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AD2S100 Scheda tecnica(PDF) 7 Page - Analog Devices |
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AD2S100 Scheda tecnica(HTML) 7 Page - Analog Devices |
7 / 12 page AD2S100 REV. A –7– Output Analog Signals There are three forms of analog output from the AD2S100. Sin/Cos orthogonal output signals are derived from the Clark/ three-to-two-phase conversion before the Park angle rotation. These signals are available on Pin 25 (Cos ) and Pin 26 (Sin ), and occur before Park angle rotation. Three-Phase Output Signals (Cos ( θ + φ), Cos (φ + θ + 120°), Cos (φ + θ + 240°)), where φ represents digital input angle. These signals are available on Pin 7 (PH/OP1), Pin 9 (PH/OP2) and Pin 8 (PH/OP3), respectively. Two-Phase (Sin ( θ + φ), Cos (θ + φ)) Signals These represent the output of the coordinate transformation. These signals are available on Pin 6 (PH/OP4, Sin ( θ + φ)) and Pin 7 (PH/OP1, Cos ( θ + φ)). HOMOPOLAR OUTPUT HOMOPOLAR Reference In a three-phase ac system, the sum of the three inputs to the converter can be used to indicate whether or not the phases are balanced. If VSUM = PH/IP1 + PH/IP2 + PH/IP3 (or PH/IPH1 + PH/IPH2 + PH/IPH3) this can be rewritten as VSUM = [Cos , + Cos ( + 120 °) + Cos ( + 240°)] = 0. Any imbalances in the line will cause the sum VSUM ≠ 0. The AD2S100 homopolar output (HPOP) goes high when VSUM > 3 × Vts. The voltage level at which the HPOP indicates an imbalance is determined by the HPREF threshold, Vts. This is set internally at ±0.5 V dc ( ±0.1 V dc). The HPOP goes high when V ts < (Cos θ+ Cos(θ+120° ) + Cos(θ+ 240° )) 3 × V where V is the nominal input voltage. With no external components VSUM must exceed ±1.5 V dc in order for HPOP to indicate an imbalance. The sensitivity of the threshold can be reduced by connecting an external resistor be- tween HPOP and ground in Figure 5 where, V ts = 0.5 R EXT R EXT + 20000 REXT = Ω Vts = V dc. 20k Ω 25µA HOMOPOLAR REFERENCE EXTERNAL RESISTOR TO TRIGGER Figure 5. The Equivalent Homopolar Reference Input Circuitry Example: From the equivalent circuit, it can be seen that the in- clusion of a 20 k Ω resistor will reduce V ts to ±0.25 V dc. This corresponds to an imbalance of ±0.75 V dc in the inputs. Homopolar Filtering The equation VSUM = Cos + Cos ( + 120°) + Cos ( + 240°) = 0 denotes an imbalance when VSUM ≠ 0. There are conditions, however, when an actual imbalance will occur and the condi- tions as defined by VSUM will be valid. For example, if the first phase was open circuit when = 90 ° or 270°, the first phase is valid at 0 V dc. VSUM is valid, therefore, when Cos is close to 0. In order to detect an imbalance has to move away from 90 ° or 270 °, i.e., when on a balanced line Cos ≠ 0. Line imbalance is detected as a function of HPREF, either set by the user or internally set at ±0.5 V dc. This corresponds to a dead zone when = 90 ° or 270° ± 30°, i.e., V SUM = 0, and, therefore, no indicated imbalance. If an external 20 k Ω resistor is added, this halves Vts and reduces the zone to ±15°. Note this example only applies if the first phase is detached. In order to prevent this false triggering an external capacitor needs to be placed from HPFILT to ground, as shown in Figure 5. This averages out the perceived imbalance over a complete cycle and will prevent the HPOP from alternatively indicating balance and imbalance over = 0 ° to 360°. For d θ dt = 1000 rpm C EXT = 200 nF d θ dt = 100 rpm C EXT = 2. 2 µF Note: The slower the input rotational speed, the larger the time constant required over which to average the HPOP output. Use of the homopolar output at slow rotational speeds becomes impractical with respect to the increased value for CEXT. AD2S100 TOP VIEW 123 12 34 HPREF HPOP HPFILT 220nF DGND CEXT REXT HPOP GND HPREF Figure 6. AD2S100 Homopolar Output Connections |
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