# 3ph AC - 1ph DC

Discussion in 'The Projects Forum' started by Shultz1e, Jul 8, 2013.

1. ### Shultz1e Thread Starter New Member

Feb 6, 2013
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Hello Everyone,
I know this is probably a fairly standard practice, but I am in the fluid power field, not the electronics field. We are going to be setting up a new hydraulic test stand in our facility using a 40HP 3ph AC motor. We will be using 24vdc controlled micro controllers and displays, valves, etc. We have 3 phase power in house, not sure if it is 208 or 240 yet.. but what is the basic procedure for taking three phase power AC and utilizing 1 leg to single phase DC? Or do you use a three phase transformer to get it all to DC, which I believe would keep all legs constant, and then rectify the 24VAC legs into individual 24vdc legs?
I probably make no sense so anyone who has a moment and would like to take me to school via the forum please by all means, make me a fool.

2. ### GopherT AAC Fanatic!

Nov 23, 2012
5,831
3,426
I don't intend to be mean but I think the best answer is to hire an electrician certified for high power applications. 40HP means 30k Watts or 140 amps at 220 volts. Those are big numbers... Especially when you are trying to interface a control circuit onto the same circuit.

Please hire some expert help.

I assume an electrician will suggest some relays controlled by a different 120 vac circuit.

3. ### wayneh Expert

Sep 9, 2010
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2,854
How much total current or power do you need at 24VDC?

I agree completely that expert help is needed for anything involving that motor.

4. ### #12 Expert

Nov 30, 2010
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6,540
From the sound of this, you want to make (3) 24VDC supplies to control an AC motor. I can't imagine why you would need that much DC power that you are thinking about it un-balancing the load on a 140 amp, 3 phase line.

5. ### wayneh Expert

Sep 9, 2010
11,914
2,854
Whoa, I had a completely different impression. I guess we need the OP to clarify.

6. ### Shultz1e Thread Starter New Member

Feb 6, 2013
8
0
Hi everyone,
I understand an electrician will be required. I am mainly on the forum to understand the concept of what is required. The 3ph AC motor will be turned on/off by a standard switch with any NEMA style starter or something there of. The hydraulic system, pump, motors, pressure sensors, flow control valves, etc will all be operated via 24vdc or by a micro operating at 24vdc which will allow regulated 5vdc sensor outputs. I was just curious to the understanding of getting a 24vdc branch circuit from a 3phase signal. #12, you are correct..on the second comment. However I do not want 3 24vdc circuits.. I only want one. The PWM signals and other controls will require very little amperage to actually operate. 8 AMPS is all the micro controller can handle anyway. I was essentially thinking if I branched off 1 leg of the 3ph it would 'unbalance' it. GopherT no need to feel like you were being mean, I was asking for it.. I know I am flirting with a territory that is the reason why electricians have careers.. I say the same when I have people asking hydraulic questions.. somethings just can't be done without knowledge/experience. Anytime something is being built, and I have the responsibility of getting it together, hydraulic system, system controls (I am also in charge of programming the micro's), so everything will be done by my hands, except the installing the power unit to our building power.. But I would certainly like to understand the process. I have done power units for customers and have been on site for the installs.. always seemed pretty basic, however I never really get to have questions answered by the on scene electrician. This site seems the best for me to ask, listen, learn. All the while maybe get harassed which is fine

7. ### elec_mech Senior Member

Nov 12, 2008
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193
I'm not an electrician, so you'll need to consult one. However, I'm making the following assumptions:
• You're using utility power, not power from a generator.
• With this 3-phase utility power, you're going to power a 3-phase 40HP motor.
• With this 3-phase utility power, you also want to obtain 24VDC to power an unspecified amount of "small" current.
The key point is you're using utility power. I don't think you need to worry about unbalancing the utility - again, double-check with an electrician. If your power is coming from a 3-phase generator, then you'd need to worry.

That said, you could use a simple 2xx isolation transformer going from two of the lines (two hot legs if you will) that steps down the voltage to 24VAC or so. Then you put the power through a bridge rectifier and some filtering capacitors and viola. Maybe a little more involved, but this gives you an idea.

Now, if you're got 3-phase wye power, meaning something like 208/120V which has a neutral connection, you could easily pick up a 120VAC -> 24VDC power supply.

You may find a power supply that accepts 2xx and outputs 24VDC, but these are probably designed for 1-phase power and the neutral may be tied to ground/housing which you don't want to power with line-to-line from a 3-phase source.

I'm assuming you're using PLC equipment? If so, I'd suggest talking to your supplier about a 24VDC power supply and what options they offer to power them.

Again, I'm not a power guy, so if someone corrects me, I'd defer to them. These are just some thoughts.

Feb 6, 2013
8
0

9. ### #12 Expert

Nov 30, 2010
16,026
6,540
It's better to buy a 24VDC supply that you can just plug in. It's quicker and will be isolated properly, UL approved, and all that.

If you wanted to roll your own, it's an 18 volt transformer you would want. The DC side charges to the peak voltage of the transformer. 18 x square root of two = 25.45 volts. You will lose a couple of volts in the rectifier and arrive at 23.5 to 24 volts under full load. If full load is 8 amps at 24 VDC, that is 192 watts. Compare that to 30,000 watts. Less than 1% of "unbalance".

10. ### John P AAC Fanatic!

Oct 14, 2008
1,611
217
I've only ever set up a DC system running off a 3-phase line once, but I remembered hearing that if you rectify each phase of the AC (following a transformer, of course) and feed all the positive outputs from the diode bridges to the same point, the ripple will be so small that for most purposes, you won't need a filter capacitor. With the peaks of the sine waves so closely spaced, there isn't time for the output to drop much before current flows through the next diode. So that's what I did, and it worked fine. Yes, 18 volts sounds right if you actually want 24, remembering that some drop will occur in the diodes.

11. ### strantor AAC Fanatic!

Oct 3, 2010
4,295
1,983
There are 3phase transformers to 18 or 24v. Then you could use a 6 diode bridge to rectify 3phase into "dc". That's your theory answer. Practical answer, do what #12 said. As long you're only pulling a few amps, it won't matter if you only use one of the 3 available phases.

#12 likes this.
12. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
step down 480/120 single phase trans, with a purchased DC supply of the required wattage.