is either constructed with an SCR or IGBT/FWD bridge, which will allow a bidirectional
flowof energy. Also theDC-bus can beboosted, which is usually realisedwith IGBTs.
The DC-bus follows the input circuit, and it is either set up as a current or a voltage
source. While the first provides a constant current within the application requirements,
the latter will provide a constant voltage for the output. If the input section cannot
regenerate into themains, it is common to integrate a chopper circuit (dynamic brake) in
the DC-bus. This circuit, also referred to as a brake chopper, dissipates the energy fed
back into the DC-bus from the load using a resistor. Otherwise, in a voltage source
DC-bus for example, the voltage would rise above the maximum rated voltage of the
capacitors and the semiconductors, andmight cause thedestruction of the inverter.
Followed by the DC-bus is the output circuit, which, within the framework of this book,
consists of an IGBT bridge. The purpose of the output section is to convert theDC from
the intermediate section into a voltageand frequency adjustableAC.
To avoid distortion of the mains voltage, filters are placed at the input of the inverter.
These can be realised as passive LC or active combinations integrated into the input
circuitry. The aim is to bring the power factor of the inverter at the mains side to 1 or
close to 1.
Filters at the output side, however, have a different function. Their purpose is to limit
rapid current and voltage gradients originating from the fast switching power
semiconductors of the output section, and thus to minimise the electro-magnetic
interference. The filters also reduce the displacement currents in motors and cables,
which may lead to problems for the motor insulation and/or the bearings. Further, the
filters protect the power electronic components by reducing the reversal charge
currents. By limiting the voltage gradients
, motors are sufficiently protected against
these problems. The control of the power semiconductors in the input, DC-bus and
output section is done with the appropriate driver units. Usually the drivers are
separately designed for the individual circuits due to the specific application and the
location within the inverter. Aside from the actual control of the power semiconductors,
i.e. turning on SCRs and turning on and off IGBTs, driver units contain more or less
complex protection functions. The protection functions are there to prevent the
destruction of the semiconductor switch in case of a fault. Generally, these protection
functions only work for the associated switch, sometimes for the associated half-bridge
and rarely for the entire output bridge. To shut down the complete inverter, however, is
not the job of the protection function of the driver unit. This is in fact the job of the
control logic. The control logic collects status signals from the driver stage as well as
output signals of selected sensors. Relevant sensors in the inverter are current sensors
which pick up the load current. Also, voltage sensors that monitor the DC-bus voltage
and, depending on the application, keep an eye on the mains voltage. In addition the
temperature is monitored. This is picked up either by sensors buried in the heatsink or
by integrated temperature sensors in the power semiconductor components. The latter
are thermistors (NTC) and IGBTs with integrated temperature sensor. The control logic
evaluates all the existing data and compares it with the set values, which are
determined by an external interface to the user or possibly supervising control logic. The
control of the driver is done according to the stored algorithms in the control logic and
the driver then controls the power section.