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3
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12-pulse circuit
To minimise both the harmonic
influence the drive the supply
system and the ripple the motor
torque, rectifier and inverter normally
operate 12-pulse mode. This
technique eliminates harmonics of
the series 5th, 7th, 17th and 19th etc. The best one use for
a particular application largely
determined the power and
voltage ratings the drive.
Excitation
The excitation the synchronous machine
can the brushless slipring type.
The diagram the right shows a
simplified 6-pulse circuit and
alternative 12-pulse circuits b), c)
and d). Since the drive system is
self-controlled, cannot fall out-of-step.MEGADRIVE-LCI system
The main components adjustable speed
drive are shown the diagram the right. The following explanation is
therefore confined the components the
frequency converter and its operation. The
rectifier supply converter conjunction
with the reactor the link circuit form
a fully controllable current source.
Inverter
Thyristors the inverter electronically switch
the current such produce three-
phase system variable frequency and
voltage for supplying the motor.5 rated speed), the
motor voltage too low guarantee reliable
commutation and pulse mode commutation
has employed. very
low speeds (0.
DC link reactor
The reactor the link circuit smoothes the
DC current and limits the rate current rise
in the link circuit the event fault.
A synchronous motor for this kind of
operation equipped with two sets of
windings electrically displaced 30°.
It provides the field current over the entire
speed range and standstill.
Voltage, current and speed are
regulated closed-loop control.
Control system
The main purpose the control system to
generate the firing impulses for the thyristors
in the rectifier and inverter the right instants
to maintain the desired voltage and frequency
across the motor..
Although the transformer and the motor are
designed for operation conjunction with a
converter, they not differ essentially from
normal versions. The design
of the inverter basically the same that of
the rectifier with the exception that the inverter
phase currents are commutated the motor
voltage.
The principal applied that four-
quadrant drive which permits driving
and braking both directions.
Similarly, the supply transformer also
has two secondary windings shifted
by 30°.. inverter this type referred to
as load-commutated inverter (LCI).
Rectifier
The rectifier the controlled line-commutated
type similar those used for drives
