Influence Of The Pulse Frequency On The Inverter Output Current; Influence Of The Pulse Frequency On Losses And Efficiency Of Inverter And Motor; Influence Of The Pulse Frequency On The Motor Noise - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information
thereof. Furthermore, the current controller clock cycle must be selected such that the set bus clock cycle
also corresponds to a whole multiple of the current controller clock cycle. When SINAMICS Link is used, bus
clock cycles of 500 μs, 1000 μs or 2000 μs can be set which means that the following current controller
clock cycles can be selected: 125 μs, 250 μs and 500 μs. For further information, refer to the function
manual "SINAMICS S120 Drive Functions" and the List Manuals.
·
When the pulse frequency is set higher than the relevant factory setting, the current derating factors
applicable to the specific unit must be observed. These can be found in the chapters on specific unit types.
·
If multiple Motor Modules (axes) are to be controlled by a single CU320-2 Control Unit in SINAMICS S multi-
motor drives, it must be noted that the maximum possible number of Motor Modules (axes) is dependent
upon the current controller clock cycle. More detailed information can be found in section "Determination of
the required control performance of the CU320-2 Control Unit" in chapter "General Information about Built-in
and Cabinet Units SINAMICS S120".
1.1.3.3

Influence of the pulse frequency on the inverter output current

The pulse frequency factory setting of either 2.0 kHz or 1.25 kHz is relatively low in order to reduce inverter switching
losses. If the pulse frequency is increased the inverter switching losses and thus also the total losses in the converter
increase accordingly. The result would be overheating of the power unit when operating at full load capacity. For this
reason, the conducting losses must be lowered in order to compensate for the increase in switching losses. This can
be achieved by reducing the permissible output current (current derating). The current derating factors as a function
of pulse frequency are unit-specific values and must be taken into account when the converter is dimensioned. The
derating factors for various pulse frequencies can be found in the chapters on specific unit types. If derating factors
are required for pulse frequencies which are not included in the tables, they can be calculated by linear interpolation
between the stated table values. Under certain boundary conditions (line voltage at low end of permissible wide-
voltage range, low ambient temperature, restricted speed range), it is possible to partially or completely avoid current
derating at pulse frequencies which are twice as high as the factory setting. For further information, please refer to
section "Operation of converters at increased pulse frequency".
1.1.3.4

Influence of the pulse frequency on losses and efficiency of inverter and motor

With the factory-set pulse frequency of 2.0 kHz or 1.25 kHz, the motor current is already close to sinusoidal. The
stray losses in the motor caused by harmonic currents are low, but not negligible. Commercially available standard
motors for 50 Hz or 60 Hz operation utilized according to temperature class 130 (previously temperature class B) in
operation directly on line can be utilized according to temperature class 155 (previously temperature class F) at the
nominal working point up to rated torque when operated on a converter. The winding temperature rise is then
between 80 and 100 K.
Raising the pulse frequency on standard motors for 50 Hz or 60 Hz reduces the motor stray losses only slightly, but
results in a considerable increase in the converter switching losses. The efficiency of the overall system (converter
and motor) deteriorates as a result.
1.1.3.5

Influence of the pulse frequency on the motor noise

A higher level of magnetic motor noise is excited when three-phase motors are operated on PWM converters as
compared to operation directly on line at 50/60 Hz supply systems. This is caused by the voltage pulsing which
results in additional voltage and current harmonics.
According to
·
IEC/TS 60034-17:2006 "Rotating electrical Machines – Part 17: Cage induction motors when fed from converters
- Application guide",
and
·
IEC/TS 60034-25:2007 "Rotating electrical Machines – Part 25: Guidance for the design and performance of a.c.
motors specifically designed for converter supply",
the A-graded noise pressure level increases up to 15 dB(A) when three-phase motors are operated on a PWM
converter up to rated frequency as compared to motors of the same type operating on pure sinusoidal voltage.
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Ó Siemens AG