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Smooth Merge to Six Step
All motor drives using the three-phase voltage source inverter need to
operate in six step if they are able to process maximum power and fully utilize
the available power electronic system capability.
The minimum on/off gate pulse times that any power system must
accommodate
for reliable power circuit operation can produce undesired current transients at
the border of modulation merging in and out of six step.
Many drives can encounter difficulties as the PWM modulator merges from
quasi-six-step operation to full six-step operation (square wave 120 degree
displaced gate drives) because the pulses become smaller and smaller until they
must be dropped completely. This abrupt transition is the basic problem.
CogniSim does not suffer from any of these kinds of limitations
because the Cognitive engine includes the effects of power switch lockup at all
times. For this reason, the merge to six step is always smooth and controlled.
This attribute allows CogniSim control to be applied seamlessly to
all motor drives from the smallest (fractional horsepower) to the
largest (40,000 HP). This is in stark contrast to traditional systems that
generally require completely different control
strategies, pulse width modulators (PWM), circuitry and software in order to be practical.
Emerging high speed aerospace applications require motor control at very
high fundamental frequencies (e.g. 400-1000 Hz or higher). However, most power
circuits have practical switching frequency limits in the range of 20KHz-50KHz
because of EMI considerations and power device limitations.
Since aerospace applications are limited to using the same
power circuits as industrial motor controls, the control challenge
is significantly greater by comparison. For example, a 1000Hz motor
drive has 20 opportunities to switch per cycle if the average switching
frequency is targeted at 20KHz. (i.e. 20:1 ratio). A comparable 60 Hz
industrial drive, has 333 opportunities to switch per cycle at the same
average switching frequency (i.e. 333:1 ratio).
Therefore, the challenge of controlling a motor to similar
performance levels is about 15 times (333/20) more difficult in
aerospace than in industry.
Hence, the merge to six-step in aerospace where pulse dropping
occurs is much more difficult to manage. It is so difficult to manage
that, in many cases, operation into six step is avoided, leading to
power circuit designs that are larger and heavier than they need to be
in order to deliver the necessary control performance reliability.
CogniSim smoothly manages the transition because it considers the
pulse dropping issues in real time as part of the operation of the
cognitive predictive engine, and maintains high performance control in
the process. This offers weight savings in aerospace because the power
circuit can be driven reliably in and out of six step where the maximum
output voltage of the inverter is attained.
In general, the ability to make every switching
event contribute to the control objective, while accounting for the
power circuit limitations, is extremely important in aerospace.
Shown below is a video of high speed operation and rotating
restart of a CogniSim controlled induction motor that demonstrates
clearly, the performance of both high speed operation in field weakened
mode (approx 2:1 field weakened, machine shutoff for safety reasons)
rotating restart and smooth merge to six step operation.
MOVE MOUSE OVER IMAGE TO START VIDEO
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