AN1151 Meter CalibrationMICROCHIP’S DEMO BOARD CALIBRATION VS. PROPER METER Each phase was calibration sequentially, starting with CALIBRATION phase A. (Note 2) stepEach phase calibration process was a 4 It is important to note Microchip does not do a full cali- calibration. The steps are shown here with approximate bration/test on each meter that we ship as demo calibration times (using 128 line cycle accumulation at boards. Steps have been taken to ensure you receive 50 Hz) an energy meter ‘out of the box’ that performs to the above meter specifications, but the equipment and GAIN (5A, 220V) - 2.5 seconds (Note 1) process we use during demo board production is not DELAY (5A, 220V, PF = 0.5L) - 2.5 seconds (Note 1) true energy calibration equipment, and any testing will OFFSET for active power reflect this. (0.005A, 220V, PF = 1) - 2.5 2.5 seconds (Note 1) The results shown here were taken from a shipped OFFSET for I (0.5A, 220V, PF=1) - 2.5 seconds RMSenergy meter that was simply ‘re-calibrated’ using the (Note 1, Note 2) Fluke electrical power standard and the calibration Note 1: Important! The approach to calibration software GUI that comes with the kit. No hardware used by the PIC18F2520 firmware and ‘tweaks’, or firmware changes were done, only a true Windows GUI (“energy meter software”), calibration using accurate energy meter calibration does not require accumulation of active equipment. power pulses to generate errors and resulting calibration correction factors for Calibration used for this Application Note the meter. Instead, the software allows Figure 2 describes the calibration and test setup used. user input of the true current and voltage The electrical power standard (Fluke 6100A) was at each calibration step. The correction configured to generate balanced loads for the meter, factors are then calculated by comparing for all calibration steps.3xI this ideal to the measured active power CAL after N line cycles, 128 line cycles in the results shown here. This approach greatly Power Meter reduces meter production time. Inputs 2: This fourth calibration step can be +0° LINE skipped for active power only meters as it only applies to the RMS measurement. LOAD Total Energy When the meter is shipped the configuration of the +120° output pulse is set such that the pulse frequency is LINE proportional to the total active energy being consumed across all 3 phases. During meter calibration however, LOAD the firmware and software work together to ensure that ‘phase to phase matching’ is included. When calibrat- ing all 3 phases, one of the phases is selected (nor- mally Phase A) as the ‘reference or ‘standard’ phase, +240° and the other phases, when accumulating energy LINE during the gain calibration step, or step 1, compare their accumulation to that recorded during phase A, LOAD thus phase to phase matching is included. For equations and signal flow, see the MCP3909 3-Phase Energy Meter Reference Design using PIC18F2520neutral User’s Guide, (DS51643). ©Fluke Corporation FIGURE 2: Meter Calibration Setup. DS01151A-page 2 © 2007 Microchip Technology