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Thread: Need info on "chopped DC" used to power AC devices

  1. #1 Need info on "chopped DC" used to power AC devices 
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    Having an argument with a person who is "never wrong". In all my years of playing with electricity (I'm an industrial electrical mechanic by trade as opposed to a straight electrician) I've always been taught and told that "chopped DC" was to be used as a sort of "last resort" to keep processes running in the event of a electrical outage, both for resistive and inductive loads. The person I'm arguing with has the opinion that "chopped DC" is just fine for electronics, but would be a problem with inductive loads such as motors. This is contrary to everything I've been taught.

    Opinions?

    Rich


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    By chopped dc I take it that you mean the output of an inverter which is not filtered and smoothed into a sine wave.

    I would agree with the other guy to the extent that electronic equipment generally operates on dc. The a-c input (or chopped dc) to the electronic device would first be rectified back into d-c and the rectified output would be filtered. It would have more ripple than a rectified sine wave, so I suppose the qualtity of the filter on your rectifier would be more important.

    The chopped d-c would definitely be a problem with inductive loads.


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    Quote Originally Posted by Harold14370
    By chopped dc I take it that you mean the output of an inverter which is not filtered and smoothed into a sine wave.

    I would agree with the other guy to the extent that electronic equipment generally operates on dc. The a-c input (or chopped dc) to the electronic device would first be rectified back into d-c and the rectified output would be filtered. It would have more ripple than a rectified sine wave, so I suppose the qualtity of the filter on your rectifier would be more important.

    The chopped d-c would definitely be a problem with inductive loads.
    Let me make this clearer. The subject came up in regard to UPS systems supplying power to DVRs. I have worked on large UPS systems consisting of banks of batteries that "held the fort" until our diesel generators could come on line and supply the normal AC voltage. The batteries were only on line for a brief period. These UPS systems were used for our chemical processes and the computers that controlled them. We also used the same setups for our computer rooms.

    What we were debating was the use of simple UPS devices that take the place of house voltage during an electrical outage and use chopped DC rather than the more expensive UPS devices that produce an AC sine wave.

    I agree with your original assumption that electronic devices usually use DC, but to induce chopped DC in place of normal AC to the power supplies of Digital Video Recorders seems to me to be a crude way to simulate AC. I don't believe that the DVRs have the ability to rectify the chopped DC. The cheap UPS devices do not have the ability to produce the sine wave of AC, hence the term "chopped DC" was usually used in the derogatory sense rather than as a good thing. It's been a long time since I worked as an electrician and perhaps things have changed.

    I'm pretty sure DVRs "expect" normal house voltage and don't have the ability to cope with chopped DC for a long period of time. I'm not sure that motors, which depend on a rather fixed frequency, would function properly in chemical processes using chopped DC. At least we treated our processes so that the motors were almost always supplied with AC power. Just those very brief moments when the diesel generators were coming up to speed.

    My thoughts on the DVRs are that they don't have the ability to do much more than use a step down transformer to get the voltage down to the point it needs to be at, then rectify it into DC. I really doubt that the DVRs are made to deal with chopped DC as a relatively long term voltage supply.

    Sorry I wasn't clearer, but I wanted to get someone interested in the arguments and made my first post as brief as I could.

    Rich
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    I don't know how cost effective this would be for you, but the UPS system I like the best was for our communications system years back when I was at a military communications site. We had a Rotary UPS. It had a 50KW motor turning a 40KW alternator for site communications power. This was attached to a diesel engine with always hot glow plugs. There was also a massive flywheel and clutch. The clutch was held open by commercial power.

    Power dies...

    Clutch engages, engine starts. Immediately.

    No loss of communications power at all, no spikes, nothing. Perfectly smooth power.

    No batteries to maintain.

    I have little to offer about the chopped DC, except I have used various types of VFD's (variable frequency drive) for AC motors. The motors ran fine from the ones that produced chopped DC, but they sounded different. You could hear a high pitch whine from the harmonics, coming from the motors, but it never hurt anything. Maybe the motors were designed to handle such a input, I don't know. We did have motors rated for up to 120 hz, and ran then from 6 hz to 120 hz.

    Maybe a simple isolation transformer between the chopped DC to equipment input would suit your needs.

    wiki: Diesel Rotary Uninterrupted Power Supply
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    Now that I've thought about it, the electronic equipment would probably have a full wave rectifier in its power supply. A square wave input to a full wave rectifier would produce a nice, flat, d-c wouldn't it? Probably better than a sine wave, even.
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    Quote Originally Posted by Harold14370
    Now that I've thought about it, the electronic equipment would probably have a full wave rectifier in its power supply. A square wave input to a full wave rectifier would produce a nice, flat, d-c wouldn't it? Probably better than a sine wave, even.
    Yes, I would imagine that it would have to have a full wave rectifier in it's power supply. That makes sense. Problem is I'm not sure how those rectifiers handle chopped DC. That's why I asked the original question. Does a rectifier that "expects" a normal AC supply work properly with chopped DC? I guess that's the heart of my original question. And I really don't know the answer to that question.

    I went to school back in the early '70s and we usually had electrical engineers teaching the classes and they were adamant then about chopped DC being used for only brief periods in emergency situations. That was a long time ago and I guess the thinking has changed since then?

    Rich
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  8. #7  
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    Quote Originally Posted by Wild Cobra
    I don't know how cost effective this would be for you, but the UPS system I like the best was for our communications system years back when I was at a military communications site. We had a Rotary UPS. It had a 50KW motor turning a 40KW alternator for site communications power. This was attached to a diesel engine with always hot glow plugs. There was also a massive flywheel and clutch. The clutch was held open by commercial power.

    Power dies...

    Clutch engages, engine starts. Immediately.

    No loss of communications power at all, no spikes, nothing. Perfectly smooth power.

    No batteries to maintain.
    I was working in a plant that opened in 1936. And we were using a lot of original equipment. The Rotary UPS would have been science fiction to us. How much do one of those cost? Sure would have made my life a lot simpler.


    I have little to offer about the chopped DC, except I have used various types of VFD's (variable frequency drive) for AC motors. The motors ran fine from the ones that produced chopped DC, but they sounded different. You could hear a high pitch whine from the harmonics, coming from the motors, but it never hurt anything. Maybe the motors were designed to handle such a input, I don't know. We did have motors rated for up to 120 hz, and ran then from 6 hz to 120 hz.
    The VFDs were just coming on line when I was promoted to a management position. I did do some work with them, but at the time, everyone was in learning mode. I do know it enabled us to get rid of all the gear reducers for speed control that were so prone to breakdowns.

    Maybe a simple isolation transformer between the chopped DC to equipment input would suit your needs.
    It was just an argument about UPS devices for home electronics that caused me to post. I use the chopped DC UPS devices to protect my DVRs, but I live in a very stable part of the National Grid and we rarely have anything more than brief blips in our power feeds.

    Rich
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