When you run a motor, does that involve installing pipe & wire?
I got ya..... I guess I should have said 'operate' instead.
Will it trip the breaker?
- Thread starter Gary11734
- Start date
- Status
When you run a motor, does that involve installing pipe & wire?
I got ya..... I guess I should have said 'operate' instead.
Adamjamma
Senior Member
- Location
- Jamaica and london
actually, based upon my usage of things here in England, I would say that cost wise, doubling the voltage has reduced the operating cost... the 240 volt toaster toasts in about 5/6th the time, and at 1/2 the amps of the 110 version so, it uses less watt hours, if I am right on my math... the same with the kettle, well, the kettle uses less amps... 11 compared to 15, and it takes 2 minutes approximately to boil, compared to three minutes in Jamaica... same style kettle.
Now, one area that makes no sense to me is cooking times on stoves as my stove in the USA is 240, my stove in UK is 240, difference is 240 three wire to 240 four wire... or one live to two live... yet water boils quicker by 3 minutes for a spaghetti pot in the USA... but eggs take one minute less time to cook the way my wife loves them.
Not everyone is so anal as me to keep track of such idiotic things like the toothbrush charging quicker or the phone charging quicker on its wall wart... same adapter but different country and the 110 to 240 being the difference... but, in almost every case the UK lets me use the AC for less time. And with me turning the switches off to limit standby current losses, it seems I use less power at 240 than at 120...
But, perhaps the more learned on here can explain to me how I am spending more here? Other than the Pound to Dollar difference?
K8MHZ
Senior Member
- Occupation
- Electrician
When you run a motor, does that involve installing pipe & wire?
I got ya..... I guess I should have said 'operate' instead.
Obviously I am playing on words because run can mean either install or operate.
K8MHZ
Senior Member
- Occupation
- Electrician
Are you sure about that?
Are the losses the same, including line loss from the meter to the unit?
That ain't an ammeter on the side of the building the power company installed.
K8MHZ
Senior Member
- Occupation
- Electrician
Of course not, but to determine if it's cheaper to operate on 240 vs. 120 we must also know how much of the energy from the poco was turned directly into cooling, as opposed to motor and conductor losses due to heat or mechanical losses.
Do you know for sure that both 120 and 240 volt air conditioners produce exactly the same BTU's per watt?
I was talking about lighting.
K8MHZ
Senior Member
- Occupation
- Electrician
:thumbsup:
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Losses in lighting circuits have no effect on cost, unless you're going to install a higher-wattage bulb to compensate.
With heat or AC, the unit will have to run longer to maintain a given thermostat setting, so losses are a minor factor.
With heat or AC, the unit will have to run longer to maintain a given thermostat setting, so losses are a minor factor.
brian john
Senior Member
- Location
- Leesburg, VA
WOW, I bet a toaster sold in 2019 won't be operating in 72 years When quality counts.
- Location
- Illinois
- Occupation
- retired electrician
Yes, I don't expect that much of anything we buy new now will still be working 72 years later. The only repair to the toaster is that I replaced the cord about 15 years ago.
Not obsolescence?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
In any training program, you are almost always forced to simplify things to get the basic concept across.
If you assume the basic simplification of a constant value resistance and OCPD that instantly trips at the handle value, then the answer is simple: the breaker trips.
The fact that these simplifications are _wrong_ does not make the question poorly worded; that is a question of context. IMHO if the instructor has been clear upfront that they are starting with simplifications (and perhaps even gives a few examples), then at the right point in the training program this is a great question. I don't think it would be fair on a test unless there is a requirement to show work, but it is fine for starting a discussion in class. As long as the students understand that they are working with a simplification, and that a more accurate answer might require better approximations.
However because of these simplifications the instructor must be ready for different answers that use better approximations to reality.
We never stop using approximations. An electrician is expected to be able to do voltage drop calculations. How many times do you simply use the NEC conductor properties for voltage drop, and how many times do you adjust your values for conductor temperature? How many times do you adjust your values for conductor cross section manufacturing tolerance? If you just use the NEC conductor properties, your answer is not going to be correct, but is likely close enough for practical purposes.
-Jon
If you assume the basic simplification of a constant value resistance and OCPD that instantly trips at the handle value, then the answer is simple: the breaker trips.
The fact that these simplifications are _wrong_ does not make the question poorly worded; that is a question of context. IMHO if the instructor has been clear upfront that they are starting with simplifications (and perhaps even gives a few examples), then at the right point in the training program this is a great question. I don't think it would be fair on a test unless there is a requirement to show work, but it is fine for starting a discussion in class. As long as the students understand that they are working with a simplification, and that a more accurate answer might require better approximations.
However because of these simplifications the instructor must be ready for different answers that use better approximations to reality.
We never stop using approximations. An electrician is expected to be able to do voltage drop calculations. How many times do you simply use the NEC conductor properties for voltage drop, and how many times do you adjust your values for conductor temperature? How many times do you adjust your values for conductor cross section manufacturing tolerance? If you just use the NEC conductor properties, your answer is not going to be correct, but is likely close enough for practical purposes.
-Jon
kwired
Electron manager
- Location
- NE Nebraska
You are making some wrong assumptions here. Newbies might get it wrong, but that can be expected. Experienced electricians that spent most of their time doing new installs may even get it wrong. Experienced troubleshooters know exactly what will happen and on top of that telling them to assume fixed resistor or that the fuse is the weakest link tells them something is wrong with the question as that isn't what they will see as typical for the situation you described.
The fuse is responding to current - but your current is only slightly over the fuse rating and still within the trip curve so time becomes a factor on when it blows. The wattage imposed on the heating element is approximately 4 times what it is designed for, approximate because it isn't exactly a fixed resistance, at the temp it will be operating it will have some resistance change. At the temp the element will be operating it likely will fail before enough time has passed to blow the fuse. This true for most common fuses that may be found in circuit supplying such load, You could have fuses designed with less time in the trip curve and then it may trip before the element fails. Standard thermal mag breakers are also going to hold longer than the heating element does.
Bottom line is it is somewhat of a loaded question - but you eventually have made the point you were trying to make clear, which is for resistive loads a change in voltage results in a reasonably equal proportional change in current.
Or one I seem to get a lot is if single vs three phase motor uses more power. Efficiency can be different between two single phase motors as easily as it can be different between a single and a three phase motor, same output power is needed to drive the load regardless of what is driving it though.
You gave us a question and we answered it. Then you started to move the goal posts. "Assume the fuse is the weak link". "Assume it is a 'fixed resistor'". And when we start making assumptions and adjusting our field position about where we are in relationship to these moving goal posts, our assumptions are summarily dismissed.
Agree 100%
What has to be remembered, this was a first-year apprentice class. What does a first-year student know? But, what are we doing in class this first year? Learning math. The teacher is saying, do the math.
This was to understand a FIXED resistance, and how to solve for it, period. But, what we found was that persons that had been in the field were claiming pretty quick that the current would drop in half immediately without doing the math. That to me was a surprise.
Understanding that the toaster, rated at 1000 watt, was a product of a fixed resistance number, and it was based on 120 volts as manufactured. Understanding the correlation of that is what is good about the problem.
Now, as I change the motor straps in a dual motor winding from one voltage to another, I can see the resistance changing because I remember this problem.
I realized we could have built-in bias from real-world experiences based on some of the answers of that class. I tend to live outside of the box, so this problem intrigued me in how we bring bias to our world of reality.
kwired
Electron manager
- Location
- NE Nebraska
Doubling the voltage to a motor also leads to high current, unless you change the winding connections - which when done correctly on a typical dual voltage motor each individual winding actually sees the same voltage regardless of what the incoming circuit voltage is. In a teaching situation you must make that clear that not only has the input voltage changed but so has the overall impedance of the load when it is intentionally done with the goal of same output power. Same can even be said for dual voltage heating elements when intended to have same output watts at either voltage.
Add: I guess what I am saying is it isn't just the math that is different it is the variables that determine the math that are different, which comes back to understanding theory and not just math.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
The crux of the biscuit here is whether one assumes the toaster to be a constant power device, which it isn't. That's what the question is testing, not what the time constant of the breaker is. It's a question for apprentices; it has to be kept simple. I think that if "seasoned electricians" often get it wrong, it's more likely because they are thinking about all the complicating details, not because they don't know how electricity works.
kwired
Electron manager
- Location
- NE Nebraska
I've seen fair amount of unintentionally getting 120 volt circuits on a 208 volt "high leg", all sorts of equipment from things containing heating elements to incandescent lamps to motors. Never have seen fuse or breaker open, seen many items burn themselves out as result of the excess voltage being applied. One case was POCO's fault, changing things on their side of meter - they checked rotation when done but never verified high leg was in proper position. This was at a cattle feed yard, a lot of Merc vapor lights (this was many years ago now) failed, and some items in a shop building. Two that I recall was a waste oil furnace and an overhead door operator - both those was lucky enough damage was just to control transformers.
- Status