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Old 09-01-14, 10:30 PM   #1
ICanHas
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Default Low cost DIY AC/fridge duty cycle and dryer and water heater kWh estimator

This novel idea uses simple household devices within easy reach of students, hobbyists and conservation enthusiasts to accurately collect the average run-time percentage of refrigeration or air conditioning compressors and other utility operated cyclic devices. This implement utilizes very simple, yet tried and true principle of synchronous motor electromechanical analog integration.
It does not use digital circuitry of any kind. The device is electromechanical and the run time is accumulated very accurately in an entirely analog process.

This allows the accumulation of run time over a fairly long period to provide you a good, repeatable and accurate run time and cycle percentage.

It can also be used to estimate the kWh consumption of devices such as electric dryers, water heaters and electric furnaces with a greater accuracy than estimating from the whole-house kWh reading. More than one sets can be used when multiple points must be observed.

It involves accessing hazardous voltage and a great care is required as with any electrical projects. It is very robust, simple and do not require specialized training in computer programs, digital electronics or specialized laboratory tools. The principle of operation is very simple and easily understood by the average people.

You need a plug-in AC analog or a flip plate type clock, which you can find at thrift stores or maybe even new at Walmart.

To allow measurement of 240v sources, a 50W transformer type 240v to 120v travel adapter is used which is economically available at department stores. The transformer is not necessary when measuring 120v equipment.

The clock runs when energized and stops when deenergized. Unlike digital microcontroller embedded systems based clock, it is easily stopped and started without special knowledge of electronics.

The advancement of time relative to elapsed time is the duty cycle. For example, if you set the clock to current time and you observe it has elapsed 4 hrs 30 minutes after 10 hours, the run duty cycle is 45%.

For something like a dryer, one set would be wired to the heating element which is thermostatically controlled and another set wired to the motor.
the advance of time on motor represents the total run time and heater clock represents the heater run time during cycle. The motor and heater power can be easily determined by the ampere measurement.

120v x amps through motor = motor watts
240 x heater amps = heater watts.



Let's say the two clock are set at 12:00 and you place one across the motor, and another across the heater through a travel transformer.

At the end of cycle, the motor clock reads 1:15, heater clock reads 12:45

Using previously measured data, you can estimate that motor: 3A = 360W (the series wound type motor or PSC type have pretty close to 1.0 PF)
Say heater measures 20A =4800W
360W x 1.25 hrs = 450Wh

(45min/60min/hr = 0.75 hours)
0.75hr x 4800W = 3,600Wh

3600+450. Estimated power use for that drying cycle is 4.05kWh or 13,822 BTU
To gather power use with a greater accuracy, a two-wattmeter real time integrating energy meter such as a revenue grade kWh must be used, however for educational and estimation purpose, I find this setup to be adequate.

The starting weight of wet load minus the weight of clothes + lint(hopefully not much. that's fabric wear) will allow you to calculate the latent heat of water in BTU.

The latent BTU divided by BTU of kWh usage is the dryer efficiency.

You could use also use two setup to independently monitor upper and lower elements of water heater, so you know if upper element ever gets used. If the upper element almost never sees use, the water heater is over sized.


Last edited by ICanHas; 09-01-14 at 10:42 PM..
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Old 09-01-14, 11:29 PM   #2
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That works nicely for loads that stay constant (when on) like most resistive loads, but not so well for heat pump type loads where the power draw can vary a lot with conditions.
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Old 09-01-14, 11:47 PM   #3
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Quote:
That works nicely for loads that stay constant (when on) like most resistive loads, but not so well for heat pump type loads where the power draw can vary a lot with conditions.
It will work perfectly with heat pump. Read first, then post. I never advocated for kWh estimation of heat pump, but a duty cycle measurement in those applications.

It will accurately measure DUTY CYCLE in percent in any application
It is not susceptible to lock-up, surge glitch or reboot, or failure as common with the digital embedded system microcontroller design.

It will estimate kWh fairly well in constant load.

If you need a cumulative kWh, a decommissioned REVENUE GRADE kWh meter like this can be used, although the wiring complexity puts it out of casual DIY experiments as it presents additional safety concerns wiring up 240v through it.


Actually, the kWh used together with the AC clock will let you get average duty cycle as well as kWh.

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Old 09-02-14, 12:51 AM   #4
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But duty cycle is usually not the end goal, instead, it's the average wattage.
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Old 09-02-14, 03:06 AM   #5
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Read first, then post. Post #3 clearly addresses that.

1.) The GE meter shown in the aforementioned post is a certified revenue grade meter. It has the ability to log the amount of energy passed through in the form of kilowatt-hrs.

2.) kWh is defined as the 3.6MJ, which is derived by the fact that 1kW = 1kJ per second and running that load for one hour which is equal 3,600 seconds adds up to 3,600kJ or 3.6MJ.

It can be concluded that by writing down the starting date and time, the kWh reading, then dividing the accumulated kWh by elapsed time in seconds it will give you the "average watt"

The duty cycle(the percentage of time it runs during the duration of test) is a useful indicator in estimating roughly how much of the AC's capacity is used. I never said that it will estimate kWh of an AC unit.

I said that it will measure the duty cycle of anything, and it will ESTIMATE the kWh of water heater or electric furnaces by measuring the exact duration of usage. It's not a revenue grade or energy performance certification laboratory grade instrumentation.

Last edited by ICanHas; 09-02-14 at 03:12 AM..
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Old 09-04-14, 03:24 PM   #6
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Since I bought my house, I've worked a shift that requires a 7 day(as opposed to a weekday/weekend thermostat) because of a 4 day workweek. When shopping for a thermostat I found models that count the call for heating and cooling in hours and minutes. It shows today, yesterday, this week, last week, and since last reset. I reset it at the start of the billing cycle and can get spot on with the heating cost and for the cooling I can get a pretty close guess.

Low cost TED units, I got mine for $20 on ebay, are good for cumulative kwh metering. I'll be installing an e-gauge for my solar system and will also install additional sensors to monitor the AC compressor(will soon integrate a more efficient mini-split heat pump that will be measured). If you want a revenue grade meter, you can get them from Hialeah Meter Company I like the TED, e-gauge, etc. type unites though because you can see real time usage which helps with working down your base load at the whole house power use level which works great in conjunction with Kill-a-watt type single outlet meters.

I used a kill-a-watt to determine the duty cycle for my fridge and it works just fine, the compressor is always between 145-150 watts after it has been running for its initial minute. It seems remarkably consistent with the on and off times if the door remains closed with the same ambient temperatures so measuring 4 cycles or so with a stop watch and you've got the average duty cycle down pretty close. As an example, my fridge runs for 10 minutes and the cycle starts every 38 minutes. It consumes just under a kwh a day. Depending on the indoor dew point and how often I open the fridge, I'll kick the defrost on every other week which consumes about 225wh to defrost with the 600 watt element usually running for 15 minutes and a 30 minute compressor cycle to catch up. This is a side-by-side ice through the door, non energy star unit. Disabling the 10 hour automatic defrost(timer was running off of clock time, not compressor time) was the best thing I've done for it because it has cut the energy use down to about 80% of where it was before.

Hope this helps.

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