Paul's Posts — 18 February 2012
By Paul McGowan
Do you think the gas meter on your car lies? You know the one – it tells you there are 10 miles to empty. I am guessing it’s “conservative” which is a kind way to say inaccurate.
I think this because I don’t believe the car makers want me to run out of gas so they intentionally pad the measurements in case I decide to push the envelope.
I think this is what they do because we do exactly the same thing on power warnings.
When you look at the percentage available warning on a Power Plant, we’ve padded it slightly so you’ll not hit the wall and crash. Our wattage measurements are entirely accurate but our cautionary systems are “conservative”.
I think it’s a good thing that manufacturers pad their warnings so consumers can push the envelope just a little.
If you see me with a gas can along the side of the road, you’ll know I was wrong.
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Soundminded
Consumers want a single simple number to compare things. Life is rarely so simple. Remember the days of the power amplifier ratings wars? There was continuous power, IHF power, EIA power, and then there was the power the amplifier drew from the wall outlet. In some systems you could replace the amplifer’s own power supply with one of equal voltage but able to supply much more current and well regulated for determining the amplifier’s output power rating. Some manufacturers specified power bandwidth, some didn’t. Every one of them could give you a different number for the same amplifier. They could range over a factor of 10 to 1 or more. Which one was right? The answer is that all of them were within the constraints of how they were measured. Which number was most valuable to the consumer? That depended on the particular circumstance and what aspct of it you needed to know.
What is the rating of a power supply system? That depends in part on the load connected to it and how well it is designed to deal with the variables of the load. What is the load’s power factor? How much harmonic distortion is kicked back from the load at the power supply? What is its crest factor K? I had a 75KVA transformer so hot you couldn’t touch it at less than 50% rated load because of harmonics from the SMPS in an early office photocopy machine.
Can it be dangerous to under-rate the output capabilities of a power source product? When engineers talk to each other the answer is definitely yes. What if the engineer specifying an automobile transmission got much too conservative a rating for the maximum torque the engine that would drive it could deliver? It could be easily destroyed when you floored the gas pedal and one manufacture’s line of cars did just that. I can buy an 800 amp 480 volt circuit breaker for $1000 or for $10,000. Which one is right? If the source of power is a 1000 KVA transformer with normal impedance then the first is satisfactory and the more expensive one is unnecessary. Change the transformer to 2500 KVA but rate it at 1000 KVA to be “conservative” and installing the less expensive breaker could result in catastrophe.
The tendency for consumers and even some engineers to look at things in isolation and/or only one aspect of them without understanding the larger context of a system can lead to disappointment and failure. A rose may be a rose may be a rose but a power source is not a power source is not a power source.