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12 Functional Descriptions 10.94
12.7.2 Description of the spindle modes
At speeds (n) above rated speed (nn), the drive acceleration capability decreases in relation to
the speed. Immediately above rated speed nn, there is a range of "constant power" in which
the torque (and thus also the acceleration capability) drop in proportion to 1/n. When speed n
increases further, there is a range ("breakdown limit") immediately above speed nk in which
the torque decreases in proportion to 1/(n*n).
The required parameters are selected and set in the MSD drive unit.
Fig. 2 Speed characteristic during acceleration (thick line)
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The obtainable linear ramp-up time constants
increase out of proportion for higher speeds
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T1
Figure 2 shows the speed characteristics which are obtained when the acceleration capability
is fully utilized.
In speed-controlled operation, the drive accelerates either according to the acceleration rate
(dashed line for T2 in Fig. 2) set in MD 419* (ramp-up time constant T in open-loop control
mode) or, when the ramp-up time constant is shorter, optimally in terms of time along the
current limit.
With software versions up to and including SW 3, the ramp-up time constant in MD 478* for
position-controlled operation must be set such that the current limit must not be reached
when the drive accelerates to maximum speed to avoid the risk of instability or overshoot on
positioning. The maximum acceleration capability of the drive cannot be fully utilized.
In the new software version, a speed-dependent acceleration characteristic, which can be
adapted according to the prevalent physical conditions (speed-dependent adaptation), is
implemented for the position-controlled spindle operating modes (most important application:
Open-loop controlled operation of a leading spindle in the synchronous spindle or ELG
grouping).
12–50
© Siemens AG 1992 All Rights Reserved 6FC5197- AA50
SINUMERIK 840C (IA)
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