Hi, Ian. Ditto on everything Phil said. If you'd like some control of the motor speed, though, you can drive the primary of the transformer with the output of a Variac like this (view in fixed font or M$ Notepad):
Some more to think about . . . if you filter it you raise the voltage because with raw RMS DC 24V=24V, with filtered DC you approach the peak voltage which is 1.4142 X the RMS value, or about 36 volts (unloaded - the transformer will drop a little voltage as will the rectifier diodes) Transformer ratings are for some specified current and at some nominal voltage - Voltage can be plus or minus 10%. off nominal.
And, you are using 20 amps. each diode in a four diode bridge will drop .6 volts for 1.2 volts in each half cycle at 20 amps that's 24 watts wasted power and requires a good heatsink. Use a center-tapped transformer and you cut the waste power in half - ditto the heatsink and size, and likely as not cost as well.
I use an adjustable 24 VDC 10 amp to run some fans - the SCR's doing the controlling also rectify the AC to save on wasted power and keep the heatsink small.
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I see the errors of my thinking. Intrinsic lossses in the transformer cause the I sq R loss ot increase faster than the diode drop losses in that (24V) voltage range. At lower voltages the diodes waste more power than the transformer and a center tap makes more sense.
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The "full wave, center tap" ( two diode) arrangement REQUIRES that a transformer's VA be DE- RATED compared to using full wave bridge.The reason is as I posted earlier - only HALF the secondary copper is being used at any moment.
For a resistive load and the same output voltage, current, and regulation, both transformers must use the same gauge of wire on their secondaries. However, since the CT arrangement requires that each half of its secondary conduct only half the time, it will _inherently_ have a higher VA rating than the FWB transformer, but some of its capacity will be unused. That is, it will run cooler than the FWB transformer for the same load.
The same is true if the load includes a reservoir capacitor, as indicated by example 1 at:
Why would a designer use the same size wire in the CT secondary? Each winding is working at a 50% duty cycle so the required ampacity of the wire is cut by half - things being what they are, the transformer maker would save copper by using smaller wire.
AND Looking at transformer ratings where they provide two independant secondaries the output will be rated 12V @ 2A , and 24V @ 1A (same game I would think)
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If we want to use the _same_ transformer to feed a full-wave bridge or as a center-tapped transformer feeding a 2 diode full-wave rectifier, then we need to have a transformer with two secondaries which can be paralleled or connected in series.
Just to nail things down, let's say that we want to feed the rectified output of the transformer into a 12 ohm resistive load and that we need 1 amp through that load.
That means that we need a 12VA transformer, and in the case of the FWB, the entire secondary needs to pass 1ARMS continuously. For a regulation of 10%, that means that the no-load secondary voltage will be 13.2V, the secondary's resistance will be:
13.2V - 12V R = ------------- = 1.2 ohm 1A
and it'll be dissipating:
P = I²R = 1A² * 1.2R = 1.2W
Since both secondaries will be wired in parallel, that means that each one will have a resistance of 2.4 ohms and will be dissipating
0.6 watts. Now, if we wire the secondaries in series in order to use them center-tapped, when either side of the secondary is delivering current to the load its resistance will be 2.4 ohms,
So, instead of:
13.2V | 1.2R | +-->12V | 12R | 0V
the circuit now looks like this:
13.2V | 2.4R | +-->11V | 12R | 0V
Notice that now, instead of 12V into the load, we'll have 11V, with one half of the secondary dropping the other volt.
That means that we're now looking at about 18% regulation instead of
10%, and to get the regulation down to 10% we'll have to double the area of the wire in the secondary, which means _increasing_ the VA rating of the transformer using the center-tapped full-wave arrangement.
No only that, each half of the secondary will be dissipating:
But for only 50% of the time, so that's about 1 watt.
Since the paralleled secondaries will only be dissipating about 0.6 watts each, that means the series-connected secondaries will be running hotter that the parallel connected ones, requiring the series connected arrangement to be rated for greater, not less, than 12VA.
--
If you meant that one would have to run the transformer at a lower
VA rating than it was rated for if the center-tapped arrangement was
used, then I agree with you.
However, the argument you gave, that the reason for that was that
only half the copper was being used at any given instant doesn\'t
seem to support that, since half the copper is being used, but only
for half the time.
Had you delved into it a little deeper, as I did, (since this is
seb) your meaning would have been clearer and this exchange would
not have been necessary.
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