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AC Drives Provide Precise Control Critical
to Next Generation Test Stands
Direct Torque Control (DTC) is widely used as a method
for controlling AC motors in many demanding applications.
It is a unique method for controlling AC motors. In
pulse-width modulation (PWM) drives, the output frequency
and voltage are the primary control reference signals
for the power switches, rather than the desired torque
in/of the motor shaft. For those who are not familiar
with inverter technology, the DTC principle can be illustrated
most accurately via this mechanical analogy: the continuous
calculation of the best angle at which to rotate a shaft,
with a given arm length and the forces available. These
electrical “force vectors” are generated
with the help of semiconductor switches called Integrated
Gate Bipolar Transistors (IGBT). Testing of rotating
machines like gears, engines, and complete cars is a
demanding task. High accuracy and dynamic load control
— that is, control of torque — is needed
both for day-inday- out testing, but excels, specifically,
on those tests with new complex electronic functions,
such as ABS, EPS — or electromechanical innovations
like dual — clutch transmissions being introduced
in the current generation of automobiles. AC motors
drive these test rigs. When manufacturing test rigs
for engines, transmission, or chassis dynamometers with
high-performance requirements, careful consideration
must be given to the AC machines and drives used in
such applications (control of speed and torque are paramount).
The way the AC motor is controlled by the drive has
a primary effect on these considerations.
Testing Processes
Chassis dynamometers are typically used to test the
performance of vehicle, exhaust emission, fuel consumption,
noise, and fine-tuning of exhaust, catalyst, and motor
fuel-injection system. It is well known that dynamometers
with AC motor technology offer the best platform to
realize high accuracy, dynamics, and energy savings.
Dynamometers should simulate the real highway precisely.
This requires that, during acceleration and deceleration,
the roll inertia be compensated dynamically to match
the mass of the tested vehicle and the real road. To
be able to realize such high-dynamic online compensation
(real-world, real use), the load torque of the roll
motor must be controlled accurately and with extreme
precision at every speed point. Testing gear-shifting-and-synchronization,
calibration of automatic transmissions, clutching, and
durability — these are typical testing needs.
Inherently, these cases require a capability to change
load torque very quickly. And, transmission test-stand
configuration can include several motors — one
simulating the engine, and two or even more for the
simulation of the load. This requires mutual coordination
of drives operation; the faster and more accurate it
is, the better it simulates real-world conditions like
differential-gear operation. Drives with DTC technology
can transfer speed/ torque information via ultrafast
optical links to each other. The speed torque signals
can be used as reference to follower drives, to assess/
react to desired load share (or as additional inputs
to the main drive speed/ torque reference). Additional
complex functions to calculate speed/torque references
to individual drives can be achieved within the DTC
drive. This is useful in setting up and delivering testing
in time-critical operations. The torque is calculated
as a cross product between the stator flux and the stator
current: T – = ?— s × i - s . The
stator flux is estimated from the stator voltage vector
and the stator current: (2) ?— s = ?(U —
s – Rs i - s ) dt . Six voltage vectors and two
zero-vectors control the stator flux and the torque.
The stator-flux amplitude is controlled to be constant.
For engine dynamometers, the dynamic performance is
the key issue to ensure that you can simulate real systems
dynamically and accurately. DTC drive technology answers
this challenge directly. The test system’s overall
dynamic performance can be quantified by looking at
the delay from reference change to change in AC motor
torque. The dynamics of the electrical system are defined
by several fundamental elements: the electrical and
mechanical characteristic of the AC machine (leakage
inductance and inertia); the torque-control cycle of
the AC drive; and any delay from speed/torque reference
via any drive interface to the controlcycle loop itself.
DTC controls motor torque every 25 µs (microseconds).
Once the torque reference is changed, DTC automatically
selects the best voltage vector to achieve the desired
torque, and checks every 25 µs if this vector
is still the best one to maximize torque rise, or whether
another vector should be used. The actual rise then
is solely dependent on motor characteristics, such as
leakage inductance. The torque rise time with a standard
motor is between 1-2 milliseconds (ms), measured from
current rise. For high-speed dynos, the leakage inductances
are typically smaller, thus requiring shorter current-
and torque-rise times. The repeatability of testing
is always important, but it is of particular importance
when testing engines and complete cars for emissions.
DTC has torque repeatability of less than ±0.5%
of nominal torque. This means that the load applied
will be the same time after time, and the measurement
results from a test cell are comparable with each other.
The benefits are reduced testing requirements, improved
test data quality, i.e., reduced overall testing time.
Load Torque Accuracy and Linearity
In the DTC method, the status of the motor is evaluated
every 25 µs by using measured signals and an advanced
motor model. The AC current fed to the motor is measured
every 5 µs, which means the torque calculated
by formula is exact and up to date. DTC incorporates
methods to identify motor design characteristics for
each specific motor. These characteristics are the values
for leakage and magnetizing inductances, stator resistances,
and their saturation behavior. Thus, the motor model
guarantees that the dynamics, accuracy, and repeatability
are optimized, although the operational point might
vary significantly (according to testing needs). DTC
has a torque accuracy of ±1.5% of nominal torque
or better when encoder feedback is used, which is typical
in these applications. Operating at low speed and torque,
DTC allows the flux to be reduced, thus giving higher
resolution at fractional torque levels. The speed accuracy
is 0.01%. In DTC, all IGBT switch changes are based
directly to the electromagnetic state of the motor.
Optimal switching is determined for every control cycle
at 25 µs intervals. That is also the main difference
between Direct Torque Control and the traditional AC
drive control methods. In DTC, there is no separate
voltage and frequency controlled PWM modulator. Via
DTC technology, the torque and flux references are compared
to actual values in hysteresis (digital switching between
a defined range) controllers every 25 µs. The
width of the hysteresis is very small and is controlled
by the desired average switching frequency. This method
generates no specific harmonic spectrum content with
high amplitudes at a certain frequency but, rather,
provides an evenly distributed power spectrum. Linearity
of torque means that, with a certain torque reference,
the actual shaft torque must remain the same, regardless
of the drive speed and torque (motoring or generating
torque). For DTC, the non-linearity is ±1% of
motor nominal torque, when encoder feedback is used.
Energy Savings and Safety
Regeneration also is an inherent part of test stands
(energy generated from the motor, when the momentum
of a load continues to turn the motor shaft). A common
DC bus construction/ installation facilitates connection
of several inverters to the same DC bus — so that
another inverter also running the equipment under the
test can directly use the power regenerated by the regenerating
inverter. Fully regenerative inverters also can feed
the energy back to the supplying AC network (grid or
self-standing generating equipment). This means significant
savings in energy cost. When high speeds and high masses
are involved, the safety of a complete test cell is
an essential design aspect. A drive equipped with DTC
ensures access to and operation of several built-in
safety functions: over speed limits in torque control
mode; limitation of torque; prevention of unexpected
start; emergency stop; et al. For example, if the speed
feedback signal from the encoder is lost, the DTC drive
is able to recognize this within a few ms and automatically
switches over to motormodel estimated speed —
and gives either an alarm or fault signal to the operator.
Most importantly, when this happens, motor acceleration
to overspeed is prevented.
More Information
For more information, contact Ken Graber, marketing
communications manager, ABB Inc., Low Voltage Drives,
at ken.j.graber@ us.abb.com or visit http://info.ims.ca/5786-322.
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