AC or DC?

AC, DC, amps, volts and kilowatt. It's all discussed in here
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Post by antiscab » Wed, 11 Feb 2009, 07:21

the curtis 1231C has been in production for at least 15 years.
ditto for the adc 9" (adc fb4001).
144v pack is dictated by the curtis.

that adc 9" is actually a forklift motor, we just happen to be able to order them in brand new, rather than dig them up out of old greasy forklifts.

if you have a more powerful controller, you can do direct drive (as rob has done with his bmw 318i, zilla 1k, 160AH 144v TS pack, fb4001).

AC either involves big $$ or same $$ and big voltage and knowledge for the same performance.
thats why you dont see many

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Post by Tonic » Fri, 13 Feb 2009, 23:01

OK. I think I have most of that now. Thanks heaps Matt, Coulomb and Woody for your explanations. I'm sure it will sink in more over time.   From this discussion, and a little bit of WikiPedia reading, my understanding is basically as follows:

Energy is the amount of work that can be performed by a force.

A Coulomb (C) is the basic unit of charge of electricity. Thus if I have a fully charged battery then as the electricity is drawn out of it then the quantity of coulombs will decrease until the charge in the battery is fully extracted, at which point there will be zero coulombs remaining (but this 100% Depth of Discharge is bad). A Coulomb is not a measure of the amount energy; it is more like a packet of electricity.

A Volt (V) is a measure of the amount of Energy that one Coulomb contains; the higher the Volts, the greater the Energy.

An Amp (A) is a measure of the flow of Coulombs past a particular point in one second, where 1A = 1C per second.

An Amp-Hour (Ah) is simply the number of Amps per hour. Thus 1Ah = 1A x 60 secs x 60 mins = 3600A; or 3600C/s for one second, 360C/s for 10 seconds, or 1C/s for 3,600 seconds, etc. Therefore, a battery with a charge capacity of 40Ah, can deliver 3600Amps for 40 hours; or 144,000C to be drawn at whatever flow rate (A) that you desire.

A Joule (J) is the amount of Energy required to move one Coulomb through an electrical potential difference of one Volt.

A Watt (W) is a measure of the flow of Energy where 1W = 1 Joule per second; or 1W = V x A. Thus if I am drawing 1A from a 12V battery I have 1W of energy being drawn.

A Kilowatt (kW) is simply 1,000W, and is typically used to state the power output of engines and the power consumption of tools and machines. A kilowatt is approximately equivalent to 1.34 horsepower (hp).

A Kilowatt-Hour (kWh) is a unit of Energy and exactly equals 3.6 megajoules (3,600,000J).

So ... now that I have this knowledge how does this equate what I need to know in the selection of a motor? Do I start with the weight of the car and the distance I need to travel to work out how much energy I need to propel my vehicle, and from there pick a motor and from there pick a battery configuration? How will this knowledge help e decide whether I need AC or DC?
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Post by EVLearner » Sat, 14 Feb 2009, 00:23

Hi Tonic

That is a great summation of the basic electrical measures, just the Volt needs a bit of clarification.

What you had was: "A Volt (V) is a measure of the amount of Energy that one Coulomb contains; the higher the Volts, the greater the Energy." and this is a bit misleading.

The volt is the measure of potential difference (not a measure of electrical energy (coulombs), nor of power (watts) ).

When a current of 1 A flows through a resisitor of 1 ohm, the voltage (potential difference) across the resistor will be 1 V. In this case, the power dissapated from the resistor is P = I * I * R = 1 watt.

Where P = power (in watts), I = Current (in amps), and R = resistance (in ohms)

If the resistor was 1 k ohms, and the current was 1 mA, then that resistor would also have 1 volt across it. In this case the power dissapated will be 1000 * 0.001 * 0.001 = 1 mW

This shows that the potential difference (V) is not related to energy nor power, (in this sense) but you will see battries that have a (no-load) potential difference of say 13.4 V at their terminals.

If we series connect say 16 of these batteries then the total voltage will be 214.4 V.

Now the fun starts! If each of these batteries had a capacity of 40 Ah, then 16 batteries in series will have a total capacity of 40 Ah, but the potential difference is 214.4 V and not 13.4 V. In simple theory, this series battery could provide 40 A at 214.4 V and that power (watts) (I * E) is 40 * 214.4 = 8576 W, and it could (in simple theory) provide this for 1 hour!

If the 16 batteries were connected in parallel, then the total capacity would be 640 Ah, but the potential difference would only be 13.4 V. In simple theory, this parallel connected battery could provide 16 * 40 A (640 A) at 13.4 V and that power (watts) (I * E) is 16 * 40 * 13.4 = 8576 W, and it could (in simple theory) provide this for 1 hour!

The difference in these situations is the value of the load resistance and the wire thickness required to connect the battery to the motor. In the first case the total load resistance would be 214.4 / 40 = 5.36 ohms. In the second case, the total load resistance would be 13.4 / 40 /16 = 0.0209 ohms (both cases inluding the wiring)

Clearly, with these currents, and the same current densities in both cases, in the parallel case the wire will have to be 16 times thicker than in the series case to prevent being a "fuse"!


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Post by coulomb » Sat, 14 Feb 2009, 17:24

Tonic wrote: OK. I think I have most of that now. Thanks heaps Matt, Coulomb and Woody for your explanations. I'm sure it will sink in more over time.   From this discussion, and a little bit of WikiPedia reading, my understanding is basically as follows ...
Overall, really good. Just a few errors I'll correct below.
An Amp-Hour (Ah) is simply the number of Amps per hour.
No, it's amps multiplied by hours. So 2 amps for 3 hours or 3 amps for 2 hours is the same Ah, even though they are 2/3 and 1.5 A/h respectively (/ always means "per" or "divided by"). "A/h" is very different to "Ah".
Thus 1Ah = 1A x 60 secs x 60 mins = 3600A;
Strictly, 1Ah = 3600As (amp.seconds). Here, I use "." for "multiplied by"; I hope it's less confusing than x or * or - (which can be confused with subtraction).
or 3600C/s for one second, 360C/s for 10 seconds, or 1C/s for 3,600 seconds, etc. Therefore, a battery with a charge capacity of 40Ah, can deliver 3600Amps for 40 hours;
Whoa! That's 3600C (Coulombs; unfortunately C is also used for Capacity, the unitless quantity that is 40 for a 40AH battery). A battery that can deliver 3600A for 40 hours would have 144,000 Ah, which is like tens of submarine batteries in parallel.
or 144,000C to be drawn at whatever flow rate (A) that you desire.
Exactly. You were just confusing coulombs with amps.
A Watt (W) is a measure of the flow of Energy where 1W = 1 Joule per second; or 1W = V x A. Thus if I am drawing 1A from a 12V battery I have 1W of energy being drawn.
Perhaps that one was a typo: it would be 12W.

Just one wrinkle I'd point out here: with AC where inductors (or capacitors) are present, V x A strictly gives VA, "voltamps". The reason for this is that some AC power is actually "reactive", meaning that the energy you put in comes back out again, for an overall zero power. So you can connect a big capacitor across the mains and get 1A of current to flow, and have a power meter show zero. Usually, the terms "power" and "W / kW / MW" are reserved for real (non reactive) power, since this is the only kind of power that does real work, and which you pay for. But for DC, V x A does indeed always give real power, so the above is all correct for DC.
How will this knowledge help e decide whether I need AC or DC?

Unfortunately, the AC verses DC is more based on cost, and factors that are hard to define, like how much work you like to do with the gearstick. There are few situations where AC will work and DC is impractical; you can usually get it to work for either. At this stage of EVolution, it's probably safe to say that DC is best for the less experienced converter. Even if you have the money to buy a complete solution (say an AC24LS with its controller, or a Siemens motor with matching controller), the higher voltage (320v+) complicates the conversion a little.

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Post by weber » Mon, 16 Feb 2009, 04:20

coulomb wrote: Actually 415/sqrt(2) ~= 293v, the minimum bus voltage needed to generate 415v 3-phase. I may have incorrectly described that as 340v, i.e. 240*sqrt(2).
Er. The minimum DC bus voltage to generate 415 V 3-phase is 415 * sqrt(2) = 587 V, as someone else already said. I presume you meant +-293 V. But DC bus voltages are not usually described in that way. The VF drive only has 2 DC bus connections, not 3.

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Post by woody » Mon, 16 Feb 2009, 07:24

coulomb wrote: It turns out that because a motor wound for 240v delta can take 415v, then a nominally 15kW 50Hz motor is really also a 26kW 87Hz motor, and it will take that 26kW all day every day, just as it will take 15kW at 50Hz. The reason is that the current is essentially the same, and the heating is almost only proportional to the current. So even if you have a low voltage motor, assuming you have the inverter drive current, you still want a high voltage.
There's a few issues here this raises:

So the efficiency is up also at higher frequency? i.e. your 90% efficient 15kW motor @ 50Hz (1.5kW heat) is probably a 94% efficient 26kW motor @ 87Hz (still 1.5kW heat)?

Is your motor therefore less efficient in delta than star since the currents are higher?

Is rewinding a 400V motor to 100V going to kill your efficiency?

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Post by antiscab » Mon, 16 Feb 2009, 07:35

increasing rpm and frequency increases core losses and friction losses
but doesnt affect ohmic resistance losses

i spose the answer is "it depends" on the motor

i spose some dyno time would give a clearer view.

rewinding i wouldnt have thought would kill the efficiency, since the current density within the windings should stay the same (if not exactly then approximately).

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Post by woody » Mon, 16 Feb 2009, 07:52

antiscab wrote: increasing rpm and frequency increases core losses and friction losses
but doesnt affect ohmic resistance losses

i spose the answer is "it depends" on the motor

i spose some dyno time would give a clearer view.

rewinding i wouldnt have thought would kill the efficiency, since the current density within the windings should stay the same (if not exactly then approximately).

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Post by EVLearner » Mon, 16 Feb 2009, 14:29

Good questions Woody

I was actually considering that the frequency is independent and that in the low frequency range (say 20 Hz to say 60 Hz) the motor would be Star (Wye) connected and from say 50 Hz to say 150 Hz the motor would be Delta connected!

Now as far as current goes, much of the current that you are speaking about will be 'reactive' current, and because the (star connected windings would have a) greater inductance in the lower frequency range, (in Star config), this reactive current should be reactively limited. Because with the upper frequency range the motor is in Delta structure the reactive current is again limited by the frequency!

At the higher frequencies, the motor should be equally efficient by being Delta connected - or even more than equal as you point out. So you get the best of the whole drive frequency range.

BTW I had this 'stroke of genius' about switching the winding configuration during motor use from winding several hundred different communications based transformers some decades ago. At the low frequency end of the spectrum, the transmission efficiency of these transformers is highly dependent on having enough self inductance so as not to noticably shunt the load resistance. At the high frequency end of the spectrum, the efficiency is limited by the amount of leakage inductance between the windings (so the windings need to be low inductance).

Star connected motor windings have a much higher self inductance making them ideal for the lower frequency band, and switching the same windings into Delta substantially reduces the self inductance, so the torque range should be considerably raised at these higher frequencies.

From my view, the Star - Delta switch would be like an electric overdrive gear! I believe that we would get the extra torque in the high revs range where the wind drag is the limiting factor.

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Post by Tonic » Mon, 16 Feb 2009, 16:56

Thanks for that clarification. Does that mean a battery is a form of resistor? And I guess the name "resistor" implies "resistence", therefore it is resisting the flow of electrons?

I'm pretty sure now that I have a reasonable grasp of the mathematics behind things. I have also decided that I will go the DC route so that I can control costs and keep the wife onside ( Image ). Should I be looking for a motor based on horsepower or volt range? If not then what should I look for?
Last edited by Tonic on Mon, 16 Feb 2009, 06:04, edited 1 time in total.
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Post by Richo » Tue, 17 Feb 2009, 02:17

http://www.evmotors.com.au/products/appguide.html
That should help you choose a DC motor.
So the short answer is NO but the long answer is YES.
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Post by weber » Tue, 17 Feb 2009, 04:06

I was looking for the thread entitled "Induction motors or custard? and couldn't find it, so this thread will have to do. :-)

"What the hell is he on about?", I hear you say. Well it happened like this:

I was sitting on a stool in the kitchen eating some delicious firm vanilla custard and jelly while my sister washed the dishes -- as you do. And I said to her, by way of distracting her from the fact that she was slaving away and I was doing absolutely nothing useful for the future of mankind, "Which do you think is the greater invention, the induction motor or custard?".

Predictably, she replied, "What the hell is an induction motor?"

And I said, "It's the kind of electric motor that powers nearly all industry. The thing about it is, it seems so unlikely. You just make a certain geometrical arrangement of two different metals, feed it electricity and it rotates! Converting electrical energy to mechanical with extraordinary efficiency (more efficient the more powerful it is). There are no magnets in it and no electrical connections to the rotating part. It must have taken some kind of genius to realise such a thing was possible."

"Whatever you reckon", she says as she slops some more suds.

"But", I reply, "you could say exactly the same thing about custard! There is absolutely nothing about milk or eggs that even _hints_ at that wonderful flavour or texture, and apparently a temperature range of only 3 degrees Celsius separates custard from a useless curdled mess."

"What do you think Einstein would say", she said.

I thought for a moment, and proffered the opinion that he would go for custard, as having given the greatest pleasure to the most people. "However", I said, "you can't make a car go with custard."

"Why not", she said. And we all cracked up laughing, apparently with images of custard going glug glug glug into fuel tanks.

Why not indeed. The Australian Electric Custard Association?

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Post by Nutz » Tue, 17 Feb 2009, 05:58

Why oh why am I writing this reply?? Fatigue I think.

Considdering the high joule energy output, and it's need for no oils, it's pavlova that wins hands down.

There goes any cred I may have had!

Thanks for that link Richo, just the sort of info I needed. The 203-06-4001 looks loke the sort of thing I want to motivate a 1000kg donor car to be an ample performer. Should I keep the old geerbox or will it only provide un necessary weight and mechanical resistance?
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Post by Nutz » Tue, 17 Feb 2009, 06:09

Hey does anyone know if anyone has tried putting an electric motor (or 2) between the axles of a car with an independant rear end, eliminating the diff and it's mechanical resistance. How might weaccount for the wheels turning at different speeds in corners if there were only on motor?
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Post by antiscab » Tue, 17 Feb 2009, 06:15

yes its been thought of.

if you do away with the diff, you do away with its torque multiplication.
so where as 500Nm might be the minimum for direct to diff, 2000Nm would be the minimum for direct to wheel

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Post by Nutz » Tue, 17 Feb 2009, 06:24

Ok, so the benefits of a diff exceed the weight and loss of power.
Especially if it means you don't need a bigger motor or battery pack right? Does the same go for the geerbox?
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Post by Richo » Tue, 17 Feb 2009, 06:36

Mmmm harness the awesome power of custard to power the induction motor.
I'll think about that one another time Image

Depends on the car but using 203-06-4001 @ 60kW keep the gearbox.


Unfortunately there is not much room widthways to fit 2 motors.
However if the motor was vertical it could work.
But I doubt an existing production car could be registered with this arrangement.
And is likely to be as lossy as a diff anyway.

One motor is unlikely to work as you would need unidirectional floating hubs.
Which would mean you couldn't reverse.

Hub motors would be ok if you could put up with the unsprung weight.


Ditch the Star-Delta switching.
It adds alot of complexity for almost no benefit.
The small advantage would be if the DC caps are too small for the controller.
And you'll loose the extra benefit in powring contactors, extra cables or cost of 6 half bridges.
So the short answer is NO but the long answer is YES.
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Post by antiscab » Tue, 17 Feb 2009, 06:42

it depends on the motor controller combination in your particular car.

eliminating the gearbox does reduce weight, complexity and losses.
however, you would ideally need a motor that can put out as much torque at low rpm to match what the original engine behind a gearbox would put out in first, and still have a reasonable amount of power at speed.

DC conversions backed by a 1000A or bigger controller do this fairly successfully.
AC industrial component based conversions can do it aswell

if you are planning on going for a high revving low voltage custom AC setup (the AC24LS comes to mind) then you will *need* to keep the gearbox.

to make the decision, work out how many rpm the input shaft to your diff would be turning at your required top speed.
then figure out how much power your motor and controller (and battery?) setup can put out at that speed.
by overlaying speed on top of rpm (since we're talking single ratio) on the power vs rpm graph, you can see what your max power is at different speeds.

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Post by Nutz » Tue, 17 Feb 2009, 07:31

I'll have to do a bit of research on diff ratios in Mazda 1500s now, and hopfully find gear ratios too.

"Hub motors would be ok if you could put up with the unsprung weight."

I'm a little confused on this matter, in my mind the weight is removed from the suspended vehicle and put directly on the road, why is this a disadvantage?
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Post by Richo » Tue, 17 Feb 2009, 07:50

So the short answer is NO but the long answer is YES.
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Post by Tonic » Tue, 17 Feb 2009, 16:14

Do all DC motors contain brushes? I read here that brushes in DC motors are bad because they obviously wear out over time, but also because they produce ozone, cause heat loss, etc. This site makes AC motors look a much more efficient option than DC, and three phase AC looks even better still.
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Post by coulomb » Tue, 17 Feb 2009, 16:32

Tonic wrote: Do all DC motors contain brushes? I read here that brushes in DC motors are bad because they obviously wear out over time, but also because they produce ozone, cause heat loss, etc. This site makes AC motors look a much more efficient option than DC, and three phase AC looks even better still.

There are motors called "brushless DC" (BDC), same as the tiny fan in your computer's power supply. They go up to about 5kW, but not often to EV sizes (only a factor of 2-2.5, so they might not be far off). The problem seems to be the large fields de- or re-magnetising the permanent magnet, which is a special rare earth type. But for our purposes, these are really AC motors, and the same sort of controller (but not identical; slip is always zero for these motors!) is required. The BDC motor has three wires, just like an induction motor, not 2 like a brushed DC motor (or 4 for a separately excited DC motor). BDCs are also called synchronous AC motors.

So yes, for our purposes, all DC motors are brushed, and so have the efficiency and other problems associated with commutators and brushes. The commutator limits them to about 144v (else you can get flashover, not nice), so they will always be relatively low voltage (<144VDC nominal) and high (500-2000A) current.

The wear of brushes is not a huge deal, and commutators seem to last surprisingly long. If you are really harsh with them, they might need some maintenance every few years (sand paper or actual cutting of the copper by a very small amount). So really I don't think that the presence of brushes per se rules out DC.

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Post by Johny » Tue, 17 Feb 2009, 16:33

There are DC motors that do not have brushes. They are called "brushless DC motors" and are really AC motors in disguise - that is AC motors with permanent magnets. The control requirements are similar to a 3 phase AC motor. Most RC planes use these kind of motors. They are available in EV power levels but are very expensive and difficult to get.

IMO
Those running DC EVs say that the brush wear is not a very big issue but I would rather not have them for many reasons:
1/ Brush dust needs to be removed from the motor either by design or maintenance (by design is forced air and an exit point for dust).
2/ DC motors brush advance tends to produce a motor that arcs more when not running at design RPM or running in reverse.
3/ DC motors are not dust and moisture sealed.

Notwithstanding those points, DC is definitely simpler to implement in an EV at this stage. When keeping the gearbox a DC motor appears to be very effective.

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Post by antiscab » Tue, 17 Feb 2009, 19:53

Tonic wrote: Do all DC motors contain brushes? I read here that brushes in DC motors are bad because they obviously wear out over time, but also because they produce ozone, cause heat loss, etc. This site makes AC motors look a much more efficient option than DC, and three phase AC looks even better still.


re brush wear,
this happens *very* slowly
at Robs workshop, there is a electric forklift built in the 60's
the traction motor still has its original brushes from the 60's, and theyre still in good condition (wish i could say the same for the rest of the electrics)

they only produce ozone when they arc, which *shouldnt* happen.
arcing results in accelerated brush wear.

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Post by weber » Tue, 17 Feb 2009, 21:20

Richo wrote: Mmmm harness the awesome power of custard to power the induction motor.
I'll think about that one another time Image
Nutz wrote: Considering the high joule energy output, and it's need for no oils, it's pavlova that wins hands down.
But Nutz,

Think of the fun the kiddies would have when the occasional _custard_tanker_ overturned and spilled its load. But wait a sec. They might start causing it deliberately! Oh the horror of it all. It might be like the final scene of Mad Max 2. Image

Sorry. Just couldn't help myself. Yes I really am 49 years old.

For me the AC versus DC decision is really one of electronics versus mechanical sliding contacts, and regen versus no regen, and high voltage low current versus low voltage high current. I go for the first in each case. But a lot depends on what expertise you have on tap.

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