generation (e.g. BJTs or GTOs). The voltage gradients of IGBTs are in the region of
several kilovolts permicrosecond.
hows a roughestimation.
Line voltage DC-link voltage
Examples of voltagegradients resulting from IGBT switching
is calculated usin
To arrive at the voltage gradient,
s used as an approximation. The 150ns timeframe is freely chosen and
serves as a cross section of switching speeds of different IGBTs. This timeframe is then
longer or shorter, as the actual casemay be.
The voltage pulses with a gradient
du coming from the output of the inverter are fed to
the load machine via the connecting cables. The peak value of these voltage pulses
equals the DC-bus voltage U
of the inverter. The motor sees a voltage with a peak
value, however, which is higher than the output voltage of the inverter and thismay lead
tomotor damage. The actual amplitude of the voltage peak value at themotor terminals
is defined by several parameters:
Impedance of the cableand themotor
Generally with a VSI controlled by PWM and connected to the motor via cables, there
will be a (partial) reflection of the voltage at the motor terminals. The reason is the
mismatch of cable and motor impedances. The wave impedance of a cable can be
calculated from the inductanceper unit of length