Modern Trends of Electric Drives
In Electric Traction System
• Traction motors can use both ac/dc currents with suitable control
electronics. They are operating in high voltage current.
• They can widely used for vehicles, elevators, locomotives etc.
• Traction motor refers to an electric motor providing the primary
rotational torque of a machine.
• Traction motors are used in electrically powers rail vehicles such
as electric multiple units and electric locomotives.
Requirements of Traction System
• High starting pulling effort in order to have rapid acceleration.
• Equipment capable of withstanding large temporary overloads for
• The locomotive should be self contained and able to run on any
• Capability of withstanding voltage fluctuations and temporary
interruption of supply.
• Parallel running usually more than one motor (two or four motors)
should be possible.
Electronic Speed Control Method
for DC Motor
• Compared to the electric and electro-mechanical systems of speed
control, the electronic methods have
Recent Trends in Electrical Traction System
• The multi-stage converter has a chain of 4 converters per phase (4-
Stage Converter), with a “Main converter” that manages more than
80% of the total power.
• Three “Slave converters” that take the rest of the power (less than
• It can generate almost perfect current & voltage waveforms,
because it is modulated by amplitude instead of pulse-width
• It can generate near sinusoidal voltages with only fundamental
• It have almost no electromagnetic interference of common mode
• They are suitable for large volt-ampere rated motor drives & high
voltages and it is lower frequency devices.
• S2 and S1’ are switched on the output is –V dc.
• Either pair S1 and S2 or S1’ and S2’ are on the
output is zero.
• S1 and S2’ are on the output is +V dc.
4 Level Multi Stage Converter
Main components of Multistage Converter
• The two-level converter is the simplest type of three-phase voltage-source
• It is a six pulse bridge in which the thyristors have been replaced by IGBTs with
• The voltage at the AC output of each phase is switched between two discrete
• Corresponding to the electrical potentials of the positive and negative DC
• When the upper of the two valves in a phase is turned on, the AC output terminal
is connected to the positive DC terminal.
• Resulting in an output voltage of +½ Ud with respect to the midpoint potential of
• When the lower valve in a phase is turned on, the AC output terminal is
connected to the negative DC terminal, resulting in an output voltage of -½ Ud.
Three Level Converter
• To improve the poor harmonic performance of the two-level converter, some
systems have been built with three level converters.
• Three-level converters can only two discrete voltage levels at the AC terminal
of each phase: +½ Ud, 0 and -½ Ud.
• A common type of three-level converter is the diode-clamped (or neutral-point-
clamped) converter, where each phase contains four IGBT valves.
• Each rated at half of the DC line to line voltage, along with two clamping diode
• The DC capacitor is split into two series-connected branches, with the
• To obtain a positive output voltage (+½ Ud) the top two IGBT valves are turned
• To obtain a negative output voltage (-½ Ud) the bottom two IGBT valves are
• To obtain zero output voltage the middle two IGBT valves are turned on
Modular Multi-Level Converter
• The MMC differs from other types of converters.
• In that the current flows continuously in all six valves of the converter
throughout the mains-frequency cycle.
• As a result, concepts such as “on-state” and “off-state” have no meaning in the
• The direct current splits equally into the three phases.
• The alternating current splits equally into the upper and lower valve of each
• The current in each valve is therefore related to the direct current Id and
alternating current Iac as follows:
• It can generate perfect current & voltages waveforms.
• The power losses into the machine due to harmonic current
are also eliminates.
• Reducing power losses at minimum value.
• Power rating of semiconductor is very small.
• Multi stage converter working at low switching frequency.
• Operation is fast & quick response.
• Active power filters.
• Sinusoidal current rectifiers.
• Machine drives.
• Power factor compensators.
• Back to back frequency link system.
• Traction drive system.
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