Voltage Regulation
- Thread starter kvramesh
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winnie
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- Springfield, MA, USA
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- Electric motor research
Transformer impedance is very roughly a measure of the 'voltage drop' that is built into the transformer.
It is measured by applying a variable voltage to the primary, shorting the secondary, and then adjusting the primary voltage until nominal full secondary current flows. Since the output is shorted, it is at essentially _zero_ volts, so you get a measurement of the voltage needed to push full current through the transformer.
The aspect that gets confusing is that this voltage drop is predominantly inductive. If your load has a good power factor, then it won't 'see' most of this voltage drop. If your load is capacitive, then it might even see the voltage increase.
If the load is relatively constant, then transformer impedance will simply cause a constant offset in your supply voltage.
-Jon
It is measured by applying a variable voltage to the primary, shorting the secondary, and then adjusting the primary voltage until nominal full secondary current flows. Since the output is shorted, it is at essentially _zero_ volts, so you get a measurement of the voltage needed to push full current through the transformer.
The aspect that gets confusing is that this voltage drop is predominantly inductive. If your load has a good power factor, then it won't 'see' most of this voltage drop. If your load is capacitive, then it might even see the voltage increase.
If the load is relatively constant, then transformer impedance will simply cause a constant offset in your supply voltage.
-Jon
coulter said:
I'm concerned about the voltage regulation to the motors...isnt that going to be really bad. the secondary voltage at full load will be much lower than 480V due to this 7.5% impedance.
what % of voltage drops are allowed from tranfo ->LV Switchgear ->MCC ->motor according to ANSI? red book is little confusing to me
You are focused on just one piece of the system. You haven't given us much information about the rest system. You will need to look at the rest of the xfm data, motor sizes, kva load, feeder sizes, feedr lengths, might even have to call the mfg and get the xfm X/R ratio. You have a whole system to look at.kvramesh said:
Your concern is voltage regulation, so sketch out the system, put in the known data, calculate the Vd out to the motors of concern. Could be the feeders cause more Vd that the xfm. To understand, you will need a vector diagram for the xfm and maybe the feeders as well. Chapter 9(?) of the NEC has a pretty good section on the feeder Vd out to normal industrial loads.
Transformer impedance is just one piece. Just because it is 7.5%Z doesn't mean it's bad. I have seen 2%Z and 12%Z. They all have their purpose.
I don't know what ANSI says. You may have to dig out the Specs and read them.kvramesh said:
Most 480V motors are rated at 460V. Generally running 5% low (440V)doesn't hurt anything. But that could depend on your loadiing. A motor loaded right up into the service factor could easily run hot if the voltage is low
Yeah, that is a problem - takes me additional research sometimes. Get some more books, talk to your senior engineers, might consider looking into some seminars. Even after 35 years I try to go to school every year on something.kvramesh said:
carl
The voltage regulation for any given kW loading will depend on load power factor.kvramesh said:
Run at max current (thus max kVA) with a 0.8 pf and you might expect regulation of around 5.6%.
Correct the same kW load to say, 0.95 pf and of course you reduce current and kVA. The regulation would then be about 3.4%.
At max kVA rating and 0.95 pf, the regulation would be about 4%.
A little aside:
In UK the standard LV distribution used to be 415/240 ? 6%.
In order to "harmonise" with Europe the nominal voltages were changed to 400/230, supposedly a a compromise between Continental European 220V and UK 240V. It was purely a beaurocratic fudge. Nobody actually did anything about it. Except that the rules on tolerances were altered to accommodate the fudge. Brilliant waste of time. Rant over.
Don't know if this helps. It's about supply tolerances but connected equipment would generally be designed to work with those tolerances.
http://users.metro2000.net/~purwinc/seec2_2.htm
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