1.) Why does the Power-Mate show a lower power reading than the multiplication of volts and current ?
2.) Why does the Power-Mate show a lower current reading than the specification plate of the appliance ?
3.) Will the Power-Mate work with a 60Hz line frequency ?
4.) Can you please tell me the time it takes for a Power-Mate to read the usage cost of an appliance such as a refrigerator. Is it immediate or does it have to be left connected for a certain time ?
5.) I need to measure the total energy consumed by a piece of equipment over a short period of time (in the order of a minute). The instantaneous power levels vary over that time from a few watts to about 300W, and the energy source being measured is the 240V mains. I would expect to measure something like 0.02kWh in a minute, so we need a resolution well under that. I'm sure your power resolution is adequate, but because of the short time and low power, the energy resolution may not be. What do you think ?
1.) Why does the Power-Mate show a lower power reading than the multiplication of volts and current?
The Power-Mate displays True Power - ie what the electricity company bills you for.
If you wish to know the apparent power (VA), simply multiply the RMS Voltage by the RMS current
readings.
For a purely resistive load the results between true and apparent powers should be
identical. For a capacitive or inductive load the apparent power will be higher than the
real power - because of the phase difference between voltage and current.
To calculate power factor, simply divide the true power by the apparent power. You can
also then calculate the phase angle if you wish.
When resistive loads are on the mains, I x E x pf = W (assume E = 240V, I = 10A, power
factor pf = 1), therefore W=2400
When reactive loads are present, pf is not equal to one, Irms x Erms = VA (Apparent Power)
assume E=240V, I = 20A, VA=4800
BUT we must take Power-Factor into account so although the apparent power is 4.8kW, due
to say a pf = 0.5 the true power consumed would be actually half or 2400W, which is the
same wattage as the first example with only 10A flowing ie. 240V x 20A x 0.5 = 2400W.
In the second case, the electricity company still has to supply the 20A of current,
although 10A of the 20A current is actually consumed by the load, the other 10A is
circulating current which is flowing through the energy suppliers conductors, mains
transformers etc.
Because this extra current flows to the load and then returns from the load back to the
mains supply, the supplier can't charge for extra power as it was not consumed, it was
supplied and then partly returned. The energy suppliers hate this because they have to
generate the higher current required in the first place, and send extra current through
their distribution system, transformers etc causing losses heat & stress etc. but they
can't actually charge for it so instead they charge a penalty rate (in other words they
find a way to charge you for it !) due to the consumers poor power factor. I understand
this does not apply to small domestic consumers yet, but does apply to larger industries.
Click here for more information on Power Factor.
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2.) Why does the Power-Mate show a lower current reading than the specification plate of the appliance?
The specification plate will usually show the maximum current for the appliance, this may occur momentarily only at switch on. For instance my monitor specification plate shows a current rating of 1.5Amps but it uses only around 250mA, or about one sixth of the amount shown during normal use.
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3.) Will the Power-Mate work with a 60Hz line frequency ?
The Power-Mate works perfectly well on 50 & 60Hz. The IC that does the Power-Calculations was designed & produced in the USA to work over a wide range of frequencies. Please note though, the Power-Mate is designed primarily for 240V The Power-Mate works fine all the way down to ~ 150V but would need modification for use at 110VAC.
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4.) Can you please tell me the time it takes for a Power-Mate to read the usage cost of an appliance such as a refrigerator. Is it immediate or does it have to be left connected for a certain time. ?
The Power-Mate should be left connected to a refrigerator (or any timed or thermostatically controlled appliance) for a complete diurnal (24 Hour - or multiple of ) measurement, if you wish to ascertain accurate quarterly or yearly running cost figures.
The fridge cycles on and off, the Power-Mate has no way on knowing in advance how long the fridge will run for, as the ambient temperature, number of times the door is opened, the heat load of items put into the fridge or freezer, the thermostat setting, the efficiency of the cooling system etc all contribute to the total power used.
If the Power-Mate was connected to a fridge during its compressors rest or off state, it would appear as if the fridge cost nothing to run, conversely if the compressor was running when the Power-Mate was first connected, and you checked the quarterly or yearly cost it would indicate a much higher figure than the 24 hour measurement will yield, due to the number of stopped or rest cycles that will occur.
The measurement ideally should be done during winter and again during summer and the results averaged to give a more accurate yearly figure.
From my experience after being connected to a fridge for 6 hours, the results will typically be within 10% of the 24 hour measurement, so estimated quarterly and yearly results may well be accurate enough for your purposes.
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5.) I need to measure the total energy consumed by a piece of equipment over a short period of time (in the order of a minute). The instantaneous power levels vary over that time from a few watts to about 300W, and the energy source being measured is the 240V mains. I would expect to measure something like 0.02kWh in a minute, so we need a resolution well under that. I'm sure your power resolution is adequate, but because of the short time and low power, the energy resolution may not be. What do you think ?
The Power-Mate resolves & displays to 0.0001 kWh in standard form so it should easily meet & exceed your requirements, the kWh is measured/calculated and updated each second. It is good to see an application that initially sounds difficult, but which exploits to some degree the capabilities that the Power-Mate unit was originally designed for.
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