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Changing an induction motor voltage

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cts_casemod View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 09 October 2012 at 9:43am
Originally posted by coulomb coulomb wrote:

Originally posted by Huub35 Huub35 wrote:

My question is, how to recognize the several windings. Should I post a picture here, or is there some general knowhow that I can immediately use to determine the type of winding?


One quick indication is if you see wires from the junction box joining with two or more windings wires. That means that some windings are already in parallel, so it's not going to be trivial to arrange them for lower voltage.



Every motor of reasonable size that I've seen seems to have windings already in parallel. So that "for free" gain doesn't seem to arise very often.


Do You have a link for this corsa with the 5.5Kw?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 09 October 2012 at 9:45am
Originally posted by acmotor acmotor wrote:

.... intercom crackles... Have you seen my kitchen scales ??



Ummm, no ,.......... but you can borrow my electronic torque meter.



Your what what ? .........Don't go bringing any of those greasy bits in from the shed.



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AC motor, on your car this 11KW motor is running as standard? (delta 415V) or are you using delta at 208V with a 415V VFD?

Thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote acmotor Quote  Post ReplyReply Direct Link To This Post Posted: 09 October 2012 at 1:11pm
Delta at 200V (rewound motor) with 380-500VAC VFD, 576VDC battery pack. In theory limit is 66kW at 3000RPM being 11kW x 3 (max torque) x 2 revs (1500 x 2 = 3000RPM) but VFD is the limit at 30 x 1.6 (rated overload) = 42kW.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 09 October 2012 at 6:01pm
Originally posted by acmotor acmotor wrote:

Delta at 200V (rewound motor) with 380-500VAC VFD, 576VDC battery pack. In theory limit is 66kW at 3000RPM being 11kW x 3 (max torque) x 2 revs (1500 x 2 = 3000RPM) but VFD is the limit at 30 x 1.6 (rated overload) = 42kW.


So you are saying that my stock motor can safely hande 3x7.5KW (22KW or 30HP) as stock? That would be enought to keep 100Kph on motorway
My diesel can keep 80KMH at 2000RPM with 10% hills in 5th gear. Its rated at only a 64HP at 4000RPM.

I am just wondering if this would work okay with over plate speed. Its a 4 Pole motor at runs at default at 1500RPM, so I would like it to be able to go up to 4000RPM to avoid shifting.

Also what have you used to cool the motor? The fan spining at 3000RPM looks like a jet ;)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richo Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2012 at 12:31pm
22kW peak 7.5kW continuous
For keeping at 100kph you need to take into account the motor continuous rating.
So I doubt it.
The 22kW is for acceleration/over-taking and used for short intervals.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2012 at 1:18pm
Originally posted by cts_casemod cts_casemod wrote:

So you are saying that my stock motor can safely hande 3x7.5KW (22KW or 30HP) as stock? That would be enough to keep 100Kph on motorway ...

As Richo pointed out, you need continuous power for highway driving. But as this post summarises, the continuous power only goes up at about the 0.7th power of the over-voltage or over-speed ratio.

So if you are doing 4000 RPM at 100 km/h, and base speed is 1480 RPM say, then the overvoltage ratio is about 4000/1480 = 2.7, so the continuous power would go up by a factor of about 2.7 ^ 0.7 (2.7 raised to the 0.7th power), or about 2.0x, or about 15 kW continuous for the nominal 7.5 kW motor.

15 kW would get our MX-5 to an estimated 90 km/h from the graph in this post (there are some other power verses speed graphs in other AEVA posts).

[ Edit: another way to look at this: if you really needed 22 kW continuous from a 7.5 kW motor, that's a ratio of 22/7.5 = 2.93x in continuous power, so you'd need something like 2.93 ^ (1/0.7) = 4.65x overvoltaging, so you'd need a speed of 4.65 x 1480 = 6885 RPM. This is almost certainly too fast for a stock motor, even with balancing, and such an over-voltage ratio has not been attempted, at least as far as I know. ]

Edited by coulomb - 10 October 2012 at 1:29pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 10 October 2012 at 7:14pm
Originally posted by coulomb coulomb wrote:

Originally posted by cts_casemod cts_casemod wrote:

So you are saying that my stock motor can safely hande 3x7.5KW (22KW or 30HP) as stock? That would be enough to keep 100Kph on motorway ...

As Richo pointed out, you need continuous power for highway driving. But as this post summarises, the continuous power only goes up at about the 0.7th power of the over-voltage or over-speed ratio.

So if you are doing 4000 RPM at 100 km/h, and base speed is 1480 RPM say, then the overvoltage ratio is about 4000/1480 = 2.7, so the continuous power would go up by a factor of about 2.7 ^ 0.7 (2.7 raised to the 0.7th power), or about 2.0x, or about 15 kW continuous for the nominal 7.5 kW motor.

15 kW would get our MX-5 to an estimated 90 km/h from the graph in this post (there are some other power verses speed graphs in other AEVA posts).

[ Edit: another way to look at this: if you really needed 22 kW continuous from a 7.5 kW motor, that's a ratio of 22/7.5 = 2.93x in continuous power, so you'd need something like 2.93 ^ (1/0.7) = 4.65x overvoltaging, so you'd need a speed of 4.65 x 1480 = 6885 RPM. This is almost certainly too fast for a stock motor, even with balancing, and such an over-voltage ratio has not been attempted, at least as far as I know. ]


Thank you coloumb,

At the moment I cant get my hands on a 208V motor. I have a 6KW one in Portugal but only god knows when I will get back there.
So the idea is to use the stock 7.5KW at 415V WITHOUT ANY OVERVOLTAGE that should be good for what i am looking at now: a city car.

But this 2.93x the stock power is valid at any speed? I am going to feed my motor with 15KW (double the power) so will I be able to use this 15KW at any speed from nameplate to 4000RPM?
Also if I am on overdrive (1500 to 4000) how much efficiency will I loose? Is it beter to keep the same gear and rev the motor or use the gearbox to keep the motor within its operating speed?

Thanks

Edited by cts_casemod - 10 October 2012 at 7:15pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 11 October 2012 at 6:01am
What voltage battery pack are you planning?
With a 600VDC pack you will only get rated torque (from a 4 pole 415V motor) to 1500 RPM and it will drop off rapidly after that. Coulomb was giving you figures based on being able to supply correct V/Hz to the motor which implies a rewind - therby more current - then your 15kW VFD isn't big enough (think current not kW for the VFD).

Eg. If a 7.5kW motor draws 15Amps per phase at rated kW then lets say 45A at rated torque times 3. The 15kW VFD will probably supply 30 Amps and with any luck 1.5 times that for 60 seconds (check this) - so 45 Amps. With these values you have good torque to about 45km/h with an overall drive ratio of 3:1. You can probably make 70km/h after a minute or so on the flat and level. Hill climbs can only be about 60 seconds travel time.

You would have to keep the gearbox or you will be stranded on slight inclined driveways etc.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 11 October 2012 at 6:01am
Originally posted by cts_casemod cts_casemod wrote:

So the idea is to use the stock 7.5KW at 415V WITHOUT ANY OVERVOLTAGE that should be good for what i am looking at now: a city car.

Oh. Then you have about 7.5 kW at base speed (near 1500 RPM), and slightly less out to maximum speed.

Quote But this 2.93x the stock power is valid at any speed?

The 2.93x is the approximate ratio of continuous power with over-voltaging by a factor of 2.93 ^ (1/0.7) = 4.65 (6885 RPM). For a motor with 4000 RPM max, the over-voltage ratio can only be 4000/1480 = 2.70x, for a continuous power ratio of about 2.7 ^ 0.7 = 2.00. The 3x that is sometimes mentioned is the common ratio of stall torque divided by nominal torque, which gives a very approximate idea of the peak torque of the motor, and hence peak power.

To answer the question, no. Don't forget that at a speed of 1000 RPM, a 7.5 kW motor only has a continuous rating of 5.0 kW, since you have about the same torque, but the speed is lower than base speed. If this is rewound by a factor of 2.70x, say, then the continuous power at 1000 RPM will still be 5.0 kW, and continuous power at 1480 RPM will still be 7.5 kW. But now you have the voltage to keep going at peak torque, all the way through to 4000 RPM. However, you have to derate the torque to allow for iron losses, so at 4000 RPM you don't have 4000/1480 x 7.5 = 20.3 kW, you only have 2.00 x 7.5 kW = 15 kW. In between, at say 2800 RPM: 2800/1480 = 1.89, and 1.89 ^ (0.7) = 1.56, so you get 1.56 x 7.5 kW = 11.7 kW continuous.

Your controller probably has a "torque boost" feature, where it does an over-voltage at lower RPM, which may improve the continuous power at lower speeds, by reducing the current for the same torque. Actually, as I write this, it seems unlikely to be useful, except the motor may overheat a little less than you might otherwise expect at low speeds.

Quote I am going to feed my motor with 15KW (double the power) so will I be able to use this 15KW at any speed from nameplate to 4000RPM?

Yes, but not continuously. You'll only get 5 kW at 1000 RPM, 7.5 kW at 1500 RPM, and 7.5 kW thereafter (all as stock). If you don't over-voltage, you don't get higher continuous power.

Quote Also if I am on overdrive (1500 to 4000) how much efficiency will I loose?

It's hard to say, but generally iron losses are negligible at low speed, and increase rapidly at higher speed. So it is probably worth using your gearbox to keep the motor below about 3000 RPM most of the time, for maximum efficiency.

[ Edit: I didn't check the 2.93x figure; this should be 2.0 because (4000/1480) ^ 0.7 = 2.0. I've corrected the figures to reflect this now. ]

Edited by coulomb - 11 October 2012 at 10:33am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 11 October 2012 at 9:11am
Two comments on that otherwise excellent summary by Coulomb, from way back.

1. When you run out of voltage, maximum torque drops of as 1/f^2, not 1/f, except possibly for some very special induction motors (not industrial types) with very small air gaps, where it does fall off at 1/f initially, before going to 1/f^2. The Tesla Roadster motor may be like this.

2. The 0.7-th power rule-of-thumb for continuous power rating was little more than a guess, based on one paper I read. ACmotor's tests cast serious doubt on this and suggested it might be closer to 1.0. However these tests were with a much smaller motor than would be used in an EV, and gave some apparently conflicting results between temperature rise measurements and power loss measurements. So it would probably still be prudent to assume the 0.7-th power when designing, until demonstrated otherwise.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stiive Quote  Post ReplyReply Direct Link To This Post Posted: 15 October 2012 at 1:59pm
Hey guys,
I've got this as-new 1.1kW 2pole 415V Teco motor which i'm intent on destroying with the controller im building.

Anyway, thinking it might be a good candidate for 1/2 voltage or even 1/4 voltage if possible. Is it doable? Seems to have a decent amount of back-iron for the stator size and also room for plenty of connections.

What you think?


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Windings (Front)


Windings (Back)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 15 October 2012 at 5:50pm
Originally posted by Stiive Stiive wrote:

... it might be a good candidate for 1/2 voltage or even 1/4 voltage if possible. Is it doable?

Since it's a two pole, I assume you're not attempting to run it at 4x frequency, since that would imply about 11400 RPM. Assuming you want to keep it as a two-pole motor.

It seems to have overlapped windings, which I suspect might be easier to modify, either with rewiring or rewinding at home.

Quote Seems to have a decent amount of back-iron for the stator size

Well, it needs the thick back-iron, being a two-pole. (Mentioned in recent posts.) [/QUOTE]
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stiive Quote  Post ReplyReply Direct Link To This Post Posted: 15 October 2012 at 6:05pm
Originally posted by coulomb coulomb wrote:


Since it's a two pole, I assume you're not attempting to run it at 4x frequency, since that would imply about 11400 RPM. Assuming you want to keep it as a two-pole motor.

Yeh, dont want to completely rewind it, not worth the hassle or money... Id be happy to parallel some windings for 1/4 voltage AND THEN put 415V in, haha. As i said, i want to see how far i can push it. Problem is at 13ohm per phase, its a bit hard to smash in serious current. Doing some paralleling should be good.
Also I want to start testing my controller with lower voltages, therefore reducing the boost voltage to spin the thing would be nice.

Originally posted by coulomb coulomb wrote:


It seems to have overlapped windings, which I suspect might be easier to modify, either with rewiring or rewinding at home.

Looks to be wound with 1 wire in-hand. Guess that makes it easier to chop and change. Though I cant follow the wires without cutting the straps and breaking up some of the thinly coated resin.
Is there any tell-tale hints that its doable before I do this? I'd like to be able to destroy this motor with current, not with scissors


Originally posted by coulomb coulomb wrote:


Well, it needs the thick back-iron, being a two-pole. (Mentioned in recent posts.)

Fair enough. I'm also getting a 4-pole motor of the same size/brand, though i think it'll be identical apart from the windings. We'll see.

Edited by Stiive - 15 October 2012 at 6:11pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 31 October 2012 at 5:50pm
Hi all,



This may be useful to prove you can actually open a motor covered in insulation varnish with a lot of TLC



I have opened my 415V Motor and have found that all 3 phases have 2 windings in series, giving me a total of 6 coils

I believe I could parallel them to have a 208V motor, am I right?

Edited by cts_casemod - 01 November 2012 at 4:38pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote coulomb Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2012 at 5:44am
Originally posted by cts_casemod cts_casemod wrote:

I have opened my 415V Motor to find I have 3 of the windings to be in series, giving me a total of 12 wires (6 for the output and 6 for the 3 series connected pairs)

I believe I could place them in parallel to have a 208V motor, am I right?

No, that would be 415/3 ~= 138 V nominal. Each winding would drop a third of the voltage, assuming overlapped windings (I think; whichever of the two main types has windings with the same number of turns).

Edit: If the first photo is supposed to represent what you found, they you have three pairs in series; then yes, you will end up with a 415 V / 2 = 208 V nominal motor. Sorry, I think I misinterpreted what you said.

Edited by coulomb - 01 November 2012 at 5:48am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2012 at 6:26am
Thanks coulomb,



Here is a more clear drawing.

Now the thing is, I can have it as a 208V Motor as you say, but this is now a 2 pole motor!!

Will I have a 4 pole motor again if instead of paralleling the windings I actually parallel them but connect them opposed to each other? lets say instead of

S...S                                 

w1a w2a                               
¦   ¦                                
¦   ¦   
¦   ¦
w1b w2b
                            
N...N      

I connect them like this

S.....N                          

w1a w2b
¦   ¦
¦   ¦
¦   ¦
w1b w2a

N.....S

Just wondering :S

Edited by cts_casemod - 01 November 2012 at 6:34am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote weber Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2012 at 1:54pm
There seems to be some serious numerical confusion going on here. A photo of a notepad implies that 208+208 = 440, and shows 2 windings per phase while Coulomb has taken you at your word as "find[ing] I have 3 of the windings to be in series".

I think you mean that all 3 phases have 2 windings (actually 2 pole-phase-groups, PPGs) in series. And let's call it a 415 V 50 Hz motor. And I take it to be originally 4 pole with consequent poles.

So yes, if you parallel the PPGs you will have a 415/2 = 208 V 50 Hz motor (not quibbling about 0.5 V). If you parallel them in one way, i.e. leaving the existing series connection and just joining the two outer ends, then you will have a totally crappy 2-pole motor, with incredibly poor power factor. If you parallel them the other way, i.e. breaking the original series connection, then you will have just as good a 4-pole as it was before, just at half the voltage and twice the current.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2012 at 4:32pm
Originally posted by weber weber wrote:

There seems to be some serious numerical confusion going on here. A photo of a notepad implies that 208+208 = 440, and shows 2 windings per phase while Coulomb has taken you at your word as "find[ing] I have 3 of the windings to be in series".

I think you mean that all 3 phases have 2 windings (actually 2 pole-phase-groups, PPGs) in series. And let's call it a 415 V 50 Hz motor. And I take it to be originally 4 pole with consequent poles.

So yes, if you parallel the PPGs you will have a 415/2 = 208 V 50 Hz motor (not quibbling about 0.5 V). If you parallel them in one way, i.e. leaving the existing series connection and just joining the two outer ends, then you will have a totally crappy 2-pole motor, with incredibly poor power factor. If you parallel them the other way, i.e. breaking the original series connection, then you will have just as good a 4-pole as it was before, just at half the voltage and twice the current.


Thank you webber.

I believe coloumb was right, but I may not have explaied myself clearly so I will revise the upper post.

So you are confirming my previous post. If I parallel both windings with the same polarity NN;SS(leaving the existing series connection and just joining the two outer ends) I have a 2 pole, if I paralel them in reverse from each other NS:NS (breaking the original series connection) I have a 4 pole motor.

What I dont understand is why will this end up being a poor power factor 2 phase motor, if I wish to wire it as a 2 pole? I was thinking in having a circuit to change poles at a certain speed for better high speed economy. Do you think is not worth to bother?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 01 November 2012 at 4:42pm
By the way, since I have the motor open it might be a good idea to place some thermal sensors on it to have a reading of the windings temperature. What value do you guys recomend? 10K?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Huub35 Quote  Post ReplyReply Direct Link To This Post Posted: 03 November 2012 at 9:50pm
Originally posted by cts_casemod cts_casemod wrote:



Do You have a link for this corsa with the 5.5Kw?

Regards


Hi CTS,

the original link is gone, via archive.org I found:
http://web.archive.org/web/20101113030222/http://rc-autopilot.de/wiki/index.php/Opel_Corsa_Project

Furtermore these might be interesting (though in german):
http://www.youtube.com/watch?v=fnoH_QdXSyo
http://elektro-pkw.de/forum/index.php
http://www.mikrocontroller.net/topic/139365

Best regards, and good luck!


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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 03 November 2012 at 11:12pm
Originally posted by Huub35 Huub35 wrote:

Originally posted by cts_casemod cts_casemod wrote:



Do You have a link for this corsa with the 5.5Kw?

Regards


Hi CTS,

the original link is gone, via archive.org I found:
http://web.archive.org/web/20101113030222/http://rc-autopilot.de/wiki/index.php/Opel_Corsa_Project

Furtermore these might be interesting (though in german):
http://www.youtube.com/watch?v=fnoH_QdXSyo
http://elektro-pkw.de/forum/index.php
http://www.mikrocontroller.net/topic/139365

Best regards, and good luck!


Huub


That will come very handy, thanks!
You could almost tell my conversion is a copy of the Corsa. He also used a 15KW VFD with a 7.5KW 4 Pole motor lol ;)

Take a look at my conversion. Waiting for parts at the moment.

http://www.diyelectriccar.com/forums/showthread.php?p=327846&posted=1#post327846

You need to be a registered member to see the pictures.

Edited by cts_casemod - 03 November 2012 at 11:29pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stiive Quote  Post ReplyReply Direct Link To This Post Posted: 13 November 2012 at 7:31am
In the middle of doing a similar mod to a 4-pole 1.5kW motor.


I'm hoping to finish it today, but the windings are unusual to me with 1 pole pair in each phase having double the windings of the other pole pair.

Does someone want to look over my theory before i cut all the other phase interconnecting wires (have only done W atm). I'm not so sure 1/6 voltage is a good idea anymore.

http://forums.aeva.asn.au/forum_posts.asp?TID=3289&title=15kw-induction-motor-overclock

Hopefully I can fit 36 wires amongst the stator windings and out the terminal box, then it wont matter so much - trial and error :)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 13 November 2012 at 10:07am
I got mine converted and it was a sucess.

Here is a video of the motor operating in Star mode at 415V

http://www.youtube.com/watch?v=hS9iO6BNNRE&feature=player_embedded

Now my question is, would it be worth it to reduce my pack voltage back to 320V and change the IGBT modules to a 200Amp at 600V?

I am having some trouble with the batteries. I need a 192Cell array and the BMS itself costs almost as much as the batteries themselves.
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Johny View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johny Quote  Post ReplyReply Direct Link To This Post Posted: 13 November 2012 at 10:58am
Originally posted by cts_casemod cts_casemod wrote:

I need a 192Cell array and the BMS itself costs almost as much as the batteries themselves.
Have you looked into BMSs that handle 16 cells per board?
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cts_casemod View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cts_casemod Quote  Post ReplyReply Direct Link To This Post Posted: 13 November 2012 at 8:42pm
Originally posted by Johny Johny wrote:

Originally posted by cts_casemod cts_casemod wrote:

I need a 192Cell array and the BMS itself costs almost as much as the batteries themselves.
Have you looked into BMSs that handle 16 cells per board?


I have. The most simple ones are about 80$ each = 1300$ + Shipping + 20% VAT + Import taxes.

What I need is something "simple" with an output to an external contactor and without balancing, but I would like to have a reading of the individual cell voltages
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