
P
P K
1.2kJ
0.5
---------------
2.4kW
=
=
P
av
E
K
tr
-------
=
P
av
1200
7
-------------
171W
=
=
1
2
3
4
5
6
7
8
9
10
0.1 0.2
0.5 1
2
5
10 20
50
Repeat cycle times
7s 1min 5min
30s
30min
Overload
factor
Deceleration time
