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Since phase lag is such a major problem at higher speeds, many
drives attempt to solve the phase lag problem by introducing a
“phase advance” factor into their voltage vector calculation. This
advances the three rotating voltage vectors by an amount equal to
the expected current phase lag for a given speed.
In a perfect world, this voltage vector phase advance would
accurately compensate for current phase lag. In the real world,
however, factors such as motor winding resistance and inductance
tolerances come into play. Also, variations in motor back EMF, cable
resistance and winding resistance changes due to temperature all
add variability to the actual current vector produced in any given
motor at a given speed. Consequently, any phase advance factor is
only a “best-guess” value. Due to these tolerance issues, there will
still be some phase current error in any real motor drive
combination. This will result in some loss of torque at higher
speeds, additional motor heating and torque variability with current
(ripple).
Some of the most modern sine commutation drives attempt to solve
these problems by taking individual current phase readings and
running multiple current loops. These drives attain a higher level of
performance than traditional sine drives, but still do not provide the
full closed loop control of TORQUE PRODUCING current possible
with state-of-the-art vector drives which thereby provide higher
efficiency and high bandwidth torque response over the full speed
range.
Vector Drives — Field-Oriented Control
The term Vector Drive refers to a class of drives that sense motor
current in each individual motor phase and resolves these readings
into two current vectors. One vector is the torque producing current
(Iq). The other is the waste current (Id). The current control
algorithm then works to drive the non-torque producing component
(Id) to zero. This method gives high bandwidth torque response over
the full speed range without the phase lag and tolerance issues that
plague older drive technologies.
The technology in these drives is made possible due to the
availability of high speed DSP technology. Because the control
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