# Electricity noob



## Coulomb (Apr 22, 2009)

Bertzie said:


> Volts=RPMs. More volts=higher RPM
> Amperes=Torque. More Amperes=more torque.


A bit simplistic, but basically correct.



> Watts=fuel. Increasing Volts or Amperes increased Watt consumption.


Watts are more like fuel flow, say in fluid ounces per minute, or whatever fuel flow is measured in in imperial units.



> I understand Watt Hours, (or rather Killowatt hours) from the electric bill, I'm just not entirely sure how those fit into an electric motor consumption.


Right. This is electrical energy, not power.



> Now, 1 Ampere at 1 Volt would consume 1 Watt, I just don't know what the time frame would be of that. Would be it be over an hour? A minute? A second?


It depends on how large your energy source is. If you have a button cell, it might produce a watt for only a few seconds. An EV pack can produce an average of some tens of thousands of watts for about an hour.

That's why the capacity of a pack is measured in energy units, like kWh (kilowatt-hours). Say the capacity is 20 kWh; it can produce 20 kW for an hour, or 10 kW for 2 hours, or 40 kW for half an hour. If your average power consumption is 20 kW, you can drive for an hour. I'm ignoring a thing called the Peukert effect, which means at 40 kW you might get a bit less than half an hour, and at 10 kW you might get a bit more than an hour. It's a limitation of the batteries; in effect, the capacity of the pack reduces with faster discharge. Modern lithium iron batteries have much less of this effect than the older lead acid packs.



> The mechanical stuff is dead simple compared to this >.<


It's just what you are used to. I find a lot of the mechanical side difficult, because it's not as familiar to me.


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## tomofreno (Mar 3, 2009)

> Now, 1 Ampere at 1 Volt would consume 1 Watt, I just don't know what the time frame would be of that. Would be it be over an hour? A minute? A second?


 Are you asking how long it would take to consume 1 Watt? Power is energy per unit time, so energy being used or produced at a given rate. A typical small sedan might use 250 Wh (Watt-hour) energy per mile going 60 mph, or a power of 250 Wh per minute, or 15 kW.


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## Sunking (Aug 10, 2009)

Bertzie said:


> I understand Watt Hours, (or rather Killowatt hours) from the electric bill, I'm just not entirely sure how those fit into an electric motor consumption.
> 
> Now, 1 Ampere at 1 Volt would consume 1 Watt, I just don't know what the time frame would be of that. Would be it be over an hour? A minute? A second?


There is no time frame to a watt. The unit Watt is just a measurement of electrical power at a given moment in time. Watt Hour(s) is a measurement of energy over a period of time and is a very complicated math formula that takes 16 years of public education to learn or 30 seconds in a private school. 

The formula: Watt Hours = watts x hours. 

That is why they call it watt-hours to confuse the Russians and Chi-Comms

You have a 100 watt light bulb and turn it on for 10 hours and you use 100 watts x 10 hours = 1000 watt hours or 1 Kwh.

Shall we move on to Amps and Amp Hours?


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## Bertzie (Oct 13, 2011)

Sunking said:


> There is no time frame to a watt. The unit Watt is just a measurement of electrical power at a given moment in time. Watt Hour(s) is a measurement of energy over a period of time and is a very complicated math formula that takes 16 years of public education to learn or 30 seconds in a private school.
> 
> The formula: Watt Hours = watts x hours.
> 
> ...


But then wouldn't that 100watt lightbulb be consuming 100 watts an hour?

And we can move onto amps and amp-hours. I likes to learn.


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## lithiumlogic (Aug 24, 2011)

The charger that Jack Rickard showed us on EVTV this week was rated at 5kw (5000 watts). It's sucking energy out of the mains at a* rate* of 5000 watts. Nobody cares about how long it can do this because it's mains... it should do it forever.

The battery pack is rated at 40 kw hours (40,000 watts for 1 hour. Or 1 watt for 40,000 hours, theoretically). killowatt-hours is a measure of *capacity*

Current is flowing through the power cord to my fridge at a *rate* of 5 amps

My car battery has a *capacity *of 50 amp hour. This means it can theoretically supply 50 amps for 1 hour. Or 1 amp for 50 hours.


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## nxp (Apr 6, 2011)

Bertzie said:


> But then wouldn't that 100watt lightbulb be consuming 100 watts an hour?


Not exactly - power (watts) is how quickly you are using up energy - the "standard" unit of energy is Joules. Watt-hours is (confusingly for many people) NOT a measure of power, but a measure of energy. You might ask, "why use two different units for energy?"

A 100 Watt lightbulb consumes 100 Joules every second, or 200 Joules in two seconds, and so on.

If you leave the 100Watt bulb on for an hour, it has consumed 100 Joules x 3600 seconds (the number of seconds in an hour). This give a "whopping" 360000 Joules used. It's not a really big amount of energy, but since the Joule is a "small" quantity of energy people need a "bigger" unit to be able to describe things with reasonable numbers. (This is similar to miles per hour - you can describe speed with yards per hour, or inches per hour of you want, but you start using silly numbers then!)

Now scientists often use "1000 Joules" as a unit of measure since it keeps a certain type of maths simple, and they call it a "kilojoule".

Engineers often prefer to use a unit of measure of "3600 Joules" since this is the amount of energy a 1 Watt bulb (or anything else pulling 1 Watt) consumes in 1 hour. This helps keep some maths (completely different to that used by the scientists) simple and they call this unit of 3600 Joules a "Watt-hour".

If instead of a 100W bulb you left a 1000W bulb (okay, a room heater) on for one hour, it would have consumed a total of 3600000 (3.6 million) Joules, or in much more manageable units 1000 Watt-hours (also called 1 kilowatt-hour). This quantity of energy is what power companies all over the world use as a "unit" of energy for sending you a bill.

Did I overcomplicate things or is it a bit clearer?


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## Bertzie (Oct 13, 2011)

Technically speaking then, wouldn't Watt-hours and Joules be interchangeable to a certain degree?


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## nxp (Apr 6, 2011)

Yes they are - you just have to be VERY careful to multiply or divide by 3600 when using them both in the same calculation.

I can't think of any example better than the inches-yards-miles example I mentioned above - you're measuring the same stuff, but at a different "size".


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## Coulomb (Apr 22, 2009)

Bertzie said:


> Technically speaking then, wouldn't Watt-hours and Joules be interchangeable to a certain degree?


Joules are in fact exactly interchangeable with watt-seconds. So 1/3600th of a watt-hour. BTW, the "-" in watt-seconds should be interpreted as a multiplication, not a subtraction (which it looks like). Some people use watt.seconds or W.s (and similarly W.h) for this reason.

A 100 watt bulb consumes 100 Wh (watt-_hours_ per hour, not 100 watts. One watt-hour per hour is more commonly called a watt.

So one last time: a 100 W bulb turned on for 2 hours consumes 100 W (watts) for the whole time, but it uses 200 Wh (watt.hours) over the course of the two hours. Over the first half hour, for example, it was still consuming 100 watts, but had only "clocked up" 50 Wh, as could be read by an energy meter.


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## nxp (Apr 6, 2011)

Coulomb, wouldn't that be 200 Wh total, rather than 2Wh total?


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## Duncan (Dec 8, 2008)

The problem is that watt hours (Wh) is a bastard measure

as is Ampere hours (Ah) - charge! 

We should not be using these units as they simply lead to confusion 

Power is in Watts
Energy is in Joules
Charge is in Coulombs 

Using bastard units is the road to NASA space probes ramming into Mars!


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## Coulomb (Apr 22, 2009)

nxp said:


> Coulomb, wouldn't that be 200 Wh total, rather than 2Wh total?


Eek! Just like me to try to help, and end up confusing more. Sigh.

Corrected now; my apologies to all for the confusion.


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## Bertzie (Oct 13, 2011)

Nother question, more specifically about electric motors, since that's what subforum this is. 

If a motors max RPM is 5500, and it's max voltage is 170, does that mean at 170 volts it will spin at 5500rpm? 

I'm looking at the netgain motors right now, (Not set on them, their site just has a lot of information) and that's the limits given on the WarP 9.

Now, assuming that what I'm assuming is correct, then at half the volts, you would get half the RPMs. (2750). That would allow me to run more amperes through for higher torque while keeping electric consumption down. (And, even those RPMs are high, which could be then turned down for even more torque!)

Am I anywhere near right on this?


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## major (Apr 4, 2008)

Bertzie said:


> Nother question, more specifically about electric motors, since that's what subforum this is.
> 
> If a motors max RPM is 5500, and it's max voltage is 170, does that mean at 170 volts it will spin at 5500rpm?
> 
> ...


Hi Bert,

These are series wound DC motors. The RPM is load dependent as well as proportional (approximately) to applied voltage. The specified maximum RPM for the motor applies regardless of voltage. It is the mechanical or construction aspect of the machine.

The rating and cooling method for the motor determine the load (and time at the load) at which the motor can be run, to a large degree independent of voltage. Load is the torque (related to current).

These aspects are depicted on the motor performance characteristic curve. Often called the speed torque curve. The motor supplier should have the curve available for you. You can also find examples and discussion regarding the proper interpretation of these curves on the forum. Learn to use that forum search tool 

Regards,

major


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## Bertzie (Oct 13, 2011)

Well, since I have all you nice people coming into my thread, it's easier for me to just ask here. 

Plus, then all the information I need is in one spot. Plus, most of the stuff out there is in complicated form I don't understand. But back to the topic at hand. 

To increase RPMs, you increase voltage, don't you? And to increase torque, you increase amperes? (Yes, I really want it to be that simple, cus if it is that simple, I'll have an idea of what I need.)


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## dougingraham (Jul 26, 2011)

Bertzie said:


> To increase RPMs, you increase voltage, don't you? And to increase torque, you increase amperes? (Yes, I really want it to be that simple, cus if it is that simple, I'll have an idea of what I need.)


Not quite that simple. Voltage is proportional to RPM. Current is proportional to torque. The problem is that the only thing you can control directly is the voltage. The motor controller has the ability to present a varying voltage to the motor from zero up to the traction pack voltage. The current is controlled by the load. Lets pretend that we have an ideal motor and it has a Kv (RPM per volt constant) of 100 This means that an unloaded motor would turn 5000 rpm on 50 volts. So if you suddenly applied 50 volts to the motor it would do its best to reach the commanded RPM and if this was an ideal motor it would draw an infinite amount of amps and the next instant the motor would be turning 5000 RPM. The current it would continue to draw after it reached the 5000 RPM would depend on the load placed on the shaft. Unfortunately we don't have ideal motors. Real motors have resistance and inductance and drag and rotating mass. All of which affect these things. Resistance in the windings, brushes, and cables will cause a voltage drop such that the motor does not see the 50 volts and therefor does not get the command to turn at 5000 RPM. The voltage the motor sees will depend on the current because of the resistances. Since applying that 50 volts when the motor is not turning results in a very high initial current the voltage drop is also very high. This is why the motor cannot spin up instantly. Probably a good thing too since an ideal motor would break something every time you commanded a change in the voltage.

The way we change the current is to command a different RPM, either higher or lower. That way the motor sort of has a goal it is trying to reach. The RPM will change until the actual voltage the motor sees is in balance with the voltage drop due to resistance from the wires in the system.

It is more complicated than this but that is the gist of it.


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## Bertzie (Oct 13, 2011)

But then how do you get excessive torque or RPMs? 

Say I want 240ft lbs of torque, at 1500rpms. How do I figure out what to give a motor to get that?


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## Coulomb (Apr 22, 2009)

Bertzie said:


> To increase RPMs, you increase voltage, don't you? And to increase torque, you increase amperes?


More or less. The problem is, you don't get to choose those two independently.

Controllers generally have one input, the pedal (throttle) setting. On many controllers, this basically sets the PWM ratio (Pulse Width Modulation), which effectively sets the voltage. Example: you have a 120 V pack, the pedal is at 75%, and the controller maps this to a PWM ratio of 83.3% (= 5/6). The output voltage is then 5/6 x 120 = 100 V.

Other controllers set the current. 75% throttle might map to 80% current, so if you have a 500 A controller, that would mean 400 A to the motor.

What will the speed of your motor be at say 100 V or 400 A ? It depends on the load. I'll assume a standard series wound DC motor here. If the motor is unloaded, e.g. in neutral, then likely the motor will spin to very high speeds and destroy itself. I'm not kidding; this is a hazard of series wound motors.

But suppose we have a moderate load. Suppose further that the current speed of the motor, say 2500 RPM, results in a back EMF of 95 V. This 95 V subtracts from the 100 V applied by the controller, so that 5 V appears across the field. This will produce a certain current through the field, say 300 A. This will produce a certain torque, say 100 ft.lbs (sorry, I'm metric, so this is a wild guess). Let's say that the load on the vehicle results in a torque load of 90 ft.lbs at the motor. That means that the motor will have 10 ft.lbs left over which will cause a gentle acceleration.

So now the motor is spinning faster. Remember how we said that RPM is proportional to voltage? Well, it's really the other way around: back EMF is proportional to RPM. So now the back EMF gently increases to 97 V, so the voltage across the field reduces from 5 to 3 V, so now the field current is only 60% of 300 A, so that's 180 A, so the torque is also only 60% of what it was before, or 60 fl.lbs. So now there is a net torque output of -30 ft.lbs, so the motor starts decelerating. It will hunt a little and settle at a speed such that the back EMF is enough to generate enough field voltage and hence field current and hence motor torque to balance the vehicle load. Let's say that is 2600 RPM.

Now the vehicle hits a hill, and the load doubles from 100 to 200 ft.lbs. Now the speed will have to adjust to a new value, such that the motor produces 200 ft.lbs. This will be a lower speed, but not half the 2600 RPM from before the hill. It might be say 2400 RPM, such that the back EMF is say 92 V, so there is some 8 V across the field, so the motor tries to draw 600 A. Ah, but we have only a 500 A controller, so the controller will output less voltage and maintain only 500 A output. So in reality it might settle at say 2200 RPM, with 80 V out of the controller, and 72.5 V back EMF, for 500 A of field current.

So you can see that the behavior is a bit complex, and it's hard to say what voltage to output to achieve a certain speed. Fortunately, there is a human in the loop, and s/he can regulate the pedal to achieve the desired speed. More pedal will give more voltage, which will tend to make the motor go faster, all things being equal. Just as with a piston engine car. We're all used to coming up to a hill, noting the speed drop, putting the pedal down, and still seeing the speed drop (just not dropping as fast as before) because it's a big hill. Similarly going down the hill, even though we take our foot off the pedal, the car might still go faster, and we might have to use the brakes to keep the speed reasonable.

Then there are AC motors and their controllers. The principles are much the same, except that the controller can control the field current independently of the armature current. That means that the controller can actually arrange things so that the torque output from the motor is negative, i.e. the motor takes power from the wheels and transfers it as electrical power to the battery pack (this is called regen, short for regeneration, I think). There are also DC motors where the field can be controlled separately from the armature; these motors are called "sepex" (for separate excitation). These can also offer regen.

In summary: you don't have to worry about setting the voltage and current to the motor. You can control one or the other; the motor controller will do this for you based on the pedal input. The other variable will sort itself out based on the current speed, load, and the motor characteristics.

[ Edit: oops, I've also been ignoring the fact that as the field current reduces, the back EMF per RPM also reduces, further complicating things. ]


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## Bertzie (Oct 13, 2011)

I'm not worried about setting it, I'm worried about choosing the right motors and controllers for a specific performance level. In order to decide that you need to know which electrical doo-dad controls which force, and then pick one accordingly. 

Is a motor of X amps and Y volts going to put out enough RPMs and Torque to do the job. Then you have to make sure you have a controller that puts out enough volts at enough amps to get said torque at said RPMs.


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## Coulomb (Apr 22, 2009)

Bertzie said:


> If a motors max RPM is 5500, and it's max voltage is 170, does that mean at 170 volts it will spin at 5500rpm?


This would be the case for a permanent magnet DC motor, with no load. With a load, the speed would drop slightly, perhaps to 5000 RPM with 170 V applied. This is so that there is enough voltage drop across the armature resistance to cause enough current to flow.

But permanent magnet DC motors are not common in electric vehicles. There are a few brushless DC motors with permanent magnets around, but despite their name they are really AC motors, and we can ignore them for the purposes of this discussion.

Most EVs use series wound DC motors, or various kinds of AC motors. For this common case, the motor voltage splits between the armature and the field. As I tried to explain in the post above, this makes predicting the current somewhat tricky.

However, at low speeds and high loads (e.g. accelerating from a stop), these motors draw a lot of current, which produces high starting torque. At higher speeds, the back EMF causes less voltage drop across the field, so less field current, which reduces the back EMF, effectively changing gears, at least to a degree. So the series DC motor can provide high torque at low speed, and now torque at high speed, somewhat like a piston engined vehicle with a gearbox. That's why some EVs don't need a gearbox; they can be "direct drive".


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## Bertzie (Oct 13, 2011)

So how do you find a motors maximum torque then?


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## Coulomb (Apr 22, 2009)

Bertzie said:


> I'm not worried about setting it, I'm worried about choosing the right motors and controllers for a specific performance level. In order to decide that you need to know which electrical doo-dad controls which force, and then pick one accordingly.


Ok, fair enough. The motor will have a "characteristic curve" which relates important variables such as voltage, current, torque, speed, and efficiency. So you choose a motor with the right torque and speed, and their product, mechanical power; this will necessitate a certain voltage and current, and their product, electrical power.

As you say, you need to make sure that you have enough voltage for the motor to go fast enough, and enough current so that you will have enough torque.


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## Coulomb (Apr 22, 2009)

Bertzie said:


> So how do you find a motors maximum torque then?


The manufacturer should provide that value. You may choose to exceed that value, risking excessive arcing of the commutator, or overheating of the motor windings.


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## Bertzie (Oct 13, 2011)

This is the performance chart from the motor I'm looking at. Is it safe to assume that those lines continue beyond the chart? It says this is at 72volts. What if I wanted to run it at 120volts? Cus a site I'm looking at says it's got a max of 170 volts, and at 1k amperes puts out 237ft lbs of torque. But the manufacturer site doesn't say anything about that.


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## Coulomb (Apr 22, 2009)

Bertzie said:


> This is the performance chart from the motor I'm looking at. Is it safe to assume that those lines continue beyond the chart? It says this is at 72volts. What if I wanted to run it at 120volts?


This has been a continuing problem; for whatever reason the manufacturers don't seem to be forthcoming with higher performance data. Perhaps they think that if they provide the data, people might actually use them at reasonable power levels, and they might get more failures. It's especially frustrating from NetGain, since their motors are supposed to be designed for EVs, not golf carts.

It sounds like you're ready to digest this thread:

torque of 9" dc with 144v pack?

This isn't necessarily talking about your Transwarp model, but the principles will be the same.

Note that the above chart is all for 72 V; you don't "extend" this chart for 120 V. You should really be able to find another chart for 120 V or 144 V; a very quick search didn't turn one up. I'm pretty sure that some users have published dynamometer results at more typical EV power levels. T\I believe that the torque verses current will be much the same at higher voltage, only the RPM verses torque curve will change significantly. But I'm suddenly unsure about that.

If you want to use the motor at above 500 A, then you'll have to extrapolate, or find better data. As you can see, extrapolating to 1000 A from the 340 A shown will be largely guesswork.


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## Bertzie (Oct 13, 2011)

Coulomb said:


> This has been a continuing problem; for whatever reason the manufacturers don't seem to be forthcoming with higher performance data. Perhaps they think that if they provide the data, people might actually use them at reasonable power levels, and they might get more failures. It's especially frustrating from NetGain, since their motors are supposed to be designed for EVs, not golf carts.
> 
> It sounds like you're ready to digest this thread:
> 
> ...


Well my original torque/rpm/volt data came from this website. http://www.evsource.com/tls_transwarp9.php

This thread was more about understanding how the different forces of electricity work to do what they do. It's not strictly necessary to know these things, I just want to know as much as I can. So thank you, (and all the other posters) for being patient with my ignorance.


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