Siemens SIMOVERT MASTERDRIVES Manual page 874

06.2005
Encoders
11.5 Selection of converters or inverters
Siemens AG 6SE7087-6QX70 (Version AE)
SIMOVERT MASTERDRIVES
Which encoder is selected depends on the requirements in each case.
Encoders provide high resolution and extremely true running at the
lowest speeds. They are thus especially suitable for highly accurate
positioning tasks. Resolvers are robust and inexpensive and provide
good resolution. Absolute-value encoders maintain the absolute
position even after the power supply is de-energized which means that
a new approach to reference point is not necessary with positioning
drives. Contrary to synchronous servomotors, induction servomotors do
not require a rotor position sensor such as an encoder or a resolver for
motor control. A pulse encoder is adequate here.
With single-axis drives, a converter now has to be selected and, with
multi-axis drives, an inverter is necessary. The selection criteria are the
same for both:
♦ The maximum motor current must be smaller than the maximum
permissible output current of the converter/inverter. If three times
the rated current is utilized in the case of the Compact PLUS unit
(3 times at a 5 kHz pulse frequency, 2.1 times at a 10 kHz pulse
frequency), this current must not flow for longer than 250 ms and
then a pause of 750 ms has to be observed with only 0.91 times the
rated current, otherwise 1.6 times the rated current is permissible for
60 s (see technical data).
♦ The arithmetic mean value of the motor current must be smaller
than the rated current of the converter/inverter with a maximum
cycle time of 300 s.
The second condition arises from the fact that the switching losses and
forward losses in the inverter are approximately proportional to the
output current. The rms value can also be calculated instead of the
arithmetic mean value. One is then more on the safe side, but it
requires more calculation work.
In order to determine the motor current at a given motor torque, the
following formula is used:
♦ For synchronous servomotors
M
Mot
=
I for M
Mot
kTn
kTn Torque constant in Nm/A
M Standstill torque
0
Compendium Motion Control
≤ M
Mot 0
Engineering Information
11-17