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AD2S105 Datasheet(PDF) 9 Page - Analog Devices |
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AD2S105 Datasheet(HTML) 9 Page - Analog Devices |
9 / 12 page AD2S105 REV. 0 –9– 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. LOW PASS FILTER PARK TRANSFORMATION 12-BIT UP/DOWN COUNTER AD2S105 a k HOMOPOLAR OUTPUT PULSE INPUTS DIRECTION Va Vb Vc THREE -TO-TWO CLARK TRANSFORMATION Vd Vq Vd1 Vq1 e j φ 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 2 π . 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 Figure 11. Field Oriented Control of AC Induction Motors 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. ADSP-2105 AD2S105 INV + PWM MOTOR AD2S90 R / D CONVERTER RESOLVER ROTOR POSITION DATA ROTOR FLUX ANGLE Ids Iqs Is2 Is3 Is1 2 CHANNEL 12 Bit A/D CONVERTER SPORT ROTOR FLUX MODEL STATOR CURRENT SIGNALS |
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