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A standard VFD (lets call it
a Scalar Drive) puts out a PWM pattern designed to maintain a constant V/Hz
pattern to the motor under ideal conditions. How the motor reacts to that PWM
pattern is very dependent upon the load conditions. The Scalar drive knows
nothing about that, it only tells the motor what to do. If for example it
provides 43Hz to the motor, and the motor spins at a speed equivalent to 40Hz,
the Scalar Drive doesn’t know. You can’t do true torque control with a scalar
drive because it has no way of knowing what the motor output torque is (beyond
an educated guess).
These problems associated with the scalar VFDs inability to alter it’s output
with changes in the load gets worse as the speed reference goes down, so the
"rule of thumb" in determining the need for which technology to use
is that scalar drives work OK at speed ranges between 5:1 (50Hz applications)
or 6:1 (60Hz applications). So if your application will need accurate control
below 10Hz, scalar may not work for you.
A Vector Drive uses feedback of various real world information (more on that
later) to further modify the PWM pattern to maintain more precise control of
the desired operating parameter, be it speed or torque. Using a more powerful
and faster microprocessor, it uses the feedback information to calculate the
exact vector of voltage and frequency to attain the goal. In a true closed loop
fashion, it goes on to constantly update that vector to maintain it. It tells
the motor what to do, then checks to see if it did it, then changes its command
to correct for any error. Vector drives come in 2 types, Open Loop and Closed
Loop, based upon the way they get their feedback information.
A true Closed Loop Vector Drive uses a shaft encoder on the motor to give
positive shaft position indication back to the microprocessor (mP). So when the
mP says move x radians, the encoder says “it only moved x-2 radians”.
The mP then alters the PWM signature on the fly to make up for the error. For
torque control, the feedback allows the mP to adjust the pattern so that a
constant level of torque can be maintained regardless of speed, i.e. a winder
application where diameters are constantly changing. If the shaft moves one way
or the other too much, the torque requirement is wrong and the error is
corrected. A true closed Loop Vector Drive can also make an AC motor develop
continuous full torque at zero speed, something that previously only DC drives
were capable of. That makes them suitable for crane and hoist applications
where the motor must produce full torque before the brake is released or else
the load begins dropping and it can’t be stopped. Closed Loop is also so close
to being a servo drive that some people use them as such. The shaft encoder can
be used to provide precise travel feedback by counting pulses. (Note: See
Addendum below for additional information)
Open Loop is actually a misnomer because it is actually a closed loop system,
but the feedback loop comes from within the VFD itself instead of an external
encoder. For this reason there is a trend to refer to them as “Sensorless
Vector” drives. The mP creates a mathematical “model” of the
motor operating parameters and keeps it in memory. As the motor operates, the
mP monitors the output current (mainly), compares it to the model and
determines from experience what the different current effects mean in terms of
the motor performance. Then the mP executes the necessary error corrections
just as the closed Loop Vector Drive does. The only drawback is that as the
motor gets slower, the ability of the mP to detect the subtle changes in
magnetics becomes more difficult. At zero speed it is generally accepted that
an Open loop Vector Drive is not reliable enough to use on cranes and hoists.
For most other applications though it is just fine.
This is all done at very high speeds, that is why you did not see Vector Drives
as available earlier on. The cost of the high speed mP technology has now come
down to every day availability.
skogsgurra (Electrical) May 14, 2004
jraef is absolutely correct in his “crash course” on VFD. One might
add that the “vector” that pops up in the description and the name of
this drive technology is the rotating space vector that describes the flux in
the motor. Since flux and current are in phase, it also describes the current
in the stator.
An induction motor is very similar to a DC motor. It needs a magnetizing
current and a torque producing current. In a DC motor, these two currents are
fed to two different windings; the field winding and the armature winding. In
an induction motor, there is only one set of windings: the stator winding. So
the vector drive has to separate the two components some other way.
It does this by keeping in mind that magnetizing current always lags
(inductive) the voltage by 90 degrees and that the torque producing current is
always in phase with the voltage. It controls the magnetizing current (usually
named Id) in one control loop and the torque producing current (Iq) in another
control loop. The two vectors Id and Iq, which are always 90 degrees apart, are
then added (vector sum) and sent to the modulator, which turns the vector
information into a rotating PWM modulated three-phase system with the correct
frequency and voltage.
As soon as a deviation from correct speed or torque or magnetizing current is
detected by the control loops the corresponding variable will be changed by the
controller to correct the variable.
If - for example - the speed is wrong, the output frequency will be corrected
and also the voltage so that the correct magnetizing current is maintained. And
correspondingly, if the stator winding heats up the magnetizing current would
go down if the decrease wasn’t detected and corrected by the controller. The
action in this latter case is that the voltage goes up (PWM adjusted), but not
the frequency (the speed was already correct.
Vector drives are among the most complex standard equipments that exist. But
keeping in mind that there are always two control loops, one for magnetizing
current and one for speed/torque will help thinking about them.
Follow up: It might be worth noting that microprocessor technology is rapidly
making scalar drives obsolete except in the smallest of sizes. The low cost for
processing power has made the issue of having and maintaining separate designs
untenable for many manufacturers.
Addendum regarding an additional important issue on the importance of encoder
feedback with Closed loop Vector drives by member DickDV on 4/18/08:
When talking about speed control and error in a drive/motor system, it is
important to understand that there are several different aspects to the issue.
First, speed error is generally due to changes in torque demand. In
an induction motor, this error is mostly slip. So the question
becomes, how well does the drive compensate for torque induced slip speed
changes. With a good vector drive, this can get down in the range of
one-tenth of motor slip without an encoder. If you need better than
that, an encoder is required. Note here that the error is a result
of torque changes. If your torque doesn’t change, you won’t have
much speed error to start with.
Second, in some applications, especially those involving web products and
tension control, cumulative error is just as important as actual
error. For example, even if you are very accurate with actual error,
if it is all negative or all positive, eventually you are going to have too
much or too little tension. No encoderless system will assure
non-cumulative error. For that you need an encoder.
Third, speed reference error is often overlooked. That is error
either in the speed signal going into the drive or error in the drive
translating the input command into an actual output speed. Usually,
the majority of this error is due to the analog input terminal analog-to-digital
conversion. A 10 bit resolution A/D input will not be nearly as
accurate as a 14 bit resolution input. This is a matter of
purchasing a drive with the input resolution adequate for the intended purpose.
Thanks to Skogsgura and DickDV for their contributions to this FAQ. I am
posting a follow-up version that adds information regarding comparing this to