Electrical Question from Mechanical Engineer 3

[JohnBreen]

Hello all,

First I hope this it the right forum - please advise me if I need to post at another forum.

I am a fair Mechanical Engineer with a specialty in pressure technology (piping and pressure vessels). My familiarity with things electrical is limited to my 66 years of experiences with electrical shocks.

My only remaining vice is radio. I like to participate in short wave listening. This hobby covers utility broadcasts, police, fire, amateurs, aircraft, and most frequencies between 150 khz and 2000 mhz. Many of my receivers and transceivers are powered by 13.8 volts DC. The required amperage for the receivers is less than 7 amps but some of the transceivers require 30 amps.

I have been given 10 (originally quite pricey) rack mount regulated power supplies that I would like to try to use. They are all Electronic Measurements Incorporated models with variable voltages (e.g., model TRC 20S30 allows voltage to be set between 0 volts and 20 volts at 30 amps).

http://www.used-line.com/c6121926s0-a_7.htm

So I had a 220 line (10 gage wire) installed to my radio room for one of the power supplies. So far all is well, that EMI 20S30 supply is a "single phase" supply. But then I "discovered" (duhhh) that some of the bigger (100 lbs!!) TRC 7.5S300 (yes, 300 amps!) power supplies (two 7.5 volt supplies in parallel will give me 15 volt capability) are three phase and I do not have three phase into the house.

All of that boils down to my question - can I somehow "convert" three phase power supplies to use my single phase 220 circuit?

Keep in mind that I am a Mechanical Engineer with shock induced curley hair (and not much of that left). There is not much of a market around home for boat anchors.

Regards, John.

 

[davidbeach]

DC is inherently single phase, although a power supply could have multiple outputs or a balanced -0+ output. The AC input into the rectifier could be 1-phase, 3-phase, 6-phase, or more; though more then 3-phase are really large rectifiers. So, just because your output is "single phase" doesn't mean that the input isn't designed for three phase.

 

[JohnBreen]

David,

So, just because your output is "single phase" doesn't mean that the input isn't designed for three phase.

Thank you for your response but the power supplies ARE designed for single phase 220 volt AC input (refer to the data sheet). Therefore I am confused about GS3's comments about these SINGLE PHASE power supplies including 3 phase transformers. I have made no comments about the output at all except that is is 0 through 7.5 volts DC at 0 through 300 amp current. The data sheet clearly describes the TRC Series Model 7.5S300 power supply as single phase AC input.

These power supplies each have one output - a positive post and negative post. Our more recent discussion above was not about the AC input as that has already been determined by IRStuff's observations from the data sheet - the TRC Series Model 7.5S300 is single phase 220 volt AC input. What we have been discussing in the last few days is the issue of how the DC posts should be connected together to get the 15 volt DC output. There was some thought expressed that there might need to be a "feed back loop" involved somehow and I was looking for some additional comment on my assumption that I could simply limit the current to 30 amps (by adjustment on both power supplies), use 10 gage wire to connect the positive DC output post of one PS to the negative DC post of the other PS and take my 15 volt DC, 30 amp feed off the remaining two DC output posts.

Regards, John.

 

[IRstuff]

It's unclear from the datasheet how the voltage sense lines are connected. The sense lines are supposed to get the supply to compensate for the IR drop in the connections, by boosting the output voltage.

Ostensibly, if you're looking at stacking 2 7.5 V outputs to get 15 V, AND you're running a relatively large current, then you do want to run the individual supply voltage sense lines such that outermost sense lines go to the load, as usual. Both of the innermost sense lines should go the same output on one of the two supplies.

This will force one of the supplies to drive to a larger voltage than the other, but the net effect is to ensure that 7.5 VDC exists across each set of sense lines, thereby resulting in 15 VDC at the load.

TTFN

FAQ731-376

 

[waross]

Usually 13.8 volt rated equipment implies equipment suitable for use in a vehicle with a lead acid battery and an alternator. The equipment will be able to accept a wide range of voltage fluctuation. There should be no need to run voltage sense wires to the load, just strap the voltage sense terminals to the output terminals on the back of the power supply.
The description of the SCR action and connections (it looks like an inside the delta connection) are not completely applicable to the single phase model. I am sure that the single phase transformer will be fitted with higher rated SCRs and larger capacitors to deal with the higher currents and with the greater ripple on single phase.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JohnBreen]

Thanks Bill,

Most amateur radio equipment is 13.8 volt DC even though it will never see an automobile. So, most of us have a bunch of smaller power supplies that we place on the floor under our operating desks (even those smaller supplies are heavy). The multitude of power supplies also generates a lot of heat (so we refer to these power supplies and the linear amplifiers as "foot warmers".

What I am trying to do is make things neater by mounting many tranceivers and receivers in an electronic equipment rack with power buses running inside the rack to power the equipment. All the radios have there own exhaust fans (and each has its own fuse in the input power line) and the cooling air will be coming in the front panels. The air flow through the rack is controlled and the hot air will be drawn out the top of the rack by an array of 4, six inch fans then up through the ceiling vent to the attic (and the attic has its own thermostatically controlled exhaust fans.

I wanted to use these rack-mount power supplies because I can put them in the bottom of the rack and because they seem to provide VERY stable quality power. So I had the 220 volt single phase AC line installed with 10 gage wire all the way back to their own breaker. The variety of small power supplies that I am now using are all 13.8 volt, 20 amp units. Obviously, the entire rack of equipment will not need 300 amps - the receivers draw very little current and only when I transmit do I need 20 amps. I would only transmit on one radio at a time. I figured I could place two 7.5 volt power supplies in the bottom of the rack, wired in series, and they would never be challenged by the radio equipment. Also the two supplies together would weigh 200 pounds so they would add ballast and make the rack stable.

"...just strap the voltage sense terminals to the output terminals on the back of the power supply...."

I have no idea what this mean or how to do it. The power supplies are like "black boxes" to me and everything inside is a mystery. I soon will have the users manual and I assume detailed schematics. Presumably, the voltage sense "terminals" would run from somewhere inside each power supply out to one of the DC outlet terminals (positive or negative?) of each power supply(?).

Regards, John.

 

[IRstuff]

The voltage sense terminals should be adjacent to their respective output voltage terminals, typically, V-sense, V-, V+, V+sense. However, if there's ANY length in the power cables, the sense lines would need to go with the power lines and terminate closer to the load. Otherwise, you'd wind up with voltages substantially lower than commanded.

TTFN

FAQ731-376

 

[JohnBreen]

Thanks TTFN,

I will wait until the user manual get here and have a look.

Regards, John.