# Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Related

1. ## Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Related

I am doing a automatic water level controller using PIC18F46K22 which also monitors mains voltage and pump current and controls the Pump.

I thought it is better to implement dynamic dry-run and overload current calculation method. So, Can I implement this method in the code?

Target = 2-HP Pump, 1-Phase, 220V, 50Hz

2 HP = 1471W

Lets take 1500W for calculation purpose as it will be easier.

1500W/220V = 6.8181A nominal pump current

Dry-Run Current = nominal current - 20% of nominal current
Overload Current = nominal current + 20% of nominal current

Dry-Run Current = 6.8181A - (6.8181A * 20 / 100) = 5.4545A

Overload Current = 6.8181A + (6.8181A * 20 / 100) = 8.1818A

20/100 = 0.2 (this is known that is 20% value)

1500W is known

Voltage is known by measuring through ADC

Current is known as measured through ADC

What I want is a an equation to find out

dynamic dry-run and overload currents that is when mains voltage supplied to pump varies then current also varies and so I need to get dry-run and overload currents in run-time.

So, how to do that?

Should I find out dynamic power from voltage and pump currents and then calculate?

Code:
```	mainsVoltageMeasuredInRunTime * pumpCurrentMeasuredInRunTime = pumpPowerInWatts

pumpPowerInWatts / mainsVoltageMeasuredInRunTime = pumpRunTimeNominalCurrent

pumpRunTimeDryRunCurrent = pumpRunTimeNominalCurrent - (20% pumpRunTimeNominalCurrent)

pumpRunTimeOverloadCurrent = pumpRunTimeNominalCurrent + (20% pumpRunTimeNominalCurrent)```
Is the above method correct?

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2. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

I think you also need to consider power factor.

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3. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

When starting up your motor is likely to draw several more Amperes than it does during normal running.

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4. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

Yes, I am considering 7 times nominal current and so 6A * 7 = 42A and so I am using a 50A type ACS758 sensor. Is my method correct?

How to use the Power Factor in the calculation?

5. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

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6. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

In my opinion, Dry run current and overload current should be set by end user.

Nandhu

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7. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

Is the above method correct?
Looks OK.

For dry run (pump running without pumping anything), the actual current will depend on the type of motor. For simple capacitor start induction motors, 20% will be a conservative value.

You need to see the boilerplate.

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8. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

In a simple system with one power source and one load it probably isn't necessary to take power factor into account as the measured values will change according to conditions anyway and could just be monitored. I would agree that making the '20%' user programmable and independently for dry and overload would be a good idea as these may have to be found empirically. Additionally, it might be useful to add another user programmable parameter, the 'ignore' time when power is first applied as the start current time will depend somewhat on whether it starts dry or under loaded conditions.

If the 'normal' values are calculated, or better still actually measured, they can be treated as constants, then all the program has to do is a comparison against real time values to determine the pump status.

In a job I did recently, I used a moving target principle, the running current was averaged over a long period (several days in this application) and deviations from normal were used to detect fault conditions rather than absolute limits.

Brian.

9. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

Finished the simulation. Have to test more.

Made a quick simulation video.

Will make PCB Layout today and get one assembled board manufactured and do the hardware testing. I Will update this thread after hardware test is done.

10. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

Forgot to mention. My code works for mains frequency 40Hz to 80Hz.

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11. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

the running current was averaged over a long period...
Sounds a good strategy. But...

If you see a trend (current slowly increasing or decreasing over a long period), perhaps the motor is getting overheated or due to retire...

If you see a periodicity, perhaps the power line supply voltage is changing or something like that...

If you see voltage or current spikes, then perhaps you have a warning...

Data is goldmine these days.

12. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

Why there will be voltage / current spikes ifa MOV is put in parallel across the power line? I have one but not shown in the simulation. One MOV is used across transformer primary and also across the power supply which goes to motor.

13. ## Re: Automatic Water Level Controller - Dynamic Dry-Run and OverLoad Current Calc Rela

One big problem is induction motor current does not drop much with no or light loads. The magentization current is always high. You need to measure true power to the motor to determine if it has no load.

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