Drives With Permanent-Magnet Three-Phase Synchronous Motors - Siemens SINAMICS G130 Engineering Manual

Another factor to consider is that the higher the rated motor current as compared to the converter rated current, the
lower the leakage inductance of the motor and thus the greater the harmonic content of the motor current as a result
of voltage modulation. This can result in converter tripping on overcurrent or, if the high harmonic content causes the
total rms value of the motor current to rise too sharply, in a reduction / limitation of the motor current by the internal
protection mechanisms in the converter (overload reaction triggered by the I
model).
In view of the interrelationships explained above, the rated current of the motor should where possible be less than or
equal to the maximum output current I
applies:
I = 1.5 x base load current I
max
In individual cases where, for example, the motor is to be operated at no load for test purposes, the motor rated
current may be higher. However, if the motor rated current exceeds twice the maximum output current of the
converter (2 x I ≈ 2.9 x I
max
increase in harmonics in the motor current caused by voltage modulation.
Rated motor current significantly lower than the rated current of the converter or Motor Module
In typical applications with vector control (with or without speed encoder), the rated motor current should equal at
least 25 % of the rated current of the converter or Motor Module. The greater the difference between the rated
currents of motor and converter, the less accurate will be the actual current sensing circuit and the lower the quality
of the vector control. For applications with very high control requirements which use a speed encoder and demand a
very accurate vector control, it is advisable to select a motor rated current which equals at least 50 % of the rated
current of the converter or Motor Module.
No restrictions of this type apply basically to V/f control mode, i.e. motors with very low rated currents can be
operated on the converter. It must be noted, however, that when motors of very low output power are operated on a
very powerful converter, the very low currents make automatic motor identification impossible. For the same reason,
it is impossible to implement motor overload protection on the basis of a specifically adapted overcurrent limit
parameter setting in the converter.
Typical applications with V/f control mode and very low-output motors fed by powerful converters are roller conveyor
drives in which up to 100 small motors might be supplied by a single converter. In this case, the motors connected to
the converter output must be individually protected by circuit breakers with a thermal overload release because the
converter itself is unable to provide overload protection for the reasons described above. With long motor cables and
motors with rated currents in the single-digit ampere range, the setting scale of the thermal overload release should
cover a range corresponding to approximately 2 to 3 times the motor rated current in order to provide protection
against the harmonic content in the motor current caused by the converter. For further details, refer also to section
"Special issues relating to motor-side contactors and circuit breakers" in chapter "Fundamental Principles and
System Description".
10.5

Drives with permanent-magnet three-phase synchronous motors

With SINAMICS converters permanent-magnet, three-phase synchronous motors can also be used alongside three-
phase asynchronous motors. The multi-pole, high-torque 1FW4 motors from the SIMOTICS HT series HT-direct
range are suitable for this application. They are designed for use with SINAMICS converters as low-speed direct
drives and can replace favorably conventional motor-gearbox combinations. In addition to the SIMOTICS HT series
HT-direct 1FW4 motors, permanent-magnet synchronous motors of other makes can also be operated on SINAMICS
converters.
Closed-loop control of permanent-magnet synchronous motors
The standard firmware of SINAMICS converters of type G130, G150, S120 and S150 provides a closed-loop control
function for permanent-magnet synchronous motors:
·
SINAMICS G130 and G150 are designed for sensorless vector control. With these converters, regenerative
energy cannot be regenerated to the supply system. For this reason, these drives are suitable only for
standard applications with low requirements regarding dynamic performance and accuracy. If the drive has
to be capable of flying restart, i.e. switching onto a rotating motor, a Voltage Sensing Module (VSM) must be
integrated in the converter instead of an encoder module.
·
SINAMICS S120 and S150 are designed for both sensorless vector control and for vector control with speed
encoder. Servo control mode is also available for SINAMICS S120 converters. With these converters,
regenerative energy can be regenerated to the supply system. These drives are therefore suitable for use
with demanding applications with the highest requirements in terms of dynamic performance and accuracy.
Sensorless vector control is possible for simple, standard applications. Vector control with speed encoder is
always required when high dynamic performance and accuracy are needed. The highest dynamic
performance is achieved using the servo control mode.
of the converter or Motor Module. In this respect, the following equation
max
≈ 1.45 x rated output current I
L
), the converter maximum current will be reduced automatically due to the steep
rated
Engineering Information
2
t monitoring or the thermal monitoring
rated
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Drive Dimensioning
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