Capacitance

[bsparky]

I have a 110vac transformer capable of 40amps going thru a bridge rectifier and driving a dc motor which cocks a skeet machine. Voltage before start of motor is 35vdc and drops to 7vdc during the cocking sequence which takes about 3 seconds to complete. I know I need a capaciter across the dc line but not sure of size and rating and help. Thanks

[jwax]

Sounds like the motor is maxing out the capacity of that power supply, during the full load.
What is it you're trying to achieve? Faster cocking? If so, you'd be better off with battery-assist than a capacitor since the energy you need is so large.
Any info you can supply about the motor would help guesstimate the battery size.

[dyarker]

I worked part-time at a Rod & Gun club many moons ago; never saw a DC target launcher. But hey why not one meant to run on car batteries that you now want to run on the AC mains?<p>What nominal voltage of the DC motor?<p>How much current when cocking? Must know this and how how much ripple is acceptable to calc the capacitance.<p>35V no capacitor at all, then:
35V * 1.414 = 49.5V, so 60V or 75V volt rating for capacitor.<p>35V no load with some capacitance already there, then:
50V rated capacitor.<p>35V down to 7V is A LOT of drop. The diodes in the bridge don't blow, so they're alright. Either the transformer isn't as strong as you think, or the wire and/or switch isn't large enough.<p>If the diodes are near max current while cocking, adding a capacitor could blow the diodes.<p>Just a guess, you're using 25ft, or more, of 18ga wire from the transformer/rectifier to the motor.<p>C U L -

[bsparky]

Yes these are new trap machine made to run off of car batteries to be placed in the field for this new field shooting sport. They wanted to replace there old ac machines. Motors run on 12vdc and they wanted them to cock as fast as they do on the batteries. I had no info on the actuall current of the motors. Some one made a call and they were told that the motors are fused for 40amps. Bridge rectifiers are 50amps and mounted on heat sinks with about a 6 foot run to the machines on #12 wire. No heat what so ever on the rectifiers after 1/2 dozen launches.
Thanks Guys

[dyarker]

Something isn't as it seems. 35V no load to 7V loaded, is too much drop! You're right, 6ft of 12AWG won't drop that much. (though 10AWG or 8AWG would be better cause this is a low voltage setup)<p>Now I suspect the transformer. What gauge are the secondary wires? (primary side has 110VAC)<p>If primary side has 4 wires, and you've only got 2 connected, one primary winding isn't being used. (Some transformers have two 110VAC windings, connect in parallel for 110V, connect in series for 220V.)<p>How many secondary wires?
..If 2, what gauge wire to rectifier?
..IF 3, disconnect the center tap.
..If 4 and only 2 connected, you're not getting current from one of the secondaries.
..If 4 and they're in series, you've doubled the voltage but halved the available current. 35V @ 20A which is overloaded. Put the windings in parallel (careful about phase) for 17V @ 40A!!!!<p>I would agree with jwax about batteries except at 35V they'll get cooked.

[MrAl]

Hello there,<p>WOW, i agree that 35v to 7v is WAYYY too much
drop! The #12 wire will drop about 0.6 volts,
so something else is surely wrong.<p>Without knowing the output voltage of the
transformer, since you measured 35v that would
make that transformer secondary wayyy too high
for that motor, if it's really a 12v motor.
What may have happened is that the first time
the unit was run up the transformer putting
out 35v into a 12v motor caused a current much
above say 30 amps which may have blown a diode
in the bridge rectifier. Although just a guess
from where im standing, this would explain the
big drop in voltage if you're now only getting
half wave rectification. Do you know how
to check diodes with a meter, that might help?<p>What's missing in this picture is output voltage
rating of the transformer secondary, and of course
if it's connected properly as per Dale's reply.<p>What's also important is that the output secondary
voltage match the requirements of the motor or
else the motor may draw too much current.<p>It's true that adding a capacitor increases
the average output voltage available but in
this case it doesnt sound like that's the correct
solution. When you finally find out the true
cause of the problem you'll find you dont need
a capacitor at all.<p>As a side note, i ran a 12vdc clutch from a
120vac fw rectified line in series with a
light bulb! The initial surge engaged the
clutch and the 'holding' current of about 1
amp was just enough so i didnt even need a
transformer...certainly no caps <p>Take care and good luck with it,
Al

[rshayes]

You might look at the primary side a little more closely. The transformer is about a 3:1 ratio, so the primary current will be about 1/3 the secondary current. With a 40 amp load, that will be about 13 amps. If you have a long extension cord with #14 or #16 wire, this could be a problem.<p>Motor starting current may be much higher than running current. Batteries can supply this, the power line might have a little trouble if the circuit is too long.

[toejam]

you should find out how much current the motor uses by powrering it with a battery in series with an ammeter I think you need a shop type 50 amp battery charger to run it,a transformer that can deliver 40 amps at 35 volts is 1400 watts if it don't weigh 50 pounds or thereabouts it aint gonna deliver that much power.
good luck
tj

[toejam]

make that 20 or so pounds

[bsparky]

Transformer has a 4 wire 120/240 prim. and 4 wire 12/24 sec. 12 ac before rect. and 35vdc after rect. No voltage drop of the ac line during cocking. All wires prim. and sec. being used. Something sure doesn't seem right

[Edd]

How much filtering capacitance are you currently
trying to use across the secondary's raw DC supply? With your other supplied info and adequate wire guaging/w/short runs , just sounds like insufficient filtering/reserve to me.<p>73's de Edd
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[dyarker]

Thanx<p>24V * 1.414 = 33.936<p>So I'm convinced that the two secondary windings are connected in series (adds the voltage) instead of parallel (adds the currents). And, there is already some capacitance after the rectifier. 1.414 is the factor to get peak voltage from RMS (sine wave) voltage. The capacitor charges to peak during no load.<p>The secondaries need to be reconnected in parallel. Be carefull to get the phases right or you could blow the transformer. Do you need help with that?<p>Cheers,

[bsparky]

Thanks guys, will get more info, voltage checks , diode checks and trans. wiring.

[bsparky]

Hi Guys was wondering when I bought these Trans. I ask for 110v input and 12v @ 40 amps output. What they sold me was a buck-boost Trans. Now this is rated for 40 amps AC would I still get 40 amps DC after bridge rectification?

[jwax]

Yes, providing your diodes can handle 40 amps.