PDA

View Full Version : What is a KVA

lane
11-30-2012, 06:02 PM
Are better what is a KVA equal to are mean . I have a rotary phase converter rated at 6 KVA .Is that equal to some amount of Horse power are what . Some one please ex plane in non electrical language.

DFMiller
11-30-2012, 06:08 PM
That Six thousand Volts Amps. Memory serves me that 746 Volt Amps is a 1 HP. So that's makes 8 HP. But I am not a EE.
But the online calculate says its is 6.4 HP. Darn reactive factor that I must have the wrong conversion. I guess it depends on if you are using power factor of 1.0 or .8
http://www.tvss.net/train/tools/08.htm
Does that help ?
Dave

PSD KEN
11-30-2012, 06:13 PM
http://coresite.com/resources-whatsakva.php

http://www.rapidtables.com/convert/electric/Amp_to_kVA.htm

Doozer
11-30-2012, 09:38 PM
Ok, this might not be 100% inclusive on the subject, but hope this helps.

Volts X Amps = Watts.
This is true for DC and for AC with a pure Resistive load (think light bulb
or heater).
VoltAmps is the same as Watts, but it is for Inductive loads (think motors
and transformers).
Inductive loads Can have a leading or lagging power factor. This means
voltage and amps are not in phase. If these are not in perfect phase,
there is a Power Factor to multiply by to get the Apparent power (the total
power needed from the Utility to power your Inductive device.
I know the concept of volts and amps out of phase seems odd, but don't
forget, waves are three dimensional. Looking at an oscilloscope shows you
the "side" of the wave. Looking at a Spectrum analyzer you can see the
"top looking down" view of a wave. This is just a means to describe how
to think of a wave, not totally great way of explaining it, maybe others can
say it better. Don't forget, just like a capacitor, an inductor stores energy.
The iron of a transformer takes some time to charge and discharge (hysteresis).
The Iron stores the electrical charge magnetically. The capacitor stores the
electrical charge chemically. Both of these devises offset the power factor.
VoltAmps is Wattage X power factor. Power companies like a power
factor of 1 ideally. Hope this helps. Others may explain it better.

--Doozer

LKeithR
11-30-2012, 10:01 PM
Others may explain it better.

--Doozer

Nah, I think you did a pretty damn good job; one of the most understandable explanations that I've seen...

lakeside53
11-30-2012, 10:02 PM
6000va is a bit meanlingless on an RPC without more data. Are they talking total connected load or single motor starting power? Do you have a picture of the nameplate?

btw., the power factor of an idling motor may be as lows as 0.15, and as high as 0.9 fully loaded. Idle current can be as high as 50% of full power, but real power is low because of the small power factor.

12-01-2012, 04:11 AM
VA is Volt * Amps KVA is Kilo volt amps

watts is Volts * Amps * PF ( power factor )

and as has been quoted 1 HP = 746 watts

with this in mind you cannot directly convert KVA to HP ( unless its a dc motor untiy PF )

as you do not know the PF any comments to the HP is meaningless

a point often lost is that the power you use is watts or KW the meter measures VA or KVA that what you get charged on

therefore the worst the PF is the more you pay for your power thats why firms pay good money to install PF correcting capacitors to get the PF up to approx. 0.9

Stuart

12-01-2012, 10:19 AM
What we are missing is a description of sine wave, frequency, phase and inductor.

I had to break this into two parts because the forum software – clearly not under the tender guidance of an instructor – gives the poster a miserly four image limit.

Sine wave: the simplest possible repeating waveform. The path of a point on a rotating object, traced in the form of a graph:

the key component of a spiral notebook
a coil spring

Frequency: the rate at which the waveform repeats. 60 times per second, in the case of AC power. renamed Hertz in the early 60s, because CPS had no meaning outside english speaking countries

Phase: The timing relationship between two waveforms at the same frequency. when you start a screw into a nut, you get them in phase. consider this image:

The blue wave crosses the center before the red wave. By concensus crossing through the center in the positive direction is 0 degrees, the positive peak is 90 degrees, et cetera

the blue wave and the red wave are 90 degrees apart. they are 90 degrees out of phase. the red wave LAGS, the blue wave LEADS

240 volt power is two phases, 180 degrees apart. 120 VAC + 120 VAC 180 out of phase = 240 VAC.
three phase power is three phases 120 degrees apart. the sum if Phase A plus either other phase at any moment is 208 VAC.

12-01-2012, 10:26 AM
Inductor:

If you pass a wire through a magnetic field you induce a current through that wire
If you induce a current through a wire you produce a magnetic field around that wire.

If you wind that wire into a coil, the current rotating around one coil butts heads with the current atround the neighboring coils:

http://www.tpub.com/neets/book2/32NE0136.GIF

the induced current from the neighboring coil is delayed by the transit time through the coil, and the transit time through the air. this delayed magnetic field induces a current in the neighboring coils which is out of phase with the current in the wire:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/aind2.gif

Bonus lesson: Commutating diodes to save DC switches:

when you disconnect an inductive load - a motor - you get a big fat blue spark.

you run a current through a coil. This creates a magnetic field in an inductor.
you disconnect that current. the magnetic field collapses. the collapse of the magnetic field is a magnetic field in motion. inward is a motion.
the collapsing magnetic field induces a current as it moves through the wire. this magnetic field is in the opposite direction of the current that induced the magnetic field. therefore the polarity is reversed.
the collapse of the magnetic field is nearly instantaneous. the voltage induced is dramatically out of proportion to the voltage that produced the magnetic field. I have measured -321 VDC spikes from a magnetic firld induced by +12 VDC.

A reverse biased diode connected across the inductive load in a DC current can not conduct when + VDC is applied across the motor or relay solenoid - it's reverse biased - but it does conduct when the collapsing magnetic field induces a massive voltage spike in excess of the rated capacity of the switch. The diode eats the vast blue spark. Use a diode rated for 400 PIV - Peak Inverse Voltage - in automotive applications.

The black waveform is without the suppressing diode – called a commutating diode in this application - the barely visible gray waveform labeled e is the suppressed diode waveform.

Ford does this with starter solenoids. you see Diode Suppressed on a Ford starter solenoid - now you know. Imagine that negative spike traveling through your car electronics.

Jaakko Fagerlund
12-01-2012, 10:30 AM
a point often lost is that the power you use is watts or KW the meter measures VA or KVA that what you get charged on
Depends where you are and what you do. In here, private persons only pay the kWh used and not by kVA. The meter reads kW(h) or kVA, can be changed. If you are a business or some such, then you get charged by the kVA and bigger businesses install PF correction caps on site.

lakeside53
12-01-2012, 12:32 PM
As an example of pf correction, my 15hp rpc without correction caps would draw about 22 amps at idle. With caps (l1-l2) I was able to get it down to about 3 amps. In my area, corrected or not, I would be charged for the 650-700 W/hr of true power it consumed.

What was more interesting.... my power meter is remote sensed (CT coils). The 15 hp motor really messed with it, and without pf correction it ran SLOWER (less power shown) with the motor on than off! I repeated the experiment several time to be sure.. lol... if I could have put up with the noise I'd have a lower power bill by leaving it on. YMMV.

If I was metered kva, I'd sure as heck correct my "idler"!

The problem with simply adding static caps for pf correction is that it's only vaild for a particualar condition. To accomplish correction over widely varying loads or mutiple motors you need to be able to switch the caps dynamically. In typicial shop use most motors have a lousy pf; they are rarely used at full load for meaningfully long periods.

The Artful Bodger
12-01-2012, 01:42 PM
AD5MB, nice post, however you may wish to review this bit..

240 volt power is two phases, 180 degrees apart. 120 VAC + 120 VAC 180 out of phase = 240 VAC.
three phase power is three phases 120 degrees apart. the sum if Phase A plus either other phase at any moment is 208 VAC.

That may be the case in North America but not universally true.

12-01-2012, 02:15 PM
240 volt power is two phases, 180 degrees apart. 120 VAC + 120 VAC 180 out of phase = 240 VAC.
three phase power is three phases 120 degrees apart. the sum if Phase A plus either other phase at any moment is 208 VAC.

The 240v centre tapped supply is considered 240v split phase, NOT 2 phase, this is something else with a phase angle other than 180°.
Max.

lakeside53
12-01-2012, 05:07 PM
240 volt power is two phases, 180 degrees apart. 120 VAC + 120 VAC 180 out of phase = 240 VAC.

This is not true. 240 is single phase that is center-tapped at the local transfomer to produce the 120, not the other way around!

lane
12-01-2012, 07:31 PM
Well I did get my answer . But like always you got over my head real Fast.I can teach you more than you want to know about machine work ,but electrical I loose it real fast. So my converter is good for 5are 6 H.P. Good enough. Thanks again Lane S.

lakeside53
12-02-2012, 01:07 AM
Maybe... there is usually a big difference in Starting Capabilty and Total Connected Load. Some manf spell it out; some don't... can yours start 5-6hp (usually a 10hp based rpc), or start say multiple 2hp one at a time up to 6?

lane
12-02-2012, 08:36 PM
The largest motor I have to run is a 3HP . and I think T can run 2 3`s at the same time but start one at a time . Don`t thank you can start more than one at a time . Have to walk across shop to start 2nd machine any way.