- Joined
- 17 Aug 2010
- Messages
- 2,889
- Reaction score
- 507
- Country
Good grief! Well I can't stop you. Crack on.
What amp fuse for a fridge freezer
- Thread starter Ihavenojob
- Start date
Sponsored Links
I will
Where does that idea come from?
Are you really suggesting that the power produced by a motor is independent of the voltage it is supplied with, and that if one increases the voltage of the supply, the current drawn from that supply will fall?
You appear to be talking about a 'notional voltage'. As far as the supply is concerned, its voltage is its voltage. Within the 'black box' that we call a motor, the effect of the back EMF impacts on the behaviour of the motor, and essentially changes the 'effective impedance/resistance'. The 'effective voltage' (supply voltage MINUS back EMF) which the innards of the motor 'sees' is not something that one can usually measure.
Provided currents didn't get so high that they 'blew up' the supply or caused a protective device to operate, If you measured the voltage being supplied to that 'black box', it would obviously be the same whether the motor was stalled (hence no back EMF) or running at high speed (high back EMF).
Kind Regards, John
Last edited:
Fair enough - but, regardless of whose name one associates with it, would it make any less sense for a statement that power was proportional to voltage multiplied by current to be regarded as 'a law'?
Although I made some attempt to rationalise/explain this, I'm really completely 'with you' about this. As I said, until two days ago I'd never seen or heard any reference to "Watt's Law" - and I confess that when I started replying to CBW's post, my very sentence was something like "There is, of course, no such thing as Watt's Law ...". However, I then looked at the calculator he had linked to and found them using the term, so decided not to post that
Although I'd never previously heard of it, as you say, it is 'all over the Internet (as are so many iffy things!) - a search for it (in quotes) gets about 30,000 hits. However, it's not limited to 'the Internet' For example, the "SAE International's Dictionary for Automotive Engineers" includes it (click here) .
However, my second shock in the last couple of days came when, in the "Academic Press Dictionary of Science and Technology", I found this ...
... which again is a totally new one on me, and maybe the start of a whole new discussion
Kind Regards, John
Sponsored Links
- Joined
- 27 Jan 2008
- Messages
- 25,288
- Reaction score
- 2,956
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
I must admit the use of electronic motor control has changed things. But the main problem is we have no idea how the motor is controlled, I first had this problem with school laptops, input 100 - 240 volt at 1.5 amp. Output 20 volt at 3.25 amp.
So input 150 - 360 watt assuming PF = 1, but output is 65 watt, it is clear the little package is not producing even 90 watt of heat, so likely current in is just 0.3 amp, not 1.5 amp, even at 100 volt only 0.65 amp, not 1.5 amp. So the trolley could have 20 laptops in it all charged with a single 13 amp plug, not as the spec sheet suggests max of 8 laptops.
Yes we have in rush, but I know the 13 amp fuse never ruptured with 20 laptops.
Had the same problem with refrigeration units, a accommodation camp with may be 200 AC's, swapping generators to check oil, so power turned off for 5 minutes, at 8 amp each, that's 1600 amp, but it never drew that much, if off for an hour OK a problem, all would start together, but for 5 minutes could simply turn it back on again.
Had the motors been three phase then it would have been different, but single phase motors may take three times run current on start or more, but no where near what a three phase would draw, a energy meter set to record peak with a non inverter motor on a freezer would show around 250 watt, and 65 watt run, a pain as the defrost heater took around 120 watt, with an inverter motor the max can show if auto defrost is working, but with a non inverter needed it to data log to see if heater switching on or not.
Some times theroy does not seem to work, you need to suck it and see.
So input 150 - 360 watt assuming PF = 1, but output is 65 watt, it is clear the little package is not producing even 90 watt of heat, so likely current in is just 0.3 amp, not 1.5 amp, even at 100 volt only 0.65 amp, not 1.5 amp. So the trolley could have 20 laptops in it all charged with a single 13 amp plug, not as the spec sheet suggests max of 8 laptops.
Yes we have in rush, but I know the 13 amp fuse never ruptured with 20 laptops.
Had the same problem with refrigeration units, a accommodation camp with may be 200 AC's, swapping generators to check oil, so power turned off for 5 minutes, at 8 amp each, that's 1600 amp, but it never drew that much, if off for an hour OK a problem, all would start together, but for 5 minutes could simply turn it back on again.
Had the motors been three phase then it would have been different, but single phase motors may take three times run current on start or more, but no where near what a three phase would draw, a energy meter set to record peak with a non inverter motor on a freezer would show around 250 watt, and 65 watt run, a pain as the defrost heater took around 120 watt, with an inverter motor the max can show if auto defrost is working, but with a non inverter needed it to data log to see if heater switching on or not.
Some times theroy does not seem to work, you need to suck it and see.
Have you never plugged a SMPS into a live socket and it sparked? even left a black burn?
That inrush is likely to be a decent number of amps.
Thankfully W1 is not here to peddle his incorrect statements about SMPSs.
My guess is your set of 20 laptops is at least 30A, maybe 100A but as you say it's so quick the fuse doesn't stand a chance.
That inrush is likely to be a decent number of amps.
Thankfully W1 is not here to peddle his incorrect statements about SMPSs.
My guess is your set of 20 laptops is at least 30A, maybe 100A but as you say it's so quick the fuse doesn't stand a chance.
Last edited:
It's not just my idea, it's published in motor catalogues. I'll try to scan a couple of sheets and post them.
Yes, within the design voltage range, and probably some way outside it in practice. I've tried to explain several times. The motor manufacturer is testing his machine. He supplies it at say 400v and adjusts the load (a dynanometer say) to rated power. He then increases the voltage to 415v. The speed increases (see #27) and so does the load. So he readjusts the load back to the rated power, and hey presto the current is then lower than before, in accordance with I*V = W. That of course is input power whereas rated power is shaft output power, but eff and PF won't change enough to alter things.
Yes, there's a nominal voltage, but the motor can accept nominal +/-5%. Just as well, as otherwise if there were any problem and the volts weren't exactly right they'd say sorry, not my problem guv.
I think you're agreeing with what I said.
No, inside the black box is a pre-determined load, so no-load or locked rotor conditions do not arise.
Last edited:
Data sheets attached, as photo and pdf. Information might be on the internet these days
Attachments
Doesn't always work like that.
I've explained the circumstances when it does
Many of the motors I came across, had the +/-5% stamped on the plate.
Yes you have but not explained the circumstances when it doesn't
There is a difference between the maximum power a motor can produce, and the amount it's actually producing.
For the most part, the power drawn by an AC induction motor during normal continuous operation is determined by the mechanical load placed on that motor. On an induction motor, Increasing the voltage will increase speed,but only slightly because the motor is already running at close to it's maximum speed. The result is that voltage rises faster than power and current drops.
If the voltage is too low, the motor will not be able to produce enough power and will likely stall. A stalled motor is *BAD* news, it's power draw will be significantly higher than when running normally, but there is nowhere for that power to go except producing heat in the motor.
Historically I've have lots of pics of motor plates but changing a PC recently I had a major clean out so think all gone. My recollection of more recent motors- which may be totally discombobulated - is something like; 380-420V 10HP 15A (not a real motor!). My domestic heating pump shows 220-240V 70W-0.29A, 60W-0.26A, 50W-0.23A (I haven't done any calcs but those current figures feel like a narrow spread).
EDIT: Forgot to mention older motors plates used to carry more info but then again may be totally discombobulated.
That's up to anybody else who can be bothered.Yes you have but not explained the circumstances when it doesn't
All I said was a motor doesn't obey Ohm's law.DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
