Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 106)

3.10.1.2 Intermediate circuit
The intermediate circuit stores and smoothes the direct voltage. The motor connected to the
frequency converter obtains energy from it and thereby partially discharges the capacitor . This
is recharged when the supply voltage is higher than the intermediate circuit voltage. The energy
is thus derived from the supply, when the supply voltage is close to the maximum. This pro-
duces current peaks that should be taken into account in the selection of switchgear connected
upstream (contactors or circuit breakers), as the ratings of these devices relate to sinusoidal
currents. At larger power ratings (from approx. 5.5 kW), intermediate circuit chokes L are
provided to extend the current flow time on the supply side and hence to reduce the current
peaks.
3.10.1.3 Inverter
The inverter again generates a three-phase supply with the desired frequency and voltage for
the connected motor. For this purpose the components of the inverter – controlled by the
control-logic – connect positive and negative voltage pulses to the motor coil. Due to the high
switching frequency, which may be many times above the audible range, a largely sinusoidal
motor current is created in interaction with the inductances of the motor. The control of the
frequency and voltage in most frequency converters is based on the principle of pulse width
modulation PWM (
Fig. 3.10-2 ).
Fig. 3.10-2
Principle of pulse width modulation
3.10.2 Operational performance
Squirrel-cage induction motors require as their basic operating characteristic a so-called U/f-
characteristic curve, which reduces the motor voltage in proportion to the frequency
(
Fig. 3.10-3 ). This is because the magnetic field in the motor is critical for the development of
torque and therefore saturation of the iron core must be avoided. Such saturation would occur if
at reduced frequency the magnitude of the voltage were not also reduced (constant volt-
age/time-area of a half cycle). In the area up to synchronous speed the drive can thus produce
the rated torque (
Fig. 1.7-8 Section 1.7.1.4 ).
Frequency converters are usually designed so that the motor voltage reaches its maximum
value at the synchronous speed and remains constant with higher frequencies. At speeds above
the synchronous speed, the available torque thus falls and the drive can be operated at
constant power.
LVSAM-WP001A-EN-P - April 2009 3-30