# High Power DC-DC Controllers



## Siwastaja (Aug 1, 2012)

Makes no sense. The controller _is_ a buck DC-DC converter. The point is that you don't need a separate inductor, as the motor already is one, thus saving the most expensive and largest component of a DC-DC.

Putting two DC-DCs after each other only increases losses and requires that extra inductor which is heavy and expensive.

If your point is to use higher battery voltage than your controller of choice supports, then get another controller or make your own that supports the higher voltage. The second DC/DC is redundant.

Another point is that your proposed 250V FETs are not going to work for a 180V pack. Use your 144V controller as is with a 144V pack and be happy, or get/design a really higher voltage controller (like the Solitons and most smaller brand AC controllers) - this means using 600V IGBTs.

Also, physical design with small paralleled FETs is very difficult. 600V IGBT bricks are easily available to almost any current, at least 600A.


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## evmetro (Apr 9, 2012)

I have wondered what the deal with pack sag is as well. If I understand correcly, a pack that has too too many volts is only a problem when it is not sagging, but under heavy load it would be great if it sagged to the controllers max volt rating? It does look temping to figure out a way to make an extra high voltage pack work. The only thing I know how to do at my level is to buy more AHs so that my correct voltage pack does not sag as much...


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## Qer (May 7, 2008)

evmetro said:


> I have wondered what the deal with pack sag is as well. If I understand correcly, a pack that has too too many volts is only a problem when it is not sagging, but under heavy load it would be great if it sagged to the controllers max volt rating? It does look temping to figure out a way to make an extra high voltage pack work. The only thing I know how to do at my level is to buy more AHs so that my correct voltage pack does not sag as much...


A modern controller design can handle much more than 144 Volt pack voltage but most motors used by DIYers (Kostov, WarP etc) can't handle more than 100-something Volts and pretty much all of them will blow up if you try to run 200 Volts or more into them.

Solution; get a pack that is dimensioned to deliver the full motor voltage even with sag (say a pack of 200-250 Volt) and get a controller that can handle the pack voltage but also limit the max motor voltage. This will likely require a modern digital design rather than an old analogue controller like the Curtis.

If you, for example, have a motor that can handle 170 Volt, have a pack that has a nominal voltage of 250 Volt and set the controller to limit motor voltage to max 150 Volt (just to be on the safe side...) your pack can sag 100 Volt without affecting top RPM. All done in one single unit, no pre-DC/DC needed.


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## Bald Eagle (Aug 21, 2013)

I apologize. I was not sufficiently clear about the constraints I am operating under. Let me set some context for my initial post.

For the application, worst case I need a peak power of over 200kW that then falls steadily to about 30kW over 45 seconds. The total energy delivered during a cycle is about 1.5 kWh. Then there are a number of minutes, e.g. 5, where the system rests and can cool and the batteries are replenished by around 10 kW of chargers. This is analogous to a series hybrid EV application. 


The motors need to be capable of 2 quadrant operation, i.e., regenerative braking, and operating under torque control. To meet these requirements, the baseline plan is to employ ac induction motors with iFOC vector controller/inverters. There is a very high value on low cost, both capital and life-cycle. The most cost effective solution identified so far uses 3 HPEVS AC-76 motors with 3 144V/500A Curtis controller/inverters with combining and speed reduction using a Gates cogged belt drive system. A HPEVS/Curtis motor/controller can be purchased for around $4500 a set. This is the lowest $/kW solution that I have identified in a vector controlled ac motor system. If anyone can point me at a more cost effective solution I am all ears. I do not want to design/implement a custom vector controller/inverter.

Long term, the battery system will go to lithium-ion but early prototypes will use lead-acid batteries. Cost consideration dictate that commodity starter batteries be employed. Three 12S1P strings would support the nominal 144V controller. A 60 Ah 12V starter battery holds about 0.7 kWh - 36 would hold about 25 kWh. The batteries would only see a very shallow discharge, worst case 6%, to provide the worst case 1.5 kWh. Thus battery requirements are dictated by power requirements, not energy storage. 10,000 cycles over 5 years without battery replacement is the goal. This limits batteries to about 800 CCA. At 500A and with some age, these batteries could easily sag below 10V/battery and the string voltage could fall below 120V. 120V and 500A is only 60 kW per motor and this is before losses. Being able to hold the controller voltage around 160 V would provide 80 kW input power and, with losses, might just satisfy my 200 kW aggregated power requirement. Paralleling additional batteries (in sets of 12 per string) to reduce the voltage sag at peak power is not cost effective and would greatly increase the system weight.

Hence my exploration of using a high power dc-dc converter. Then I can stack a few additional batteries in series, e.g 16 to reach 192 V nominal, and a dc-dc converter would both reduce the battery string current (improving battery durability) while providing a constant input voltage to the controller for high power. If anyone can suggest a better solution within the constraint of using the Curtis controller I would welcome the input. 

I found some International Rectifier 250V FETs, IRFP4768PbF, for under $5 each that can support over 60A continuous. 8 or 10 of these in parallel could readily realize a main buck converter switch for 500A. With the high duty cycle of the converter, only a few additional ones could be used in shunt to form a synchronous rectifier. The gate drivers for these switches run under $1.50 each so well under $100 of power electronics would suffice. These voltages are getting near the crossover point for going from FETs to IGBTs. If anyone can point me at IGBT components that are more cost effective again I am all ears.

An embedded processor with PWM outputs I am already planning for battery monitoring and charger control could also control this converter. So what is left for me to make this converter workable is the inductor. At a 40 kHz PWM rate, I think I need about 50 uH at 500A with under 50A pk-pk ripple. Air core is prohibitively large and the wire (4/0 gauge at $5/foot) costs far more than the electronics. What is indicated here is using a magnetic core but I do not have significant magnetics design experience. This is what really prompted my post. – I thought others in the DIY EV community might have developed similar high power dc-dc converters and I could benefit from their experience.


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## Siwastaja (Aug 1, 2012)

You do realize that what you propose seems to be much over your engineering skills, so you'll need an outside expert which will cost you a lot more than just buying the right motor and controller from the start?

For example, the FET you propose won't work at all but they blow up instantly. As I stated before, go with higher voltage rating. Consider going with 600V parts in which case you could increase the battery voltage more.

You do realize that a 200 kW DC-DC needs a really HUGE inductor? The one the EMW guys go with for a 12 kW design already is big. You cannot just buy these kind of components. If you could find a suitable one, it will be expensive as hell.

So you need to start by investing to magnet wire and set of toroids. Go with at least $1000.

Your 200 kW DC-DC would, in fact, be at least a three-phase design, possibly more. It could weight 30-50 kg and the component cost would be at least $5000. It would require countless design hours. If you cannot do it yourself, then it's at least $50000-100000.

And, all for a system where nothing makes sense.

Just use right tools for the job without kludging it.

As you can see, you cannot get the parts you need on the market at an affordable price. It won't help to develop an expensive kludge that would magically allow you to use the parts not designed for the job, if this kludge will bring the total price to more than what it would have been to use the right tools from the beginning.

What you do need is a motor that's up to the power level and an inverter that's up to both the voltage and power ratings you want. If you cannot afford such a system, then the only option is to do it yourself. It won't be much more difficult than designing that DC/DC would have been.

One option is upgrading the power stage of your inverter of choice. Then, you could keep the algorithms. This has been done by many, but they have usually used industrial VFDs with algorithms not optimized for vehicle traction.

I need to add that we fought with the same kind of problem; no cheap AC motors and controllers on the market. We thought that even the cheapest ones you propose were too expensive. We went on rewinding an industrial motor for more power, and designing our own controller. It has been a great success so far with surprisingly little design hours spent. We haven't implemented FOC as of yet, but we are doing great with a really simple control scheme. There is a few related threads going on on this section.


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## Bald Eagle (Aug 21, 2013)

Siwastaja,

I actually am a very experienced Electrical Engineer with significant experience in Class D amplifier circuit design - just not in high power magnetics. In my day job, I am a systems engineer with lots of controls, embedded microprocessor, and signal processing skills and experience.

As for using the right tools for the job, there are commercial HEVs that use dc-dc converters between the batteries and controllers for just this purpose - I did not originate this concept.

So far in my analysis, for a buck converter I think I need about 250 uF of input capacitance (4 60 uF film capacitors at $16 each), 16 300V FETs and drivers at $6 each, and about a 30 uH inductor with a 40 kHz switching frequency (if I can go higher without the switching losses getting too high, then the passives get easier, lower worse). I can do a single layer air core toroidal inductor of this inductance with about 12 - 15 m of wire, probably between 1/0 and 3 gauge depending on how much efficiency loss I want to accept. That wire should cost under $60. So without requiring a magnetic core, I see less than $250 in parts costs. With the right core, maybe I could do the inductor cheaper, but undoubtedly smaller. (Size is not critical in my application.) This is with the components I have identified, I really haven’t tried to find more cost effective ones yet (like maybe going to 400 or 600 volt IGBTs). Alternatively, if I buy 12 additional batteries to parallel for just one of my three 144V strings, I’m looking at about $1200 and another 600 lbs. With such a dc-dc converter, I add maybe 4 batteries to my string (now a 16S1P 192V nominal string ) and I save over $500 and 400 lbs. For three motor/controller sets, I could be ahead over $1500 and 1200 lbs. Sounds like a pretty good opportunity to explore further to me. I may well be overlooking something important but so far I have identified no show stoppers and still see significant potential for lower cost. If you can point out something I am overlooking, I would be most appreciative.

What I need to do now is model this on Spice and get a better idea of what frequency I should actually switch this thing at. In parallel, I will try to get more educated in high power magnetics.

Again, if you or anyone can identify a more cost effective motor and/or controller, that would be very welcome news. But I need fast/accurate vector torque control to close a 1 Hz feedback control loop to deal with the system dynamics in my application and that can operate through 0 speed into the regenerative region. (Note that with synchronous rectification a buck converter is bi-directional and can transfer the regeneration power back to the batteries. The amount of regenerative power that must be dealt with in my application is relatively small, <25 kW.) The torque response time of the Curtis iFOC controller/inverter is reported to be in the 10 to 20 ms range for reference.


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## frodus (Apr 12, 2008)

While I agree you didn't originate this concept, you are going where many others have failed. That's mostly because there's nothing on the market, and that's where you differ a little. But, It's not as easy as you make it out to be, and it will need to be able to supply that full power you need into the controller. I seriously doubt you can build a DC-DC that has a 75kW output for under $2500, let alone $250. I'd just build a better battery pack of paralleled cells for that extra money... Or move to a larger controller from another company.


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## Siwastaja (Aug 1, 2012)

The problem with such a DC/DC that needs to carry the whole traction power is that it needs to be sized for the peak power, which I think was 200 kW in your design.

Some HEVs indeed use DC/DC's, but these are parallel hybrid systems that are very power-limited. Check the numbers, but I'd say it's not much more than 15 kW. A far cry from 200 kW! They start up the ICE when they need power.

Still, it's a bit odd design choice IMO from Toyota. The low cell voltage of NiMH and the usage of a very small pack may be the reason. Separate buck pre-stage for a 3-phase inverter also have some point in it and can get part of the losses back in reduced losses at the 3-phase stage and motor, but you would not have that benefit as you use an off-the-shelf fixed voltage inverter, not an integrated system.

Start designing by getting FETs with at least 2x the voltage rating of your planned input voltage, and 3-4x the current rating of the output current. You need a lot of leeway. Think about regenerating into almost full battery pack. The voltage will rise. Your 180V pack will peak at 215 volts. So use FETs rated for at least 400V.


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## Bald Eagle (Aug 21, 2013)

Thank you Travis and Siwastaja for your constructive comments. This may well be fraught with difficulties and may not work out at all. But I am about a year away from starting actual construction on my project and exploring this further in the interim will be useful if, for nothing else, increasing my understanding of the pitfalls.

I don't think this comes up much in this forum because the focus of most members are EVs and I think this concept applies more to HEVs. EVs are more generally limited by energy storage than the power limitations of their battery systems as they get the power capability for free as a byproduct of getting enough energy for practical ranges. HEVs are typically more power limited in their battery systems. But this exploration may be of some benefit for members so I will keep the group informed.

I will learn a lot more about this when I get a basic model running in LTSpice. My requirements for a single converter are for 75 kW but this is only for a few seconds tapering down to less than 10 kW within a minute and then several minutes to cool. The 200 kW is for the entire system employing three separate motor/controller/converter/battery strings mechanically combined. The design for this low duty cycle operation is nowhere near that that would be required for continuous operation at full power. The requirements on the inductor and input capacitor are much less strenuous when the step down range is relatively small as exists in my case, e.g. 192V down to 150V. Take a look at the volt-seconds required of the inductor and the ripple current on the input capacitor when the buck converter duty cycle is above 80%. Looking at the EMW charger, their inductors are 70 or 100A at around 250 uH. Does anyone know what the core material is and/or who the vendor for these cores is? Rewound with heavier wire, such cores might be in the right ballpark for what I need. The Curtis controller's dc link capacitor provides the output capacitance for the converter. Does anyone have an idea what the capacitance of the Curtis 144V controller is? That would be useful for modeling. 

As noted previously, in my application operation in the regenerative range only involves modest powers, e.g. 8 kW, compared to the peak powers in the forward direction. This would occur after the high forward power portion of the cycle and the amount of energy returned would be far below that already delivered. This is not like heavy regenerative braking in an EV. 8kW at 200V is only 40A back into the batteries and this is only for maybe 20 seconds. While having similarities to HEV applications, my application is generally less demanding.


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## PStechPaul (May 1, 2012)

If you plan to use automotive starter batteries for 200 kW peak and 30 kW after 45 seconds, you will probably need at least 50 kWh of batteries. Starter batteries are typically designed for fairly short duration high current discharge of maybe 5C for 10 seconds and then a relatively long period of charging at about 0.2-0.5C. Even under those conditions, 5 years and 10,000 cycles is highly optimistic. A car might be started 10 times a day, but except on first start after a cold night, the discharge is only a few seconds. That would be about 10,000 cycles in 3 years, and many batteries fail at that point.

This seems to be something other than a normal EV, and if it is for a stationary application then perhaps AC grid power might be available. But I will assume there is a reason for the batteries. You can get 105 Ah FLAs designed for deep discharge for about $80 each, with a weight of 65 lb each. For 50 kWh you would need 42 of them. If you connect all of them in series, you will have a nominal 504 VDC supply with a maximum of 600 VDC and a minimum of 420 VDC at 10V each at 5C for your 200 kW requirement. This voltage range is perfect for a 460V standard VFD, which can be used for overclocking a 20 kW 230 VAC motor at 2x frequency and 2.5x torque for a short duration 100kW output. Since you will already have over 2700 lb of lead batteries to deal with, a 300 lb three phase induction motor and a 100 lb 75kW 100HP drive should not be an issue. 

I think this would be far preferable to designing and building a 200kW peak buck converter, on top of using multiple lower voltage DC drives and ACIMs. A high power DC-DC converter is not a trivial design, and unless you are very lucky and highly skilled, you will probably blow up several prototypes especially if you use marginally rated components. Putting multiple components in parallel means making sure current is properly shared, and if one component fails, it will usually trigger a chain reaction that takes out all of them. Also keep in mind that when you are dealing with 200 kW of DC power at several hundred volts, short circuits can draw in excess of a megawatt, and you need proper overcurrent protection as well as blast containment. Failure to do so could be fatal. Check out some of the high voltage arc failures on YouTube, and most of them are AC, which is much easier for circuit breakers and fuses to handle.

If you insist on proceeding with this project as you are planning, I would urge you to first build something like a 1.5 kVA DC-DC converter and drive a 2 HP motor with it. Also consider using a transformer rather than an inductor to achieve the voltage differential. You can even do it without using really high frequencies and ferrite or powdered iron cores. I have used an ordinary silicon steel tape wound toroid rewound for higher current and lower voltage at up to 16 kHz, at which you may possibly be able to get 100-200 times the rated power at 60 Hz. So a 1.4 kVA toroid core, which is about 8" dia, might be able to give you 100-200 kW, at least for short durations. Transformers do not have the same saturation problems that are involved with energy storage and release using inductors.

You may be able to design a bidirectional DC-DC converter to recoup some energy from regeneration, but it is much easier to implement with a direct battery connection to a DC bus link of a VFD as I suggested. And the overall system will be much less expensive with all standard components which can easily be serviced or replaced.


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## Bald Eagle (Aug 21, 2013)

PStechPaul,

Thank you for your comments - I generally concur with most of them. Some additional ones follow.

The 200 kW peak power with 1.5 kWh extracted in a cycle is a worst case scenario. A more typical cycle will require under 130 kW with less energy extracted. But I have to design the electronics to support the worst case. 

Again, the buck converter I am looking at is not for 200kW, it is for 75kW with a separate converter for each motor/controller/battery string. Three of these would be used to obtain the 200kW system power. 

On batteries: a similar application to mine employed 50 88 Ah 12V starter batteries with similar cycling. This is almost exactly the 50 kWh you suggested. In that case, the motor was a three phase 440Vac motor and they needed the 50 batteries to get the necessary string voltage for the inverter. The 88Ah rating was chosen conservatively lacking experience. The experience with that system has been that the batteries were replaced after 6 years just because of their calendar lifetime expectations - no real operational problems had been observed. My study of commodity 12V FLA batteries indicates that it is difficult to get high-volume, low cost batteries with much more than about 800CCA. (In my application, the system will commonly be operated between 20 and 40C so hot cranking ratings are more applicable but not as readily available. Storage will be in an un-airconditioned building year round.) These seem to be in the 50-60Ah regime. When you go to larger Ah you are generally moving to deep-cycle or combined cycle batteries with thicker plates but the CCA, which I take as indication of power capability, does not increase much. As noted, my application is power limited, not energy storage limited, and my cycling could be more described as micro-cycling. It seems the AGM batteries are on a declining price curve and I am seriously considering using them over FLA. But still, in the commodity sizes with good cost, these are still in the 800-900 CCA range. 

With 12 of such batteries, I am concerned about the voltage sag (limiting my power output) and battery durability. Going to 16S1P with a dc-dc converter would allow me to provide a continuous 150-160V input to the Curtis controller and the batteries would see a current reduction increasing durability. Three such streams would constitute 48 batteries and, other than my thinking of use slightly smaller Ah than the other system, I am in the ballpark of what you have suggested. Could you provide sources for the $80 105Ah batteries? I have been targeting more like $100 for 60 Ah FLA starter batteries. At the prices you indicate, I could certainly consider using larger batteries for more durability.

This is not an EV application and I cannot provide more specific details in a public forum. (I can share more details in confidence if you are interested.) But it does have many similarities to a series HEV system - batteries provide the peak power with a lower power generator providing the average power. In some cases, grid power may be available and would simply replace the generator; but again only at the 10-15kW level.

This battery-electric system is intended to displace existing solutions that employ automotive drive trains. Given the commodity pricing of these components and availability from junk yards, capital cost (but also lifecycle cost) is a driving issue to drive this evolution. Long term, I expect EV purposed components (motors/controllers/batteries) to become very economically competitive but that is still several years away. Combining three HPEVS AC-76 motors with Curtis controllers is the the most $/kW cost effective solution I have identified. I have been holding off on actual construction of a prototype for years because suitable single motor/controller sets (e.g. Parker, Remy) are still at low volume evaluation pricing, if available to me at all. Pricing for a suitable motor/controller set comes in around $30K. For my initial prototype, I can build a limited capability system using only 1 HPEVS/Curtis motor/controller (<$5K) with only 1 battery string. But it would have barely the power needed for demonstration so I am going to have to push that motor/controller/battery sting hard. Hence my interest in a dc-dc converter to maximize power output and a 16S1P string to keep the stresses on the batteries tolerable. Battery lifetime on this prototype is not a high priority. A second motor/controller/battery string would be added relatively quickly once the prototype demonstrates the value of a battery-electric drive. This would expand the system's operating envelope and reduce the stresses on the components. Adding the third motor set might be deferred until a succeeding prototype.


Just yesterday, I realized the value of using a multi-phase converter design. The independent inductor per half-bridge (synchronous rectification allowing bi-directional operation) reduces the issues with paralleling the FETs in a single phase design but, more importantly, reduces the passive component requirements. I expect to need at least 10 half-bridges. My baseline FETs are the 250V or 300V (if needed for ringing headroom) IRPF 4768 or IRFP4868 FEts that should be able to operate at the worst case 50A per leg levels. (I have not surveyed other vendors so there may well be better options.) With 50A, 15 uH inductors and a staggered 10 phase drive, my initial estimates are for output current ripple are around 2%. Two of these FETs and a gate driver should come in at under $15 so under $150 for the power electronics. I am still searching for suitable inductors in the 15uH, 50A area. My biggest knowlege/experience hole is in the magnetics and help in this area and pointers to available core vendors would be greatly appreciated - Digikey and Mouser do not seem to carry any magnetic core components.

I do not have a lot of experience with really high power design and I do not take this development lightly. I am very uncomfortable above 48V and with lots of current available. In general, there are bigger system level problems that I need to deal with and I would prefer to use off-the-shelf components wherever possible. But this converter makes the use of the AC-76 motor/controller much more viable which is why I am considering it. I have read the 'Plasma Boy' stores and am very cognizant of the dangers involved. This would be a progressive development starting with Spice simulations, then single phase prototypes starting with low voltages and current limits, then progressing to higher voltage testing, and then finally to the multi-phase system - first at low voltage/currents and slowly working my way up the power curve. I fully expect to blow up some components along the way and use lots of protection, both electrical and physical. 

Finally, the bi-directional regeneration capability is not to recoup energy. It is to allow the relatively rare occasions when operation moves a little into the second quadrant and the motor becomes a generator. I just need the bi-directional capability to keep the controller dc link voltage under control. Again, the regenerative system power level should be under 25kW so each controller is only operating with about 8kW reverse power.

Thanks again for your comments and I would appreciate help in some of the areas indicated above or just general guidance.


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## Bald Eagle (Aug 21, 2013)

To provide additional context for this discussion a simulation, plot of the motor speed, torque, and power for a high nominal cycle is illustrated below.
View attachment PubTorqueSpeedPower.pdf

Sorry for the poor presentation but I am new to this site and have not learned how to best post images.

The power climbs quickly to about 170kW and then drops to about 30kW at about 50 seconds. Under some conditions, the speed curve can shift downward and can go negative with torque positive resulting in negative power, i.e., regenerative action.

The speed and torque are dimensionless depending on motor speed-torque characteristics and gearing but the power is in kW. This is mechanical power out of the system so the electrical power is higher due to controller, motor, and mechanical system efficiencies.

Some of the dynamics involved are apparent. Over most of the cycle the torque is constant while the motor speed is varying in response to the load's behavior. This is why a vector controller operating in torque mode is desired.


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## TEV (Nov 25, 2011)

Due to A.D. bankruptcie you can get some high voltage/high power systems at a low price this is just an example there are other sources out there 
http://www.diyelectriccar.com/forums/showthread.php?t=83460

Have fun


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## PStechPaul (May 1, 2012)

It's best to use an image in JPG or PNG format:










I still think you would do better with a single large pack of batteries for a high DC voltage and one or more standard controller/motor pairs could run from that source. 

As for your previous question about deep cycle batteries, they are available from:
http://www.tractorsupply.com/en/store/stowaway-battery-st27dc180#desc-tab ($89, 675 Cranking Amps, 60 pounds)
http://www.walmart.com/ip/EverStart-27DC-6-Marine-Battery/16795212#Specifications ($78, 720 Amps, 115 Ah, 50 pounds)
http://www.walmart.com/ip/EverStart-Maxx-Group-Size-29-Marine-Battery/20531539 ($99, 845 Amps, 114 Ah, 60 pounds)


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## Bald Eagle (Aug 21, 2013)

TEV said:


> Due to A.D. bankruptcie you can get some high voltage/high power systems at a low price this is just an example there are other sources out there
> http://www.diyelectriccar.com/forums/showthread.php?t=83460
> 
> Tempting. One of these motor/controller sets would work well for my prototype and two would be sweet for a full up system. I've liked the Siemens motors for a long time but the $8K prices I had found had been a deterrent. I do intend to command the inverters using CANbus.
> ...


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## Bald Eagle (Aug 21, 2013)

PStechPaul,

Walmart has been one of the lowest cost battery resellers I have identified. Sam's club is even a bit better. I have also found good prices at Big R but they are regional and don't have anything posted online. Tractor Supply looks like another good option to add to my list. 

The battery you identified on Tractor Supply illustrates the issues I was describing with deep cycle batteries. It has lots of Ah but its CCA is only 675A. In general, my criteria for candidate batteries has been >750CCA and at least a 3 year warranty (to help insure I'm not buying junk). There are some better candidates on the TS site so I will certainly keep them in mind.

It seems like the some good candidates in FLA are in the Group Size 27, e.g.,

http://www.samsclub.com/sams/energizer-12-volt-automotive-battery-group-size-27/prod6770018.ip

This one is $90.

Again, if the price comes down a bit, AGM batteries look very good. Something like the Group Size H8 LN5 would be sweet,

http://www.samsclub.com/sams/h8-ln5-agm-36-mo-free/prod7700286.ip


Any pointers to good magnetics information or vendors?

PS

Thanks for putting a good image of my power requirements up. Also, long term I fully agree a high voltage single motor/controller will be the way to go. I am just trying to find a viable low cost solution from components readily available today.


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## PStechPaul (May 1, 2012)

If you will be buying a large quantity of batteries you might look at local manufacturers. Here is one that is only a couple hour drive from my home:
http://www.dekabatteries.com/

They manufacture SLAs that are sold locally under the Werker brand name by: http://www.batteriesplus.com/
I am considering them for my tractor project which might use 20-24 of the 12V 7-8Ah batteries which normally go for about $35 but they said they could sell them to me for about $17 each in quantity.

I also found a large list of manufacturers and information:
http://jgdarden.com/batteryfaq/batbrand.htm

If you want to use SLAs I have purchased UB12120 SLAs for about $22 each (including shipping) from: http://stores.ebay.com/Ecomelectronics

If cost and availability of components is critical then I still think standard industrial motors and VFDs are the way to go. I would roughly estimate that you can get 30 kW (40 HP) motors for about $800 each and matching drives might be $1000 each, especially if you are OK with a 600V DC bus. It might take 50 12V batteries at $80 each but you would have a solid 50-60 kWh pack for about $4000 and your entire system would be about $10k.

I am looking at an electric tractor/utility vehicle conversion that may be used in developing third world countries where cost, simplicity, ruggedness, and serviceability are primary concerns, and I think the way to go is three phase motors and controllers and a full voltage (240-720VDC) lead acid battery pack. For my smaller tractor project which I have already had running on 12V and 24V batteries, I may just use one or more 24VDC to 220 VAC 1000W DC-DC converters which internally produce about 270 VDC and I can probably use banks of 300 VDC capacitors to absorb the regeneration. You can get a 1000-1500W converter for about $50 or less:
http://www.ebay.com/itm/151073597579

It may be worthwhile to get one and reverse engineer it for your needs. Or you could even use 20 of these in parallel to get 270 VDC at 30 kW for $1000. And you can get 5000W converters or even higher:
http://www.ebay.com/itm/5000w-sine-wave-power-inverter-24V-DC-110V-AC-converter-/220820384299
http://www.ebay.com/itm/16000w-8000...110V-AC-converter-power-tools-RV/170673543134

I really doubt something like 8000/16000 watts with a 12VDC input because it will draw at least 667/1333 amps which is a lot even for the heavy bolt connections as shown in the pictures. And I think these are rated so high in order to handle starting surge currents for motors. I have one which is supposed to be 1000W but I found it is really 400W continuous. However it may be enough to power the 2 HP motor on my small tractor if I just use the DC part. 

For the magnetics, I am not really an expert, and mostly I have made boost converters. The one I used on my tractor has a silicon steel toroid transformer to generate 300 VDC from 12V or 24V, and it worked OK driving the VFD and motor with about 400 watts. It was supposed to handle up to 1500-3000W, but that would probably require a 48V input and wiring for at least 60 amps.

You can find some big ferrite cores and bobbins on eBay:
http://www.ebay.com/itm/310576600002?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
http://www.ebay.com/itm/E80-Large-Transformer-Core-500mT-Bsat-Power-Ferrite-EE-80-x4/121152583150
http://www.ebay.com/itm/251019409204?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

I have also ordered samples from:
http://lodestonepacific.com/

They have everything, and you can select the core material and bobbin type that is suitable for your design application. But the devil's in the details and there are many things to consider for a successful design. There are some excellent books, such as:
http://www.amazon.com/Transformer-I...TF8&qid=1377321168&sr=1-1&keywords=0824751159

But I would suggest finding someone with hands-on experience at the power levels you require. Maybe Valerun can help you.


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## Bald Eagle (Aug 21, 2013)

PStechPaul,

Thanks for the links on battery manufacturers - I had no idea there were so many. I would have expected major consolidation by now in such a mature industry.

Not quite sure were you are going with the 30 kW motor/controllers. I need 200 kW total so, unless these are average powers and you expect to exploit their overdrive capability, this would take a lot of motors. I would like to keep the solution at 3 motors or less. Same thing for the controllers, is this a peak power or continuous with over 2:1 peaking capability for a minute? When I hear VFD drives, I think of scalar V/Hz control. I need the dynamic responsivity of vector control and their ability to operate near zero speed. When I priced single industrial motors in the 120 kW continuous range (capable of supporting my 200 kW peak requirement) and suitable controllers a year or so back, the prices were coming in over $30K. But maybe I'm missing something here.

The link to Lodestone Pacific was the key I was looking for for the inductor design. It linked to Micormetals-Arnold and, between the catalog information there and a design calculator, it was easy to establish that my magnetics requirements are easily satisfied at low cost.I did a design on a 25 uH inductor at 50 kHz with 50A dc and it gave lots of suitable cores ranging from 2.5 to 4 inch OD - the larger cores having lower core losses as would be expected. It also give reference prices and, strangely, the 3 inch cores are cheaper than the 2.5 inch cores. I’m sure these are large quantities numbers but the 3 inch cores are listed around $2.75 to $3. (2.5 inch are about $4.75 and 4 inch about $6.70.) The design for the 3 inch, 26 u_r core needed 32 turns of 6 gauge and this yielded a 3.5 mohm dc resistance and about 8.5 W copper loss (core loss was 5.5W). The wire length is about 7.6 feet. Given my short duty cycle requirements, I could easily go to 9 gauge of even smaller. Bottom line, inductors for each phase could cost under $5. The cost per phase (10 needed) for FETs, gate driver, and inductor should be under $20 - $200 for all the major components. 

With the strong immune response I got when I first broached the idea of this dc-dc converter, I thought I must be missing something fundamental. But so far, I do not see any real show-stoppers.


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## PStechPaul (May 1, 2012)

I was considering the price of used/surplus drives on eBay. I found a 100 HP drive for $1800:
http://www.ebay.com/itm/ABB-ACS-600...00801-3AUA489999N546-100-HP-VFD-/111102736688

A new (Chinese) 150 HP VFD is about $5000:
http://www.ebay.com/itm/Delta-AC-Mo...50HP-3-phase-VARIABLE-FREQUENCY-/111127993752

Most drives permit an overload of 150% so drives like this could provide at least 100-150 kW peak. Motors can typically run at 2x torque and a 2x overclocking will give you 120 HP from a 30 HP motor. It's best to use separate motor/controller pairs, but it's possible to run two motors from one controller. However it may not work well with sensorless vector control.


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## Siwastaja (Aug 1, 2012)

Bald Eagle said:


> When I priced single industrial motors in the 120 kW continuous range (capable of supporting my 200 kW peak requirement) and suitable controllers a year or so back, the prices were coming in over $30K. But maybe I'm missing something here.


Yes, you are missing how these motors are rated and which frequency you run them at.

They are rated to run at 50/60 Hz which is highly unoptimal in using all the power the motors are really capable of.

In addition, they are rated to run for decades with possibly hindered cooling and inadequate drive.

For example, what Azure Dynamics did was that they bought a normal 5 kW (or so, IIRC) industrial AC motor and rated it for almost 10x power and sold as a specialized "EV motor".

The trick is to increase the frequency and rpm of those motors to get more power out of them. Cooling may need to be increased at the same time, but not necessarily. You see, the weight of iron pretty much defines how much torque you get out of them so you increase the rpm to get more power. If you go too high, eddy current losses increase, but otherwise, the efficiency is not affected.

You of course need to increase voltage at the same time, so, unfortunately, this means that you need to rewind these motors for a lower voltage or order them with special windings. For example, we rewound our 380VAC motor for 76VAC.

This is a very basic stuff and you can find discussions on the motor section.

This may or may not apply to your project, but yes, if you were looking for 120 kW rated industrial motors, those are well capable of outputting at least 500 kW continuous and could be used in a train. That's why they weigh and cost; they are really overkill.

Specialized EV motors are nothing fundamentally different, they are just even more optimized for the job.

The need for tedious rewinding process or special order is the only reason hindering people from using industrial AC motors in EV conversions all the time.


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## Siwastaja (Aug 1, 2012)

Bald Eagle said:


> With the strong immune response I got when I first broached the idea of this dc-dc converter, I thought I must be missing something fundamental. But so far, I do not see any real show-stoppers.


Nah, it's doable if you kind of know what you are doing. But, it's completely redundant and, as said before, makes no sense. So if you really know what you are doing, you know there are better ways. That's no show stopper if you decide that the show must go on.

But that being said, people are doing stupid engineering all the time just for fun. I'm also doing it sometimes.

Just look at all those discussions about "pros and cons" of using automatic transmission in an EV, and all the effort people put in using them, when there is zero technical sense in what they are doing. Some people even use a belt-driven alternator and idling function in the controller to charge the 12V battery!
But of course these projects still work and they like to do it that way so what the heck.

But I'd say a powerful DC/DC is harder than those examples. It will be a good design experiment you can later apply to some more meaningful projects; for example, if you need to build a charger, there's a lot in common.

(I'd rather use the effort to upgrade the power stage of your inverter of choice to support higher voltage. It will probably be a smaller or a similar project to the DC/DC and the result is smaller and has better efficiency and less parts. I'd also ditch the idea of lead acid and go with lithium. Even ultracapacitor might be viable in this particular case.)

You have clearly made your decision already on this, so I guess it's best to get to the work. Good luck!


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## Bald Eagle (Aug 21, 2013)

PStechPaul,

As this is an open source design, I am trying not to use motors that others cannot obtain new, E-bay certainly provides opportunities to save a lot of money but the goal is to produce something that can be readily duplicated by others.

The motors I was looking at were rated to support 150-200% or their continuous torque rating for a minute. That seemed conservative but I did not chose to push that. This is not a dragster application but one where long term durability is a key requirement. All I need is for motors to start blowing to get the naysayers on my application to say "see, I told you so." 

I am certain that thermally they are capable of much more than that for my minute cycle given how massive they are. But motors can also become magnetics flux limited and I did not know if that was the limiting factor.

I cannot use sensorless vector as those controllers do not work well when the motor operates near zero speed (or even negative) which can happen in my application. The HPEVS/Curtis baseline motor/controllers employ sensored iFOC vector.


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## Bald Eagle (Aug 21, 2013)

PS

This is primarily in response to a post that was up momentarily and then taken down. Not sure why it was retracted.

Siwastaja,

Your statements from a couple of days ago sure seemed to say differently,



"For example, the FET you propose won't work at all but they blow up instantly."

Operating at 2/3 the semiconductor voltage rating is standard practice. 



"You do realize that a 200 kW DC-DC needs a really HUGE inductor? The one the EMW guys go with for a 12 kW design already is big. You cannot just buy these kind of components. If you could find a suitable one, it will be expensive as hell.

So you need to start by investing to magnet wire and set of toroids. Go with at least $1000."


If you had taken the time to read my post, it was clear that the dc-dc converter was for 75 kW. Three were to be used to get over 200 kW. It turns out that I can easily construct the 10 inductors I need for $5 each weighing around 300 grams each.



"Your 200 kW DC-DC would, in fact, be at least a three-phase design, possibly more. It could weight 30-50 kg and the component cost would be at least $5000. It would require countless design hours. If you cannot do it yourself, then it's at least $50000-100000."

I'm actually going with 10 phases. All the power components so far only come in under 5 kg. Because of my short, low duty cycle operation, I really don't need heat sinks so much as heat mass to absorb the losses for the cycle duration with plenty of time to cool in between. At this time, my total costs for all the FETs, the gate drivers, and the inductors is under $200.



For someone that does not understand the application and the constraints I am operating under including cost, durability, component availability, and ease of duplication by those less skilled than I, you certainly seem to be very judgmental. I apologize for not being able to share publicly he application but the reason is not economic or commercial. If you want to actually understand what I am doing, send me a message and I can likely share why I cannot disclose publicly and give you a better understanding of the application. Then what I am doing may not seem like such "stupid engineering." 

I really did not ask for comments on the merits of using this converter in my application - just if it had been explored before and what are some of the pitfalls.


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## Siwastaja (Aug 1, 2012)

Bald Eagle said:


> The motors I was looking at were rated to support 150-200% or their continuous torque rating for a minute. That seemed conservative but I did not chose to push that.


Hi,

The trick is not to overrate the _torque_ rating as that _will_ decrease efficiency due to the I^2R losses and driving the motor nearer to magnetic saturation. Of course you can push it a little, too, but most of the power is achievable by overclocking. This is the same principle how it is possible to get an order of magnitude more power from a 1 kg transformer when you run it at higher frequency (the idea behind switch mode PSUs)


Hey, I'm trying to help. My motto is; always question the whole design. It makes no sense to refine fine details when the underlying big idea is flawed. Well, it's not _that_ much flawed in your case, and as I said, it will work, but the same principle applies nevertheless.

In your case, I admit I was a bit harsh first. But the components you first proposed were unusable in that application. How do I know? I have blown FETs. That's why I though you might need outside help to decide which components to use and how. It also looked like you were under an assumption that the short duty cycle would magically make the design so much easier, whereas it won't be much of help in reality. It helps a bit in heatsinking, indeed.

10-phase sounds good. Divide it to small modules instead of trying to just parallel up small fets to drive a big inductor.

It is certainly doable.

I'm just saying that it could be done more easily, with lower cost and with better efficiency without the DC DC. I'll say it again: it's redundant.

The fact that Toyota has used it is very different because they have it integrated in the power inverter which has several benefits, and because they have lower power levels.

OTOH, I'm also saying that the DC DC idea is valid and you can learn a lot by doing it and utilize that design later. So I wasn't sarcastic in my previous post.

BTW, I spend a lot of time pondering these things and writing a reply, giving valuable information and hints. I'd appreciate if you did appreciate it even if I missed some design details in these long posts.

And I'm sometimes wrong. You of course make the decision. Use all information available.

Always question the big application!

I was investigating a commercial EV conversion where their BMS had destroyed some of the cells (sounds familiar, Jack-listeners ?) The BMS was overly complex and the cell module had a component count of over 130. The BOM per cell module was at least $50. They had been solving many "problems" that do not exists with li-ion cells at all, and indeed used a _lot_ of design effort to design a system that is so complex. All that design work was completely unnecessary and caused only damage because the risk to miss something important in a complex system increases compared to a simple system. They also could have bought 10 kWh extra batteries with the cost of that BMS. So, think simple, and don't overmodularize your design as it will hinder you from seeing the big picture.


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## PStechPaul (May 1, 2012)

Good points. I always try to think "outside the box" to consider new and sometimes radical or even crazy ideas, and the discussion the ensues is very valuable for looking at details as well as the big picture of a design. I have fallen victim to choosing a particular design path without fully researching and understanding alternate ways to accomplish the same thing, and it has resulted in products and systems that soon (well, in ten years or so) became unsupportable and essentially obsolete. 

For this application, I looked at the speed-torque-power curve and the duty cycle involved, and it looked like alternate means of energy storage and release might be advantageous. So I think it is worthwhile to consider LiFePO4 and their cheaper Li-Ion cousins, as well ultracapacitors, and mechanical means such as flywheels and compressed air. It appears that the total energy of the 50 second cycle is about 3 kWh, but the peak power of 200 kW would demand a peak discharge of about 70C, which would be far out of the range for lead-acid technology and even most LiFePO4, but possibly doable using Li-Ion or ultracapacitors. The cost of a 4 kWh 50C Li-Ion pack would be in the order of $1600, while for ultracapacitors it would be about $15,000. A lead-acid pack would only be able to produce about 5-10C so you need about 40 kWh which would cost about $2500-$5000, and may need replacement in 2-3 years. LiFePO4 would probably also do 5-10C without Peukert derating but you'd probably need a 20 kWh pack which would be about $12,000.

A flywheel UPS costs about $300/kW (maybe also $300/kWh) so a 5 kW system able to provide 40C would cost about $1500, good for 100,000+ cycles, and capable of 15 second charging, so it is well worth considering, and might be implemented with mechanical connection to the load which could eliminate the motor and controller as well as the batteries. But such decisions depend on the actual application details.

It will be interesting to follow the development of this project, and particularly the DC-DC converter which is the subject of this thread. If the design pans out as about $100 for a 5 kW unit then it should be of interest to almost everyone, and if it can be modified to do boost as well as buck (using a transformer or Cuk design), then it would have a large potential market among EV enthusiasts and many others. So perhaps it would be good to focus on that rather than the overall design at this point.


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## Bald Eagle (Aug 21, 2013)

Paul,

My peak power is 200 kW but that is only for a brief portion of the cycle with the power falling to a low level over the remainder of the cycle. Integrating over a worst case cycle, I am comfortable with specifying 2 kWh as my worst case cycle energy requirement. I have looked at flywheels and ultracapacitors before but had not found any economically attractive options. 

Ultracapacitors are almost ideally matched to my application. The peak power draw occurs early in the cycle when the voltage is still high and the falling motor speed means my voltage requirements fall throughout the cycle. I usually think of about 3 kWh when I consider capacitive storage. The 0.5 CV^2 energy relation for capacitive storage means that the voltage remaining after 2 kWh had been extracted would be over 55% of the initial voltage - more than enough. But as you note, the cost for ultracapacitors for even this small amount of energy is not attractive.

I would really like further information on the flywheel storage you referenced. For storage in the few kWh range that I need, I have never seen prices anywhere near what you quoted. Generally I have assumed that flywheel storage would require a generator attached to the flywheel that also could support my peak 200 kW requirement. Effectively this means both a motor and generator capable of 200 kW peak power levels. I find it hard to imagine that economically viable systems exist for the few kWh energy storage I need that can deliver 200 kWh peak.

Right now, lithium is where I expect to end up but, to keep capital costs low and avoid the complexity of a sophisticated BMS, I am baselining lead-acid for prototype work. I have little confidence in the durability of the 50C lithium cells out there. The best technical solution that I believe exists today for lithium are the high power A123 cells which run more around 30C. I think about 8 kWh of these cells would be a good solution. But the cost of these cells is still too high for me to consider today. Long term, I fully expect this to change but I think this is still several years away.


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## PStechPaul (May 1, 2012)

If you must use full-price off-the-shelf components, a good source is Automation Direct. A 100 HP premium efficiency 1800 RPM motor is about $3200:
http://www.automationdirect.com/adc...Premium_Efficiency_(1_-_300HP)/MTCP-100-3BD18

Their matching 100 HP drive is about $3500:
http://www.automationdirect.com/adc...S3_Drive_Units_(230_-z-_460_VAC_SLV)/GS3-4100

I have yet to find any readily available commercial flywheel systems in the energy range you need, but I found a paper that characterizes small scale flywheels as being those that can store up to 6 kWh, and they are available in low speed (<10,000 RPM) and high speed versions. And they gave one example where a full installation cost was $92,000, which supposedly realized a Life Cycle Cost (LLC) about 2/3 that of a comparable lead-acid battery system:
http://www.erc.uct.ac.za/jesa/volume20/20-1jesa-okouetal.pdf

They also show an example for a 300Wh home solar system. They show a battery LCC of about $2200 over a 20 year period which assumes replacement every 3 years, while the flywheel lifetime is assumed to be 20 years, and the flywheel was 2/3, or about $1500. Thus I deduce $1500/300wH or about $4500/kWh, and your 4 kWh system would be $18,000. But for the very short peak power you need you might be able to use a much smaller motor and controller (about 20 HP or 15 kW) to drive a flywheel directly, bringing it up to speed in the much longer time intervals between events (duty cycle 5-10%), and then use mechanical means to transfer that rotational energy to the load at the speed, torque, and power levels you need. Even if you used a frictional clutch or hydraulic torque converter or variable speed mechanical belt drive, the lower efficiency during the high power cycle might be offset by the lower overall cost. 

It may even be possible to build your own flywheel and attach it to the motor shaft and slowly ramp up to full speed, and then extract the stored energy over the short time period you require. You can get special high speed bearings for motors that allow them to spin safely up to 5000 and even 10,000 RPM. Just as an exercise, assume that you can use a 100 kg disk about 1 meter in diameter, and spin it at 6000 RPM or 100 RPS. If most of the mass is at the outer portion of the flywheel, then the energy should be the same as linear kinetic energy which is 0.5*m*V^2. So the velocity is 3.14*100 and the energy should be almost 5 MJ (if I have my units correct), and this is about 1.37 kWh. So you could achieve the 3 kWh by using a 250 kg flywheel or by spinning the 100 kg flywheel at about 8500 RPM. Check my calculations, but I think they are at least "in the ballpark".


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## Bald Eagle (Aug 21, 2013)

I have thought about how to package this dc-dc converter and have come up with some concepts. Below is an illustration of what I have been thinking. Pardon the limitations of my drawing program to properly depict the cores (Seeing the cores behind...) 

Blooming Onion.jpg

This is a 10-phase bi-directional buck converter using 10 half-bridges, each with its own inductor wrapped on the yellow 3" powder cores. Obviously, this concept could be scaled to different numbers of phases. Each half-bridge’s FETs and gate driver are mounted between segments of concentric decagons. These decagons are simply formed of bent copper sheet. The inner is at the battery potential, the outer is at ground. The FETs are soldered into tiny gate driver boards providing low inductance gate drives. The input capacitance across the supply leads is distributed around the ring to keep the supply impedance as low as possible. Differential signaling is employed between the controller and each gate driver for noise immunity.

These polygon rings also act as heat sinks, or more properly heat masses considering my application allows time for cooling between the short 1 minute operational cycles. Additional sheets span the inner diagonals as fins to provide additional heat sink/mass. In my application, the PWM duty factor is probably above 75% so most of the conduction losses are on these high-side switches attached to the inner ring. Fins could also be attached to the outer ring if additional cooling was needed for the low-side switches, e.g., for lower PWM duty cycles. The unit could be housed in a 12x12 inch box 6 inches high with a fan(s) on the top pulling air through a grilled bottom. The polygon heights can be increased to provide additional area/mass. If more continuous operation was required and/or air cooling was insufficient, one of more loops of copper tubing could be brazed to the polygons for liquid cooling with proper provisions for electrical isolation. 

Shown below is a variation on this concept that could be housed in a tubular housing. The power electronics configuration is conceptually the same as above but the inductor cores are arraigned in a stack above them. This unit could be housed in a 6 inch PVC pipe with a fan at the top pulling air through the stack.

Tubular.jpg


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## PStechPaul (May 1, 2012)

Well, you earn points for style, and possibly the "Area 51" award for emulating alien technology. 

Since we appear to be going ahead with the buck converter idea for now, rather than examining other approaches for the overall concept, it would help to reiterate the precise specifications for this device. AIUI, this will take as input a pack of lead-acid (or other technology) batteries, with a high voltage such as 300-500 VDC, and step it down to about 150-200 VDC for use with EV type controllers and motors that are limited to that voltage. It needs to supply that voltage at a peak power of 200 kW for several seconds, and then a nearly linear reduction of power to 30 kW after 50 seconds, from your chart.

I don't recall exactly why you need to use lower voltage EV type motors and controllers, and why you can't just match the battery pack directly to that requirement. The only things I can think of are size, weight, cost, controller design, and availability, and in most of those categories standard industrial drives and motors should be competitive and even advantageous. Once we understand this requirement, it may be easier to discuss the details of the DC-DC controller.

[edit] Looking back at your earlier posts I see that just want a 192V (16S) nominal battery pack so that it can still provide at least 160V at maximum sag of 10 vpc when the batteries are near their end of life. Your peak current at 144V for the Curtis controller at 200 kW is about 1400A, which will be split among three motor/controller systems at 500A current limit each. So what you really need is a buck converter that can handle peak current of 1400A and a voltage differential of 192 -144 = 48 VDC, or a maximum power of 67 kW. So that makes the design 3 times cheaper. In fact, maybe you can use one DC-DC module for each motor/controller, and you have a 23 kW design. It's better to make this as modular as possible, and the more units you build, the lower the cost/kw becomes, and also it makes it more practical to keep a spare on hand to swap out.


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## Bald Eagle (Aug 21, 2013)

PStechPaul said:


> Well, you earn points for style, and possibly the "Area 51" award for emulating alien technology.
> 
> Since we appear to be going ahead with the buck converter idea for now, rather than examining other approaches for the overall concept, it would help to reiterate the precise specifications for this device. AIUI, this will take as input a pack of lead-acid (or other technology) batteries, with a high voltage such as 300-500 VDC, and step it down to about 150-200 VDC for use with EV type controllers and motors that are limited to that voltage. It needs to supply that voltage at a peak power of 200 kW for several seconds, and then a nearly linear reduction of power to 30 kW after 50 seconds, from your chart.
> 
> I don't recall exactly why you need to use lower voltage EV type motors and controllers, and why you can't just match the battery pack directly to that requirement. The only things I can think of are size, weight, cost, controller design, and availability, and in most of those categories standard industrial drives and motors should be competitive and even advantageous. Once we understand this requirement, it may be easier to discuss the details of the DC-DC controller.


For all the reasons you state, this is the way I am going for now but am always looking for other options. I have some that you have suggested that I will be considering going forward but that will be in parallel. For now, let's keep the discussion on high-power dc-dc converters.



> [edit] Looking back at your earlier posts I see that just want a 192V (16S) nominal battery pack so that it can still provide at least 160V at maximum sag of 10 vpc when the batteries are near their end of life. Your peak current at 144V for the Curtis controller at 200 kW is about 1400A, which will be split among three motor/controller systems at 500A current limit each. So what you really need is a buck converter that can handle peak current of 1400A and a voltage differential of 192 -144 = 48 VDC, or a maximum power of 67 kW. So that makes the design 3 times cheaper. In fact, maybe you can use one DC-DC module for each motor/controller, and you have a 23 kW design. It's better to make this as modular as possible, and the more units you build, the lower the cost/kw becomes, and also it makes it more practical to keep a spare on hand to swap out.


You pretty much have the requirements down now: I do want a separate 75 kW dc-dc converter for each motor/controller/16S1P lead-acid battery string. I have never said I wanted a 200 kW converter, only 75 kW. Each of the 10 phases in the concept drawing is supporting 50A peak for 500 A total at an output voltage of 150-160 V (75 - 80 kW).

I have been saying the same thing differently but your statement that I am only reducing the voltage about 40 V is what makes this a much simpler converter. I have been saying the same thing by saying my PWM duty cycle is generally going to be 0.8 or higher under full load when the batteries are sagging. That is why all the power components (FETs, inductors, capacitors, and gate drivers) should cost around $200 - add that again for the pwbs, copper sheet, wiring, connectors all the other miscellaneous stuff...


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