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Posted by on March 15, 2006, 5:21 am
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On Tue, 14 Mar 2006 11:35:08 -0600, Don Foreman
>On Tue, 14 Mar 2006 16:00:28 +0000, pentagrid@yahoo.com wrote:
>> With the coil fed by a bridge rectifier, when the AC input
>>is disconnected, the inductive overshoot drives all four diodes
>>into forward conduction and this safely dissipates the stored
>>energy in the resistance of the magnet coil. A protective
>>capacitor is not necessary on the input or output of the bridge.
>>
>> With no output capacitor the DC output voltage will be the
>>MEAN value of the input voltage - 0.9 x RMS. less the diode
>>drops.
>>
>> With a large output capacitor the output approaches the
>>PEAK value of the input voltage, again less the diode drops.
>>
>> Jim
>
>Yup, the diodes will snub the coil on turnoff.
>
>Most DC voltmeters do respond to average value. However, neglecting
>diode drops, the RMS value of the rectified DC must be the same as
>the RMS value of the AC by definition. The rectified voltage
>waveform is the same as the AC wave except that alternate
>half-cycles are reversed in polarity. RMS means root mean squared,
>and the squaring operation makes polarity irrelevant. Therefore, the
>RMS values (neglecting diode drops) are identical.
Just a bit more:-
With no output capacitor the DC output voltage will be the
MEAN value of the input voltage - 0.9 x RMS. less the diode
drops.
The RMS value of the output voltage is equal to the RMS value
of the input voltage (less diode drops)
However it is the MEAN value of the applied voltage that
determines the DC component of the output current and it is the
value of this current that determines the current drawn from the
supply and the power dissipated in the magnet.
If,as is usual with this type of electromagnet,the
inductance is large, the magnet current with be almost pure DC
with only a small ripple component. This means that (neglecting
the effect of the residual ripple component) the waveform of the
current drawn from the supply will be a square wave of amplitude
equal to the DC output current.
Jim
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Posted by Tom Gardner on March 14, 2006, 11:33 am
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After reading your post, I started wondering about the mag on my Arter
Grinder. It has a clockwork rotary switch that one has to turn 270 degrees
then it locks on. To turn it off, one turns the switch a few degrees and it
unlocks and turns off slowly. I think it pulses the mag.
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Posted by john on March 14, 2006, 9:01 pm
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Gunner wrote:
> This is a bit embaressing. Sigh..its been a looong time since I needed
> to figure this sort of thing out.....Im a board changer/wrench
> turner/relay and plc type...
>
> I have an elderly electromagnetic chuck that Ive been sitting on for a
> couple years until I found a decent usable 6x8 surface grinder..which
> I recently did (Covel), but it has no power supply. Rated 110vts DC.
>
> So I dug out a bridge rectifier, added a power cord, and powered up
> the chuck with the output. Chuck does indeed work, but its a smidge
> weaker than I think it should be..or would like it to be. Its half
> wave DC..so Im only applying a magnetic field half the time, right?
>
> Now if I added a simple capacitor to the output, it should give me a
> longer signal time as it fills in the gaps. This should result in a
> slightly stronger effective magnetic field. Right?
> One that is not forming and collapsing 60 times a second
>
> Assuming Ive not shit in my messkit at this point..what MFD cap should
> I use? Cringe.....blush
>
> Current draw is about 1 amp
>
> Clean DC would be nice..but its only a magnetic chuck....
>
> Shrug..
>
> Now the last thing....some electo chucks tend to magnitize over
> time..or bleed off the field over a number of seconds.
>
> If I put a maintained On/off/Momentary On toggle switch in...I should
> be able to run a momentary bit of 110vAC though the chuck to
> demagnitize it, right?
>
> Thanks
>
> Gunner, hanging his head in shame...
>
>
> "A prudent man foresees the difficulties ahead and prepares for them;
> the simpleton goes blindly on and suffers the consequences."
> - Proverbs 22:3
If you dont have a transformer for isolation I would check for a short
to ground in the magnet. As far as needing a cap or ballast resistor,
it is not necessary. Most of the magnetic chucks Ive repaired dont have
any. Your diode bridge may be the problem. I would never reuse a bridge
diode unless you have a very good method of testing it.
This method wil work.
Put a light bulb across it and see if you get full brillance on the
bulb. Then put a diode in series with the light bulb and see if you get
half brillance. Reverse the diode and do the same thing. This will
check the complete bridge diode. Many diodes get "weak" they get an
abnormal voltage drop across them under any load but test good with a
diode tester. I've changed more of them than I can remember that had
abnormal voltage drops. In avionics the KN-65 DME was notorious for
bad diode bridges.
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Posted by DoN. Nichols on March 15, 2006, 12:48 am
Please log in for more thread options > This is a bit embaressing. Sigh..its been a looong time since I needed
> to figure this sort of thing out.....Im a board changer/wrench
> turner/relay and plc type...
>
> I have an elderly electromagnetic chuck that Ive been sitting on for a
> couple years until I found a decent usable 6x8 surface grinder..which
> I recently did (Covel), but it has no power supply. Rated 110vts DC.
O.K. The mag chuck, I presume you mean, is rated 110 VDC?
> So I dug out a bridge rectifier, added a power cord, and powered up
> the chuck with the output. Chuck does indeed work, but its a smidge
> weaker than I think it should be..or would like it to be. Its half
> wave DC..so Im only applying a magnetic field half the time, right?
Huh? How do you get half-wave with a bridge rectifier? That
should give you full-wave rectification.
> Now if I added a simple capacitor to the output, it should give me a
> longer signal time as it fills in the gaps.
Yes.
> This should result in a
> slightly stronger effective magnetic field. Right?
> One that is not forming and collapsing 60 times a second
>
> Assuming Ive not shit in my messkit at this point..what MFD cap should
> I use? Cringe.....blush
>
> Current draw is about 1 amp
Well -- it could be calculated to determine what the minimum
capacitance needed would be -- but I simply used a 250V 1000 uF computer
grade electrolytic. (At least 200V is needed because the peak voltage
from that bridge should be on the order of 170 V, and with minimum
ripple, the voltage across the capacitor will be that pretty much all of
the time.
You can use larger capacitance if you want, or significantly
less. The 200V is a minimum, and higher won't hurt.
You also want a resistor to limit the surge current. Let's see,
with 1 Amp current, a 5 ohm resistor would drop 5 volts, and will need
to be capable of handling 5 Watts. Probably a 10 Watt wire-wound would
do nicely.
> Clean DC would be nice..but its only a magnetic chuck....
>
> Shrug..
>
> Now the last thing....some electo chucks tend to magnitize over
> time..or bleed off the field over a number of seconds.
Right.
> If I put a maintained On/off/Momentary On toggle switch in...I should
> be able to run a momentary bit of 110vAC though the chuck to
> demagnitize it, right?
Yes -- if you have the switching right, so you don't apply that
while the capacitor and bridge are connected to the chuck.
I have a schematic of one which I designed to replace what was
in the base of my Sanford grinder (about a bit smaller -- 6x4 inch,
IIRC).
http://www2.d-and-d.com/PROJECTS/SANFORD-GRINDER/index.html
an click on the link for the pdf version of the drawing. It is easier
to read when printed. The .gif is probably not as good, and certainly
the reduced size schematic tends to have lines vanish, depending on the
browser in use.
The switch which I used has a center off position, and is stable
in either on position, though a momentary to the "degauss" position
could be used. I consider the switch which I have better, as you get
better demagnitization of the workpiece if it is removed while the AC is
on -- akin to moving a tape head demagnetizer away from the head before
switching off the power, which could otherwise switch off during a peak
and thus leave more residual magnetism.
The chuck had a plug which fit into the side wall of the base of
the grinder, and that is what is shown in the schematic -- not the chuck
itself.
Note that when using this design, you *don't* want to use
coolant. If you want to do that, add an isolation transformer between
the power line and the bridge rectifier.
Good Luck,
DoN.
--
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
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