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[Fwd: Solid State Wattmeter - Test Results]




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Chip, All

I ran tests tonight on the EDN Solid State Wattmeter I've been using
in a 1.0KVA wattmeter.  The experimental design was basically use the
controller to power a 120VAC,1.56kW floor heater.  Inductance is trival
(shaded pole blower) and 1500W heater.  The calibration instrument used
to calibrate and test this meter was a Yokogawa WT2010 1ph wattmeter.
This unit is NIST traceable, has a 100Khz sampling, 12bit A/D conversion
and has a measurement accuracy significantly better than +/- 1%.

After inital unit calibration, 3 separate power runs were conducted,
using
the panel mounted analog voltmeter and current meter on the controller.
The SS wattmeter (SSP) circuit voltage output (1V = 100W) was measured
with a
Fluke 87 true RMS digital voltmeter.  The input voltage, current and
power
was likewise monitored on the Yokogawa.  All instruments had been
calibrated (either factory or less than 6 months) using NIST traceable
instrumentation.  SSP values listed as watts (Vmeas * 100W scaling).

Run 1
Controller			Yokogawa	
V	I	V*I	SSP	V	I	P	%Deviation (SSP to Yokogawa)
20	2.5	50	39.7	19.69	2.29	45.0	-11.78
30	3.0	90	87.9	28.80	3.33	95.7	 -8.15
40	4.1	164	151.4	37.37	4.32	162.6	 -6.89
50	5.4	270	266.0	49.10	5.57	273.2	 -2.63
60	6.5	390	388.5	59.10	6.64	392.3	 -0.97
70	7.5	525	513.0	67.80	7.53	510.3	  0.53
80	8.5	680	665.0	77.00	8.48	652.0	  1.99
90	9.6	864	835.0	86.30	9.44	813.9	  2.59
100	10.7	1070	1050	96.93	10.54	1022.7	  2.67
110	11.7	1287	1267	106.6	11.50	1230.0	  3.00
113	12.2	1379	1387	111.0	12.00	1335.0	  3.89

Run 2
Controller			Yokogawa	
V	I	V*I	SSP	V	I	P	%Deviation (SSP to Yokogawa)
20	2.5	50	39.5	19.53	2.26	44.1	-10.43
30	3.3	99	100.9	30.70	3.54	108.9	 -7.35
40	4.3	172	171.0	39.85	4.55	181.0	 -5.52
50	5.5	275	271.0	49.80	5.64	280.4	 -3.35
60	6.5	390	381.0	58.70	6.61	391.0	 -2.56
70	7.5	525	507.0	67.80	7.52	510.0	 -0.59
80	8.5	680	653.0	76.70	8.45	648.0	  0.77
90	9.6	864	840.0	86.90	9.50	825.0	  1.82
100	10.7	1070	1042	96.76	10.52	1018.0	  2.36
110	11.7	1287	1264	106.6	11.54	1230.0	  2.76
113	12.2	1379	1375	111.2	12.01	1339.0	  2.69

Run 3
Controller			Yokogawa	
V	I	V*I	SSP	V	I	P	%Deviation (SSP to Yokogawa)
20	2.5	50	34.5	18.45	2.14	39.5	-12.66
30	3.3	99	82.0	28.00	3.23	108.9	 -9.29
40	4.4	176	174.0	40.28	4.59	181.0	 -5.94
50	5.3	265	253.0	48.07	5.45	265.0	 -4.53
60	6.5	390	368.0	58.02	6.54	379.0	 -2.90
70	7.5	525	507.0	67.86	7.53	510.0	 -0.58
80	8.5	680	650.0	76.80	8.45	649.0	  0.15
90	9.5	855	815.0	85.87	9.39	806.0	  1.12
100	10.7	1070	1038	96.84	10.53	1018.0	  1.96
110	11.7	1287	1257	106.5	11.53	1226.0	  2.52
113	12.2	1379	1370	111.2	12.01	1334.0	  2.70

Discussion:

-Calibration time 30 minutes, time per run approximately 15 minutes, no
delay
 between runs
-Variation in system readings due to setpoint being controlled by analog
 panel meter (+/- 5% accuracy)on controller.
-Variation in Yokogawa from calculated due to integration times, manual
 data entry, and TCR (temp coeff. of resistance) effects of heater
element(s).

Conclusions:

-Solid-state wattmeter is reasonably accurate and repeatable over upper
 60% of span (40-113 VAC).
-Voltage not adjusted higher due to article upper measurement limit of
1.3KVA.
 In actual use, upper limit was not reached until 741 op-amp "bumped"
the +
 rail at indicated 1.408KW.
-Higher accuracy maybe possible by additional coordinated trimming of
'0' and 'span'
 (and a lot of patience! :^( ), and possible replacement of 741 op-amp
with a higher
 performance unit.  +/- 5% accuracy as originally claimed in previous
postings
 was achieved.

DAVE SHARPE, TCBOR

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