Ohms law in theory and practice

[ant101]

Hi to all,

I understand Ohms law in theory, I=V/R. I have tested a fan I have salvaged from an old boiler using some flex, with a 3 amp fuse fitted into the plug (same rating fuse would be in boiler). All is fine and fan works. 240V is going to the fan and the fan has a resistance of 40 ohms.
According to ohms law this gives a current of 6 amps! Why dosent the fuse blow? Also if this was in a boiler there would be a 2 amp quick blow fuse fitted making it more likely to blow! I understand that fuses must have some tollerence but logic tells me it woudnt reach 6 amps or have I missed something.

Thanks for any help in advance

 

[ban-all-sheds]

You've missed the fact that motor windings are inductive.

The motor may well have a resistance of only 40Ω, but its impedance will be a lot higher.

 

[JohnW2]

In addition to what BAS has said, which applies to any inductive load (in practice, mainly things containing motors/pumps etc.), you will also find an apparently anomoly if you try the same exercise on, say, a traditional (incandescent, not CFL/'energy saving') lamp (light bulb). In this case, the inductance is negligible, so the impedance (which is what determines the current in an AC circuit) will be almost exactly the same as the resistance you measure. The problem in this case is that the resistance is much lower when the lamp is cold (i.e.when you measure it) than when it is operating and hot - so your resistance measurement will give you the impression that it will draw a lot more current than it actuially does when hot (it does carry the high current you would calculate from Ohm's law, but only for a fraction of a second when first switched on, before it heats up).

Kind Regards, John.

 

[Space cat]

BAS is correct. Your meter is using DC to measure resistance, not impedance. Try the same thing with the primary winding of a mains transformer and see what you get.

To add further complications, a spinning motor is also a generator. The voltage it generates is called back emf and it acts in opposition to the applied voltage. The effect of this is to reduce the voltage actually 'seen' by the motor. There are two experiments you might like to try:

1) Insert an ammeter in series with the fan and measure its running current.

2) Stall the shaft then QUICKLY repeat the measurement.

The second test will give you a higher current because there is no back emf. I emphasize the need to be quick because you risk burning out the motor with this higher current.

 

[ant101]

Thanks everyone for taking the time to reply. I got more info from here and in a lot less time than I did trying to google it. You have all pointed me in the right directon and I will read more when I have the time, but am really satisfied now. I have more questons though.
I am curious about what happens to electrons once they have done their work, ie lit the bulb etc. Do they lose their charge? thanks

 

[JohnW2]

I am curious about what happens to electrons once they have done their work, ie lit the bulb etc. Do they lose their charge? thanks

They are always there, and unaltered (aleays have their charge) - they just move around the circuit.

Think of an electron as one of the links in a bicycle chain. After a particular link has 'done its work' in turning the bicycle's wheel, it just moves to a position in the 'circuit' beyond the gear wheel - but it's still there, and part of the 'current flow' around the circuit. When flow stops (i.e. when you stop peddling), it just remains stationary, wherever it is - the same with electrons in a circuit.

Kind Regards, John.

 

[ant101]

Thanks for that, that was a good analogy. Is it possible to still get as much of a shock then? I ve heard people say that you cant get a shock off a neutral wire once the electons have passed through saya bulb. Maybe AC is totally different.

 

[Paul_C]

To add further complications, a spinning motor is also a generator. The voltage it generates is called back emf and it acts in opposition to the applied voltage.

This is also why there is a switch-on surge of current when you start a motor which is many times greater than the running current: At the moment the power is applied the motor is stationary, hence no counter-e.m.f. is being generated until the motor starts to turn and gets up to speed. If you've ever noticed a lamp dim momentarily when starting a motor of any substantial size (e.g. a circular saw), that's why - The motor is drawing much more current for that brief moment, resulting in the voltage dropping. It's especially noticeable at the end of a long extension cord, etc.

I ve heard people say that you cant get a shock off a neutral wire once the electons have passed through saya bulb.

It has nothing to do with that at all. It's because neutral conductor (at least in U.K. public supply systems and in most other countries too) is earthed. In order to sustain an electrical shock, there needs to be a potential difference between the two points of contact. If one of the points is earth (because you're standing on it), then the other point you touch must be at some voltage relative to earth for you to feel a shock. Under normal operation, the neutral is at or very near to earth potential (no more than a few volts at most).

If there is a break in the neutral conductor, however, the side which is then not connected to earth will rise to a high voltage by way of the circuit from the live side - Then you can most certainly get a shock from it.

 

[ban-all-sheds]

Think of an electron as one of the links in a bicycle chain. After a particular link has 'done its work' in turning the bicycle's wheel, it just moves to a position in the 'circuit' beyond the gear wheel - but it's still there, and part of the 'current flow' around the circuit. When flow stops (i.e. when you stop peddling), it just remains stationary, wherever it is - the same with electrons in a circuit.

Electric currents in solids typically flow very slowly. For example, in a copper wire of cross-section 0.5 mm2, carrying a current of 5 A, the drift velocity of the electrons is on the order of a millimetre per second.

 

[JohnW2]

Thanks for that, that was a good analogy. Is it possible to still get as much of a shock then? I ve heard people say that you cant get a shock off a neutral wire once the electons have passed through saya bulb. Maybe AC is totally different.

As Paul has said, that's nothing to do with the reason. There does not have to be a movement of electrons (i.e. a current flowing) for there to be a risk of a shock - although there will be a flow of electrons (through you) if you do receive a shock. Voltage is the driving force that can make electrons move (hence current flow) if there is a circuit for it to flow through (and that includes a circuit through you, if you get a shock).

You ought to try some book about basic electricity - maybe someone here can recommend one.

Kind Regards, John.

 

[Risteard]

It's because neutral conductor (at least in U.K. public supply systems and in most other countries too) is earthed.

Surely a neutral conductor is always earthed, otherwise it ceases to be a neutral conductor.

 

[JohnW2]

Electric currents in solids typically flow very slowly. For example, in a copper wire of cross-section 0.5 mm2, carrying a current of 5 A, the drift velocity of the electrons is on the order of a millimetre per second.

I know that, but please bear in mind the level of the OP's knowledge and try to avoid confusing him/her by trying to teach how to run before how to walk.

Kind Regards, John.

 

[Paul_C]

Surely a neutral conductor is always earthed, otherwise it ceases to be a neutral conductor.

Not at all - It would still function as a neutral without being earthed. Norway uses such an arrangement, for example.

 

[Space cat]

Surely a neutral conductor is always earthed, otherwise it ceases to be a neutral conductor.

True. If you break the connection between a neutral wire and earth, it will cease to be neutral and you could get a shock from it. The problem is that, from its colour and its position in the circuit, it still LOOKS like a neutral conductor. So beware, things are not always what they seem.

Edit: The connection from neutral to earth probably isn't in your house; it'll most likely be at the substation. Depending on your distance from the substation, you might find a small voltage difference between neutral and earth. Try measuring it.

 

[Paul_C]

True. If you break the connection between a neutral wire and earth, it will cease to be neutral and you could get a shock from it.

Neutral does not mean "having negligible or no voltage to earth."

There is perhaps a little confusion surrounding this because the Wiring Regs. have long contained a very poor, and technically incorrect, definition which says that the earthed circuit conductor of a simple 2-wire system is also to be called a neutral conductor. But that inexactitude aside, you can still have something like a 3-phase wye system in which the common point of the transformer windings is not earthed, or is earthed only by way of an impedance. The conductor extended to installations from that point is still a neutral, because it carries only the imbalance current of the connected loads.