How to detect Dry Run and Overload using PIC

[BradtheRad]

Please provide link for full-color led/lcd hi-resolution display.

I suspect you'll have your hands full if you make your display single color. You still need to consider ease of viewing in daytime vs nighttime. That is a factor in deciding between led or lcd. Also whether to multiplex digits. Also how to make the enclosure and mount it.

Also how often the readout gets updated. How quickly does supply voltage change?

Also number of digits. I think 3 is sufficient. No need to read fractions of a volt.

If you wish you can find articles by builders who create websites and/or videos telling how a microcontroller can drive an lcd display. From what I can see, it's a major challenge. I do not have hands-on experience with microcontrollers or lcd screens. I made a frequency counter with six red led digits and it's a challenge the first time around.

 

[Okada]

The cost of RGB 7 Segment displays I found are very high and so I decided to go with Red color 7 Segment display that is mono color. Also a screen filter cannot be used if RGB 7 Segment Display is used. I have decided to use Red filter for the display.

The readout get updated only when the adc value changes compared to previous value. Flags are used throughout the code so that unnecessary instructions doesn't keep on getting executed like. If I need to turn ON the relay then a flag is used to turn ON/OFF the relay and the instructions for that gets toggled only once when state changes that is the controller doesn't keep on executing RELAY = 1 or RELAY = 0 instruction.

I have used same display for voltage and pump current.

I can't use LCD display. Ofcourse I know how to use it but the cost of LCD display is more compared to 3 Digit Seven Display.

 

[betwixt]

Okada, this thread is titled "How to detect Dry Run and Overload using PIC" but you seem now to be asking for precision measurements rather than "too little/too much" decisions. Do you need the precision if you are only checking for dry run and overload, there will be a big difference in current between the motor states.

Brian.

 

[Okada]

@betwixt

I need to be able to measure currents between 0 and 20A using the AC1020 C.T.. Please provide a circuit for it with or without OpAmp. If OpAmp is used then I will be using OPA342. I am going to use the same circuit with little modification to the code for 4 to 6 different HP rated pumps and hence the dry run currents, normal currents and overload currents will be different but they will be between 0 and 20A.

Yes, I need precision current measurement. If my pump normal current is 6.2A then I will consider 5.5 A or less as dry run current and 8 A or more as overload current. Starting current of the pump will be more but for a short period. When pump turns ON, I will make itl wait for 3 seconds and then measure the current and if current is not in normal range of 5.5A to 8A then the pump will be turned OFF and the error parameter will be flashing on the display till it is rectified. After error parameter is rectified the pump turns ON automatically.

I posted link for Khyatee single phase submersible pump controller. I am making an exact product other than the current product being designed. Even that will need a current measuring range of 0 to 20A. The Khyatee panel shows auto and manual current calibration. In auto calibration mode it measures the current and flashes the measured current on the display and if there is no user input for 10 seconds then it will store the current in eeprom as normal operating current for that pump. It will take some 2 A or less current than this normal current as dry run current and 2A or more current than the normal current as overload current.

Dry run and overload currents are measured only after normal current is calibrated. After calibration if pump is turned ON it waits 5 seconds and if the current measured after 5 seconds is in the dry run or overload current range then it shuts of the pump.

 

[betwixt]

If you want best accuract using those parts, I would suggest the CT and burden resistor, dropping the resulting voltage a little with a potential divider (maybe a potentiometer for calibration) then using a precision rectifier. The rectifier can be the OPA342 with one of your low Vf diodes in the feedback path and another in series with the amplifier output. That will give best linearity in the measurement but the output voltage can never go higher than the supply voltage minus one diode Vf. I doubt that would be a problem in real life, it just means the maximum ADC output range would be restricted slightly. You could drop the ADC reference using another diode of the same type (gives best temperature stability) or apply a small multiplication to the ADC result in software to correct the result.

I suggest looking a precision rectifier circuits to see what I mean, they are simple and effective, especially at low frequencies.

Brian.

 

[Okada]

I can't use special precision rectifiers made by LTC (opamp based) because I have to keep the manufacturing cost of the product not more than 20 USD. Already C.T. and OPA342 have increased the cost.

 

[betwixt]

You can make a precision rectifier using almost any op-amp, it's a type of circuit, not a type of IC. You can use the OPA342 to make one with only two other components!

As cost is important, I would suggest you forget absolute accuracy and use the simplest circuit possible. Just rectify the CT output with a single diode half-wave rectifier, filter it with a small capacitor and feed the resulting DC to the PIC ADC input. It will not be super accurate but you should still be able to see differences of a few mA in motor current, more than enough to determine the motor state and accommodate different motor types.

If you want it to 'self calibrate' the best you can do without a reference load is average the readings over several seconds of normal (pumping) operation and instead of setting absolute limits for the current, use a percentage change. For example, instead of saying 5A = normal, 1A = runng dry and 10A = overload, say "dry = <(normal - 20%)" and "overload = >(normal + 20%)". As long as you can find a value for 'normal' the other limits adapt around it.

Brian.

 

[Okada]

Thank you betwixt. I will try to design the C.T. circuit.