You do realize that for AC motor watts, you need an actual wattmeter?
Measuring amps and volts, then multiplying, doesn’t work when coils are involved. Or capacitors. It might get you close, but you won’t know exactly how close.
The problem is that an ideal coil or capacitor does not use any energy, since it gives back all energy that was stored during an earlier AC pulse. The current is there in a coil or capacitor, and the voltage is there, but the energy just sloshes back and forth with an average flow of zero. This can be very confusing because the direction of current is supposed to slosh back and forth. But electrical energy doesn’t usually do that. For light bulbs and heaters, electrical energy flows continuously forward while the electric charges inside the wires wiggle back and forth. Think of air molecules vibrating in a pipe while the sound waves move forward. (And it doesn’t help that most simple textbooks pretend that charges and energy are the same thing, as if coulombs were a kind of joule, or amperes were a kind of watt.)
In my experience, people hate electricity because grade-school textbooks always get it wrong, and college-level books employ mathematical reasoning while avoiding verbal descriptions. My original background is science museum exhibit designs, where neither option is acceptable.
Simultaneously multiplying the instantaneous current (i) and instantaneous voltage (v) will give you instantaneous power regardless of whether it is a coil, cap, resistor, or something in-between. Assuming you’re sampling at greater than twice the Nyquist frequency, taking a bunch of iv samples over time – and then averaging them over an appropriate period – will give you the average power, regardless of whether it is a coil, cap, resistor, or something in-between. That’s what power analyzers do.
You can totally go with the modified extension cord for those appliances also. When you clamp the ampmeter around the wires, you will either get a reading or you won’t. So if at first you get no reading, clamp the other wire (no reading means you clamped the neutral or ground). You’ll be able to detect current the same way with them that you do with the dryer. However…most clamp on meters have a margin of error around .2 amps and a 200amp scale. So good luck with low current draw detection. Or…just get the Kill A Watt It should be pretty self explanatory on how to use it.
But again, if you’re doing an experiment, the important information to keep in mind is the “maximum” amp draw listed for the device…and it’s helpful information to have whether you’re experimenting or just diagnosing a problem. Gotta have something to compare your data to ya know?
I recall seeing a device that was manufactured specifically as an accessory for clamp-on ammeters for measuring the current draw of appliances, etc. It had a male plug on one end, a female outlet on the other, and in between the conductors were separated by a gap large enough to accomodate the pincers of a clamp-on ammeter. Does this ring a bell, and if so, any idea what it’s called and where I might buy one? Thanks.
I was using a 600 A “clamp around” meter last year to measure the current between a generator and dummy load. I was also measuring generator voltage and load voltage. Measurements were acquired using a 12-bit, high speed data acquisition system. The load was a “known” resistance.
The following day I analyzed the data in Excel. The data didn’t make any sense. I messed with the data for days, and it was obvious one of the measurements was wrong. But I couldn’t determine which channel it was.
So I went in the lab and hooked our 400 A power supply to a resistive load. I clamped the ammeter around one of the wires and it read… 400 A. So far so good. I then I touched the outside surface of the ammeter’s clamp to the other wire. It read 350 A! I moved the ammeter an inch away from the other wire, and it read correctly again (400 A).
I looked at the photos we took when we were testing the generator. The other wire to the load was touching the outside surface of the ammeter’s clamp. :smack:
Lesson learned: clamp-around ammeters are susceptible to external magnetic fields. Keep all other current-carrying wires away from the ammeter! At least an inch or two away.
I’m no EE but I have have tweaked an appliance or two. I don’t know the OP’s skill level, I sure don’t know what the Nyquist frequency is (was that Kenneth’s last name?) but I can share some fun facts about home appliances and their wiring.
In the USA dryers and ranges used to have a 3 wire connection. Line 1, Line 2 and Ground. No neutral. Your clothes dryer drive motor and timer motor ran one line and ground, not neutral. Heaters across L1 and L2. Your big old shiny O’Keefe and Merritt Electric Range had a coffee pot outlet on a timer; Yep the neutral prong and the ground prong were both connected to that ground wire. The Frigidaire Speed Heat Element used a 115 Volt heating element hooked to 230 Volts until it heated up and then switched over to 115 using that good old ground wire. The other surface and oven heating elements were across L1 - L2. This went on for quite a while and meantime the Phone Guy and the Plumber were getting shocked while working on Mobile Homes. Complained about it too. So the Code Guys made Mobile Homes use a 4 wire connection with a neutral. The Code Guys tried to make regular houses use that system too, but the Huge Evil Home Appliance Cartel kept it from happening. Only in the last 10 years or so has every jurisdiction adopted the NEC 4 wire requirement.
Electric Water Heaters are strictly L1 L2, no neutral needed. Same with your Heat Pump, Electric Furnace, Central A/C. All loads are 230*, no neutral required.
Exceptions and Work Arounds exist and are quite interesting to encounter.
Now that all appliances have that neutral in them the Huge Evil Home Appliance Cartel has been busy finding uses for it. That’s why your dryer can text the local Wally World and have them send you a box of Cling Free Sheets.
Progress.
*(208, 230, 240, whatever your local utility supplies, that’s another topic)
Yep it gets rather frustrating. Standard pressure switches for pumping equipment don’t have any room to work with, so it can be rather annoying to clip on a meter without interference. On the smaller submersibles the control box is also a disconnect so you can’t clip in there because it needs to be closed to run. Thankfully on the larger equipment where accurate amp draws become really important I can clip on in the spacious control boxes or on on the magnetic contractors.
I’m always looking for new close quarters ammeters. The always end up garbage or too expensive to have in my hand tool bag, no middle ground.
[QUOTE=Crafter_Man]
Lesson learned: clamp-around ammeters are susceptible to external magnetic fields. Keep all other current-carrying wires away from the ammeter! At least an inch or two away.
[/QUOTE]
I know in the past, it used to be important for the wire being measured to be as close as possible to dead-centrer in the meter’s “jaw” for best accuracy. Is that still the case, or have they been improved to the point where the reading is good or “good enough” as long as there’s one and only one wire in the jaw?
You should probably note that the OP was posted in 2011, over four years ago. She’s probably done with whatever project the question was posted for by now, but it doesn’t hurt to share for general knowledge.
