AC or DC?

AC, DC, amps, volts and kilowatt. It's all discussed in here
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AC or DC?

Post by Tonic » Fri, 06 Feb 2009, 21:04

Hi to all. This is my first post and I am in need of an answer to which type of motor is best given my application. I must point out that I know little about electric motors and hope to learn as I go along.

I have obtained a Hyundai Excel in great condition for next to nothing and intend to pull the petrol motor and related parts out this weekend. My immediate problem is in deciding which motor is best as I would like to order one soon (cost is not really a factor), and the fact that I live on top of Mount Tamborine on the Gold Coast and drive down to Broadbeach every weekday to go to work (around 40kms each way). I will be able to charge the EV at work so hopefully batteries will have plenty of power for the home trip, but it is the last 10kms that will be a killer. The grades of most of the roads up Mt Tamborine are around 10-14%, and the up-hill climb usually takes around 10 mins to get to the top.

So my questions are:
1. Which type of motor: AC or DC? (and is regenerative braking an option coming down the mountain)
2. How much horsepower will I need?
3. What sort of voltage is optimal?
4. Single or 3 Phase (and why)?

If anyone could set me in the right direction I will be most grateful.
Last edited by Tonic on Fri, 06 Feb 2009, 10:11, edited 1 time in total.
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AC or DC?

Post by Richo » Fri, 06 Feb 2009, 23:38

1. Both are good but DC is more commonly available at the moment.
You won't get regen with DC.

2. Look at the original motor spec and try to match that.
Typically an 8-9" motor would ne good.

3. The higher the voltage the higher the top speed.
Typically a DC conversion is usually 120-144V.

4. Single phase should be ok unless you work really short days Image
harder to get 3 phase chargers.

So the short answer is NO but the long answer is YES.
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AC or DC?

Post by Richo » Fri, 06 Feb 2009, 23:42

excuse my manners...

Welcome to AEVA Tonic

Also as the excel is FWD I'm assuming you are leaving the gearbox in.
So the short answer is NO but the long answer is YES.
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AC or DC?

Post by Tonic » Sat, 07 Feb 2009, 00:01

Hi Richo,

Thanks for ur swift reply. Yes I was intending to leave the gearbox in.

Would it be foolish to pursue the AC option? If so is the 120-144V range still applicable for a suitable motor?

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Post by Richo » Sat, 07 Feb 2009, 01:18

Genrally the AC motors require a higher voltage.
The azure motors they use in the Getz conversions is just over 300Vdc.
I'll be using around 700Vdc for industrial AC conversion.

It's up to you with what you feel confident installing/using.
So the short answer is NO but the long answer is YES.
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AC or DC?

Post by antiscab » Sat, 07 Feb 2009, 06:18

Gday Tonic,

welcome to the forum.

for a conversion such as yours, i would suggest using the AC24LS made by azure dynamics, and sold here by blade electrc vehicles:
http://www.azuredynamics.com/products/f ... tSheet.pdf

i would suggest using a 312v pack, and wiring the motor in delta (gives 70kw peak at 8000rpm when you do this, but i still have to confirm with azure whether the controller will let you do this).

the AC24LS is reasonably compact, and is meant for FWD applications.

40km on the flats is achievable with lead acid, but in the hills id suggest going with lithium.

im assuming you use this vehicle for your commute already?
how much fuel does it use now? (we can use this to extrapolate an approximate energy requirement).

i would think a 40AH 312v (104 cell) TS pack would satisfy your range requirements.
at 12kw discharge rate, you have 12kwh avail.
max power would only be around 50kw (at the batteries, 50-60hp at the wheels)

what speed do you normally climb those 14% hills at?
i need this to determine your minimum power requirement.

keeping the gearbox is a good idea for fwd cars.
i know im having trouble sorting out a diff for my mr2.

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AC or DC?

Post by Nutz » Sun, 08 Feb 2009, 00:43

Another newbie here, trying to get my head around the whole ac/dc thing myself. How do you get ac current from a battery pack? If an inverter is used, what loss of power is involved in the process? Is a bigger motor better if it fits (or will it just draw more power than it needs eaven whist coasting?
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Post by EVLearner » Sun, 08 Feb 2009, 02:26

I am a bit puzzled why we need to have a higher voltage for AC motors. From my side I have been seriously considering 4 banks of 48 V = 200 V nominally, and that is about 70 V rms neutral to phase - or about 120 V rms between phases.

Now I know that standard motors (to date) are made for 415 V rms between phases and that the US standard is about 115 V rms neutral to phase or about 200 V rms betwen phases, but there is nothing wrong (other than the cost) with getting a motor rewound and have the stator coils connected parallel instead of series connections, and this will keep the wire relatively thin and easy to bend in the formers.

The motor conditioners could also use bifilar, trifilar, quad (parallel) windings etc and that will also have a definite positive for wire flexibility, making the manufacturing process much faster (and cheaper).

Who knows, this could be a major development in AC motor manufacturing in the very near future (a few months), if and when electric cars get really popular.

Sure, when it comes to lower voltages it also means higher currents, but again I don't see that as a problem as the currents (I believe) are to a large degree circulatory and even then we are only talking about the same amount of current as in a DC motor (a few hundred amperes being high frequency switched) - but the braking efect can be used to recharge the batteries, and that is a real definite plus compared to DC motors which really struggle to be efficient series wound generators!

Comments appreciated..

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Post by Richo » Sun, 08 Feb 2009, 02:31

Yep that's right nutz and inverter, Variable frequency drive (VFD), converts the DC voltage into the AC voltage.
It changes the AC based on your accelerator and settings.
Inverter losses are around 98%.
AC motors are more efficient than DC - permanent magnet motors excepted.

Bigger motors tend to have a better torque/current ratio.
A motor will only draw what is needed regardless of size.
However a bigger motor controller may have a higher operational idle current.
But it won't be much.

But you have to check the motor, controller and battery are compatible.
No point getting a bigger motor if the controller is too small.
Or getting a good motor and controller then be limited by the battery output.
So the short answer is NO but the long answer is YES.
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AC or DC?

Post by Nutz » Sun, 08 Feb 2009, 02:50

"Inverter losses are around 98%".Im sure you mean approx 2% right?
The other thing I have no knowledge of , but would like to know more about is regenerative braking. It is stated above that you need to run AC, what else do you need? Is it worth the extra expense??
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Post by Nutz » Sun, 08 Feb 2009, 03:02

Not having done a lot of research an prices yet, might it be worth doorknocking a few elec. motor rewind joints to look for "lost treasure" in the form of an industrial motor to do the job, or do we inherit a whole lot of problems when we move away from the purpose built jobbies.
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AC or DC?

Post by woody » Sun, 08 Feb 2009, 03:22

@Tonic: Blade Electric Vehicles sell the commercial AC gear (AC24LS + DMOC445) controllers, but they also sell converted Hyundai Getz, so they may be able to sell you adapter plates and other bits they use in their Getz conversion if they suit the excel.

@Nutz:

The purpose built jobbies are usually sold as a motor + controller pair which are suited to each other, and are simple to connect: 2 wires for battery, 3 wires for motor, 2/3 wires for speedo etc.

The industrial route is not as well trodden, acmotor on this forum has pioneered the way and shown it is very doable. a4x4kiwi has nearly finished his hilux using the same plan, and has documented his journey very well.

Johnny, Sparky Brother and myself are looking to go along very similar paths.

I see the upside of industrial AC is:
controllers and motors are usually available cheap on ebay.
controllers are very flexible
wide range of motors new and used

The downside is:
everything is air cooled (minor)
the controllers aren't as compact as they could be
the controllers are a very fiddly to set up and tune

cheers,
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AC or DC?

Post by Richo » Sun, 08 Feb 2009, 07:07

Nutz wrote: "Inverter losses are around 98%".Im sure you mean approx 2% right
Sorry my freudian Image
Nutz wrote: The other thing I have no knowledge of , but would like to know more about is regenerative braking. It is stated above that you need to run AC, what else do you need? Is it worth the extra expense??


Batteries - Inverter - Motor - Vehicle.

Normally in an induction motor the controller AC signal is a bit fatser than the rotor (moving middle bit).
This difference is the "slip".
This causes the power from the batteries to flow into the motor causing it to move.

When regen is oprating the opposite happens.
The controller makes sure that the signal to the motor is slower than the rotor.
So the vehicles kenetic energy turns the rotor which creates power to flow back into the battery.
The inverter is configured to do this as per the configuration settings.
No extra bits are required.

However if the regen is too much or the batteries are full the regen power needs to be diverted to a braking resistor or the controller needs to reduce/stop regen.
Without the braking resistor the regen should cut out and the braking effects will stop.
So the machanical brakes still need to be used. Image

It should come standard with an AC setup so there is no extra cost.
Some people say that typically it is only providing the same effect as having 1-2 extra batteries.
There is another thread around that discusses the worth of regen.

Personally I won't be spending any more on my AC system than the DC equivelent of a warp 9" with suitable controller.
But unless you can design and build your own AC controller this won't be an option for the majority. Image
So the short answer is NO but the long answer is YES.
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Post by Nutz » Sun, 08 Feb 2009, 07:28

My interest in regen comes from the 69 model Mazda 1500 i want to convert only having "stomp harder" brakes (drums/no vac.assist)and me not wanting to have the whole front end re engineered, so a braking resistor may be looking cheap. Hey I won't need a vacuum pump either.
Looks like I might be looking to a new AC system and LFP batteries, with little (if any) change from $15,000 Image
Last edited by Nutz on Sat, 07 Feb 2009, 20:41, edited 1 time in total.
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Post by coulomb » Sun, 08 Feb 2009, 07:42

EVLearner wrote: I am a bit puzzled why we need to have a higher voltage for AC motors.
It has to do with the fact that industrial motors can generally stand around 415v ac (or a little more). You can get them at nominally lower voltages. For example, ABB motors have can be ordered with an "S" in position 13 of their code number instead of "D", for 220-240v delta / 380-415 star.

Edit: I meant to suggest that the "D" stands for delta 400v operation, and the "S" for star operation at around 400v.

(There are other code letters, but they are all higher voltages than 400v delta, except for "X" which is custom, and may be much more expensive). 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.

You could of course just use a 26kW 50Hz motor and a lower voltage, higher current inverter, but then the motor weighs a lot more, and industrial VFDs are mostly expecting 380-440vac input. To get the extra current, you will need a much bigger VFD, and acmotor's is already almost the width of his truck!

So for industrial drives, you get best value out of a high voltage pack. The AC24LS series, which expect around 320-350v packs, are actually low voltage (about half) of what conversions using industrial drives will use (600-720 vdc).

Also, the industrial controllers often have a quite high bus voltage at which their braking resistor transistor attempts to keep the bus voltage at; it's around 750-780 vDC. You want that to be around the charging voltage for your cells, say 3.6-4v for Lithium, or 14.4-15v for lead acid. That will dictate a high voltage pack. You could ignore the braking resistor option on the VFD (dangerous), or provide your own.
From my side I have been seriously considering 4 banks of 48 V = 200 V nominally, and that is about 70 V rms neutral to phase - or about 120 V rms between phases.
Heh Image I've been waiting for an excuse to show off my neutral wobble image:
Image

This is supposed to show two phases of the VFD output (red and green), and the difference between these is the blue sine wave. Note how the blue wave has a higher amplitude (587v peak, 415v RMS) than the flattened sinewave outputs from the VFD (293v peak, sqrt(2) less than 415v).

Edit: was "Note how the blue wave has a higher amplitude (576v peak, 415v RMS) than the flattened sinewave outputs from the VFD (340v peak, same as 240v RMS sine wave)."

All the drive manufacturers do this; otherwise, you can't get a 415v sine wave from a 415v rectified input! So in fact, for 200v DC input, you can (neglecting losses and real-world voltage drops across IGBTs etc), you can get 200 / sqrt(2) ~= 141v AC RMS output (phase to phase, as 3 phase is usually specified). If you don't do the neutral wobble thing, you'd only get 200v / 2 / sqrt(2) * sqrt(3) ~= 122v RMS phase to phase. (Edit: was 115v via spurious math). For true disbelievers, I can provide the C program that generated the data for this graph.
[Using lower voltage motors] Who knows, this could be a major development in AC motor manufacturing in the very near future (a few months), if and when electric cars get really popular.
Yes, indeed, I expect that using a 240v delta motor could well become standard for conversions. But I don't expect lower pack voltages.
Sure, when it comes to lower voltages it also means higher currents, but again I don't see that as a problem as the currents (I believe) are to a large degree circulatory
Image Circulatory? You mean reactive current? Generally, the reactive current is less than the real current, by a factor of at least 2:1, often 3:1 (please correct me if I'm wrong).
and even then we are only talking about the same amount of current as in a DC motor (a few hundred amperes being high frequency switched)
Yes, but the DC inverter has just 1 switch, and a big diode. The AC controller has 6 switches. That's one reason that the AC controllers like to keep the current down, and the voltage up: less devices paralleled. I read today that a common Curtis DC controller has 35 MOSFETS paralleled.
- but the braking effect can be used to recharge the batteries, and that is a real definite plus compared to DC motors which really struggle to be efficient series wound generators!
Yes, a series wound DC motor doesn't like to regenerate, while an AC induction motor does it naturally. Separately excited DC motors can regenerate (2 switches are then needed, so the controller is more complex). AC inverters almost can't help providing regeneration as a feature; the bridges are inherently bidirectional.

However, I would not say that regeneration by itself is enough to justify AC on its own. Depending on the application, you might get 15% extra range from regen (acmotor may dispute this figure), but the built for EV controllers and motors are perhaps twice the price of the DC controllers and motors. The main advantage of AC (in my opinion) is the torque at high speed, sometimes obviating the need for the gearbox and clutch. Also, when you have a low voltage motor, there is a cheap sort of gearbox available in the form of the star/delta switch. It's like a fixed 1.73:1 (sqrt(3):1) gearbox, available by using a big contactor and some smarts with the inverter (you have to tell it that the 50Hz motor has gone away, and now there is this 87Hz motor with different characteristics, and would you please do a flying start in a fraction of a second, thank you).

You may have figured out that I'm in the AC camp Image
Last edited by coulomb on Sun, 08 Feb 2009, 18:21, edited 1 time in total.

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Post by Nutz » Sun, 08 Feb 2009, 08:03

So let me see if i get this right, DC is cheap to build, but gives the performance of a golf cart. If I want any sort of street cred. i will need to go for a mid-large sized AC set up (gearbox optional)
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Post by coulomb » Sun, 08 Feb 2009, 17:47

Nutz wrote: So let me see if i get this right, DC is cheap to build, but gives the performance of a golf cart. If I want any sort of street cred. I will need to go for a mid-large sized AC set up (gearbox optional)
Close, but some of the hottest racing machines are still DC (e.g. John Wayland in the USA). At the very top end, with dual motors and batteries capable of 2000A for the duration of a 400m (.25 mile) race, DC can be quite competitive.

Edit: I should have said that unless you design it that way, you will get way better than "golf cart performance" from DC.

I'm sure that the DC supporters will say, quite rightly, that you can still do a DC conversion that won't lose all your street cred. If you have the money, AC will give you more options, and probably more range, and probably more performance at high speed, but DC is still very impressive at street (as opposed to highway) speeds.
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Post by EVLearner » Sun, 08 Feb 2009, 19:00

"You could of course just use a 26kW 50Hz motor and a lower voltage, higher current inverter, but then the motor weighs a lot more,..."

I don't understand. Surely the motor magnetic circuit would be the same but the winding turns would be less with thicker wire - (or parallelled thinner wire to fill the winding area) ... so the motor should be the same physical size??

"then a nominally 15kW 50Hz motor is really also a 26kW 87Hz motor,"

So if I had the VFC runing at say 400 Hz, then the motor could be rated at say 120 kW, does that follow?

It may be possible to run at say 400 Hz or faster as the magnetic flux is not heading to saturation because of the frequency, but I am not so sure how fast the rotor can safely spin before parts of it start to land on the stator and slow things down!! (Can you tell me what the safe rotor spin limit is?)

"Yes, indeed, I expect that using a 240v delta motor could well become standard for conversions. But I don't expect lower pack voltages."

Let's assume that the 240 V Delta windings are 4 windings, series connected per 'inter-phase'. If this was rewired as 2 sets of 2 series windings (parallel connected) then this motor would be a 120 V Delta winding (inter-phase), and that arrangement would fit the bill almost perfectly.

"Heh I've been waiting for an excuse to show off my neutral wobble image: "

Love the graph, and it took a few minutes to comprehend, and the penny dropped - the BLUE line is the absolute difference between the two driving lines RED and GREEN, and the actual feed is +/- 300 V nominally, so the pp level would be 600 V nominally between phases as you have shown and this only comes about by having the input waveforms 'flattened' for about 60deg, where otherwise the intrsecting levels would have been at about 0.866, boosting the output level by about 30% - else the max (peak) would have been about 420 V.

This waveform patern would go a long way to keep the switching efficiency up and the switching currents down




Image

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Post by Nutz » Sun, 08 Feb 2009, 20:47

O.K. then, time for me to really show my ignorance.
All this talk of kilowatts, amps, inches, hertz, horsepower etc. is doing my head in. I'm afraid that I am one of those people that learn by seeing and doing rather than crunching a whole lot of number crunching and calculating.
To add to the confusion I find that we not only have AC and DC (which I do understand) but there are also EC motors out there too Image Please explain??? (see [http://www.ebmpapst.com/en/products/mot ... otors.html])
Here are a few examples from Ebay, they look big enough to me, but I have no idea about the numbers involved and how they compare toeach other or eaven to an I.C.E. (I am after similar perfomance to a mazda MX5)
Item number: 180326778391 Fully Blown!
Item number: 370154424734 Hmmm alloy must be light!
Item number: 380101572722 Looks the right size.
Item number: 260355415660 10 Horses that should get the cart moving.
I'll stop here, but there are more.
(I acknowledge that noe of these have controllers)


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Post by Nutz » Sun, 08 Feb 2009, 20:57

Then there is this, Is that a controller on top or just a big J box
Item number: 260355415660
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Post by EVLearner » Sun, 08 Feb 2009, 21:14

No you are not ignorant, as I am rather confused too and that it why I purchased a little 1.5 kW 3 phase motor last week to get a better underatanding of what is going on here! It's cheap and the connectors will cost more than the motor - but at least I will be able to connect a small controller and find out what happens without busting the bank.

BTW by the time you read this the last of the motors (260355415660 10 Horses that should get the cart moving. )will have left Ebay!!

My gut feeling is that the motor should be about 10 to 20 kW at 50 Hz and that the useful power is normalised on the frequency - but I can't see how or why (so that makes this dilemma obviously simple to somebody else)!

From my side - I have started to work backwards - just like with electronics - from the output to the input. So with this project; the weight of the vehicle and the required torque to the revs; and clearing distractions; I should have that somewhat sorted out in metric terms in a few weeks!!

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Post by Richo » Mon, 09 Feb 2009, 03:33

Some of those motors in your e-bay list are single phase.
Typically they will be limited to 2.4kW peak.
Also the advantage of using 3-phases is it is more efficient.

An EC motor is a stepper motor.
They are not suitable for electric vehicles.

A car can typically use 20kW to run at 100kph.
But acceleration is where you need the power.
Most 3-phase motors have a peak power roughly triple its nominal power.
SO if you want to run at 100kph then you would need 20kW nominal.
and so the peak would be around 60kW.

DC is the same cost or cheaper and likely to provide the same or more power than AC.
So the short answer is NO but the long answer is YES.
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Post by Nutz » Mon, 09 Feb 2009, 03:54

Ok so what will i need to look for to identfy the motor that will do the Job I want. Assuming I am looking at 20kw 3 phase motor, what are the other clues that I will find in adverts etc. that will let me know that I am onto the right thing?
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Post by bga » Mon, 09 Feb 2009, 05:01

If that's single phase, the box is probably full of capactor.

ACMotor's Red Suzi uses an older 11kW ABB, which works well. It illustrates a problem with ACIMs in that the industrial motors are fairly bulky and may have fit problems in many vehicles, especially East-West types.

For ACIMs, the best size option looks to be ABB, although there's a cost penalty and some oddness with MEPS (Minimum Energy Performance Standards) causing supply issues with the high output (small size) versions.

The ACIM's high voltage (600V) DC requirement makes the battery a lot more complex than a DC system's 150V battery. I see the advantages being in the motor availability (same day shipping), competitive cost, efficiency (5% better than DC), sealed operation, robustness (horrible industrial environments), lower current, regeneration capability and the disadvantages being high voltage, larger size and complex controllers.

edit : typo & twiddle
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Post by coulomb » Mon, 09 Feb 2009, 05:03

EVLearner wrote: "You could of course just use a 26kW 50Hz motor and a lower voltage, higher current inverter, but then the motor weighs a lot more,..."

I don't understand. Surely the motor magnetic circuit would be the same but the winding turns would be less with thicker wire - (or parallelled thinner wire to fill the winding area) ... so the motor should be the same physical size??
It's because a 26kW 50Hz motor has to draw more current than a 15kW 50Hz motor. The extra current requires thicker windings and bigger iron to hold the bigger slots, and generates extra heat, necessitating more bulk to dissipate that heat.
"then a nominally 15kW 50Hz motor is really also a 26kW 87Hz motor,"

So if I had the VFC runing at say 400 Hz, then the motor could be rated at say 120 kW, does that follow?
Yes, if your VFD could output 1920v RMS phase to phase, and the motor's insulation could stand that voltage. Of course, neither is true, so you can only get 415/240 = 1.73x more power from a 415v nominal controller.

To get more power from a 415v controller, you could go to a motor with a lower voltage winding, e.g. 415/8 = 51.9v delta. Your controller would need to output 120kW at 415v, or 167A (before losses; a 15kW motor means 15kW mechanical, so perhaps 180A per phase).
It may be possible to run at say 400 Hz or faster as the magnetic flux is not heading to saturation because of the frequency, but I am not so sure how fast the rotor can safely spin before parts of it start to land on the stator and slow things down!! (Can you tell me what the safe rotor spin limit is?)
Good point. The other problem is that industrial motors seem to be limited to 4500rpm (75Hz, 2 pole, or 150Hz 4 pole). So the practical limit for a 4 pole motor is three times the nominal power, by winding for 1/3 of the nominal voltage (400/3 = 133.3v). So a true 120v delta motor would be interesting, as you state:
Let's assume that the 240 V Delta windings are 4 windings, series connected per 'inter-phase'. If this was rewired as 2 sets of 2 series windings (parallel connected) then this motor would be a 120 V Delta winding (inter-phase), and that arrangement would fit the bill almost perfectly.
"Heh I've been waiting for an excuse to show off my neutral wobble image: "

Love the graph, and it took a few minutes to comprehend,
Yes, sorry about that; I confused myself there.
and the penny dropped - the BLUE line is the absolute difference between the two driving lines RED and GREEN, and the actual feed is +/- 300 V nominally,
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).
so the pp level would be 600 V nominally between phases as you have shown and this only comes about by having the input waveforms 'flattened' for about 60deg, where otherwise the intersecting levels would have been at about 0.866, boosting the output level by about 30% - else the max (peak) would have been about 420 V.
Yes, I think you have the right idea. Well done; it takes most of us a fair while to grok that.
This waveform pattern would go a long way to keep the switching efficiency up and the switching currents down
I guess so, but I believe that all the VFD manufacturers do this. Otherwise, from 415v rectified (=415 * sqrt(2) = 587v), you'd only be able to get 415/2 = 207.5v sine waves (wrt a non-wobbling neutral), or 207.5 * sqrt(3) = 360v delta, some 13% short of where you started with.

My apologies to anyone I confused. Edit: with my 340v error last post.
Last edited by coulomb on Sun, 08 Feb 2009, 18:41, edited 1 time in total.

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