AC DC?

Have you made or bought a converted vehicle if so this is for you
TTmartin
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AC DC?

Postby TTmartin » Sat Jan 14, 2012 9:38 pm

Hi all,

With all the EV world knowing AC is the so called (better system) why is it that the only easily available motor/controllers for AC are only rated to 96v, this is ok for really light cars but anything over say 1000kg will I think struggle, am I right?
With a DC system most conversions seem to aim for 120 to 144v to achieve adequate performance and range.


Measure twice cut once.

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Night Train
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Re: AC DC?

Postby Night Train » Sun Jan 15, 2012 5:26 pm

IIRC DC series motors suffer from back EMF requiring higher voltages to make them spin faster against a load.

AC motors rely on the frequency of the three phase supply to control their speed. Higher speed needs a higher frequency.

mattcarr
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Re: AC DC?

Postby mattcarr » Sun Jan 15, 2012 8:52 pm

One thing to consider is that although the Curtis 1268r controller is rated at 96 volts, it's operational voltage range is, I believe up to 128 volts. So as long as your packs nominal votlage is below this mark you should be okay. The people who converted my car do offer an option to take the battery pack up to 32 cells, which is 7 more than I currently have. This would give a maximum voltage of 128 volts for the pack off charge = 4 vpc fully charged or, 3.3 volts per cell nominal votage.

TTmartin
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Location: Bath

Re: AC DC?

Postby TTmartin » Mon Jan 16, 2012 10:51 pm

Night train,

My mechanical capabilities are well? Ok, :D electrical well? Basic. :(
I know the difference between AC and DC but when it comes to the in,s and outs of motors all I know is they spin when powered up through suitable control, I leave the rest to higher beings than myself.
That's why I've joined this forum to absorb some knowledge about things I know little of.

In a simplistic way (humor me)
If you have two identical cars one powered by a 144v DC motor/controller and the other with a 96v AC motor/controller which would out perform the other?

mattcarr
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Re: AC DC?

Postby mattcarr » Tue Jan 17, 2012 7:26 am

The performance would depend on the controller, regardless of the motor. The more amps a controller can put out to the motor the more power you would have.

My Citroen has got an 82.5 volt battery and a Curtis 1268r AC controller. Even though the controller is rated for up to 500 amps, it is set to be within the paramters of my battery. I have got lipo batteries that are rated for maximum continuous discharge of 2C - or 2 times their AH capacity. Mine are 160ah - so the maximum current draw is 320 amps. I only ever see this briefly on my display as the car only ever really displays around 240 amps maximum. Which is plenty enough performance for my car.
So I have 82.5 volts x 240 amps = 19.6kw. Now if I had a 128 volt pack, then for the same current my power would be 30kw

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Night Train
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Re: AC DC?

Postby Night Train » Tue Jan 17, 2012 11:44 am

In simple terms the power of a motor is in watts (or kilowatts).
Volts x Amps = Watts
Therefore to get the same power with a lower voltage you will need some more amps. That implies that a lower voltage battery pack will draw more current (amps) then a higher voltage pack.

The controller will deal with all that and deliver the maximum power it can.
Simply put, and this is very simply, the maximum voltage x the maximum current of the controller gives the maximum power it can deliver. You will need to consider controller design, efficiency, duty cycle, etc. but it is a sort of idea.

The difference between AC and DC is the way they work.

An AC motor rotates because the sinusoidal wave form of the current flowing into it creates a moving magnetic field that makes the armature turn round. Like a surfer on a wave, the faster the wave travels the faster the armature will turn. To increase the speed of the wave is to increase the frequency of the wave. The input voltage doesn't really matter too much except that the higher the voltage the lower the current.

A DC series wound motor is very different. The main drawback is that its speed is dependent on the input voltage and the load on the shaft. If you apply a voltage the motor will spin. The only thing stopping it from spinning faster and faster is the drag from the bearings and air resistance and any load you apply to the shaft. If you don't have a load then the motor can spin very fast in deed!
This is the reason why a DC series wound motor must NEVER be tested with more then 12v without a shaft load. I have a motor that will happily spin to 1800rpm on 12v. 24v will probably spin it to distruction.
When installed in a car the controller will limit the voltage to control the speed of the car. However, if you were driving along with your foot on the accelerator and decided to put your foot on the clutch, or select neutral, then your motor will spin to death.*

Anyway, the load on the motor will cause the motor to slow down and so to maintain the speed more voltage will need to be applied. The thing that causes the motor to reach and hold a speed is that the armature spining is causing a magnetic field to oppose the one that is driving the motor. This is the 'back EMF'. When the back EMF equals the input voltage the motor cannot spin any faster. To make it go faster you need to apply a higher voltage that will make the motor spin faster still until the back EMF balances it again. And so on. That is why you need high voltage to get speed in a DC car.

There is another way to get high speed in a DC car and that is to gear it for lower motor speed. This does work but you then need more torque from the motor which means more current.
You can look at a DC series motor like this: Voltage = speed, Current = torque.
The downside of this is that current also equates to heat and losses.

Running at higher current means the motor will get hotter. A hot motor is converting your battery power to heat instead of motion so the efficiency is reduced.

This is another area where AC and DC differ.
In a DC motor the heat is created in the armature, the spinning bit in the centre of the motor. That makes it difficult to keep cool and often forced air cooling is used.
In an AC motor the heat is generated in the static field coils that are around the inside of the motor casing. That means it can be cooled with big fins on the outside or a water jacket wraped around the motor.

AC is the 'ideal' situation reading back on this with the added benefit that it easily regenerates.
However, the downside is the cost of high power controllers. As mentioned above regarding the wave form that drives the motor around, it is actually three waves running 60deg apart in rotation, three phase. That means that the controller is really three controllers being controlled to make three waves exactly 60deg apart at anything from 0Hz (cycles per sec) to whatever maximum 1000'sHz is required. That makes it a difficult and expensive bit of kit to produce.

A DC controller is much, much simpler in comparison and so it is easier to get high power capacity and reliability in a cost effective package.

If anyone more knowledgeable can see errors in this, other then over simplification, then please flag them up and make corrections. :wink:
:)




*Spinning a motor to death is a very bad thing. The centripetal forces on the armature are very high and when it spins too fast the copper coils and the commutator bars will fly outwards explosively sending shrapnal flying out of the motor explosively. On the DIY electric ar forum one chap was having his car towed home on a rope. The vibration on the road caused the gear stick to drop into first gear and the road wheels spun the motor so fast it exploded and buried bits of copper in the bonnet and in the road surface.

TTmartin
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Location: Bath

Re: AC DC?

Postby TTmartin » Wed Jan 18, 2012 12:04 am

Mattcarr, Night train,

Thank you,
Those replies have really helped me, I know bits and pieces and you have just helped put the pieces together, very helpful indeed, :D

So, :lol: back to the original question, (see below)

If you have two identical cars one powered by a 144v DC motor/controller and the other with a 96v AC motor/controller which would out perform the other?

And I will add, controllers designed to get the best from each motor,
and batteries were lithium 160ah identical packs in each car except voltage eg: 96v AC / 144v DC :?:

No politician type answers please :lol:

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retepsnikrep
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Re: AC DC?

Postby retepsnikrep » Wed Jan 18, 2012 3:27 am

The dc system would perform better in a purely performance/acceleration scenario but it's like comparing apples to oranges as the power available from the battery in the 144v setup is considerably more than the 96v setup. If both were running 96v and identical current then the AC system would be better and certainly more effcient IMHO.
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.

TTmartin
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Joined: Fri Jan 06, 2012 10:10 am
Location: Bath

Re: AC DC?

Postby TTmartin » Wed Jan 18, 2012 6:42 pm

Thank you Peter,
That's what I thought but not really qualified to say it.
The problem is there are not any motor / controllers of higher voltage than 96v in AC readily available for DIY projects.

Regards

Martin.

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Night Train
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Re: AC DC?

Postby Night Train » Wed Jan 18, 2012 11:23 pm

TTmartin wrote:T
The problem is there are not any motor / controllers of higher voltage than 96v in AC readily available for DIY projects.

That is one of the very best reasons for so many DIY cars having DC set ups.

There is this VW Passat though.


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