Siemens SINAMICS G130 Engineering Manual page 315

Drive Dimensioning
Engineering Information
Derating in crawling mode with low speed resp. low converter output frequency
Water-cooled and externally-ventilated synchronous motors of series 1FW4 can be use 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.
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-Magnetical 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 nRated, the output voltage V of the converter increases in proportion to
the 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 nRated of the motor, the converter output voltage V remains constant because, with
SINAMICS 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 dependent on the speed
If the converter trips during operation in the field-weakening range, the reactive current I in the stator, which weakens
the rotor field, is no longer present and therefore also the voltage drop ΔV. The voltage V at the motor terminals and
at the converter output thus increases within a few 10 ms to the value of the EMF depending on the field-weakening
speed of the motor. As a result, the DC link is charged via the freewheeling diodes of the inverter to the amplitude of
the EMF of the motor.
SINAMICS Engineering Manual - May 2008
315/396
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