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3.1.3.
Thermal equivalent torque (rms torque)
The selection of the right motor can be made through the calculation of the rms torque
M
(i.e. root mean squared torque) (sometimes called equivalent torque).
rms
This calculation does not take into account the thermal time constant. It can be used
only if the overload time is much shorter than the copper thermal time constant.
The rms torque M
rms
Let us consider:
- the period of the cycle T [s],
- the successively samples of movements i characterized each ones by the maximal
torque M
[Nm] reached during the duration
i
So, the rms torque M
M
rms
Example:
For a cycle of 2s at 0 Nm and 2s at 10Nm and a period of 4 s, the rms torque is
M
rms
Illustration :
Acceleration-deceleration torque:
Resistant torque:
Max-min speed:
Max torque provided by the motor:
rms torque:
20
15
10
5
0
0.0
0.2
-5
-10
-15
motor torque [Nm]
speed [rpm]
-20
The maximal torque M
obtained by the algebric sum of the acceleration-deceleration torque and the resistant
torque. Therefore, M
reflects the heating of the motor during its duty cycle.
can be calculated through the following basic formula:
rms
n
1
2
*
M
t
i
i
T
i
1
1
2
*
10
*
2
, 7
07
Nm
4
10 Nm for 0,1 s.
1 Nm during all the movement.
2800 rpm during 0,2 s.
11 Nm.
6 Nm.
0.4
0.6
rms average torque [Nm]
rms average speed [rpm]
delivered by the motor at each segment i of movement is
i
corresponds to the maximal value of M
max
14 - PVD 3664_GB_NK-November 2016.Docx
t
[s].
i
0.8
1.0
.
i
4000
3000
2000
1000
Time [s]
0
1.2
-1000
-2000
-3000
-4000
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