White Paper

DTC
A motor control
technique forall seasons

DTC
A motor control technique for all seasons

Variable-speed drives (VSDs) have enabled unprecedented
performance in electric motors and delivered dramaticenergy savings by matching motor speed and torque toactual requirements of the driven load. Most VSDs in
the market rely on a modulator stage that conditions voltage
and frequency inputs to the motor, but causes inherent
time delay in processing control signals. In contrast,
the premium VSDs from ABB employ direct torque control(DTC) — an innovative technology originated by ABB —greatly increasing motor torque response. In addition,
DTC provides further benefits and has grown into a larger
technology brand which includes drive hardware, control
software, and numerous system-level features.
Electric motors are often at the spearhead of modern
production systems, whether in metal processing lines,
robotic machining cells, or building and office automation
systems. The motors we see today have certainly benefitted
from advances in electric materials, manufacturing
efficiencies, and analytical tools. However, their designprinciples have remained the same for over 100 years in
the case of the workhorse asynchronous (or induction)
alternating current (AC) motor. Rather, the remarkable
performance of these motors in today’s applications
comes from modern electronic controls — variable-speed
drives (VSDs) — and accurate motor models whose
sophisticated control algorithms can be rapidly executed byhigh-performance digital signal processors. Moreover, thedevelopment of VSDs has enabled the use of new
AC motor technologies such as permanent magnet
synchronous motors and synchronous reluctance motors.

motor torque by using armature current. Over time,
AC drive designs have evolved offering improved dynamic
performance. (One recent, noteworthy discussion of various
available AC drive control methods appears in Ref. 1.)
Most high-performance VSDs in 1980s relied on pulse-widthmodulation (PWM). However, one consequence of usinga modulator stage is the delay and a need to filter
the measured currents when executing motor control
commands — hence slowing down motor torque response.
In contrast, ABB took a different approach to highperformance AC motor control. AC drives from ABB
intended for demanding applications use an innovativetechnology called direct torque control (DTC). The method
directly controls motor torque instead of trying to control
the currents analogously to DC drives. This means better
accuracy in matching the load requirements of the driven
system. Originated by one of the founding companies of
ABB and patented in the mid-1980s, DTC further eliminates
the need for an extra modulator stage and thus achievescontrol dynamics that are close to the theoretical maximum.
ABB introduced its first AC industrial drive with direct torquecontrol to the market in 1995 (Ref. 2).
In principle, DTC was already a leading technology backin 1995, but subsequent developments in processor
computational power, communication interfaces, application
programming, etc., have enabled higher performance,
providing premium motor control for a broad range ofapplications.

Initially, direct current (DC) motors drew the attention of drive
developers. With an even longer history than their
AC cousins, DC motors offered inherently simple speed andtorque control. However, higher motor cost, more complex
construction with a mechanical commutator, and brush
maintenance issues were some tradeoffs associated withDC motors.
AC induction motors offered simpler, rugged construction,
lower cost, and posed fewer maintenance concerns —characteristics that have led to their wide usage witha huge installed base worldwide. On the other hand, control
of induction motors proved to be complex. Accurate speed
control, and particularly torque control, remained elusive
for early AC drives. Naturally the goal of the early designers
was to emulate in AC drives the DC drive’s simple control of

ABB White Paper | DTC 3

Figure 1: DTC’s operation principle.

DTC’s core is the torque control loop, where a sophisticatedadaptive motor model applies advanced mathematicalalgorithms to predict motor status. Primary controlled
variables—stator flux and motor torque—are accurately
estimated by the motor model using inputs of motor phasecurrents and DC bus voltage measurements, plus the statesof the power-switching transistors in the drive.
The motor model also calculates shaft speed. Temperaturecompensation helps to enhance calculation accuracy
without an encoder.
Why use DTC?
Superior torque response is just one feature of DTC.
The technology offers further customer benefits, including:- No need for motor speed or position feedback in 95% 		
		 of applications. Thus, installation of costly encoders
		 or other feedback devices can be avoided.
- DTC control is available for different types of motors,		 including permanent magnet and synchronous		 reluctance motors.
- Accurate torque and speed control down to low speeds, 				 as well as full startup torque down to zero speed.
- Excellent torque linearity.
- High static and dynamic speed accuracy.
- No preset switching frequency. Optimal transistor 				 switching is determined for every control cycle, allowing
		 the drive to more readily match driven load requirements.

4 DTC | ABB White Paper

In a larger view, benefits of DTC extend to software, userinterfaces, maintenance, and system-level features.
As the name suggests, DTC seeks to control motor flux andtorque directly, instead of trying to control these variables
indirectly like DC drives and vector-controlled AC drives do.
Separate torque and speed control loops make up the full
DTC system but work together in an integrated way(see Fig. 1, DTC block diagram).
Additional motor parameters are automatically fed tothe adaptive model during a motor identification run when
the drive is commissioned. In many cases, appropriate
model parameter identification can be done without
rotating the motor shaft. For fine tuning of the motor
model, which is only needed for few high-demand
applications, the motor has to be run, but then only fora short time and without load.
Stator resistance (voltage drop) is the only and easily
measurable parameter needed for estimating the motor’smagnetic flux. Motor torque can then be calculated as
the cross product of estimated stator flux and statorcurrent vectors. While stator resistance is the main sourceof estimation error, its influence diminishes with increasing
motor speed and voltage. Thus DTC has excellent torqueaccuracy in a wide speed range. Moreover, DTC
includes advanced ways to minimize estimation error
at low motor speeds.

Output signals from the motor model — which represent
actual stator flux and motor torque — go to a flux
comparator and torque comparator, respectively (Fig.1).
These separate control units compare their inputs toa flux and torque reference value. Already in mid 1990s
the first DTC controlled drives performed these functions
every 25 microseconds (µs) using a high-power digital
signal processor (DSP). In the latest control generation
the interval is reduced down to 12.5 µs, thus further
enhancing control performance. Each comparator seeks
to hold its respective flux or torque vector magnitude
within a narrow hysteresis band around a reference value.
DTC’s fast torque response without overshoot comes,
in part, from ability to minimize these vector fluctuations.
Exceptional motor response is also due to the
DSP control algorithms updating the adaptive motor
model at the same high cycle rate.
Flux and torque errors — differences between estimatedand reference values — and the angular position(or sector) of the stator flux vector are used to calculate
flux and torque status in the hysteresis controllers.
Then, these status values become inputs to the optimum
pulse selector, where the optimum voltage vector isselected from the look-up table (Fig. 1). In this way,
the most appropriate signal pulses for each control cycle
can be sent to power switches in the inverter to obtainor maintain precise motor torque.
A form of programmable logic — so-called fieldprogrammable gate array (FPGA) — assists the DSP withdetermining inverter switching logic and other tasks.
The FPGA allows for control modifications or drive design
updates versus an application-specific integrated circuit(ASIC) which, if used, requires locking-in the design.
The speed-control loop, which comprises the rest of
DTC’s functional blocks, is described in Appendix 1.

Performance indicators
DTC provides customers superior performance features
over competing drive methods. Being a “sensorless”(speed estimation instead of measurement) control
method from its foundations, costly motor speed or
position feedback devices are not needed in most cases.
Depending on motor size, static speed accuracy as
low as ±0.1% is typically obtained. For higher demand
applications, a DTC drive equipped with a standard
encoder (1,024 pulses/rev) typically achieves ±0.01%
speed accuracy.
Dynamic speed accuracy (time integral of speed deviation
under a 100% load impact) is 0.3–0.4 %sec with typicalequipment driven by the motor. Using an encoder, speed
accuracy typically improves to 0.1 %sec and matches
servo drive accuracy.

Torque response time to a 100% torque reference step
is typically 1–5 milliseconds (ms), which approaches
the motor’s physical limit. Torque repeatability under
the same reference command is typically as low as 1%of nominal torque across the drive’s speed range.
As for control at very low motor speeds, DTC provides
100% torque down to zero speed—without (or with)speed feedback, as well as a position control featurewhen using an encoder. The foregoing performancevalues refer specifically to induction motor control.

Beyond induction motors
DTC was originally developed for AC induction motors
because of their popularity in myriad industrial andcommercial applications. No doubt the “workhorse role”
of induction motor technology will prevail overthe foreseeable future. However, in the quest for higher
power density and evolving international efficiency
regulations other motor topologies are drawing renewedinterest.
For example, standard IEC 60034 part 30 (Ref. 3) definesinternational efficiency (IE) classes, the highest of which —IE4 (super-premium efficiency) — is becoming difficult tomeet for induction motors. An even higher, IE5 class has
been proposed, although without further specification,in the latest approved 2nd edition of IEC 60034-30.
The good news is that DTC is equally applicable toother motor types, such as permanent magnet (PM)synchronous and synchronous reluctance (SynRM)
motors. The main difference occurs during motor starting.
Unlike induction motors, PM synchronous motors and
SynRM motors require the control system to estimate
rotor position at startup from the location of polesin the rotor, if no position sensor is used.
In these motors, absence of rotor windings and the slipspeed effect inherent to induction motors substantiallyreduce losses. Thus resulting efficiency gains areachieved. Moreover, synchronous operation means thatexcellent speed accuracy is achieved even without
a speed or position sensor. Thus, a sensor can beomitted in most cases except in applications such as
winches and hoists that require non-zero torqueat standstill for long periods.
Permanent magnets are commonly mounted on
the rotor’s outer surface. However, a PM synchronous
motor variant, the internal PM (IPM) rotor design, embeds
the magnets within the rotor structure. An extra reluctancetorque component generated in IPM synchronous motors
makes them attractive for high-demand applications.

ABB White Paper | DTC 5

In addition, embedded magnets create very pronounced
rotor-pole saliency, which allows accurate speed estimationand enhances DTC’s basic sensorless operating mode.
Due to high torque to motor size ratio, a simpler system
drive train may be possible when using PM synchronous
motors. For example, a direct-driven low speed PM motor
can eliminate the gearbox in packaging machines.
Numerous applications for PM synchronous motors include
machine tools, marine propulsion, wind turbines (generators),
and cooling tower fans for electric power plants.
One partly economic drawback of PM synchronous motors
is their reliance on so-called rare-earth (RE) magnet materials
for best performance. Most used of the RE materials is
the compound neodymium-iron-boron. Recent pricing andglobal supply issues of RE materials have created serious

concern for equipment manufacturers that reaches wellbeyond electric motors (Ref. 4). Here is where synchronous
reluctance motors provide an alternative.
ABB has included a line of SynRM motor and drive packages
in its product offerings, in part, to anticipate possible
rare-earth magnet supply problems (Ref. 5). Synchronous
reluctance motors have a stator structure similar to induction
motors. However, the rotor consists of axially stacked steellaminations shaped to provide a cross section with fourpoles — alternate high permeable (iron) axes and lowpermeable (air) axes. Importantly, no magnets are neededin the rotor.
Typical SynRM motor applications include driving pumps
and fans, where there is a quadratic torque (and thus cubic
power) relationship with speed (see below).

Figure 2: The new synchronous reluctance motor utilizes new rotor
design and is optimized for VSD operation. The technology cuts
down rotor losses, improves reliability and enables either smaller

and lighter designs (high output SynRM motor-drive packages)
or extremely high efficiency (IE4 SynRM motor-drive packages).

Versions of DTC modified for PM synchronous and
SynRM motors have been implemented by ABB.
Important to customers is that ABB’s latest DTC drives
allow easy upgrading of an existing induction motor
application to run a PM synchronous or SynRM machine
and benefit from enhanced performance.

This can be visualized from so-called “affinity laws”
associated with pumps and fans that relate variables
such as flow volume, pump speed, pressure, power, etc.
For example, pump speed versus power has a cubic
relationship, meaning that when a process sequence allows
the pump to run at ½ of full speed, only 1/8 of full power
is required. Of course, reduced motor and drive efficiencies
at partial loads would lower the “system” efficiency but
overall less energy will be used.

In addition to high dynamic motor control, DTC drives —combined with any of the efficient motor technologies
mentioned above — offer great energy savings potential
for the large number of variable-speed pump and fan
applications.

6 DTC | ABB White Paper

Recent performance measurements

In mid-2012, ABB conducted a series of measurements
to ensure that the continuing improvements in DTC
technology are keeping ABB AC drives at the top of
performance. Significant results of the test measurements
are summarized here.
Torque stability near zero speed(ACS800 vs. ACS880 drives)
Graph 1 compares the torque control accuracy of
ABB’s ACS800 and new ACS880 industrial drives insensorless (open-loop) operating mode. In the tests,
the drives run a 15 kW, four-pole induction motor with
its rated torque reference and with the load machine
controlled to make slow speed reversals near zero speed.
(Note that 90 rpm is about 6% of the nominal speedof the motor).

150

100

20
Torque reference

80

15

60
Torque error of
the synchronous
reluctance motor

Torque error of
the induction motor

40

10

5

20
Actual andreference
torques (%)

0Time (s)
0

5

10

15

20

25

30

35

-20
Induction motor
torque-40

0 Torqueerror (%)
-5

-10

Synchronous
reluctancemotor torque

-60

40

-15

-80

-20

-100

Graph 2. For both motor types the torque deviation from reference isheld to only a few percent of rated torque by DTC, both in motoring
and braking mode. Maximum torque error is slightly less for thetested synchronous reluctance motor than for the induction motor.

Torque with ACS800

100
Torque with ACS880
50

Speed (rpm), 00Torque (%)

Time (s)
1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

-50
Speed
-100

-150

Servo-class dynamic performance
Graph 3 indicates measured speed and angular position of atested 1.5 Nm, 6,000 rpm permanent magnet synchronous
motor (with 0.57 kg cm2 rotor inertia) during fast speed
reversal from - 6,000 rpm to + 6,000 rpm in less than 25
milliseconds (ms). This is very close to the theoretical limit
you can achieve with the torque limit set to twice the rated
torque. Theoretical limit refers to the motor’s mechanical time
constant of 24 ms, which is the time needed to accelerate
the motor from zero to nominal speed using nominal torque.

Graph 1. Both drives have remarkable sensorless control ability
to operate long periods in the near zero speed range. However,the new ACS880 has less deviation from the torque reference andthus can provide better motor control performance than ACS800.

6000

360
300
240

Angular position
3000

180
120

Torque accuracy during ramping(induction motor vs. SynRM motor)
Graph 2 shows the ACS880 drive’s sensorless torque control
accuracy compared for two types of 15 kW motors tested(at 50% of rated speed) — a four-pole induction motor and asynchronous reluctance motor.

60
Speed
(rpm)

0

16

21

26

31

36 time (ms)0Angularposition(degrees)

-60
Speed
-3000

-120
-180
-240
-300

-6000

-360

Graph 3. Although not a servo drive, ACS880 drive with DTC canchange motor speed very fast and accurately both with closed loop
and sensorless motor control modes. One performance measure
is torque accuracy during extremely fast acceleration, as
determined by comparing measured acceleration time to themotor’s mechanical time constant. Acceleration times of 24.4 ms(100% torque) and 12.1 ms (200% torque) were measured in open
loop mode, compared to 24 ms and 12 ms, respectively, whichcorrespond to acceleration times for absolute torque accuracy.

ABB White Paper | DTC 7

Wider applications
Another aspect of the DTC story is its expansion beyond
applications for which the technology was created.
Demanding, high-dynamic applications were targeted
early on, because they could justify costly initial softwaredevelopments and available microprocessors. That scenario
has changed greatly. Control system software has beenamortized over the growing sales volume of AC drives
and economically justified to implement in drives for more
standard applications. High-performance DSPs also havebecome common and affordable.
Ability to respond rapidly to changes in process variables
such as pressure, tension, or position using exceptional
speed and torque control dynamics has made DTC attractive
to wider industrial and process applications.
DTC can provide protective functions to connected
machinery or the motor itself (see more in Appendix 2). Tight
torque control can optimize tuning of the speed controller todamp out torsional vibrations.
DTC has also been applied to reduce harmonic distortion
from the drive, hence improving power line quality. Lowfrequency harmonics can be mitigated in the line currents
by replacing the diode rectifier of an AC drive with a DTCcontrolled IGBT supply unit (ISU). The LCL filter of the ISU
removes high-frequency harmonics and provides additional
filtering for the grid. In many cases even voltage distortionin the grid may be reduced by using a drive with an ISU.
Moreover, with an ISU it is possible to feed the braking
energy back to the grid. Thus in applications that require
frequent deceleration energy cost savings can be achieved.

DTC today, tomorrow
Resting on firm theoretical foundations, direct torquecontrol has shown a continuum of hardware and software
improvements over its more than 25-year lifespan. A
DSP-based technology from the start, DTC has overcome
limitations of the early processors for speedy calculationof control algorithms. DSP limitations also restricted thedrive’s maximum switching frequency in the past, hence its
output frequency. DTC relies on rapid switching of the drive’s
transistors for optimal performance and timely updatingof motor model parameters. Powerful processors are now
readily available.
Today, DTC drives have higher output frequency, allowing
motors to run faster. This is an important feature forcertain applications, such as test benches and machine
tools. ABB drives running induction motors in an industrial
application typically provide 2-4 kHz switching frequencies
that maximize the efficiency, while ABB machinery drives

8 DTC | ABB White Paper

powering PM synchronous motors typically supply 5-8 kHz
switching to run the motors with best possible dynamics.
Software has been another key element behind the successof DTC. Improvements and updates include redesigned andoptimized code for the whole control system (from customer
interface to motor shaft) to further enhance drive responsetime and performance.
Motor models also receive regular updating. Control
algorithms are periodically analyzed and the resulting
improvements are thoroughly verified through laboratory
testing with different motors. This can include investigatingsome new features or control ideas with an existing or
modified motor; or looking at some special customer
application requirement.
After an improvement has been confirmed, it may beincorporated into the next software release as part of normal
design flow. Each software release is likely to introduce some
new features or better control performance. If the solution
to a customer problem has enough general appeal, it could
likewise become part of a later software version.
A more robust motor identification algorithm has also been
incorporated into DTC. Enabled by the drive’s more powerfulmicroprocessor, this software improves motor identification
at standstill. As mentioned earlier, the identification algorithm
automatically finds a driven motor’s correct properties for
optimal control tuning during drive commissioning — even if
name plate values are not known or prove inaccurate.
ABB has focused its respected drive engineering heritageand invested substantial resources on the development of
direct torque control.
Today, DTC remains a living technology, having built acontinuum of advances atop a solid foundation. As a result,
DTC has grown into a brand offering larger than “torquecontrol”—incorporating intelligent user interfaces, drive
maintenance and diagnostic features, and higher levelsoftware functions, among other features.
Looking ahead, ABB intends to follow the same path with
its enduring DTC technology. Customers of ABB drives
can be confident that the benefits of direct torque control
technology in which they invest today will continue for thelong term.
For help with any technical terms in this release,please go to: www.abb.com/glossaryReferences

1) Kazmierkowski, M.P., et al, “High-Performance Motor
Drives,” IEEE Industrial Electronics Magazine, Sept. 2011,
Vol. 5, No. 3 (p. 6-26).
2) “Direct Torque Control Comes to AC Drives,” 		
Control Engineering, March 1995, Vol. 42, No. 3 (p. 9).
3) Standard IEC 60034-30, Ed.2: Rotating electricalmachines - Part 30: Efficiency classes (IE-code), International
Electrotechnical Commission. www.iec.ch

4) “Rare-earth magnet supply and cost issues,” 		
Control Engineering, Aug. 2011. http://www.controleng.com/
index.php?id=483&cHash=081010&tx_ttnews[tt_		news]=55091
5) “Super premium efficiency synchronous motor and drive
package: Taking energy efficiency to a new level,” ABB Lowvoltage AC motors and drives brochure (2011).

ABB White Paper | DTC 9Appendix 1
The rest of DTC’s function blocks story

The main article summarized the workings of DTC’s torquecontrol loop. Here is a brief description of the associatedspeed-control loop. These two loops are integrated andfunction as a holistic unit. Separate descriptions are intended
only to simplify understanding of the block diagram. So
here’s the rest of the “walk around the block.”Three main elements comprise the speed-control loop: thespeed controller block itself and separate torque reference
and flux reference controllers. The speed controller includes
a PID (proportional-integral-derivative) controller and an
acceleration compensator. Input to the speed controller isthe error found when comparing an external speed reference
signal and the actual speed signal from the adaptive motor
model — part of the torque and flux control loop (see main
article). This error signal, calculated from the speed referencechange and the derivative term, goes to both the PID unit
and acceleration compensator. Their combined outputs
become the speed controller’s output.
That output is sent to the torque reference controller where
the speed control’s output is regulated by preset torquelimits and DC bus voltage. An external (or user’s) torquereference signal can also be used instead of the speed
control as an input for this block. Torque reference controller
output is the so-called “internal torque reference” that goesto the torque comparator block in the torque and flux control
loop.
Similarly, the flux reference controller provides an “internal
flux reference” to the flux comparator block (part of torqueand flux control loop). This signal is an absolute stator flux
value, which DTC can appropriately regulate and modify toobtain useful inverter functions. Examples include energy
optimization — which minimizes motor losses and cuts
motor noise — and flux braking, which by temporarily
increasing the motor losses allows faster motor braking
when no special braking resistor is used.

10 DTC | ABB White Paper

Appendix 2
Customer benefits of DTC

AC drives with direct torque control (DTC) offer various
features that benefit specific user applications. Customers
in industries such as papermaking, web materials
production, and extruder machines for film materials can
expect DTC’s rapid torque response and accurate torquecontrol to translate into more uniform product quality andhigher process output. Torque linearity becomes a further
advantage for constant tension winding of the rolls needed inthese applications.
Cost reduction for conveyors and transfer lines, as wellas packaging machines, is possible because in many
applications, there is no need for encoders or other motor
speed/position feedback devices. Besides initial cost,encoders require maintenance and accuracy checks overtime.
Moreover, for packaging machine manufacturers it may bepossible to eliminate a mechanical brake in a section of themachine due to torque control down to zero speed provided
by DTC — and an ability to hold 100% torque at zero speed.
Note, however, that a speed or position sensor is required if
a braking (generating) torque is required close to zero speed
for more than few seconds. A braking resistor or an IGBT
supply unit is also needed in the drive if high deceleration isrequired.
AC drives with direct torque control (DTC) offer various
features that benefit specific user applications. Customers
in industries such as papermaking, web materials
production, and extruder machines for film materials can
expect DTC’s rapid torque response and accurate torquecontrol to translate into more uniform product quality andhigher process output. Torque linearity becomes a further
advantage for constant tension winding of the rolls needed inthese applications.

Moreover, for packaging machine manufacturers it may bepossible to eliminate a mechanical brake in a section of themachine due to torque control down to zero speed provided
by DTC — and an ability to hold 100% torque at zero speed.
Note, however, that a speed or position sensor is required if
a braking (generating) torque is required close to zero speed
for more than few seconds. A braking resistor or an IGBT
supply unit is also needed in the drive if high deceleration isrequired.
Ability to closely monitor the state of the motor can benefit
still other customer applications. Minimizing overloads andshock loads becomes possible through timely detectionof connected system parameter changes and DTC’s fast
control response. The concept can be extended to drivensystem failure detection. For example, sudden torque loss
might indicate a conveyor belt breakage —or higher thannormal torque needed to produce some output may indicate
binding or abnormal wear in the machine — which requires
appropriate user response to prevent further damage.
As mentioned in the main article, drives can be used as apart of process diagnostics. This becomes beneficial tocustomers with process control applications since changes
in driven-system variables such as pressure, tension,or position can be related to motor torque and speed
characteristics. Altered motor characteristics can be an early
warning of unwanted process changes.

Cost reduction for conveyors and transfer lines, as wellas packaging machines, is possible because in many
applications, there is no need for encoders or other motor
speed/position feedback devices. Besides initial cost,encoders require maintenance and accuracy checks overtime.

ABB White Paper | DTC 11

Contact

Ere Jääskeläinen
Product Line Manager
ere.jaaskelainen@fi.abb.com
Pasi PohjalainenMarketing Manager
pasi.pohjalainen@fi.abb.com
ABB Finland
P.O. Box 184
FI-00381 Helsinki
Finland