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Directly Implemented Flux / Torque Control

Induction Motor Vector Control Without an Encoder

James N. Nash

Principal Engineer

ABB Industrial Systems Inc.

New Berlin, WI 53151 USA

ABSTRACT

The basic evolution of directly implemented flux / torque control (F/T control) from other drive types is explained. Qualita-

tive comparisons with other drives are included. The basic concepts behind F/T control are clarified. An explanation of direct

self-control and the field orientation concepts implemented in the adaptive motor model block is presented. The reliance of the

control method on fast processing techniques is stressed. The theoretical foundations for the control concept are provided in

summary format. Information on the ancillary control blocks outside the basic F/T control is given. The implementation of

special functions directly related to the control approach is described. Implications for pulp and paper applications is discussed.

Finally, performance data from an actual system is presented.

I. INTRODUCTION

The evolution from DC drives to various forms of AC drives has been motivated by the continuing need for simultaneous

performance, simplicity, and reliability. In practice these needs have often proved to encompass mutually exclusive goals.

Therefore, new developments often strive to improve the available mix of advantages versus disadvantages. Directly imple-

mented flux / torque control (F/T control) represents a recent step in mix improvement. F/T control is typically referred to as

direct torque control in other literature. Fig. 1 shows drive evolution as a basic four step process. This viewpoint is simplistic

but useful for comparison purposes.

Freq. Ref.V/ f

RatioAC

Motor

Scalar Frequency Control

P W MModulator

V

f

SpeedControl

TorqueControl

A CMotor

T

Flux Vector Control

VectorControl

P W MModulator

V

f

SpeedControl

TorqueControl

ACMotor

F/T Control

HysteresisControl

SpeedControl

TorqueControl

DCMotor

T

DC Drive

Fig. 1. Evolution of Drive Control Techniques

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DC Drive Torque is directly proportional to armature current in the DC motor. By using an inner current control loop the

DC drive can directly control torque. Likewise, the constant magnetic field orientation which is achieved mechanically through

commutator action makes direct flux control a given. Thus, two primary factors towards insuring responsive control (i.e. direct

torque control and direct flux control) are both present in the DC drive. The relatively simple electronics required to implement

the DC drive represents another advantage. On the negative side, both the initial and maintenance costs of DC motors are high

and high speed accuracy performance can only be achieved if an encoder is included for feedback.

Scalar Frequency Control All of the AC drives compared here allow the use of economical, robust AC induction motors.Scalar frequency control also offers the advantage of operation without an encoder. On the negative side, torque and flux are

neither directly nor indirectly controlled. Control is instead provided by a frequency and voltage reference generator with

constant Volts per Hertz output which then drives a pulse width modulated (PWM) modulator. Although simple, this arrange-

ment provides limited speed accuracy and poor torque response. Flux and torque levels are dictated by the response of the motor

to the applied frequency and voltage and are not under the control of the drive.

Flux Vector Control Flux vector control re-establishes one of the advantages of the DC drive through implementation of

direct flux control. In this case, field orientation is controlled electronically. The spatial angular position of rotor flux is

calculated and controlled by the drive based on a relative comparison of the known stator field vector to feedback of rotor

angular position and speed. The motors electrical characteristics are mathematically modeled with microprocessor techniques

to enable processing of the data. Torque control is indirect because of its position in the control algorithm prior to the vector

control process; however, good torque response is achieved nonetheless. Inclusion of the pulse encoder insures high speed and

torque accuracy.

The biggest disadvantage of flux vector control is the mandated inclusion of the pulse encoder. Another minor disadvantageis that torque is indirectly rather than directly controlled. Finally, the inclusion of the PWM modulator, which processes the

voltage and frequency reference outputs of the vector control stage, creates a signal delay between the input references and the

resulting stator voltage vector produced. These last two factors limit the ultimate ability of flux vector control to achieve very

rapid flux and torque control.

Directly Implemented Flux / Torque Control F/T control also re-establishes direct flux control. In addition direct torque

control is implemented. Both flux and torque are controlled by a hysteresis controller. The delays associated with the PWM

modulator stage are removed since the PWM modulator is replaced by optimal switching logic. The original benefits associated

with the DC drive of direct torque control, direct flux control, and high responsiveness are thus all re-established. Torque

response is better than that available with either DC or flux vector control. In addition, assuming moderate speed accuracy is

acceptable (typically 0.1% to 0.3% or 10% of motor slip) the need for a pulse encoder is eliminated. The theory and operation

of F/T control is described in subsequent sections of this paper.

Table I provides a summary comparison of the four drives described above.

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II. DIRECTLY IMPLEMENTED FLUX/ TORQUE CONTROL

F/T control has its roots in field oriented control and direct self-control. Field oriented control uses spatial vector theory to

optimally control magnetic field orientation. It has been successfully applied to the design of flux vector controls and is well

documented. Direct self-control theory is less well known. It is a patented concept developed in Germany by Manfred Depen-

brock and has been described in several papers [2][3] which he has published. The fundamental premise of direct self-control

is:

Given a specific DC link voltage (EDC ), and a specific stator flux level (ref) , a unique frequency of inverter opera-tion (e) is established.

This is true because the time (T) required by the time integral of the voltage (EDC) to integrate up to the field flux level (ref)is unique and represents the half period time of the frequency of operation (e). Since the operational frequency (e) is estab-lished without a frequency reference, this operational mode is referred to as direct self-control. Output frequency is thus not

requested, but rather, is self controlled via the actual frequencies present. Once sensed, whether the frequency increases or

decreases depends on what the torque reference from the speed regulator requests. Differential changes to operational frequency

are determined by the torque request. Nominal operating frequency is self determined via the sensed feedbacks.

F/T control combines field oriented control theory, direct self-control theory, and recent advances in digital signal processor

(DSP) and application specific integrated circuit (ASIC) technology to achieve a practical sensorless variable frequency drive.

Fig. 2 shows the basic functional blocks used to implement the core of the F/T control scheme. The relationship of this core

to the complete control will be described in a subsequent section. Three key blocks, (1) Torque / Flux Comparators, (2) Optimal

Switching Logic, and (3) Adaptive Motor Model, interact to provide the primary control required.

Torque / Flux Comparators The Torque Comparator and the Flux Comparator are both contained in the Hysteresis Control

block. These function to compare the Torque Reference with Actual Torque and the Flux Reference with Actual Flux. Actuallevels are calculated by the Adaptive Motor Model. When Actual Torque drops below its differential hysteresis limit, the

Torque Status output goes high. Likewise, when Actual Torque rises above its differential hysteresis limit, the Torque Status

output goes low. Similarly when Actual Flux drops below its differential hysteresis limit, the Flux Status output goes high and

when Actual Flux rises above its differential hysteresis limit, the Flux Status output goes low. The upper and lower differential

limit switching points for both torque and flux are determined by the Hysteresis Window input. This input is used to vary the

differential hysteresis limit windows such that the switching frequencies of the power output devices are maintained within the

range of 1.5 kHz to 3.5 kHz.

TABLE I

COMPARISONS OF CONTROL TYPES

Control Type

Torque

Control

Flux

Control Response Advantages Disadvantages

DC Drive Direct Direct High High accuracyGood torque response

Simple

Motor maintenanceMotor cost

Encoder required for high

accuracy

Scalar Fre-

quency Con-

trol

None None Low No encoder

Simple

Low accuracy

Poor torque response

Flux Vector

Control

Indirect Direct High High accuracy

Good torque response

Encoder always required

F/T Control Direct Direct High No encoder

Moderate accuracy

Excellent torque response

Encoder required for high

accuracy

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Optimal Switching Logic Processing of the Torque Status output and the Flux Status output is handled by the Optimal

Switching Logic contained in the ASIC block. The function of the Optimal Switching Logic is to select the appropriate stator

voltage vector that will satisfy both the Torque Status output and the Flux Status output. In reality there are only six voltage

vectors and two zero voltage vectors that a voltage source inverter can produce. These are shown in Fig. 3.

The analysis performed by the Optimal Switching Logic is based on the mathematical spatial vector relationships of stator

flux, rotor flux, stator current, and stator voltage. These relationships are shown as a vector diagram in Fig. 4.

The torque developed by the motor is proportional to the cross product of the stator flux vector (s) and the rotor flux vector(r). The magnitude of stator flux is normally kept as constant as possible and torque is controlled by varying the angle ()between the stator flux vector and the rotor flux vector. This method is feasible because the rotor time constant is much larger

than the stator time constant. Thus, rotor flux is relatively stable and changes quite slowly compared to stator flux.

Adapt ive

Motor

Mode l

A C

Motor

Torque Ref

Flux Ref

Torque

Status

Flux

Status

Torque Comparator

Flux Comparator

Hysteresis Control ASIC

Opt ima l

Sw i t ch i ng

Logic

S1, S2,

S3

Inverter

Current

DC Link Vol tage

Swi tch Post ions

Actual

Flux

Actual

Torque

Ac tua l Speed

Actual Frequency

Hysteres is

W i n d o w

DS P

Fig. 2 : Basic F/T Control Scheme

M

S 1 S 2 S 3

- + - + + -

+ - -

+ - +- - +

- + +

Fig. 3 : Available Stator Voltage Vectors

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When an increase in torque is required the Optimal Switching Logic selects a stator voltage vector (Ux) that develops a tan-

gential pull on the stator flux vector (s) tending to rotate it counterclockwise with respect to the rotor flux vector (r). Theenlarged angle () created effectively increases the torque produced. When a decrease in torque is required the Optimal Switch-ing Logic selects a zero voltage vector which allows both stator flux and produced torque to decay naturally. If stator flux

decays below its normal lower limit the Flux Status output will again request an increase in stator flux. If the Torque Status

output is still low, a new stator voltage vector (U x) is selected that tends to increase stator flux while simultaneously reducing the

angle () between the stator and rotor flux vectors.

Note that the combination of the Hysteresis Control block (Torque and Flux Comparators) and the ASIC control block

(Optimal Switching Logic) eliminate the need for a traditional PWM modulator. This provides two benefits. First, small signal

delays associated with the modulator are eliminated; and second, the discrete constant carrier frequencies used by the modulator

are no longer present. The latter benefit can be readily discerned when observing a running drive since the characteristic gear

shifting sounds associated with a PWM inverter no longer exist. The sound remaining is a white noise that is less obtrusive to

the listener.

Adaptive Motor Model With reference to Fig. 2 it can be seen that the Adaptive Motor Model is responsible for generating

four internal feedback signals: (1) Actual Flux (stator), (2) Actual Torque, (3) Actual Speed, and (4) Actual Frequency. Thefirst two values which are critical to proper F/T control operation are calculated every 25 microseconds. The later two values

which are used by outer loop controllers are calculated once per millisecond.

Dynamic inputs to the Adaptive Motor Model include: (1) motor Current from two stator phases, (2) DC Link Voltage, and

(3) power Switch Positions. Static motor data is also utilized in making calculations. The static data comes from two sources,

user input data and information determined automatically from a motor identification run that occurs during commissioning.

The user input data includes motor nominal voltage, motor nominal current, motor nominal frequency, motor nominal speed,

and motor nominal power. The motor ID run data collected includes motor inductances, stator resistances, and stator saturation

effects.

The exact mathematical details of how the Adaptive Motor Model calculates its outputs are beyond the scope of this paper;

however, the subsequent section Theoretical Foundations is included to provide an overview of the mathematical foundations

upon which the calculations are based.

Torque Production The estimated air gap torque (Te) produced by the drive is shown in Fig. 5. The periods during which

torque has a positive slope such as the region indicated as T+ in the figure indicate intervals where an active stator voltage

vector is causing torque to increase. The periods during which torque has a negative slope such as the region indicated as T0 in

the figure indicate intervals where a zero voltage vector has been selected. During most periods either a simple positive slope or

simple negative slope is indicated. During these periods a single stator voltage vector selection has been able to satisfy both the

Torque Status and the Flux Status output. However, during some periods such as the region near the Te identifier, a dual slope

is present. This is indicative of a interval during which after one voltage vector has been chosen, and although additional torque

still needs to be developed (i.e. the Torque Status output isnt satisfied), a change in the Flux Status output has occurred that

requires selection of another stator voltage vector.

y

x

s

r

is

rotat ion di rect ion

U1

U2

U3

U4

U5

U6

Fig. 4 : Stator Spatial Vector Relationships

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Normally switching occurs whenever torque drops below Tref- T1 or exceeds Tref+ T1. In the special case where torqueexceeds Tref+ T2 the Optimal Switching Logic selects a stator voltage vector (Ux) that forces a decrease in torque by causing areduction in the angle () between the stator flux vector and the rotor flux vector.

Due to the hysteresis control the torque appears to have a choppy waveform. In practice this is not a concern since the aver-

age period of the waveform is under 1 millisecond and substantially less than the motors time constant.

The fact that all of the described spatial vector comparisons occur once every 25 microseconds is a key factor in the operation

of the drive. Since the maximum power device switching frequency is 3.5 kHz, 10 or more torque and flux comparisons are

possible per switching decision. Without this kind of data to decision ratio stable operation wouldnt be possible. It is only

due to recent advances in digital signal processor technology that this type of operation has become practical.

Initial Starting The statement that the operational frequency (e) is established without using a frequency reference orotherwise directly controlling frequency was made at the beginning of this section. This followed from the fact that F/T control

utilizes direct self-control as a part of its control scheme. Subsequent discussion has described how this is accomplished in a

running system. The issue of system initialization, however, has not been addressed. How the drive can establish a correct start

mode, when feedback data isnt available for the Adaptive Motor Model, is a valid question.

In reality, the drive doesnt initially know what frequency to operate at. Only after the Adaptive Motor Model analyzes the

Current and Switch Position feedbacks can a determination of flux, torque, speed, and frequency be made. To obtain the initial

feedbacks a low level DC flux is established in the motor. This level is low enough that overcurrent tripping and inappropriate

torques are not created, and high enough that meaningful feedback data is received. Once valid feedback data is availableoperation proceeds as previously described.

III. THEORETICAL FOUNDATIONS

The DSP is responsible for calculating all motor variables required by other portions of the F/T control circuitry. These

calculations are based on basic motor theory and field oriented control theory. While not intending to provide a complete

understanding of the mathematics and theory involved, Table II is included to provide a general feel for the basis of operation.

Calculated variables are listed in the order that they are typically determined. Note that many of the variables used are vector

quantities. These quantities are shown in the fundamental equations with a bar above the appropriate variable name.

Determination of Actual Flux (Stator) is always of first and primary concern. Note that two equations are listed. Both equa-

tions are utilized by the control. The first equation provides a first order estimate of the flux level present. The second equation

is used to effectively fine tune the value provided by the first. The next equation calculates Actual Torque. Since both Actual

Flux and Actual Torque are required directly by the Hysteresis Control comparators, these equations are solved every 25microseconds.

The remaining four equations are used to calculate Actual Frequency and Actual Speed which are used by the outer loops of

the control. Since the outer loops arent as time critical, these values are determined only once per millisecond. The Rotor Flux

Vector and Rotor Flux Angle variables provide values needed to calculate Actual Frequency and Actual Speed but arent used

directly elsewhere in the control scheme.

0.85

0.9

0.8

T e

T 0 T+

Tre f

T re f + T1

T re f + T2

Tre f

- T1

T re f - T2

Fig. 5 : Estimated Air Gap Torque with Hysteresis Control

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TABLE II

FUNDAMENTAL EQUATIONS FOR CALCULATING MOTOR VARIABLES

Calculated

Variable

Fundamental Equations

Actual Flux(Stator) ( )s s s su R i dt = s s s m r L i L i= +

Actual Torque T k ie s s=

Rotor Flux Vector( )

rr

m

s s s

rx ry

L

LL i

j

=

= +

Rotor Flux Angle

r

ry

rx

=

arctan

Actual Frequencye

r r rd

dt

t t

t= =

( ) ( )2 1

Actual Speed

r N e r

e

r

p RT

=

2

Nomenclature:

shown in Fig. 2

i

i

k

L

L

L

p

R

R

T

u

r

s

m

r

s

N

r

s

e

s

rotor inductance

stator inductance

number of pole pairs

rotor resistance

stator resistance

estimated torque

stator voltage

rotor current

stator current

constant

magnitizing inductance

r

r

r

rx

ry

s

e

r

rotor position angle

leakage factor

rotor power factor

rotor flux

real component of rotor flux

imag. component of rotor flux

stator flux

electrical angular speed

rotor angular speed (mech.)

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IV. THE COMPLETE DRIVE

Circuitry in addition that shown in Fig. 2 and described in section II is required is build a complete drive. Fig. 6 shows the

block diagram for a complete F/T control drive. Blocks for Speed Control, Torque Reference Control, Flux Reference Control,

and Switching Frequency Control have been added.

The Speed Control block is implemented with a traditional PID controller. The Speed Reference input is compared to the

Actual Speed feedback from the Adaptive Motor Model. The resulting output signal, Torque (Speed) Reference, becomes the

reference for the Torque Reference Control. An inertia compensator is also included although not shown. The compensator

helps reduce control deviation due to acceleration or deceleration of system inertia. It also allows the Speed Control to be

primarily tuned based on changes in static load. Automatic tuning is provided.

The Torque Reference Control has two inputs: Torque (Speed) Reference and Absolute Torque Reference. When the drive

functions as a speed control, only the Torque (Speed) Reference is used. When the drive functions as a torque control, only the

Absolute Torque Reference is used. These references are never used simultaneously. The output, Torque Reference, is proc-

essed by the F/T Control as described in section II.

The output of the Torque Reference Control provides automatic limiting based on any of four independent system occur-rences: (1) if during deceleration the DC link voltage starts to exceed the set maximum level, (2) if maximum permitted

electrical frequency is reached, (3) if requested torque reaches a level near motor pullout torque (which is dependent on motor

characteristics), and (4) if current overload is detected. The last of these limits, current overload, is a function of time where

very high current (typically 200%) is permitted for a short time period ( 2 seconds) and high current (typically 150%) ispermitted for a moderate time period ( 1 minute).

The Flux Level reference and Actual Frequency feedback are the primary inputs to the Flux Reference Control. The Flux

Level reference is a parameter which can be set by the user. The Actual Frequency feedback is used to provide frequency

sensitive flux manipulation for field weakening control, flux braking, and flux optimization. The latter two functions are

described in the next section. The output, Flux Reference, is processed by the F/T Control as described in section II.

The last additional block is the Switching Frequency Control. The power output devices have limitations on their maximum

switching frequency. Likewise, the desired output waveform places restrictions on the minimum desired switching frequency.

The function of the Switching Frequency Control is to vary the size of the hysteresis windows utilized by the Flux Comparator

and Torque Comparator such that power switching frequencies are maintained between 1.5 kHz and 3.5 kHz. The output,Hysteresis Window, is processed by the F/T Control as described in section II.

V. IMPLEMENTATION OF SPECIAL FUNCTIONS

The implementation of several special functions and control properties is made easier because F/T control works directly to

control stator flux. The included special functions are flying start, flux braking, flux optimization, and power loss ride through.

Flying Start The Adaptive Motor Model is capable of determining the status of stator flux and all other required motor

variables within 200 to 500 milliseconds assuming that all residual flux in the motor is absent. If residual flux exists, the

determination occurs much faster. This resolution is accomplished based on a low level DC current which is injected into the

SpeedControl

Torque RefControl

F/TControl

(see Fig. 2)

Flux RefControl

SwitchingFrequency

Control

ACMotor

Current

DC LinkVoltage

SwitchPost ions

S1 ,S2, S3

TorqueRe f

Flux Ref

Actual Speed

HysteresisWindow

ActualFrequency

Flux Level

Torq(Spd)Re f

Absolute Torque Ref

SpeedRe f

Inverter

Fig. 6 : Complete F/T Control Drive

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stator. Once the status of stator flux is known, full synchronization of the inverter with the motor is immediate. This makes F/T

control ideal for flying start applications. Proper operation is possible regardless of motor speed or direction prior to synchroni-

zation.

An alternate mode of starting called DC Magnetization is also available. In this mode an optimized value of DC flux of

relatively high level is immediately impressed on the stator. This minimizes the time required by the Adaptive Motor Model to

gather all needed data from a stationary motor. Full torque can typically be available within 150 to 400 milliseconds dependent

on motor size.Flux Braking It is common to inject DC into one or more stator windings to provide braking of an AC drive. This is

effective but is accompanied by a required delay to allow the flux to decay both before the DC can be applied and afterwards

before normal AC can be reapplied. F/T control uses a different method to achieve similar results. The stator is over excited in

a controlled manner to allow the braking energy to dissipate in the stator as is2Rs losses. Since F/T control directly controls

stator flux, this is a straight forward approach. In addition, because the flux continues to be applied at the appropriate excitation

frequency, there is no delay required to either initiate this method or to re-initiate the normal powering mode. Thus, this

method of braking can be used dynamically to slow the motor between any two normal operating points with immediate transfer

back to normal powering mode. It should be noted, however, this method is primarily useful at lower speeds since the necessary

voltage isnt available to appreciably over excite the stator at higher frequencies.

Flux Optimization A lightly loaded motor doesnt need full stator flux to produce the required torque. F/T control takes

advantage of this by selecting an optimal magnetizing level based on load. When full torque is required, full stator flux is

requested. At reduced load levels a reduced level of stator flux is developed. An unloaded motor may run with as little as 50%

of its nominal magnetizing current. Dependent on the application this may lead to significant reductions in motor heating andimprovements in overall efficiency.

Power Loss Ride Through If the power feed to a F/T control drive is lost, the system automatically enters a regenerative

mode that maintains the DC link voltage at its minimal operating level as long as possible. The energy to accomplish this

comes from the mechanical inertia stored in the load and motor. As long as rotational energy continues to exist the link voltage

is maintained. If the power feed comes back on line before rotation ceases, the drive will immediately return to the operating

point at which it was running prior to the loss of power. The time that this power loss ride through capability can be maintained

is extremely load dependent. A high inertia system which is lightly loaded can ride through a power loss of a minute or more.

On the other hand, a low inertia system that is fully loaded may only be able to stay on line for 200 to 300 milliseconds. This

feature can be disabled if safety concerns dictate doing so.

VI. SIGNIFICANCE TO PULP AND PAPER

The primary advances that F/T control brings to pulp and paper applications are partial elimination of pulse encoders and

improved torsional control. Although the need for pulse encoders has not yet be taken away, many sections of machines will not

require them. These include torque controlled sections (e.g. guide rolls, spreader rolls, helper drives) and followers in mas-

ter/follower combinations. Also less demanding speed applications such as winder rider roll drives are now possible without

feedback. The full possibilities for pulse encoder removal on specific applications will not be known until a broad range of pilot

testing can be completed. Overall the outlook for additional elimination is good. The attendant advantages this represents in

reduced part count, reduced wiring, and improved reliability are obvious.

The rapid torque response available from F/T control makes improved torsional control possible for all pulp and paper appli-

cations. In general there is increased immunity to mechanical oscillation. In addition, for those cases where resonance is an

issue, special self-tuning damping software can be invoked. Overall, drive tuning proceeds faster, requires a lower level of

expertise, and results in tighter load control.

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VII. PERFORMANCE DATA

Performance data was taken with the test system listed in Table III. Figures 7, 8, 9, and 10 show actual results. Note that all

results were taken without a shaft encoder. Fig. 7 shows the extremely fast torque response possible with F/T control. Torque

responses below 2 milliseconds are typical even in large motors. Speed response is also excellent as shown in Fig. 8. Note the

stability of the current throughout the entire reversing process. Fig. 9 demonstrates the smooth speed response right through

zero speed with a slow reversing reference and 80% constant torque loading. Regenerative energy was dissipated using a

braking chopper connected across the DC link. Fig. 10 demonstrates the smooth torque control possible even at zero speed.Torque linearity error is below 4%.

Table IV shows typical torque and speed performance data for F/T control with and without an encoder. Typical values for a

DC drive, scalar frequency control, and flux vector control are also included for comparison.

Torque Response

-20

0

20

40

60

80

100

1 2 3 4 5 6 7

Time (ms)

%

Estimated air-gap torque

Fig. 7 : 70% torque reference step at 25 Hz. Estimated air-

gap torque. No encoder.

Fast Reversing

-100

-80

-60

-40

-20

0

20

40

60

80

100

0 100 200 300 400 500 600 700 800

Time (ms)

%

Phase Current

Fig. 8 : Full speed fast reversing with constant torque load

(20%). Measured phase current. No encoder.

Slow Reversing

Time (s)

%

-60

-40

-20

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40 45

7 5 0 r p m

- 7 5 0 r p m

Shaft Torque

S h af t s e e d

Fig. 9 : Slow reversing with constant torque load (80%).

Measured shaft torque and speed. No encoder.

Torque Ramp at Zero Speed

-100

-80

-60

-40

-20

020

40

60

80

100

0 10 20 30 40 50 60

Time (s)

%

Measured Shaft Torque

Fig. 10 : Torque ramp from Tnom to -Tnom at zero speed.

Measured shaft torque. No encoder.

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TABLE III

TEST SYSTEM EQUIPMENT DATA

Motor Data

Power 15 kW

Current 30 A

Voltage 380 Vac

Poles 4

Frequency 50 Hz

Speed 1460 rpm

Inverter Data

Power 25 kW

Voltage 400 Vac

VII. SUMMARY

F/T control combines the benefits of direct flux and direct torque control into an encoderless variable frequency drive that

doesnt require a PWM modulator. Recent advances in DSP and ASIC technology plus the theoretical concepts developed for

direct self-control make this possible. Implementation of special functions such as flying start, flux braking, flux optimization,

and power loss ride through are all made easier due to the control approach utilized. For many applications exceptional

performance can be realized without an encoder. For those applications that require high speed accuracy an encoder can be

added to achieve speed accuracy equivalent to competitive approaches. In addition, with the encoder, dynamic speed and torque

response are better than any alternate approach. F/T control represents a definite step forward in the mix of drive controls to

choose from.

ACKNOWLEDGMENT

This paper is based extensively upon an IEEE paper previously presented by the author [4].

Major portions of the original paper are in turn based upon the research of Tiitinen, Pohjalainen, and Lalu [1]. The author

gratefully acknowledges the importance of their work.

REFERENCES

[1] P. Tiitinen, P. Pohjalainen, and J. Lalu, The Next Generation Motor Control Method - Direct Torque Control, DTC, EPE

Chapter Symposium, Lausanne, 1994.

TABLE IV

TYPICAL TORQUE AND SPEED PERFORMANCE DATA

DC Drive

with Encoder

Scalar Fre-

quency Control

Flux Vector

Control

(with En-

coder)

F/T Control F/T Control

with Encoder

Torque Control

Linearity +/- 3 % +/- 12 % +/- 4 % +/- 4 % +/- 3 %

Repeatability NA +/- 4 % +/- 1 % +/- 1 % +/- 1 %

Response Time 10..20 ms 150 ms 10..20 ms 1..2 ms 1..2 ms

Speed Control

Static Accuracy +/- 0.01 % +/- 1..3 % +/- 0.01 % +/- 0.1..0.3 % +/- 0.01 %

Dynamic Accuracy 0.3 %s 3 %s 0.3 %s 0.4 %s 0.1 %s

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[2] M. Depenbrock, Direct Self-Control (DSC) of Inverter-Fed Induction Machine, IEEE Transactions on Power Electronics,

Vol. 3, No. 4, October 1988.

[3] U. Baader, and M. Depenbrock, Direct Self Control (DSC) of Inverter-Fed Induction Machine: A Basis for Speed Control

Without Speed Measurement, IEEE Transactions on Industry Applications, Vol. 28, No. 3, 1992.

[4] J. Nash, Direct Torque Control, Induction Motor Vector Control Without an Encoder, IEEE 1996 Pulp and Paper

Industry Technical Conference, June, 1996.