EmonTx 3-phase firmware returns negative readings

Hardware emonTx
1

Hi,

So I’ve updated my EmonTx with the 3-phase firmware. I currently have the 3 CTs monitoring the same line just to make sure the values are different and they are, however the values for CT2 and CT3 are negative despite all 3 CTs are facing the same direction. I haven’t made any changes to the firmware either, so I’m not sure if I missed something in the user guide. A sample of the output can be seen below.

pi@raspberrypi:~/emontx-3phase $ pio device monitor
Warning! `env_default` configuration option in section [platformio] is deprecated and will be removed in the next release! Please use `default_envs` instead
--- Available filters and text transformations: colorize, debug, default, direct, hexlify, log2file, nocontrol, printable, send_on_enter, time
--- More details at http://bit.ly/pio-monitor-filters
--- Miniterm on /dev/ttyUSB0  9600,8,N,1 ---
--- Quit: Ctrl+C | Menu: Ctrl+T | Help: Ctrl+T followed by Ctrl+H ---
ct1:67.98,ct2:-41.89,ct3:-12.93,ct4:0.00,vrms:242.53,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
ct1:69.12,ct2:-43.32,ct3:-13.63,ct4:0.00,vrms:240.72,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
ct1:68.54,ct2:-42.42,ct3:-13.33,ct4:0.00,vrms:241.68,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
ct1:67.72,ct2:-41.96,ct3:-12.79,ct4:0.00,vrms:242.25,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
ct1:67.79,ct2:-42.08,ct3:-12.65,ct4:0.00,vrms:243.06,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
ct1:67.90,ct2:-42.34,ct3:-12.60,ct4:0.00,vrms:243.33,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
ct1:68.04,ct2:-42.35,ct3:-12.78,ct4:0.00,vrms:242.20,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
ct1:67.95,ct2:-42.38,ct3:-12.80,ct4:0.00,vrms:242.41,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
ct1:69.40,ct2:-43.66,ct3:-12.97,ct4:0.00,vrms:243.18,t1:300.00,t2:300.00,t3:300.00,t4:300.00,t5:300.00,t6:300.00,pulses:0
2

I take it the numbers you are looking at are the powers, not the currents?
Of course the powers will be, and L2 and L3 should be exactly half the value of the L1 c.t., when you have the v.t on L1. That they’re not is because you’re probably down below the working range of the c.t, where the errors are large.

Think of the 3-phase phasor diagram for your explanation.

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3

Yes, the values for ct1, ct2 and ct3 is Power.

Would the low amps result in negative readings though?

4

No, if you put a decent current (within the working range of the c.t, i.e > 10 A) you should see - as explained in the calibration instructions - in L2 & L3 exactly -0.5 × PL1

Your 3 c.t’s are all on L1. The voltage phasor for L1 is exactly in phase with the current. Power is the product of the two phasors.
Internally, the 3-phase sketch develops voltages for L2 & L3 that can be represented on the familiar phasor diagram, but the component of those I2 & I3 phasors projected onto their respective V2 & V3 phasors is the multiplier that you must use with the current phasor to give you real power on those phases.

path328-4

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