What amp fuse for a fridge freezer

[SUNRAY]

Understandable!

I must say that I usually struggle to reconcile these quoted "annual consumption" figures with the apparent reality!

Kind Regards, John

me too. surely a family will open the door more frequently than a singleton and equally the amount of stored contents will dictate the amount of stored heat.

Like so many other things 'they' have to come up with a standard measurement system (just like fuel consumption of a vehicle). I imagine, and it is only a guess, there is some standard; ambient & internal temperature, amount of contents and opening times & duration for consumption measured over the course of a specified time. Any deviation from any one of those standards will affect consumption.
A chippy we did some work in monitored their appliances, a 6ft high domestic quality fridge was way over consuming but it was being opened frequently and only 3ft from hot fryers.

 

[fixitflav]

I'm still rather confused. I agree that, to achieve a given load (hence same electrical power, if efficiency remains unchanged), if one varies voltage, then" current would vary inversely" - that's what Watt's Law tells us, but you previously suggested that it was "the opposite of what Ohm says" (I presume you meant Watt).

What am I missing?

Kind Regards, John

The equation input power = V*I*PF applies to an electric motor, so V and I are inversely proportional for a given power.

Ohm's law says I = V/R so for a given R, I varies directly as V.

If you measured the DC resistance of a motor and used it to calculate the current at normal supply volts, it would be much higher than motor FLC. I think the explanation is the motor generates a back-EMF so the effective voltage across the winding is much less than supply volts.

"the opposite of what Ohm says" (I presume you meant Watt).

No, I meant Ohm. He's the one who says I is proportional to V. The power formula says I varies inversely as V.

 

[DetlefSchmitz]

Where do people get off inventing something called Watt's Law? They seem to just be referring to a formula for power.
The only law Watt made was 'Watt's pots never boil".

 

[fixitflav]

One has to look at the motors rating plate to see the running current at different voltages, they don't all follow the same power curves.

Can you give an example? Nameplates may show different voltage and current for delta and star connections, but AFAIK at the same power.

 

[fixitflav]

Where do people get off inventing something called Watt's Law? They seem to just be referring to a formula for power.
The only law Watt made was 'Watt's pots never boil".

Fair point, this thread is the 1st time I've heard it called that. I took it as another way of saying W = I*V*PF ( if it's AC)

 

[JohnW2]

The equation input power = V*I*PF applies to an electric motor, so V and I are inversely proportional for a given power.

Yep - and it applies to any load supplied with AC, not just a motor

Ohm's law says I = V/R so for a given R, I varies directly as V.

Indeed it does, although one would have to change R to Z in the case of AC.

Where you confused things, at least for me, is when your prefaced your comment "Opposite from what Ohm says" with "for a given power output, increased voltage gives smaller current". Mr Ohm must still be obeyed - if an increase in voltage is accompanies by a decrease in current, that must mean that resistance (impedance) has increased, and if you fed Mr Ohm's equation with the correct value for R (Z) in that situation, it would inevitably give you the correct current.

If you measured the DC resistance of a motor and used it to calculate the current at normal supply volts, it would be much higher than motor FLC.

Of course. All you're saying is that the ('AC') impedance of something inductive can be (and usually is) very different from its DC resistance.

That doesn't alter the fact that all electrical systems must obey Ohm's Law (not 'behave opposite to it'), provided one uses impedance, rather than DC resistance, if it's an AC system.

Kind Regards, John

 

[JohnW2]

Where do people get off inventing something called Watt's Law? They seem to just be referring to a formula for power.

Quite so. Until yesterday I'd never heard of such a thing, and I only went along with it only because that's what the calculator that CBW linked to called it.

However, I suppose it's no less 'permissible' than it is to put Mr Ohm's name to something which is "just a formula for voltage, current or resistance" ?

Kind Regards, John

 

[JohnW2]

me too. surely a family will open the door more frequently than a singleton and equally the amount of stored contents will dictate the amount of stored heat.

Quite so.

Like so many other things 'they' have to come up with a standard measurement system (just like fuel consumption of a vehicle). I imagine, and it is only a guess, there is some standard; ambient & internal temperature, amount of contents and opening times & duration for consumption measured over the course of a specified time. Any deviation from any one of those standards will affect consumption.

Indeed - but my personal experiences seems to suggest that those assumed 'standard conditions' never seem to be very realstic in in terms of how I, for one, use the appliance/car/whatevr. I am perhaps not surprised that, more often than not, they seem to give an over-optimistic idea of what will happen in 'normal use' (at least, my 'normal use')!

Why, I wonder (even though I can guess the answer ) do virtually all heat-producing appliances sold in the UK have their 'wattages' quoted at 240V (a voltage not only higher than the UK nominal supply voltage but also, I think, higher than the 'nominal' in any nearby country) ? !

Kind Regards, John

 

[JohnW2]

Nameplates may show different voltage and current for delta and star connections, but AFAIK at the same power.

Yep, we discussed that here at length recently. As you say, they merely indicate the phase-to-phase and phase-'neutral' ('star point') voltages of the same supply that will result in the same power - so the voltage given for star (phase-phase) will be about 1.732 times the voltage given for delta (P-'N').

I think that is only really of relevance to large motors which one wants to start in star, then switch to delta once they are running (using same supply) - since I can't see why anyone would deliberately run (rather than start) a motor in star when a much smaller motor would produce the same power (with the same supply) if run in delta.

Kind Regards, John

 

[DetlefSchmitz]

Quite so. Until yesterday I'd never heard of such a thing, and I only went along with it only because that's what the calculator that CBW linked to called it.

However, I suppose it's no less 'permissible' than it is to put Mr Ohm's name to something which is "just a formula for voltage, current or resistance" ?

Kind Regards, John

Not really. Ohm never talked about resistance. He stated a 'law' (or observation) which said that the current through a conductor between two points is directly proportional to the voltage across the two points.

 

[JohnW2]

Not really. Ohm never talked about resistance. He stated a 'law' (or observation) which said that the current through a conductor between two points is directly proportional to the voltage across the two points.

Fair enough, but I imagine that it was not him who called it 'a Law', that being something that others have done subsequently.

I haven't a clue as to the answer, but did not Mr Watt perhaps make a comparable observation that 'power' (maybe heat production?) was proportional to the product of voltage and current? If he did,, I guess that someone way subsequently have decided to call it 'a Law'?

Kind Regards, John

 

[fixitflav]

Yep - and it applies to any load supplied with AC, not just a motor

No! The difference is – with a motor, if the voltage is increased, the power stays constant, so the current falls. With a passive circuit, AC or DC, the current increases, and the power rises (as V^2). Imagine the kit is in a “black box” and you can only measure volts and amps. Admittedly in that scenario the motor power might rise a little, proportional to speed if the load is constant torque, but the effect is small and doesn’t alter the conclusion

Where you confused things, at least for me, is when your prefaced your comment "Opposite from what Ohm says" with "for a given power output, increased voltage gives smaller current".

I meant that looking at the system as a whole (from outside the black box) with a motor, rising volts giving lower amps is opposite to Mr. Ohm.

Mr Ohm must still be obeyed - if an increase in voltage is accompanies by a decrease in current, that must mean that resistance (impedance) has increased, and if you fed Mr Ohm's equation with the correct value for R (Z) in that situation, it would inevitably give you the correct current.

Yes, but you must also use the correct voltage. A motor generates a back-EMF (as I mentioned before) so the voltage for Ohm is supply volts minus back-EMF. On the assumption that back-EMF = supply voltage at synchronous speed, and proportional to speed, with typical 3% slip at full load, back-EMF = 97% supply volts and Ohm only sees 3%. I did some calcs a while ago for speed variation with applied volts on above basis, and it agrees pretty well with manufacturer’s data. If you’re interested I’ll post it.

 

[SUNRAY]

I've done a fair bit of work with motors in my work as 'controls' and I can honestly say that not all motors are equal, one classic example; we equipped 3 sites with virtually identical systems, 2 were running at ~230/400V, 3rd was >>240/415V. Basically there were conveyor belts, fans and pumps, I can't remember which way round but let's say at increased voltage pumps ran at higher and fans ran at lower current.

We had some cold water booster pumps which run at vastly reduced pressure/capacity on the standby generator producing <230/400V to the point they didn't keepup with maintaining the cistern in a block of flats and had to be replaced. I don't recall taking current readings.

Some even run hotter at increased voltage while other do the same at reduced voltage.

I'll get the 'ard 'at

 

[DetlefSchmitz]

Fair enough, but I imagine that it was not him who called it 'a Law', that being something that others have done subsequently.

Yes, but with the justification that the original statement that current was proportional to applied voltage made sense as being referred to as a law. And the formulaic representation encapsulates that law into a useful tool by including the definition of the constant as 'resistance'.

I haven't a clue as to the answer, but did not Mr Watt perhaps make a comparable observation that 'power' (maybe heat production?) was proportional to the product of voltage and current? If he did,, I guess that someone way subsequently have decided to call it 'a Law'?

Oh come now. Watt was a mechanical engineer who never did an electrical experiment in his life. He died before Ohm publicised his discovery of the constant relationship between voltage and current. His name was associated with power when they started assigning 'scientists' names to units. It is disappointing when people propagate incorrect history with no facts to back it up. This is the down side of the internet.

 

[fixitflav]

Yes, but with the justification that the original statement that current was proportional to applied voltage made sense as being referred to as a law. And the formulaic representation encapsulates that law into a useful tool by including the definition of the constant as 'resistance'.

Oh come now. Watt was a mechanical engineer who never did an electrical experiment in his life. He died before Ohm publicised his discovery of the constant relationship between voltage and current. His name was associated with power when they started assigning 'scientists' names to units. It is disappointing when people propagate incorrect history with no facts to back it up. This is the down side of the internet.

Does it matter what we call it, as long we use the right formula? A rose by any other name would smell as sweet!