469

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

dt

du

230V/1~

325V

s

kV2.2

µ

400V/3~

565V

s

kV8.3

µ

690V/3~

975V

s

kV5.6

µ

Examples of voltagegradients resulting from IGBT switching

TheDC-bus voltageU

DC

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.

ns 150

U

dt

du

DC

≈

The voltage pulses with a gradient

dt

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

DC

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

•

Cable length

•

DC-bus voltage

•

Voltage gradient

•

PWMpulse pattern

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

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