General Information About Drive Dimensioning; General - Siemens SINAMICS G130 Engineering Manual

Drive Dimensioning
Engineering Information

10 General Information about Drive Dimensioning

10.1

General

When motors are operated on PWM converters rather than directly on the line supply, there are some special
aspects to be considered.
1. In converter-fed operation, the motors are supplied with pulse-width modulated, square-wave voltages.
By comparison with a sinusoidal line voltage supply, this produces
·
increased voltage stresses on the motor winding,
·
increased bearing currents in the rolling-contact bearings of the motor, and
·
harmonics in the motor currents, and as a consequence,
- stray losses in the motor,
- increased motor noise and
- torque oscillations on the shaft.
2. The converter is capable of varying the motor speed by adjusting the motor frequency. In this regard, the
following must be noted:
· At speeds below rated speed, the torque utilization limit must be observed. The useful torque must be
reduced if necessary from rated torque because the cooling efficiency of self-cooled standard and trans-
standard motors is dependent on speed and the self-cooling system becomes less effective at decreasing
speed.
· At speeds above rated speed, the useful torque must be reduced from the rated torque value, because
the magnetic flux in the motor decreases as the speed rises and the motor operates increasingly in the
field-weakening range.
The first aspect, i.e. the consequences of supplying a motor with square-wave voltages, is discussed in chapter
"Fundamental Principles and System Description". The subject will not therefore be elaborated on in this chapter.
The second aspect, i.e. the relevance of speed variation on the drive dimensioning process, will be discussed on the
following pages.
All major aspects of converter-fed operation of asynchronous motors are also described in the following standards:
· IEC/TS 60034-17:2006 "Rotating electrical Machines – Part 17: Squirrel-cage induction motors when fed from
converters – Application guide".
· IEC/TS 60034-25:2007 "Rotating electrical Machines – Part 25: Guidance for the design and performance of
squirrel-cage induction motors specifically designed for converter operation".
Typical load torques as a function of speed
The load torques M encountered in practice and their correlation with speed n can be essentially characterized by
L
four speed / torque characteristics.
Torque M as a function of speed n
L
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
Power P as a function of speed n
L