Send both AC and DC voltages down the cable

[alwaink]

I was just told by someone that you can send both AC and DC voltages down the same cable at the same time (to a Wye motor winding). There's a sensor gauge that requires DC voltage and has a lead connected to the Wye neutral. The guy's reasoning is that the motor only sees the AC components, and at the Wye neutral, both AC voltage and current should be zero (this I agree). So at neutral there's only DC components left, which feeds to the sensor gauge.

I honestly had never heard of this before...maybe it's just because I just graduated and dont' have much experience yet. Could someone validate this? Thanks.

 

[Skogsgurra]

It is correct. In theory. And if your sine is without harmonics and transients.

But industrial systems have both harmonics and transients so you need a hefty filter to make any use of this possibility. Also, you will have to send 1/3rd of the current down each motor wire so you do not DC brake the motor (of course, only if the current is more than a few percent of motor rated current).

Where would you connect the other end?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[alwaink]

Well, the DC part of it is supposed to go to the sensor gauge via the Wye neutral. I believe the thinking is so that there can be DC voltage supply to the sensor gauge at all times to monitor measurements regardless of whether the motor is running or not.

What kind of effects would harmonics & transients do to the system (either motor or sensor gauge)?

It's interesting to know it's possible to have AC in kilo-volts and DC in 120V in the same 3-phase cables.

Thanks.

 

[jraef]

Ah, you didn't mention kilovolts! That's a horse of a different color. kV potential has a way of going where it wants to. The isolation of the components in the DC system against flashover of the kVAC power would make the concept unattainable or at least impractical. If you are looking t keep a motor mounted sensor powered on a Medium Voltage motor, you need to look at a separate source IMHO.

JRaef.com
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[waross]

It was done to power block-down convertors on satelite TV systems. The DC power was fed through the coax and the signal was coupled and de-coupled with capacitors.
This technique may not work for motor currents.
respectfully

 

[Skogsgurra]

"What kind of effects would harmonics & transients do to the system (either motor or sensor gauge)?"

Answer:

Sum of harmonics is not always zero (like sum of three-phase fundamental is). For instance, sum of triplenes add.

Transients are seldom cancelling across the phases. They are more often common mode or occuring (starting) at different time, like closing and breaking transients. So, there will be heavy voltage shifts in the wye point when these occur.

Bill, I think the OP was thinking three-phase system.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[alwaink]

I'd like to superimpose AC and DC sources into one wire for simulation on a smaller scale (100-120V AC&DC). I have a DC power supply to control the DC voltage output, and perhaps get 120VAC from the wall. My load has a choke that can filter up to 1000VAC and the rest of the device can take 100VDC, 25mA. Below is how I plan to hook things up:

120VAC (wall)--->Current Regulator----|
|---Choke--Load
100VDC (DC supply) --->Diode-->Choke--|

I've never dealt with 2 power supplies at once before. So here are my questions:

1. I'm afraid some of the AC coming from the wall to the load will divert to the DC supply equipment and damage it. I don't know if having a choke in the DC line is enough to stop it or do I need that diode?
2. I'd like to regulate the current coming from AC. Any advice on what I can use for current regulation?


Thanks

 

[stevenal]

You need two points to describe a potential difference. One wire will not work.

 

[alwaink]

The load, AC, and DC are all connected with ground wire for return path. I was just trying to show a rough presentation, and the text sketch didn't show up well.

 

[stevenal]

Ground is not used as a normal AC return path per code. Also per code, neutral and ground are bonded at the service entrance. This bond will short circuit your DC, which I understand is connected neutral to ground. You sound like you're in over your head.

 

[jghrist]

I think you will have to use an isolation transformer. You could put the dc supply in series with the isolated ac source. The total voltage would be the sum of the two: varying sinusoidally between -15 and +185 volts if you use 120 VAC rms (170 volts peak-to-peak) and 100 VDC.

I have not addressed code or safety issues - these need to be considered.