Author: EeyoreDate: 12:46 15-10-07
|
|
spamfree@spam.heaven wrote:
> Eeyore wrote:
> >spamfree@spam.heaven wrote:
> >> Eeyore wrote:
>
> >> >Your load is ~ 15A.
> >>
> >> More like 20, isn't it?
> >
> >20A x 36V = 720 W !
>
> That's about right. The motor is not perfect and its 500W output will
> require something like that flat out.
Hang on. Who said 500W *output* equals 720W input ? Where did you get such an idea ? Why
do you think the motor's rated in that way ?
If it says '500W' I'd expect it to mean 500W input power.
Graham
|
|
|
|
Author: EeyoreDate: 12:52 15-10-07
|
|
spamfree@spam.heaven wrote:
> If I had say 24 of these 75c 4700uF 50V electrolytics
You would have 112,800 uF.
Far in excess of what you need.
Graham
|
|
|
|
Author: EeyoreDate: 12:55 15-10-07
|
|
BobG wrote:
> maybe just the secondary of a transformer would work?
That would probably be a poor choice actually.
Transformers are designed for AC not DC. The core will probably just saturate
and you'll merely add some DC resistance.
Graham
|
|
|
|
Author: EeyoreDate: 12:56 15-10-07
|
|
spamfree@spam.heaven wrote:
> On Mon, 15 Oct 2007 22:02:25 +0900, spamfree@spam.heaven wrote:
>
> >If I had say 24 of these 75c 4700uF 50V electrolytics arranged across
> >the rectifier output as 12 sets of series pairs, with a fan directed
> >towards them would this $20 outlay have any chance of success?
> >I could keep an eye on the temps with my handy radiation thermometer.
>
> Just to elaborate and check my calculations, I'll draw an ascii
> diagram of my propsed circuit.
>
> +36V-----------------------------------------------------------------------
> | | | | | | | | | | | |
> = = = = = = = = = = = = | |
> | | | | | | | | | | = = =
> = = = = = = = = =
> | | | | | | | | | | | |
> 0V---------------------------------------------------------------------------
>
> Each cap is 4700 uF so each pair will be 2350 uF across the output
> leads.
Eh ? They have a 50V rating. Why not use it ?
Graham
|
|
|
|
Author: BobGDate: 15:19 15-10-07
|
|
On Oct 15, 12:55 pm, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:
> BobG wrote:
> > maybe just the secondary of a transformer would work?
>
> That would probably be a poor choice actually.
>
> Transformers are designed for AC not DC. The core will probably just saturate
> and you'll merely add some DC resistance.
>
> Graham
=============================
If he needs 15A and 'as much L as he can get', seems like any trans
with a 15A secondary satisfies the amperage req, and you just live
with whatever L is avail. I think I recall guitar amps having a 5H
filter choke, but thats at 500VDC and ma instead of Amps. Whats the L
of a power trans secondary if the primary is floating?
|
|
|
|
Author: Roger DewhurstDate: 17:58 15-10-07
|
|
|
|
|
|
Author: Tim WescottDate: 18:22 15-10-07
|
|
spamfree@spam.heaven wrote:
> I'm building a power supply for a 36V, 500W DC motor to drive a small
> lathe.
>
> I have scrounged a transformer from an old microwave oven and removed
> the 2000V secondary windings leaving the 240V primary windings intact
> and undamaged (I will test with a megger before powering up.)
>
> I will wind some 30A automotive flex around the secondary core and
> test until I have around 36V (the rated voltage of motor and speed
> controller)
>
> I have found a bridge rectifier to use for the rectification of this
> secondary 50Hz current. It is rated at 400V and 35A. Will this be
> suitable? Is there a downside to using too large a BR?
> This is about the only one that would appear to handle the rated 18A
> for the motor. Is 35A rating enough for the BR?
>
> Now to smoothing. I have calculated that I will need something like a
> 56,000 microFarad capacitor to reduce my ripple to 10%.
>
> I'm not even sure if this will be smooth enough for my motor
> controller which is a Chinese scooter controller here:
>
> http://secure.oatleyelectronics.com/files/spc536notes.pdf
>
> Anyway, I can't find a cap this size in Australia (I'm sure they exist
> - they do in Digikey - very expensive but unavailable singly) but I
> can find 50V electrolytics of 4700 microF for around 75c (US) each. If
> I strung a dozen or so of these between the DC output terminals, would
> I have any major problems? Yes, I would prefer 100V caps but the 50s
> are all that are offered by this surplus goods vendor.
>
> Otherwise, I was thinking of using 3 cheap 12V car batteries across
> the DC terminals. I've heard these have ginormous capacitance, and
> hence smoothing effect. They might be short-lived however, and be
> false economy.
>
> As these caps are so cheap, would it be of advantage to hook several
> dozen up in series and parallel to increase capacitance and voltage
> handling?
>
> I'm learning furiously, but as a newbie, some things that are obvious
> to the experts are rather hard to find out about, especially when you
> are not quite sure of anything. jack
Your other questions seem to be answered, here's my two bits:
Yes, you can use the itty bitty caps. Verify their current ratings if
you can -- some of what makes the spendy caps spendy is the AC ripple
current they can deliver without overheating.
You can parallel as many caps as you want, for more capacity and
current. 50V isn't enough for a 36V supply. You can connect caps in
series as long as you use some balance resistors. You ought to have a
ballast resistor on the supply anyway, to make sure the caps discharge
by themselves -- you can combine the ballast and balance functions by
ballasting each capacitor bank separately.
Have fun.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
|
|
|
|
|
|
On Mon, 15 Oct 2007 17:43:58 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:
>
>
>spamfree@spam.heaven wrote:
>
>> Eeyore wrote:
>> >spamfree@spam.heaven wrote:
>> >>
>> >> from:
>> >> C:\Documents and Settings\Administrator\Desktop\Power Supplies.mht
>> >
>> >A curious 'cookbook' equation. Mine is from electronics first principles.
>>
>> Well, yes, I assumed that. but is it not useful? It's within an order
>> of magnitude. What would be the downside of using twice the
>> capacitance here?
>
>Unnecessary cost for one !
Fair enough, but I assume it would provide extra smoothing which may
or may not be of advantage here. I still can't find out what the
smoothness requirements are for these Chinese scooter motor
controllers. An guy on another group with experience with these things
thinks it needs a pretty smooth supply, as there is little power
conditioning in them as they are designed for batteries.
But he says the only way to find out is to try. BTW, he has given me
alternative ways to smooth the PS output as his calculations for the
required smoothness come out with a 3F capacitor which will be
expensive. jack
|
|
|
|
|
|
On Mon, 15 Oct 2007 17:46:12 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:
>
>
>spamfree@spam.heaven wrote:
>
>> Eeyore wrote:
>> >spamfree@spam.heaven wrote:
>> >> Eeyore wrote:
>>
>> >> >Your load is ~ 15A.
>> >>
>> >> More like 20, isn't it?
>> >
>> >20A x 36V = 720 W !
>>
>> That's about right. The motor is not perfect and its 500W output will
>> require something like that flat out.
>
>Hang on. Who said 500W *output* equals 720W input ? Where did you get such an idea ?
Why
>do you think the motor's rated in that way ?
>
>If it says '500W' I'd expect it to mean 500W input power.
Well you would disagree with NEMA. jack
|
|
|
|
|
|
On Mon, 15 Oct 2007 17:52:53 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:
>
>
>spamfree@spam.heaven wrote:
>
>> If I had say 24 of these 75c 4700uF 50V electrolytics
>
>You would have 112,800 uF.
>
>Far in excess of what you need.
But do you know how smooth my supply needs to be?
And of course, you are assuming these would all be connected in
parallel. jack
|
|
|
|
|
|
On Mon, 15 Oct 2007 17:55:20 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:
>
>
>BobG wrote:
>
>> maybe just the secondary of a transformer would work?
>
>That would probably be a poor choice actually.
>
>Transformers are designed for AC not DC. The core will probably just saturate
>and you'll merely add some DC resistance.
I'm not even off the starting blocks wrt smoothing inductors.
Is a coil a useful addition? jack
|
|
|
|
|
|
On Mon, 15 Oct 2007 17:56:11 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:
>
>
>spamfree@spam.heaven wrote:
>
>> On Mon, 15 Oct 2007 22:02:25 +0900, spamfree@spam.heaven wrote:
>>
>> >If I had say 24 of these 75c 4700uF 50V electrolytics arranged across
>> >the rectifier output as 12 sets of series pairs, with a fan directed
>> >towards them would this $20 outlay have any chance of success?
>> >I could keep an eye on the temps with my handy radiation thermometer.
>>
>> Just to elaborate and check my calculations, I'll draw an ascii
>> diagram of my propsed circuit.
>>
>> +36V-----------------------------------------------------------------------
>> | | | | | | | | | | | |
>> = = = = = = = = = = = = | |
>> | | | | | | | | | | = = =
>> = = = = = = = = =
>> | | | | | | | | | | | |
>> 0V---------------------------------------------------------------------------
>>
>> Each cap is 4700 uF so each pair will be 2350 uF across the output
>> leads.
>
>Eh ? They have a 50V rating. Why not use it ?
Yes, and I've seen quoted a 63V surge capability, but what I wanted
assurance of is the need or not of some multiple of a safety factor.
Thanks for that, jack
|
|
|
|
Author: HammyDate: 18:54 15-10-07
|
|
On Mon, 15 Oct 2007 22:02:25 +0900, spamfree@spam.heaven wrote:
>On Mon, 15 Oct 2007 12:44:19 GMT, Hammy <spamme@hotmail.com> wrote:
>
>>On Mon, 15 Oct 2007 20:01:33 +0900, spamfree@spam.heaven wrote:
>>
>>>On Mon, 15 Oct 2007 11:57:39 +0100, Eeyore
>>><rabbitsfriendsandrelations@hotmail.com> wrote:
>>>
>>>>spamfree@spam.heaven wrote:
>>>>
>
>Thanks, Hammy, this is the sort of thing I need to know.
>
>If I had say 24 of these 75c 4700uF 50V electrolytics arranged across
>the rectifier output as 12 sets of series pairs, with a fan directed
>towards them would this $20 outlay have any chance of success?
>I could keep an eye on the temps with my handy radiation thermometer.
>
>jack
Here is an application guide from evox rifa.
http://www.evoxrifa.com/electrolytic_cat/electrolytic_appguide.pdf
Should answer most of your capacitor related questions.
|
|
|
|
|
|
On Mon, 15 Oct 2007 15:22:26 -0700, Tim Wescott <tim@seemywebsite.com>
wrote:
>Yes, you can use the itty bitty caps. Verify their current ratings if
>you can -- some of what makes the spendy caps spendy is the AC ripple
>current they can deliver without overheating.
I'm confused. This ripple current is surely what I want the caps to
absorb, not to deliver, or am I on the wrong tack as usual? :)
Anyways, it seems that the increased surface area of many small caps
over one big one would favour heat dissipation. Just depends on what
you can get your hands on for reasonable moolah. Seventy-five cents
for 4700uF seems pretty good value?
>You can parallel as many caps as you want, for more capacity and
>current. 50V isn't enough for a 36V supply.
Someone jumped on me for suspecting this :)
>You can connect caps in
>series as long as you use some balance resistors. You ought to have a
>ballast resistor on the supply anyway, to make sure the caps discharge
>by themselves
Is this really necessary for 36V? Or is safety not the only reason to
discharge caps?
> -- you can combine the ballast and balance functions by
>ballasting each capacitor bank separately.
How does the balance work? What value and where would you put these
resistors?
>Have fun.
I am, thanks, Tim.
jack
|
|
|
|
|
|
|
|
|
|
Author: JamieDate: 19:41 15-10-07
|
|
BobG wrote:
> I took a chinese scooter speed controller apart... little plastic box
> with a couple of to220 mosfets in it.... I dont think that thing cares
> about the quality of the voltage going to the motor... it just chops
> it. A couple caps with a big L in series will smooth out the volts...
> call it a filter choke if you want... maybe just the secondary of a
> transformer would work?
>
For the motor in the scooter, it's fine. The induction in the motor
makes using a PWM drive an economical choice.
If you were to use this same drive as a DC power source, you would
then need to make some chances. You might want to consider what the
CW frequency being used on the PWM drive to properly calculate a use
able (L).
--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
|
|
|
|
Author: meDate: 20:25 15-10-07
|
|
spamfree@spam.heaven wrote in
news:cfr7h3lmkfnikv6abjuurtvegr20etj4g3@4ax.com:
>On Mon, 15 Oct 2007 17:55:20 +0100, Eeyore
><rabbitsfriendsandrelations@hotmail.com> wrote:
>
>>
>>
>>BobG wrote:
>>
>>> maybe just the secondary of a transformer would work?
>>
>>That would probably be a poor choice actually.
>>
>>Transformers are designed for AC not DC. The core will probably just
>>saturate and you'll merely add some DC resistance.
>
>I'm not even off the starting blocks wrt smoothing inductors.
>Is a coil a useful addition? jack
>
Not in your application, just go with the rectified transformer output with
smoothing capacitors.
----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
|
|
|
|
|
|
On Mon, 15 Oct 2007 19:25:59 -0500, me <me@here.net> wrote:
>spamfree@spam.heaven wrote in
>news:cfr7h3lmkfnikv6abjuurtvegr20etj4g3@4ax.com:
>
>>On Mon, 15 Oct 2007 17:55:20 +0100, Eeyore
>><rabbitsfriendsandrelations@hotmail.com> wrote:
>>
>>>
>>>
>>>BobG wrote:
>>>
>>>> maybe just the secondary of a transformer would work?
>>>
>>>That would probably be a poor choice actually.
>>>
>>>Transformers are designed for AC not DC. The core will probably just
>>>saturate and you'll merely add some DC resistance.
>>
>>I'm not even off the starting blocks wrt smoothing inductors.
>>Is a coil a useful addition? jack
>>
>
>Not in your application, just go with the rectified transformer output with
>smoothing capacitors.
Right, thanks for that. I thought as I'd not seen any inductors in any
circuits for similar power supplies they would not be of much use.
jack
|
|
|
|
|
|
On Tue, 16 Oct 2007 03:02:27 GMT, ehsjr <ehsjr@bellatlantic.net>
wrote:
>Using more capacitance increases the surge current when you
>first turn the thing on. You might be able to "split out"
>the supply to provide the controller logic with extremely
>smooth power even if there is a lot of ripple.
>Conceptually, it's like this:
>
> + 36 -----+-----------------------+
> | a |
> [D1] [Motor]
> | |
> +-----------+ |
> | | |
> [4700 uF] [Logic]---[MotorDriver]
> | | |
> Gnd ------+-----------+-----------+
>
>
>The control logic draws *much* lower current than the
>motor. The cap keeps the supply to the logic smooth -
>the diode keeps the motor from discharging the cap.
>
>Whether you can get inside the controller to add
>a diode and cap is an unknown.
>
>Actually, I suspect the existing circuit uses something
>like that already, but that is a guess - I'm not familiar
>with these scooters or their controllers.
Thanks, Ed. Food for thought.
I looked at the Evox Rifa article that Hammy pointed me to, and their
formula says I need only 11,000uF.
jack
|
|
|
|
|
|
On Tue, 16 Oct 2007 04:04:16 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:
>
>
>Tim Wescott wrote:
>
>> You can parallel as many caps as you want, for more capacity and
>> current. 50V isn't enough for a 36V supply.
>
>Utter nonsense.
>
>Why do you think they make caps with a voltage rating ?
I was worried about spikes and transients, like the Evox Rifa article
warns about. jack
|
|
|
|
|
