Something to bear in mind next time someone starts a cheap power tool thread! It is not only the motor losses that add up, there are also the mechanical ones in poorly made gearboxes etc. The upshot of which is that if input power is being quoted, the tool may perform way below the expectation that would be given from the numbers alone (PPPoo tools anyone?). If it is the output power being quoted (far less common on cheap tools), then you can expect it to draw more power than you would otherwise expect.
The only "question" not directly addressed that I can see, was the varying power factor attributable to different types of electric motor.
This will be most common tools with universal motors. Things with heaters can be somewhat inductive, so even your soldering iron may not have a power factor of 1.
Try this one:
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you can see there are in fact a large number of permutations of "electric motor" - not just "with brushes", and "induction". As you can imagine, as you start adding field coils to the load in addition to the motor windings, the electrical load starts getting ever more complex. Add to that the fact that many induction motors have a capacitor used to help them start which is switched out when running, you get a load that will have a dynamic power factor that changes as the motor starts, and runs, and also changes with (mechanical) load.
There is also the separate issue of the inrush current required by your appliance at switch on. If the generator is not able to meet the demands for that, then you may never get the thing running in the first place, even if in theory the generator should be able to supply it once it is up and running.
While in maplin the other day I picked up one of these:
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measures volt, current (and can show watts etc) as well as keeping track of kwh etc. One thing that surprised me was that it has a PF option that shows you the power factor of whatever is plugged into it. For 12quid it seemed a bargain so I picked one up. Haven't a clue how accurate it really is but its a gadget :-) The manual that comes with it is not exactly great either - nice engrish.
It seems to return about the correct values for my kettle and a couple of smaller appliances. Also, it shows the kettle to have a PF of 1 near as dammit (as I would expect) right down to my cordless drill charger which it claims has a PF of .25. Its a cheapy wickes cordless with a 1hr charger - I guess its plausible.
Not tried it on the fridge yet as that requires effort moving the fridge :)
I was curious about that, but the Maplin list (and the salesdroid) seemed to imply that it only measured immediate power. Can I set it off counting for a week and come back to a tally of kWh ? I want to track a dehumidifer that runs intermittently.
It also measures watts, volt-amps and power factor, so you can find out for yourself about all those different motors.
Although the product description doesn't mention kWh, the FAQ tab does. The specific questions and the answers are very muddled about the difference between "power" and "energy" but overall it seems fairly clear - clear enough to take a punt for £12.50, so thanks for the tip!
's interesting -- Maplin used to sell the Brennenstuhl PM230 version of this (the picture looks like a different make now). I bought the Brennenstuhl one when it was on special offer. Its measuring seemed OK for resistive loads, but for loads with high harmonic content and no phase shift (like anything with a switched mode PSU, compact fluorescents, etc), it was way off. It claims to measure phase angle and power factor and true power, but my suspicion was that it was assuming power factor < 1 would only ever be due to phase shift, which is wrong. It may be that it didn't even get that case right -- I don't think I tested it on any phase shifted loads.
Try it with a compact fluorescent with integral electronic control gear. The Brennenstuhl was wrong by a factor of 3 on such a load.
Ok, found the bit of paper now. According the the manual it can do:
Item Range Accuracy RMS Volt 180-250 Vrms (Max 1% typical 0.2%) RMS current 0-15 Arms (Max 1% typ 0.3%) Active Power 0-3750 Watts (Max 2% typ 0.5%) Apparant Power 0-3750 VA (Max 2% typ 0.5%) Line Frequency 0-63.0 Hz (Max 2% typ +/- 0.1Hz) Power Factor 0-1.00 (Max 0.03 typ 0.01) Energy Quantity 0-9999 KWh (Max 2% typ 0.5%) KWH hour 0:00-9999 (30 ppm)
Note: The typical accuracy for Volt is in the range 190V-250V and for current is in the range 0.2A to 15A
Press KWH shows the energy consumption then press key to display the time (hour) and active power for 1 second each automatically. KWH/Hour is a toggle function key, the LCD will show the sequence of KWH -> Hour -> KWH. Kilo-watt- hour is electronic energy. Hour is the duration time after poweron (ie from
No, I hadn't noticed that ! I was looking at a paper cat in Maplin's last week. Kevin the teenager was quite clear that it didn't do kWh and it wasn't clearly discernible without opening the package.
That's not something I'd ever do again - for _any_ Brennenstuhl product. I've had far too many of their multiway boards simply fall apart - they're as bad as Ring used to be.
Fairly, yes. 11/18 is nearer to 0.61 than 0.64 but it's (just) within the 0.03 accuracy limit you quoted in an earlier post. Nothing would appear to be out by a factor of 3.
I'll go and get one of these at the weekend if the local Maplin have got any. It seems too good to be true for the price...
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