Hi Everyone,
I’m from Singapore and new with this EmonTX as I just bought it and installed to measure my Induction Motor Power readings. As you can see in those 2 images below.
Image 1 Negative Value in P4
Imager 2 Positive Value in P4
I have checked my CT direction and the arrow is pointing towards the direction of the load. Besides that my E4 is positive in the beginning but changes to negative after a few minutes. Are these normal?
Please advice.
Thanks,
Sebastian
Welcome, @Sebastian_Peter to the OEM forum.
You don’t give any details of your motor or what it is driving, I see you have a “sensible” voltage on V1 only, so would it be a single phase, capacitor-start, capacitor-run motor with almost no load on it?
If so, then my first guess is the “reverse” power is due to the motor having a very low power factor under those conditions, and in that case a small phase error in the power measurement will make it appear that the motor is generating when it is not.
A quick check of your emonTx would be to measure the power of a known resistive load - say an electric kettle or convector heater (no fan) - and compare the reading with the power rating of the appliance. This should be positive and within about 5% of the “nameplate” rating of the appliance.
Greetings @Robert.Wall,
Thanks for your swift and detailed reply. After reading your reply I have rewired my system as the Induction motor is a 3 phase drive. Below is the screenshot of the sample drive I’m using. It’s a 1 HP 0.75kW drive. There is no load applied to the drive shaft.
My system setting is as follow. I’m using a 220V/380 VFD 1-phase to 3-phase Frequency Inverter AC Motor Converter Variable Frequency to power the Induction motor. I’m tapping the U V W into the Voltage sensor and clamping the CT sensor on the same wires.
Below is the latest screenshot.
Now P5 is giving me negative value besides that I used multimeter to measure the voltage channeled to the Voltage sensor. The readings are as follow:
U = 217.5V, V = 217.5V, W = 217.5V but
the sensor reading showing U = 31.31V, V = 265.3V, W = 247.1V.
Please advice @Robert.Wall much appreciated 
-Seb-
Hello @Sebastian_Peter could you record the data from P4, P5, P6, V1, V2, V3 so that we can see what they look like over time, perhaps keep the motor speed at a constant level at around mid power for a couple of minutes. It would be interesting to see how stable/unstable the readings are.
PS: I’ve double checked your order and see it’s an EmonTx5 with a 3-phase emonVs (3x 100A CT’s and 3x 50A CT’s).
The emonVs is designed for 3-phase 4-wire systems not 3 phase 3-wire systems I wonder if that’s the issue here? I assume there is no fourth/neutral wire? See: 3-Phase Power — OpenEnergyMonitor 0.0.1 documentation
Hi @TrystanLea thanks for swift reply. I have recorded for approximately 3 mins and recorded using Mqtt protocol. Attached below is the screenshot.
PS: I’ve double checked your order and see it’s an EmonTx5 with a 3-phase emonVs (3x 100A CT’s and 3x 50A CT’s). Yes you are right. I’m currently using the 50A CT on P4,P5 and P6
Cheers and best regards
@TrystanLea you are absolutely right, I used the 3 phase 3-wires to wire to the voltage sensor. Unfortunately my 220V/380 VFD 1-phase to 3-phase Frequency Inverter AC Motor Converter Variable Frequency is not providing the Neutral output. Only U V and W. I tried to used the neutral from the input wire to connect to the neutral in the voltage sensor but it’s not working.
Any advice on this?
Best regards
@Sebastian_Peter Im afraid it needs that 4th neutral wire. @Robert.Wall do you have any experience with zig-tag transformers? Zigzag transformer - Wikipedia could something like that be an option?
As Trystan says, the neutral connection is essential for the emonVs to provide the correct voltage sample for the emonTx5 to be able to calculate the power correctly. Without the neutral connection, the d.c. power supply in the emonVs - connected between L1 & N - pulls the star point towards L1 which explains the very low reading of V1 and the high readings of V2 & V3.
What are you measuring with respect to, if you do not have a neutral wire?
I have never used or had a need to specify a zigzag transformer, but using one to generate a neutral should make a significant improvement. You should find V1, V2 & V3 are then very nearly equal, likewise the power values.
I can offer no guarantee that the software, the emonVs or the current transformers will perform correctly when the supply frequency varies. All were only ever intended for use on either 50 Hz or 60 Hz mains supplies with a very limited (±1% maximum) deviation from the nominal frequency.
