# Questions about batteries and electric motors

1. Jun 27, 2013

### guyknowslittle

a concept i can't grasp is the meaning of voltage for an electric motor. it seems all the motor needs to operate is a magnetic field which is generated by current. why does the electric motor "care" whether the current is fed to it from a 100V or a 200V battery? Isn't the concept of voltage useful only when talking about whatever delivers the current to the motor, like a battery?

2. Jun 27, 2013

### Staff: Mentor

How do you think, what would happen to a motor rated at 30 V when you will connect it to 3 kV source? And why?

3. Jun 27, 2013

### guyknowslittle

well it depends by how much current does the 3 kV source offer. if it offers very little current, i dont think the motor would care that its current came from an obscenely high voltage source.

why? - im not an engineer, im a chemist, i dont understand many things about how batteries are used to power devices but i work with new battery materials. so i hope you dont mind i ask "high school level" questions for an engineer

4. Jun 27, 2013

### Staff: Mentor

Not exactly. Have you heard about Ohm law?

5. Jun 27, 2013

### guyknowslittle

V=IR, so if the resistance varies with voltage, a high voltage can offer the same intensity of current. am i correct?

6. Jun 27, 2013

### Staff: Mentor

Resistance of the coil in the motor doesn't vary with the voltage. Think in terms of I=V/R.

7. Jun 27, 2013

### guyknowslittle

i meant i make it vary, maybe a second motor. but lets talk batteries.

so if i have battery A operating at 10V, 5A and 2ohm and battery B operating at 5V, 5A and 1ohm, will an electric motor care if it receives its 5A of current from the 10V or from the 5V battery?

8. Jun 27, 2013

### Staff: Mentor

Borek is trying to give you the answer, but you seem to be trying to make it wrong! The resistance is fixed, so if the voltage changes, what happens to the current via V=IR?

9. Jun 27, 2013

### Averagesupernova

The OP doesn't understand source impedance either. I find this behavior quite common with folks that understand just a little bit.
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Guyknowslittle, what I mean by that concerns your second post. A 3KV source that is only able to source a small amount of current will lose a lot of the 3KV across its own source impedance and the motor will see a seriously reduced voltage to start with. What the other folks in this thread have said makes good sense.

10. Jun 28, 2013

### Staff: Mentor

Basically, a 3kV source "offers" and can deliver 100 times as much current as does the 30V source. If it doesn't, then it's really not a 3kV source at all! If it delivers only as much current to the motor as does a 30V source, then as far as the motor is concerned, under those conditions it is really just a 30V source.

You are correct, in that a 30V motor doesn't care where the current comes from, SO LONG AS IT'S AT A VOLTAGE OF 30V BY THE TIME IT REACHES THE MOTOR'S TERMINALS.

11. Jun 28, 2013

### guyknowslittle

thank you for the kind explanations, i am starting to understand.

is the "load" to the "source" just a resistor? so the load of moving a car provides a higher resistance to a source than the load of lighting up a light bulb?

12. Jun 28, 2013

### sophiecentaur

A typical electrical power supply is one that maintains its voltage over a wide range of loads. A low resistance load will take MORE power not less. So a traction motor is much lower resistance than a light bulb.
Read some more about basics rather than trying to learn by a series of questions which may not necessarily be too relevant. Try a (possibly online) textbook.

13. Jun 29, 2013

### guyknowslittle

I read all the time. If you have a book to suggest please do. The questions are relevant to me. That's why I ask them.
So how does a source like a battery see a load. What in a load makes the battery discharge faster or slower?

14. Jun 29, 2013

### sophiecentaur

15. Jun 29, 2013

### guyknowslittle

Why so upset guy? I'm just asking a question in a forum which should have the answers. You don't have to answer but don't get bent out of shape. I am a chemist. I work with materials for new batteries. But I don't understand much about battery use. Maybe a few of you can shed some light I would be forever grateful.

16. Jun 29, 2013

### Averagesupernova

This is pretty basic stuff which can be found in books about basic electricity. If you take the time to read them you will learn way more in the amount of time these basic things can be explained here.
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Ohms law:
V = IR
R = V/I
I = V/R
Pretty much describes the current that a voltage source such as a battery will source. If you work with battery chemistry you surely understand that the more current taken from a battery the sooner it will discharge.

17. Jun 29, 2013

### guyknowslittle

Yes that is ohms law :). How does the discharge rate of the battery pack in an electric vehicle vary from 1C to 4C for example when the vehicle accelerates? It's the same electric motor and the same battery. How does the electric motor vary it's load on the battery?

18. Jun 29, 2013

### sophiecentaur

I'm not upset - just putting you straight.
So you are "a Chemist". Are you telling me that Chemists don't know the relationship[ between power and energy, or between charge and current?
We "should have the answers", should we? And how much are you paying for this on-demand service?
If you read back through this thread, the answers are all there - if you care to use your head to recognise them. Like I said, spoonfeeding is tiresome and not the reason for most people to contribute to this forum.

19. Jun 29, 2013

### Averagesupernova

Do some reading on electric motors. A motor uses more electrical energy when it is more heavily loaded mechanically.
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It seems you are making a lot of assumptions based on a very small amount of knowledge.

20. Jun 29, 2013

### guyknowslittle

Thank you. What would you suggest is a good source to read about electric motors? How is the motor more loaded mechanically? What assumptions am I making? I don't know much. Please share :)

21. Jun 30, 2013

### sophiecentaur

You could start by looking up the terms "Mechanical Load" and "Electrical Load" and see how they are defined and how they relate to Voltage, Current, Resistance, Energy, Force and Power (you know, I mean all the basics that you will have done in School, if you are a 'practising' Chemist').

When you have come up with a question that deals with stuff other than those basics then you will be surprised at the more positive reception you will get. Some 'self drive' can earn a lot of respect here.

22. Jun 30, 2013

### guyknowslittle

ok boss, will do. thank you for the term suggestions. i need all the guidance i can get.
i still think my questions are good questions though. :) they are fundamental. for example i would not get upset if someone asked me what is chemical potential and how it relates to the activation energy even though the formulas are taught in high school, i suppose. after a while its the fundamental stuff that becomes interesting, again.

as a matter of fact i looked in the chemistry section on this forum. those questions are very basic in the chemistry field. what is vapor pressure, what is dissolving, etc, and i see some of the same people who roasted me here give proper critical answers. may i please have the same courtesy even if my (simple) questions deal with a field in which the majority of members on this forum specialize.

i think you are a bit too critical. ease up a bit. it will make the forum more friendly to newcomers. oh, and were you asking for payment a few posts up? is that forum policy? maybe a mod should take a look at that.

Last edited: Jun 30, 2013
23. Jun 30, 2013

### sophiecentaur

The mods tend to have a sense of humour on this forum. I was not touting for business haha. I have had enough of that particular way of earning money.

Looking again at your earlier posts and other people's comments, I can see I am not the only one to take issue with your way of eliciting information here. You seem to have rejected some very sound information - which is why it's been suggested that you look to text books for your information. You are less likely to argue with them, perhaps. Question and answer method is a very inefficient way to get to grips with the whole of a topic. The Internet is chock solid with good basic Physics and you can always pick the particular level to suit you - unlike in the old days, where you had a text book and one teacher as resources. As long as you don't rely on isolated 'loony' ideas and stick to sites like Hyperphysics and those with .edu URLs, you can't go far wrong.

24. Jun 30, 2013

### Staff: Mentor

You may want to to read this:

https://www.physicsforums.com/blog.php?b=3588 [Broken]

Last edited by a moderator: May 6, 2017