D R I V E S &

S W I T C H G E A R

What direct torque control (DTC) is, why and how it has evolved, the basic
theory behind its success and the features and benefits of this new technology.

Direct torque control in

AC drive technology
Variable speed drives (VSDs) control Information from ABB
the flow of energy from the mains to
the process. Energy is supplied to the
process through the motor shaft. Two
variables describe the state of the shaft: without the need for sophisticated created by the current through the field
torque and speed. We must control electronics. However, the evolution of winding in the stator. This field is always
these quantities to control the flow of AC VSD technology was aimed partly at right angles to the field created by the
energy. In practice, either one of them at combining advantages of the DC armature winding. This is known as field
is controlled and we speak of “torque motor such as its fast torque response orientation and is needed to generate
control” or “speed control”. Speed and and speed accuracy with the ruggedness maximum torque. The commutator-
torque are determined by load in both and economy of the maintenance-free brush assembly ensures this condition
torque and speed control mode. AC motor. is maintained regardless of the rotor
DC motors were initially used as VSDs position.
DC motor drives Once field orientation is achieved, the
as they could achieve the required
speed and torque with ease and In a DC motor, the magnetic field is DC motor’s torque is controlled easily

Fig. 2: Control loop of an AC drive with

Fig. 1: Control loop of a DC motor drive. requency control using PWM.

Fig. 3: Control loop of an AC drive with flux

vector control using PWM. Fig. 4: Control loop of an AC drive using DTC.

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 Drive Control variables Both voltage and frequency references
are fed into a modulator, which simulates
Armature current, IA an AC sine wave and feeds this to the
DC drives
Magnetising cuttent, IM
motor’s stator windings. This technique
AC drives (PWM)
Output volatge, U is called pulse width modulation (PWM)
Output frequency, f and uses a diode rectifier towards the
Motor torque, T mains (see Fig. 2). The intermediate DC
Direct torque control voltage is kept constant.
Motor magnetsing flux, 
The inverter controls the motor in the
Table 1: Comparison of control variables. form of a PWM pulse train, dictating both
voltage and frequency. Of signifisance is
that this method does not use a feedback
by varying the armature current and with a commutator-brush assembly.
device which takes speed or position
by keeping the magnetising current There is, therefore, no need for
complex electronic control circuitry measurements from the motor’s shaft
constant. The advantage of DC drives
which would increase the cost of the and feeds these back into the control
is that speed and torque, the two main
motor controller. loop.
concerns to the end-user, are controlled
directly through armature current: torque Such an arrangement, without a
Disadvantages feedback device, is called an “open-
is the inner control loop and speed is the
outer control loop (see Fig. 1). The disadvantages of the DC motor are loop drive”.
reduced motor reliability, the need for This controlling principle offers a low-
Advantages regular maintenance, its high purchase cost, simple solution to controlling AC
DC motor drives provide accurate and price and the fact that it needs an induction motors because there is no
fast torque control, high dynamic speed encoder for feedback. feedback device. This type of drive is
response and are simple to control. While a DC drive produces an easily- suitable for applications such as pumps
They were initially used for variable controlled torque from zero to base and fans which do not require high levels
speed control because they could easily speed and beyond, the motor ’s of accuracy or precision.
and accurately achieve good torque mechanics are more complex and In terms of disadvantages, field
and speed response. DC motors can require regular maintenance. orientation of the motor is not used
produce a torque that is: AC motor drives with this technique, also known as scalar
 D i r e c t : t h e m o t o r t o r q u e i s
control. Instead, frequency and voltage
These machines are small and robust, are the main control variables and are
proportional to the armature of simple design, compact, require low
current: the torque can therefore be applied to the stator windings. The status
maintenance and are not expensive to of the rotor is ignored, meaning that no
controlled directly and accurately. buy. AC variable speed drive technology speed or position signal is fed back.
 Rapid: torque control is fast; the was developed to combine advantages
drive system can have a very high of the DC drive (such as fast torque Therefore, torque can be controlled with
dynamic speed response. Torque can response and speed accuracy) with any degree of accuracy. The technique
be changed instantaneously if the also uses a modulator which basically
those of the standard AC motor.
motor is fed from an ideal current slows down communication between the
source. A voltage-fed drive still has Frequency control incoming voltage and frequency signals,
a fast response as this is determined using PWM causing the motor to respond to this
by the rotor’s electrical time constant The AC drive frequency control technique changing signal. The modulator slows
only (i.e. the total inductance and uses as controlling variables parameters down communication between incoming
resistance in the armature circuit). generated outside the motor, namely voltage and frequency signals
 Simple: field orientation is achieved voltage and frequency. AC drives: flux vector control
using PWM
Evolution of direct torque control
The flux-vector drive must know the
spatial angular position of the rotor
flux inside the AC induction motor
to emulate the magnetic operating
conditions of a DC motor, i.e. to perform
the field orientation process.
With flux vector PWM drives, field
orientation is achieved by electronic
means rather than with the mechanical
commutator-brush assembly of the DC
motor.
Information on the rotor status is
obtained by feeding back rotor speed
and angular position relative to the
stator field by means of a pulse encoder.
A drive which uses speed encoders is
Fig. 5: DTC comprises two key blocks: speed ontrol and torque control. known as a "closed-loop drive".

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The motors' electrical characteristics are also modelled
mathematically by means of microprocessors. The electronic
controller of a flux-vector drive creates electrical quantities
such as voltage, current and frequency which are the
controlling variables. It feeds these to the AC induction motor
through a modulator. Torque, therefore, is controlled indirectly.
Advantages include good torque response, accurate speed
control and full torque at zero speed.
Feedback and modulators are, however, needed and they are
costly to buy. A feedback device is required to achieve high
torque response and speed accuracy. This can also be costly
and adds complexity to the traditional, simple AC induction
motor. The modulator slows down communication between
incoming voltage and frequency signals, resulting in the motor
responding to this changing signal. The motor is mechanically
simple but the drive is electrically complex.
AC drives: direct torque control
Controlling variables
DTC technology developed by ABB achieves field orientation
without feedback, using advanced motor theory to calculate
the motor torque directly, without using modulation. The
controlling variables are motor magnetising flux and motor
torque.
With DTC, there is no modulator and no requirement for
a tachometer or position encoder to feed back the speed
or position of the motor shaft. DTC uses the fastest digital
signal processing hardware available and a more advanced
mathematical "understanding" of how motors work.
The result is a drive with a torque response typically ten times
faster than that of any AC or DC drive. The dynamic speed
accuracy of DTC drives will be eight times higher than those
of open loop AC drives and will be comparable to DC drives
using feedback.
DTC produces the first “universal” drive with the capability to
perform either as AC or DC drives.
VSDs: a comparison
When noting the differences between the control blocks (see
Figs. 1 – 4), the first observation is the similarity between
the control block of the DC drive (Fig. 1) and that of DTC
(Fig. 4). Both use motor parameters to control torque directly,
but DTC has added benefits. These are the fact that no
feedback device or external excitation is required, as well as
all the benefits of AC motors.
As can be seen from Table 1, both DC and DTC drives use
actual motor parameters to control torque and speed. With
DTC, no tachometer or encoder is needed to feed back speed
or position signals.
DTC: basic control theory
Fig. 5 shows the complete block diagram for direct torque
control. The block diagram shows that DTC has two
fundamental sections: the torque control loop and the speed
control loop.
Torque control loop
 Step 1: two motor phase currents and the DC bus voltage
are simply measured in normal operation, together with
the inverter’s switch positions (see Fig. 6).
 Step 2: the measured information from the motor is
fed to the adaptive motor model. The sophistication of
this motor model allows precise data about the motor
to be calculated. The motor model is fed information

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  Step 4: Both the latest 40 MHz
digital signal processor (DSP) and
ASIC hardware to determine the
switching logic of the inverter are
within the optimum pulse selector.
All control signals are transmitted
via optical links for high-speed data
transmission. This configuration brings
immense processing speed and the
inverter’s semiconductor switching
devices are supplied with optimum
pulses for reaching or maintaining
accurate motor torque every 25 ms.
The correct switch combination is
determined at every control cycle
and there is no predetermined
switching pattern. Unlike traditional
PWM drives where up to 30% of
switch changes are unnecessary,
each switching is needed and
used with DTC. This high-speed
Fig. 6: Torque control loop. switching is fundamental to the
success of DTC. The main motor
control parameters are updated
40 000 times per second, allowing
for extremely rapid response on the
shaft. This also allows the motor
model (see step 2) to update the
information. The high performance
figures including a static speed
control accuracy of some 0,5%
(without encoder) and the torque
response of less than 2 ms are thanks
to this processing speed.
Speed control
 Step 5: within the torque reference
controller, the speed control output
is limited by the torque limits and
DC bus voltage (see Fig 7). It
also includes speed control for
when external torque signal is
used. The internal torque reference
from this block is fed to the torque
comparator.
Fig. 7: Speed control loop.
 Step 6: The speed controller block
consists of a PID controller and
about the motor collected during most industrial applications. This is an acceleration compensator. The
a motor identification run before a significant advance over other AC external speed reference signal
it operates the DTC drive. This is drive technology. The motor model is compared to the actual speed
called auto tuning and data such as is, in fact, key to DTC’s unrivalled produced in the motor model. The
stator resistance, mutual inductance low speed performance. It outputs error signal is then fed to the PID
and saturation coefficients are control signals which represent controller and the acceleration
determined along with the motor’s actual motor torque and stator flux. compensator and rthe output is the
inertia. The identification of motor The motor model also calculates sum of both their outputs.
model parameters can be done shaft speed.  Step 7: An absolute stator flux value

without a rotating motor shaft. This  Step 3: The information needed to can be given from the flux reference
also makes it easy to apply DTC control power switches is produced controller to the flux comparator
technology in retrofits. The extremely in the torque and flux comparators. block. The ability to control and
fine tuning of the motor model is Both actual torque and actual flux are modify this absolute value provides
achieved when the identification fed to the comparators where they an easy way to realise many inverter
run also includes running the motor are compared to a torque and flux functions such as flux optimisation
shaft for some seconds. There is no reference value every 25 ms. Torque and flux braking.
need to feed back any shaft speed and flux status signals are calculated Contact Mark Sheldon,
or position with tachometers or by means of a level hysteresis control ABB South Africa,
encoders if the static speed accuracy method. These signals are then fed to Tel 010 202-5868,
requirement is over 5%, as it is for the optimum pulse selector. mark.sheldon@za.abb.com

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