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AD2S105AP

Three-Phase Current Conditioner

Analog Devices 

AD2S105

MEASUREMENT OF HARMONICS

In ac power systems, the quality of the electrical supply can be

affected by harmonic voltages injected into the power main by

loads, such as variable speed drive systems and computer power

supplies. These harmonics are injected into other loads through

the point of common coupling of the supply. This produces ex-

tra losses in power factor correction capacitors, power supplies

and other loads which may result in failure. It also can result in

tripping and failure of computer systems and other sensitive

equipment. In ac machines the resultant harmonic currents and

flux patterns produce extra motor losses and torque pulsations,

which can be damaging to the load.

The AD2S105 can be used to monitor and detect the presence

and magnitude of a particular harmonic on a three-phase line.

Figure 10 shows the implementation of such a scheme using the

AD2S105, where Va, Vb, Vc are the scaled line voltages.

AD2S105

Va

Vd

Vd1 LOW PASS ak

Vb

Vc

THREE -TO-TWO

jφ

e PARK

CLARK

TRANSFORMATION Vq

TRANSFORMATION

Vq1

FILTER

HOMOPOLAR

OUTPUT

12-BIT UP/DOWN

COUNTER

PULSE INPUTS

DIRECTION

Figure 10. Harmonics Measurement Using AD2S105

Selecting a harmonic is achieved by synchronizing the rotational

frequency of the park digital input, ␾, with the frequency of the

fundamental component and the integer harmonic selected. The

update rate, r, of the counters is determined by:

r

=

4096

×

n×ω

2π

.

Here, r = input clock pulse rate (pulses/second);

n = the order of harmonics to be measured;

␻ = fundamental angular frequency of the ac signal.

The magnitude of the n-th harmonic as well as the fundamental

component in the power line is represented by the output of the

low-pass filter, ak. In concert with magnitude of the harmonic

the AD2S105 homopolar output will indicate whether the three

phases are balanced or not. For more details about this applica-

tion, refer to the related application note listed in the

bibliography.

Field Oriented Control of AC Induction Motors

In ac induction motors, torque is produced through interaction

between the rotating air gap field and currents induced in the

rotor windings. The stator currents consist of two components,

the flux component which drives the air gap field, and the

torque component which is reflected from the rotor windings. A

successful field oriented control strategy must independently

control the flux component of current, referred to as direct cur-

rent (Ids), and the torque producing component of stator cur-

rent, referred to as quadrature current (Iqs).

The control architecture in Figure 11 is referred to as field ori-

ented because the control algorithms performed on the ADSP-

2105 processor operate on decoupled flux and torque current

components in a reference frame relative to the rotor flux of the

motor. The control algorithms provide fast torque response at

any speed which results in superior dynamic performance, and

consequently, load variations have minimal effect on speed or

position control.

The AD2S90 resolver-to-digital converter is used to convert the

modulated resolver position signals into a 12-bit digital position

value. This value is brought into the ADSP-2105 via the

serial port where the control algorithms calculate the rotor flux

angle. The rotor flux angle is the sum of the rotor position and

the slip angle. The relationship between the stator current fre-

quency and the slip frequency can be summarized by the follow-

ing formula:

fstat = (␻m × (p/2)) + fslip

where: fstat = Stator Current Frequency (Hz)

␻m = Mechanical Speed of the Motor ( revs/sec )

p = Number of poles

fslip = Slip Frequency (Hz)

The rotor flux angle is fed into the 12-bit position input of the

AD2S105. The AD2S105 transforms the three ac stator cur-

rents using the digital rotor flux angle into dc values represent-

ing direct current (Ids) and quadrature current (Iqs). The

transformation of the ac signals into dc values simplifies the de-

sign of the A/D converter as it avoids the bandwidth sampling

issues inherent in ac signal processing and in most cases elimi-

nates the need for a simultaneous sampling A/D converter.

STATOR Is1

CURRENT Is2

SIGNALS Is3

Ids

AD2S105

2 CHANNEL

12 Bit A/D

Iqs CONVERTER

ROTOR FLUX ANGLE

ADSP-2105

ROTOR

FLUX

MODEL

INV

+

PWM

SPORT

ROTOR POSITION DATA

MOTOR

RESOLVER

AD2S90

R/D

CONVERTER

REV. 0

Figure 11. Field Oriented Control of AC Induction Motors

–9–

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