# [EVDL] Power factor correction



## EVDL List (Jul 27, 2007)

I built a charger using an SCR based DC drive. It works great but I have
one significant problem. Power Factor. I knew the PF would be bad but I
thought I could correct it by putting some motor run capacitors on the line
side of the drive. They do improve the power factor, however, they seem to
be drawing power themselves equal to what they corrected. That sort of
defeats the purpose.
I am currently driving the 230V drive with a 115V-230V step up
auto-transformer with the 115V plugged into a Kill-a-Watt. I currently have
70uF of capacitors that I can switch in and out. The KW meters a PF of 0.3
at 16 amps and 600 watts without the capacitors. The drive is outputting
just about 600 watts. Once I switch the capacitors in, I continue to output
just under 600 watts at the drive. The line stays about 16 amps. However,
the PF improves to 0.4 and the watts increases correspondingly. The power
factor gets some correction however, the amps remain the same so it looks
like whatever power I corrected, it just went into the capacitor.

Am I trying to do something that is not attainable (correct PF on SCR based
drive)? Is it normal for motor run capacitors to draw that much power?
They are not warm so I am a little at a loss for where the power is going.
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## EVDL List (Jul 27, 2007)

> Steve Skarda wrote:
> > I built a charger using an SCR based DC drive. It works great but I have
> > one significant problem. Power Factor. I knew the PF would be bad but I
> > thought I could correct it by putting some motor run capacitors on the line
> ...


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## EVDL List (Jul 27, 2007)

Thanks for that detailed explanation Lee!! I understand now what is
happening.
Just wanted to send you a private note to let you know how much I truly
appreciate the extraordinary amount of time you spend providing detailed
answers and helping many of us out tremendously.

I guess my only option is to find a way to reduce my line voltage and hope
the drive will still work so at least I can get the SCR's turning on sooner.





> Lee Hart <[email protected]> wrote:
> 
> > Steve Skarda wrote:
> > > I built a charger using an SCR based DC drive. It works great but I have
> ...


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## EVDL List (Jul 27, 2007)

> Steve Skarda wrote:
> >> I built a charger using an SCR based DC drive. It works great but I have
> >> one significant problem. Power Factor.
> 
> ...


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## EVDL List (Jul 27, 2007)

> Lee Hart <[email protected]> wrote:
> 
> >
> > The improved power factor means the RMS current drawn from the AC line
> ...


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## EVDL List (Jul 27, 2007)

> Lee Hart <[email protected]> wrote:
> >> The improved power factor means the RMS current drawn from the AC line
> >> is less (though the Kill-A-Watt meter may not show this; I don't think
> >> it shows true RMS amps)...
> ...


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## EVDL List (Jul 27, 2007)

> Lee Hart <[email protected]> wrote:
> 
> > Lee Hart <[email protected]> wrote:
> > >> The improved power factor means the RMS current drawn from the AC line
> ...


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## EVDL List (Jul 27, 2007)

>> More inductance shouldn't hurt. It will mainly change the optimal value
>> of capacitance you need to cancel out its inductance.



> Steve Skarda wrote:
> > I am not sure if we are talking about the same thing here. The inductance
> > is at the output of the drive in order to protect the SCR's from the
> > capacitance of the batteries. That doesn't impact the capacitance I add on
> ...


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## EVDL List (Jul 27, 2007)

Thanks Lee! I am not sure if I completely understand what is happening but
I believe I have my design goal nearly met. I installed a new EKM meter on
the 230V line. After further studying, I don't believe the amp readout can
be used to calculate wattage due to the harmonics and/or non-sinusoidal
waveform. I was using the amp reading (and the incorrect kill-a-watt pf
reading) to determine pf but that was in error. My power factor was
actually better than I thought it was. The EKM readout of power factor
suggests that I actually crossed over from lagging to leading when I added
capacitance which would explain the sudden increase in amps that I was
confused about earlier. I can calculate what the power factor should be
based on the drive output voltage versus the line voltage and it calculates
to match what the EKM meter is reading, not what the killawatt reads or
amperage suggests I should have.
The part that I still don't fully understand is why I can't get an accurate
current reading even with a true RMS meter.
EKM readings
230 volts
25 amps (It adds line 1 to line 2)
1601 watts
0.85 PF

Fluke True RMS
12.45 amps

Battery side
141.3 volts
10.5 amps (mV reading off 20 amp shunt)
Calculated 1483.7 watts output

Calculations
230*12.5*.85=2443.8 watts EKM volts*amps*pf does NOT match EKM watt
readout??

Based on the wattage on the output and the drive efficiency rating of 97%,
it would seem that the 1601 watts on the EKM meter is likely correct but the
amperage must not be correct. The voltage is correct (verified with Fluke).
I believe the power factor is correct because amps increase if I add more
capacitance (likely crossing from 0.86 lagging to leading). If I calculate
the capacitance required to get 0.85, it is about exactly what I have
installed. I think the amp reading is not representative of what is really
going into the power. I sent a note to the developer of the EKM reading to
see if he has any insight into the mismatched readings. I am writing it off
to harmonics but can't explain why my True RMS meter doesn't account for
that.

This is fed by a 115V to 230V autotransformer so actual current being seen
by the circuit breaker should be 25 amps according to the true RMS meter.
This is a 20 amp circuit breaker but it does not trip after 30 minutes of
being in service. It will trip if I crank amps seen on the True RMS meter
to 13.5 so the trip point is somewhere between 25 and 26.5 amps based on
what the True RMS says. If I look at the trip curve on the circuit breaker's
website, it should trip between 20 and 25 amps but I am not tripping even at
26 amps.

At this point, I am really just sharing information for learning because I
am satisfied with the progress I have made. Considering that I am putting
close to 1500 watts into the batteries with a 115V, 20 amp circuit, I am
satisfied that I have met my design goal for opportunity charging at my
workplace. It doesn't trip until I am over 1600 watts but I still have some
more room for pf improvement. I just need to rearrange my capacitors for a
slightly higher capacitance. Overall, I am 3 times better than I was doing
with my on-board QuickCharge so as long as everything continues to work, I
should be doing well.

Steve
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## EVDL List (Jul 27, 2007)

> Steve Skarda wrote:
> 
> > The part that I still don't fully understand is why I can't
> > get an accurate current reading even with a true RMS meter.
> ...


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## EVDL List (Jul 27, 2007)

> Steve Skarda wrote:
> > Thanks Lee! ...I installed a new EKM meter on the 230V line...
> > After further studying, I don't believe the amp readout can
> > be used to calculate wattage due to the harmonics and/or non-sinusoidal
> ...


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## EVDL List (Jul 27, 2007)

>
>
> I have done some more research. Further reading supports up exactly what
you are describing. The key is understanding the difference between
"displacement power factor" and "true power factor." These are only equal
"under sinusoidal conditions."
http://users.ece.utexas.edu/~grady/POWERFAC.pdf

"It is important to point out that one cannot, in general, compensate for
poor distortion power
factor by adding shunt capacitors. Only the displacement power factor can be
improved with
capacitors. This fact is especially important in load areas that are
dominated by single-phase
power electronic loads, which tend to have high displacement power factors
but low distortion
power factors. In these instances, the addition of shunt capacitors will
likely worsen the power
factor by inducing resonances and higher harmonic levels. A better solution
is to add passive or
active filters to remove the harmonics produced by the nonlinear loads, or
to utilize lowdistortion
power electronic loads."

The EKM meter and the response I am seeing by adding capacitance is
indicative of displacement factor but the true pf is much lower due to the
distortion.

Thanks for contributing to this thread. I have learned a great deal more
about harmonics, power factor correction, and the impacts of distortion on
power factor.
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## EVDL List (Jul 27, 2007)

By the way, my charger based on an old analog drive seems to be a relative
success. Time will tell if it will last. I wouldn't suggest it for someone
not comfortable with industrial controls but it really is a low cost charger
for anyone that has experience with DC drives. I am posting in a link to
the charge profile from my first full charge. When connected to a 230V, 20A
line, I can output close to 3kw.
http://s227.photobucket.com/albums/dd106/skskarda/?action=view&current=ScreenHunter_01Aug251425.gif
This is for 20, T-105 batteries.

Parts:
-3HP FlexPak Plus Analog Drive $75 on Ebay
-6mH, 20 amp inductor. $110 new (should be able to find much cheaper used)
-mV alarm $35 Ebay
-20 amp shunt $20 Ebay
-Motor run capacitors $30 Ebay

The drive jumpers are set to control speed via armature voltage. I adjusted
the speed pot to provide the voltage that I wanted at the finish point. The
current limit pot is set to prevent tripping my AC line. The low current
alarm monitors the voltage on the shunt and is set to seal in the drive
start pushbutton with the consequence of shutting off the drive when the
output current drops below 2 amps.

The primary downfall of this setup is the power factor and harmonics per
this thread. While I haven't seen any problems in my house, there probably
is some risk due to the noise I am putting on my line. I also have some
concerns about battery heating due to ripple currents but on the charge
cycles I have seen to date, I have not had problems. A third downfall is
that the peak wattage happens closer to 80% charged since I am controlling
drive output amperage versus drive input like the Russco charger does. Since
voltage is lower at the start of the cycle, I am not pulling full amps from
my AC line at the start. That's not a huge deal but it is a downside.
Finally, the drive is not enclosed so the wiring is all exposed in my trunk
which some might not feel is safe.

If I had to do it again, I'd probably more seriously consider a PWM dc drive
but it is just so cheap to pick up an old SCR based analog DC drive, I
couldn't pass up the opportunity to try it out. I couldnt' have done it
without the guidance of the people on this list that are a lot smarter than
I am. Thanks again!
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## EVDL List (Jul 27, 2007)

> Steve Skarda wrote:
> 
> > By the way, my charger based on an old analog drive seems to
> > be a relative success.
> ...


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## EVDL List (Jul 27, 2007)

> Roger Stockton <[email protected]>wrote:
> >
> > If you haven't already, you might want to add a timer to limit the maximum
> > charge duration.
> ...


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## EVDL List (Jul 27, 2007)

> Steve Skarda wrote:
> 
> > The drive input is isolated from the output so that is not a
> > concern.
> ...


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## EVDL List (Jul 27, 2007)

Roger, thank you for correcting me. I was wrong when I suggested the drive
was isolated. I verified that it works OK from a 115V GFCI receptacle
which is a relief. I'll need to go and purchase a 230V GFCI also so I have
some added protection. I appreciate the guidance and am a little
embarrassed that I answered the question regarding isolation without
investigating a little further first.
I am using a 2000VA autotransformer to step up the 115 to 230V so I can get
the voltage I need when running off a 115V circuit. Ideally, I'd like a
full transformer so I could protect my line from the harmonics but the
autotransformer was only $19 on Ebay and it weighs less. Given that this
was (and still is) sort of experimental I didn't want to spend more money
then necessary to start with. In the future if I continue to use this setup
for charging,

It appears that there are some 230V treadmill PWM motor controllers that run
up to 3HP. Their PF and harmonics are not nearly as bad as they are on an
old SCR based drive but they are still pretty cheap. I might keep my eye
out for something like that. Would the PWM output work OK for battery
charging or would that type of signal degrade the batteries? Would I still
want an inductor on the output of the drive? I guess the safe assumption
would be to have one so the output looks more like a motor.

Thanks again!
Steve



> Roger Stockton <[email protected]>wrote:
> 
> > Steve Skarda wrote:
> >
> ...


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## EVDL List (Jul 27, 2007)

> Steve Skarda wrote:
> > I am using a 2000VA autotransformer to step up the 115 to 230V so I can get
> > the voltage I need when running off a 115V circuit. Ideally, I'd like a
> > full transformer so I could protect my line from the harmonics but the
> ...


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## EVDL List (Jul 27, 2007)

> Lee Hart <[email protected]> wrote:
> 
> >
> > > It appears that there are some 230V treadmill PWM motor controllers that
> ...


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