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How to step down 250V to 96V and dissipate 7.5 kW — Parallax Forums

How to step down 250V to 96V and dissipate 7.5 kW

FlyingFishFingerFlyingFishFinger Posts: 461
edited 2010-02-16 22:47 in General Discussion
Hi-
I know this isn't at all related to Parallax but there is so much knowledge around here I figured someone might be able to help us. We want to do bench testing of the motor for our solar car. It's an NGM SCM-150 pancake motor, runs at 96V and has a peak power output of 7.5 kW.
First question is...we have a 250V industrial outlet in our workshop, I don't remember off the top of my head what it's rated. Last time I checked though it was enough to power our motor at full load if we wanted to. But, how do we convert that to 96V and be able to draw 80A from it? Power supplies that do this exist, but cost at least $10,000.
Second question...we can also power our motor from lead acids we have. Granted, they won't last long at full load, but the question remains, how do we dissipate this much power?
Thanks

Rafael

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UC Berkeley '12 EECS
CalSol: Berkeley Solar Car team
www.calsol.berkeley.edu
KJ6AWU

Comments

  • BradCBradC Posts: 2,601
    edited 2010-02-04 08:32
    FlyingFishFinger said...
    Second question...we can also power our motor from lead acids we have. Granted, they won't last long at full load, but the question remains, how do we dissipate this much power?

    Lots of water and some cheap immersion heaters or kettle elements.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Life may be "too short", but it's the longest thing we ever do.
  • mikestefoymikestefoy Posts: 84
    edited 2010-02-04 08:58
    since the 250V will undoubtedly be ac, why not use a transformer down to 96/1.414, then bridge diodes,capacitors to give 96V D.C.

    7,5000watts @ 96volts is 75 Amps, so thats a lot of BIG capacitors !!
    you probably dont need to regulate it since its for a motor


    Mike
  • mikestefoymikestefoy Posts: 84
    edited 2010-02-04 08:59
    750 Amps, of course !!!!
  • mikestefoymikestefoy Posts: 84
    edited 2010-02-04 08:59
    750 Amps, of course !!!!
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2010-02-04 09:05
    Yeah I thought of that. Where can we get such a transformer, and how big would those diodes have to be??
    And I'm not sure how clean the motor controller wants the input voltage to be...

    Rafael

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    UC Berkeley '12 EECS
    CalSol: Berkeley Solar Car team
    www.calsol.berkeley.edu
    KJ6AWU
  • mikestefoymikestefoy Posts: 84
    edited 2010-02-04 09:05
    oops, its morning here in France, so sorry for misreading the power required.

    what does "peak" power mean. i guess startup requirement.

    if it is, then maybe you could have an 100Amp transformer/rectifier/capacitor for the average power, and use the batteries for peak delivery.

    A normal car battery of 20AH, will give the required 100amps or more when starting a car engine, so 8 car batteries in series.

    better put some good protection like fuses/circuit breakers in there though.



    hope i have helped

    Mike
  • mikestefoymikestefoy Posts: 84
    edited 2010-02-04 09:13
    something like this

    http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0504224


    its 1KW, so may work, only costs $150


    I would try to find a higher output though, probably 2 KW.

    I guess you could use 3 1KW ones, and have 3 sets of Xformers/rectifiers/capacitors.

    and just wire the outputs in parallel.

    since you would have 3 circuits, thats each diode bridge rated at say 20-30Amps continuous.


    I would put an ammeter in series with each output to monitor the load sharing

    sounds OK to me at a cursory glance


    Mike
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2010-02-04 18:50
    Vous etes en France? Tres bien [noparse]:)[/noparse]
    Anyway, where would we get diodes that big?

    Rafael

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    UC Berkeley '12 EECS
    CalSol: Berkeley Solar Car team
    www.calsol.berkeley.edu
    KJ6AWU
  • davejamesdavejames Posts: 4,047
    edited 2010-02-04 19:02
    @Fish,

    One of many suppliers for large capacity diodes...

    (data sheet)·· http://www.vishay.com/docs/93521/70hfr.pdf
    (price)·· http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=70HFR20-ND


    Good success!

    DJ

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-02-04 19:19
    This idea is a bit off-the-wall and definitely not as safe as using a transformer to isolate the motor power from the mains, but synchronous rectification comes to mind. The idea is that you could use MOSFETs to conduct at appropriate times during the AC cycle to transfer power to your filter caps. The net voltage on the caps would be timing dependent. To lower the voltage, for example, you'd cut off conduction earlier in each cycle. The cap voltage would then be used as feedback to the timing controller in order to keep it a constant 96V.

    WARNING: You'd be dealing with non-isolated mains voltage on the caps and in the motors, so take appropriate precautions!!! The switching and timing circuitry, however, can (and should be) optically isolated from the mains supply.

    -Phil
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2010-02-04 20:21
    Hm, I like the transformer + rectifier idea better I think. I don't know what our budget for extra stuff is at the moment, so the primary goal is currently how to effectively apply full load to the motor. Sounds like connecting a generator and heating water sounds like an idea, but how would we control loading like that?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    UC Berkeley '12 EECS
    CalSol: Berkeley Solar Car team
    www.calsol.berkeley.edu
    KJ6AWU
  • mikestefoymikestefoy Posts: 84
    edited 2010-02-05 07:35
    @flyingfishfinder

    this is my last reply to this thread.

    we are here to give you help and advice, not to do the job for you.

    you dont seeem to be able to google "50A bridge rectifer" on your own

    I would suggest that you dont have the skills to design this.

    a suitable bridge rectifier is 50A continuous, costs $12

    http://uk.farnell.com/multicomp/cm5002/bridge-rectifier-50a-200v/dp/9380868


    Mike
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2010-02-05 09:24
    Mike - I do not want you to do the job for me and I never intended to make that impression. I consider myself a quite capable electrical engineer, but I have not worked with this much power before (I don't think anyone on our team has) and I do not feel safe scaling up something I know works on a small scale without appropriate advice. I know very well how to build a circuit that would do the job. My question to the community was merely whether there might be a better way or what the safest way would be. Please note too that I said in my last post "the primary goal is currently how to effectively apply full load to the motor." which does not involve the power conversions you have been kind enough to provide us with links for. Rest assured we are quite capable of finding the necessary parts, if we have the budget for a transformer and the rest. After all, we are building a road-legal solar car from scratch, which is not to be taken lightly. Either way, I apologize for any misleading impression I might have made.
    However, to me the question remains: If we use an alternator and resistive heating elements, how to we control how much heat is dissipated at a given time? We can't exactly adjust the resistance of the elements as far as I'm aware of. A teammate suggested a variac with a heating element on the secondary, would this work? As you say, people are happy to furnish others with advice. I am seeking such, not instructions on how to build this test bench. If you feel like I'm asking obvious questions, by all means tell me so and don't do my work for me.

    Rafael

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    UC Berkeley '12 EECS
    CalSol: Berkeley Solar Car team
    www.calsol.berkeley.edu
    KJ6AWU
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2010-02-05 17:25
    FlyingFishFinger,

    One thought I had was to use a dynamotor approach, and then I thought what would one look like as far as physical size that could keep up with 7.5 kW?

    Then I thought a gas powered generator... the motor/generator is AC but usually that is a matter of wiring.. the size for the amount of power would be about the same. It's not a dynamotor in the conventional sense, the 'primary' is a gas engine as opposed to an electric motor and you wouldn't be able to use the 250 Volts you currently have at your disposal, but by varying the speed on the gas motor (I figure at least a 15HP gas motor) you could dial-in your 96 Volts (I presume you wanted DC) on the 'secondary' ... just to compare prices, a gas generator of that size (7.5 kW) could be had for about $1000

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • skylightskylight Posts: 1,915
    edited 2010-02-05 19:53
    you spoke of using batteries as your source if using DC power an old fashioned approach would work, using carbon piles
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2010-02-05 23:17
    Interesting. I'll take a look

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    UC Berkeley '12 EECS
    CalSol: Berkeley Solar Car team
    www.calsol.berkeley.edu
    KJ6AWU
  • TappermanTapperman Posts: 319
    edited 2010-02-14 20:39
    FlyingFishFinger said...
    Hm, I like the transformer + rectifier idea better I think. I don't know what our budget for extra stuff is at the moment, so the primary goal is currently how to effectively apply full load to the motor. Sounds like connecting a generator and heating water sounds like an idea, but how would we control loading like that?

    Try pumping water up hill ... that will give an accurate picture of work load.·

    Also, if your going to consider using Bat. in series ... I would recommend using relays to stack the bat's, provided your controller does the stacking (when no current is flowing) Also, make sure the relays are large enough to handle the current, even with the precaution just mentioned, you can weld the contacts together with the kinds of currents·available from a lead-acid bat·... Using relays is a little expensive, but when the circuit is off, no dangerous voltages are present.·· AKA, when the hood is up.
  • David BDavid B Posts: 592
    edited 2010-02-16 22:47
    If you're still interested, google load bank rentals.

    When our company needed to test our UPS batteries, our tech guy rented a load bank and hooked it up to the batteries.

    The load bank cooling fans sounded like a jet engine was running in our building, but it did the job.
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