343

power dissipation in the windings. This can damage the windings, so the

dt

du values

need to be reduced, especially for large machines. As this usually involves inverters

with IGBT modules connected in parallel, the principle of

dt

du

filters will be briefly

described in the following.

dt

du

Filters in parallel connection

As described in chapters

and

modern IGBTs are essentially not controllable when

turning off. Reducing the

dt

du

of modern IGBTs when turning off causes a massive

increase in turn-off losses. To protect the electric machinery from excessive

dt

du

when

the IGBT turns on and off,

dt

du

filters are used in the output of the inverter. A capacitor,

C

du/dt

is connected parallel to the load inductance L

Load

. If, for example, the IGBT T

1

is

now turned on, the load flows into the capacitor the instant the device is turned on, and

all the voltage is present across the inductance L

11

. Once the capacitor is charged the

voltage can increase across C

du/dt

, and therefore across the L

Load

located parallel to the

capacitor. This means that the

dt

du

on the load is determined not by the connected

IGBT, but by the relationship between L

11

, R andC

du/dt

. R limits themaximum current of

the capacityC

du/dt

. If the switching voltage of the IGBT is considered to be ideal, the

dt

du

across the load can be calculated according to

dt /du

11

DC

C L

U

dt

du

⋅

=

As well as using output inductors, there is a second method of current balancing in

parallel connection: Ring connection of chokes. This makes use of the well-known

common mode effect. The idea comes from the era when thyristors were connected in

parallel and can be applied in the same way to modern IGBT connections. Ideally, all

currents in the parallel connection should be of the same value and direction. In this

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