Siemens SINAMICS G130 Engineering Manual page 514

Drive Dimensioning
Engineering Information
Current derating factors applicable to the converter
Pulse frequency derating
It is essential that SINAMICS converters, when used with permanent-magnet synchronous motors, must be operated
with relatively high pulse frequency due to the eddy current losses in the magnets. Therefore, the factory-set pulse
frequency of 1.25 kHz or 2.0 kHz must be increased, which causes a derating of the output current. The derating
factors can be found in the corresponding tables of the unit-specific chapters.
1FW4 synchronous motors from the SIMOTICS HT series HT-direct range require a pulse frequency of at least 2.5 kHz.
Synchronous motors produced by other manufacturers often require even higher pulse frequencies of up to 4 kHz.
Derating in crawling mode with low speed or low converter output frequency
Water-cooled and forced-ventilated synchronous motors of series 1FW4 can be used for up to three hours in crawl
mode with speeds close to zero. At these operating conditions, the converter can only deliver 50 % of its rated output
current. If a higher current is required, the converter must be oversized according to the derating curves in the
chapter "Fundamental Principles and System Description", section "Dimensioning of power units for operation at low
output frequencies".
Operation in the field weakening range
Permanent-magnet synchronous motors have a permanent magnetic field as a result of the magnets in the rotor.
Thus, the motors produce a voltage, as soon as the rotor starts to turn. The EMF (Electro-Magnetic Force) induced in
the stator winding as a result of the rotation of the rotor increases in proportion to the rotor speed. The following
diagram shows the electrical circuit diagram (one phase) of a permanent-magnet synchronous motor.
Electrical diagram of a permanent-magnet synchronous motor
In the base speed range up to rated speed n
speed. As the EMF produced by the permanent magnets in the motor also increases in proportion to the speed, a
balance exists between the output voltage V of the converter and the EMF of the motor.
From the rated speed n
Rated
converters, it is limited to the value of the line supply voltage connected to the converter input. The EMF of the motor,
however, still increases in proportion to the speed. In order to restore the balance between the constant converter
output voltage V and the correspondingly higher EMF of the motor in the field-weakening range, a supplementary
reactive current I must be delivered to the stator winding by the converter, in addition to the active current which
produces the torque. This is to weaken the field induced by the rotor and to restore the voltage balance in the motor
by producing the voltage drop ΔV. The higher the speed in field-weakening range, the larger the field-weakening
reactive current must be. This reactive current must be considered at the dimensioning of the drive. At operation in
high field-weakening range, a clear over-dimensioning of the drive may be required.
Converter output voltage V and EMF of the motor as a function of speed
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
, the output voltage V of the converter increases in proportion to the
Rated
of the motor, the converter output voltage V remains constant because, with SINAMICS