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Home > Archive > Electrical Engineering > October 2007 > dc electromagnet question?
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dc electromagnet question?
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| I have constructed a DC electromagnet for a special use at my shop. Being a
electrical contractor I thought this would be simple. (NOT) Made the
electromagnet, connected it to a 12vdc power supply at 5 amps and it works
great with one problem, it starts to heat up quickly. My desired results
was for it to run on a 80% duty cycle. Will resistors keep this heating
problem from happening? DC voltage is not something I have dealt with
indepth. Can anyone help me with this problem?
wvick@cox.net
Vick
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| Palindrome 2007-09-27, 5:25 pm |
| vick wrote:
> I have constructed a DC electromagnet for a special use at my shop. Being a
> electrical contractor I thought this would be simple. (NOT) Made the
> electromagnet, connected it to a 12vdc power supply at 5 amps and it works
> great with one problem, it starts to heat up quickly. My desired results
> was for it to run on a 80% duty cycle. Will resistors keep this heating
> problem from happening? DC voltage is not something I have dealt with
> indepth. Can anyone help me with this problem?
A resistor in series will decrease the current, decrease the dissipation
in the coil and decrease the strength of the electromagnet. If that is
ok, then fine, stick a resistor in series or lower the voltage.
Otherwise, you need a way of getting rid of the 60W that you are dumping
into the coil - or redesign the electromagnet. A fan or two (or four)
might be an idea.
Another idea, that might be appropriate, is a resistor that can be
switched out. So, the full strength of the electromagnet will be
available when needed, but some form of automatic or manual switch
reduces the current when the full strength isn't.
--
Sue
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| Is there a way to reduce the wattage from the power supply or should I just
get another power supply with less wattage? Thanks for your
help!!!!!!!!!!!!!!!!!
Vick
> A resistor in series will decrease the current, decrease the dissipation
> in the coil and decrease the strength of the electromagnet. If that is ok,
> then fine, stick a resistor in series or lower the voltage.
>
> Otherwise, you need a way of getting rid of the 60W that you are dumping
> into the coil - or redesign the electromagnet. A fan or two (or four)
> might be an idea.
>
> Another idea, that might be appropriate, is a resistor that can be
> switched out. So, the full strength of the electromagnet will be available
> when needed, but some form of automatic or manual switch reduces the
> current when the full strength isn't.
>
> --
> Sue
>
>
>
| |
| Palindrome 2007-09-27, 8:25 pm |
| vick wrote:
> Is there a way to reduce the wattage from the power supply or should I just
> get another power supply with less wattage? Thanks for your
> help!!!!!!!!!!!!!!!!!
You can get a power supply with less *voltage*. That will reduce the
current and hence the power dissipated - and the strength of the
electromagnet.
> Vick
>
>
| |
| Stuart 2007-09-27, 8:25 pm |
| In article <raWKi.11352$495.7706@newsfe22.lga>,
vick <wvick@cox.net> wrote:
> Is there a way to reduce the wattage from the power supply or should I
> just get another power supply with less wattage? Thanks for your
> help!!!!!!!!!!!!!!!!! Vick
If you're an electrical contractor remind me never to employ you to do
anything electrical. You clearly haven't a clue.
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Stuart 2007-09-27, 8:25 pm |
| In article <jjWKi.574686$sR4.299654@fe08.news.easynews.com>,
Palindrome <me9@privacy.net> wrote:
> Reply-To: me@palindr0me.plus.com
> vick wrote:
[color=darkred]
> You can get a power supply with less *voltage*. That will reduce the
> current and hence the power dissipated - and the strength of the
> electromagnet.
Being "positive" this time.
Rewind your coil with a finer gauge of wire but use more turns.
The higher resistance will reduce the current and hence the heating but as
magnetic flux is the product of the number of turns and the current you
can maintain the required strength of field/
For example, if you reduce the wire size from 1mm square to 0.5mm square
and double the number of turns. The current, and hence the power
dissipation, will be halved but you magnetic flux will remain virtually
the same.
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
|
| Thanks for your help! It will be a do able solution hopefully.
Being a electrical contractor I (believe it or not) specialize in
design/build unique commerical electrical applications. I have dealt with
dc only with systems that have already been design for special equipment.
But, for me, this just did not compute in my brain compared to ac for what
ever reasons. But thats again for your help. Its people like you helping
people like me that make the world a better place.
Vick
"Stuart" <SW_NOSPAM@dsl.pipex.com> wrote in message
news:4f291f1989SW_NOSPAM@dsl.pipex.com...
> In article <raWKi.11352$495.7706@newsfe22.lga>,
> vick <wvick@cox.net> wrote:
>
> If you're an electrical contractor remind me never to employ you to do
> anything electrical. You clearly haven't a clue.
>
> --
> Stuart Winsor
>
> From is valid but subject to change without notice if it gets spammed.
>
> For Barn dances and folk evenings in the Coventry and Warwickshire area
> See: http://www.barndance.org.uk
| |
| Don Kelly 2007-09-28, 3:25 am |
| ----------------------------
"vick" <wvick@cox.net> wrote in message
news:fjVKi.333393$dA7.289211@newsfe16.lga...
>I have constructed a DC electromagnet for a special use at my shop. Being
>a electrical contractor I thought this would be simple. (NOT) Made the
>electromagnet, connected it to a 12vdc power supply at 5 amps and it works
>great with one problem, it starts to heat up quickly. My desired results
>was for it to run on a 80% duty cycle. Will resistors keep this heating
>problem from happening? DC voltage is not something I have dealt with
>indepth. Can anyone help me with this problem?
> wvick@cox.net
> Vick
>
-----
If you use resistors, you will reduce the heating-but you will also reduce
the force produced by the magnet in proportion. Using a lower voltage
source will also do the same.-
In that case increase the number of turns of wire to compensate -this will
also add some resistance.
For example, twice the number of turns (same wire) doubles the resistance
and halves the current resulting in half the heating for the same
force(proportional to ampere-turns)- but a bigger magnet. Life consists of
tradeoffs.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
| Don Kelly 2007-09-28, 3:25 am |
| ----------------------------
"Stuart" <SW_NOSPAM@dsl.pipex.com> wrote in message
news:4f29206b35SW_NOSPAM@dsl.pipex.com...
> In article <jjWKi.574686$sR4.299654@fe08.news.easynews.com>,
> Palindrome <me9@privacy.net> wrote:
>
>
>
> Being "positive" this time.
>
> Rewind your coil with a finer gauge of wire but use more turns.
>
> The higher resistance will reduce the current and hence the heating but as
> magnetic flux is the product of the number of turns and the current you
> can maintain the required strength of field/
>
> For example, if you reduce the wire size from 1mm square to 0.5mm square
> and double the number of turns. The current, and hence the power
> dissipation, will be halved but you magnetic flux will remain virtually
> the same.
>
> --
> Stuart Winsor
-----
You indicate a doubling of the wire length and half the
cross-section--resulting in 4 times the resistance. From this the current
and losses will be 1/4 the original value but the force will also be
approximately 1/4 (proportional to {NI}^2). (unless the original setup was
driving the iron into saturation ).
--
Don Kelly dhky@shawcross.ca
remove the X to answer
>
> From is valid but subject to change without notice if it gets spammed.
>
> For Barn dances and folk evenings in the Coventry and Warwickshire area
> See: http://www.barndance.org.uk
| |
| Stuart 2007-09-28, 1:25 pm |
| In article <Ae0Li.8547$x%6.7933@pd7urf2no>,
Don Kelly <dhky@shaw.ca> wrote:
> ----- You indicate a doubling of the wire length and half the
> cross-section--resulting in 4 times the resistance. From this the
> current and losses will be 1/4 the original value but the force will
> also be approximately 1/4 (proportional to {NI}^2). (unless the
> original setup was driving the iron into saturation ).
Hmm! I must remember not to post at 20 mins past midnight, when I'm
already half asleep, in future :-)
Your other post is of course the correct answer.
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Stuart 2007-09-28, 1:25 pm |
| In article <PJXKi.110258$lZ7.85645@newsfe20.lga>,
vick <wvick@cox.net> wrote:
> Thanks for your help!
<SNIP>
Glad to be of service - I trust you read my other post?
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Palindrome 2007-09-28, 1:25 pm |
| Stuart wrote:
> In article <Ae0Li.8547$x%6.7933@pd7urf2no>,
> Don Kelly <dhky@shaw.ca> wrote:
>
> Hmm! I must remember not to post at 20 mins past midnight, when I'm
> already half asleep, in future :-)
>
> Your other post is of course the correct answer.
>
That was why, at a slightly earlier time, I settled for writing, "Or
redesign the electromagnet" - without explaining how. 
I thought, if he really wants to know, he can ask and I can answer -
when the old brain cell isn't still swimming in cherry brandy..
--
Sue
| |
| Stuart 2007-09-28, 1:25 pm |
| In article <o59Li.603715$Bo7.132375@fe07.news.easynews.com>,
Palindrome <me9@privacy.net> wrote:
> I thought, if he really wants to know, he can ask and I can answer -
> when the old brain cell isn't still swimming in cherry brandy..
Nice!
Personally I prefer a single malt but cherry brandy's nice too.
--
Stuart Winsor
From is valid but subject to change without notice if it gets spammed.
For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk
| |
| Don Kelly 2007-09-29, 3:25 am |
| "Stuart" <SW_NOSPAM@dsl.pipex.com> wrote in message
news:4f2977122fSW_NOSPAM@dsl.pipex.com...
> In article <Ae0Li.8547$x%6.7933@pd7urf2no>,
> Don Kelly <dhky@shaw.ca> wrote:
>
> Hmm! I must remember not to post at 20 mins past midnight, when I'm
> already half asleep, in future :-)
>
> Your other post is of course the correct answer.
>
> --
> Stuart Winsor
>
> From is valid but subject to change without notice if it gets spammed.
>
> For Barn dances and folk evenings in the Coventry and Warwickshire area
> See: http://www.barndance.org.uk
---------------
You are not the only one to boob due to lack of sleep, surfeit of wine, or
simple haste.
I have done it more times than I want to count.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------
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Stuart wrote:
>
> The higher resistance will reduce the current and hence the heating but as
> magnetic flux is the product of the number of turns and the current you
> can maintain the required strength of field/
>
> For example, if you reduce the wire size from 1mm square to 0.5mm square
> and double the number of turns. The current, and hence the power
> dissipation, will be halved but you magnetic flux will remain virtually
> the same.
I hate to be the wet blanket here, but winding a coil with smaller
diameter wire will NOT change the heating if the field strength is
kept the same! It will change the voltage required to give that
heating.
As a hint: A coil wound with 950 feet of 20 gauge wire will weight
about 3 pounds, run on 12 volts dissipating about 15 watts in the coil
(think of a soldering pencil in there!) and produces a field of about
250 Gauss (without any iron present). The coil will draw about 1.2347
amps at 12 volts and have a resistance of roughly 9.7 Ohms. If you
change the wire gauge keeping the coil size identical everything will
remain the same except the voltage needed to get that field. Note that
coil inductance will be greatly changed but that is not of concern
here. (Inductance of the 20 ga version about .042 H.)
(Calculation of the above values left as an exercise for the
interested student!)
You can only lower the heating by lowering the voltage drive which of
course lowers the field strength of the magnet. Winding with finer
wire but keeping the voltage the same will reduce heating but also the
field. If you can get away with a lower field that is fine.
Otherwise, the usual solution is to use lower voltage and heavier
wire! Namely tiny copper tube with insulation on it. You run water
through the tube to take the heat out of the coil. Except for dealing
with water it makes a killer electromagnet. Takes a heavy current low
voltage power supply though.
| |
| BFoelsch 2007-09-29, 1:25 pm |
| On Fri, 28 Sep 2007 05:24:48 GMT, "Don Kelly" <dhky@shaw.ca> wrote:
>----------------------------
>"Stuart" <SW_NOSPAM@dsl.pipex.com> wrote in message
>news:4f29206b35SW_NOSPAM@dsl.pipex.com...
>-----
>You indicate a doubling of the wire length and half the
>cross-section--resulting in 4 times the resistance. From this the current
>and losses will be 1/4 the original value but the force will also be
>approximately 1/4 (proportional to {NI}^2). (unless the original setup was
>driving the iron into saturation ).
Why (NI)^2?
MMF is just NI!!, as in ampere-turns!!
| |
| Don Kelly 2007-09-30, 3:25 am |
| ----------------------------
"BFoelsch" <BFoelsch@comcast.ditch.this.net> wrote in message
news:8tusf3t7oj4h9idpcgog3mihvtitr644np@4ax.com...
> On Fri, 28 Sep 2007 05:24:48 GMT, "Don Kelly" <dhky@shaw.ca> wrote:
>
>
>
> Why (NI)^2?
>
> MMF is just NI!!, as in ampere-turns!!
-----------
mmf is NI but the mechanical force is dependent on (mmf)^2 .
Check it out.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
| BFoelsch 2007-09-30, 9:25 am |
| On Sun, 30 Sep 2007 02:57:41 GMT, "Don Kelly" <dhky@shaw.ca> wrote:
>----------------------------
>"BFoelsch" <BFoelsch@comcast.ditch.this.net> wrote in message
>news:8tusf3t7oj4h9idpcgog3mihvtitr644np@4ax.com...
>-----------
>
> mmf is NI but the mechanical force is dependent on (mmf)^2 .
>Check it out.
Sorry about that.
A "senior moment," I guess.
Thanks again.
| |
| Don Kelly 2007-10-03, 3:25 am |
| "BFoelsch" <BFoelsch@comcast.ditch.this.net> wrote in message
news:mo9vf31lv4eu1ctqipbtl61q25jf77eud8@4ax.com...
> On Sun, 30 Sep 2007 02:57:41 GMT, "Don Kelly" <dhky@shaw.ca> wrote:
>
>
> Sorry about that.
>
> A "senior moment," I guess.
>
> Thanks again.
No problem- join the crowd and be glad that they are not Roy or chairman
moments.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
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