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gtslabs
 
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Default Phase Converter cycling with compressor to save money.

I finally got my 15 hp phase converter working today using capacitors
and a momentary push button. I plan on using it with a 10 hp 3 phase
air compressor. However just at idle it is pulling 20 amps. I figure
I can use an additional pressure switch to have the converter turn on
before the compressor does. But what do I need to have it turn off
after the compressor cycles off?

Thanks
Steve

  #2   Report Post  
Grant Erwin
 
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gtslabs wrote:

I finally got my 15 hp phase converter working today using capacitors
and a momentary push button. I plan on using it with a 10 hp 3 phase
air compressor. However just at idle it is pulling 20 amps. I figure
I can use an additional pressure switch to have the converter turn on
before the compressor does. But what do I need to have it turn off
after the compressor cycles off?


How are you measuring this current? Be aware that there are two kinds of current
- real and imaginary - and you pay for the former but not the latter. You may
well be able to cut that apparent power down by a factor of 4 or 5 using some
run caps between L1 and L2. Your converter looks very inductive, if you put some
capacitance across it it may look more resistive.

Sorry for the EE terminology, I don't know any better.

Grant
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gtslabs
 
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I am using electrolytic caps for the startup. I think I read somewhere
NOT to use them for run capacitors. Tuning it was my next step.
However I only use this converter for this compressor and it is not
needed on all day, mainly when I am sand blasting. I was uning a tong
type meter.

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Grant Erwin
 
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gtslabs wrote:

I am using electrolytic caps for the startup. I think I read somewhere
NOT to use them for run capacitors. Tuning it was my next step.
However I only use this converter for this compressor and it is not
needed on all day, mainly when I am sand blasting. I was uning a tong
type meter.


This has nothing to do with start capacitance or tuning caps or balancing
current between legs, nothing whatever. I'm talking about power factor
correction capacitors.

Here is what Gary Coffman posted once on this subject:

"Power factor correction caps connect between L1 and L2. The value
is whatever reduces the 1 ph line current from the service entrance
to the lowest value. Again experimentation will be required. Do this
*after* you balance the converter, if you're going to balance it. The
value won't be the same for a balanced converter as it will be for
an unbalanced converter. (Usually, it'll be smaller for a balanced
converter.)

"What the power factor correction caps do is cancel the inductive
reactance due to the motor which is seen by the service entrance.
This reactance can set up high circulating currents in the primary
feed if it isn't canceled. These are wattless watts in the sense that
the current is out of phase with voltage and doesn't represent real
power being drawn from the utility, but they can cause the wires to
heat up (series R loss), and require you to use a larger than normal
breaker to prevent nuisance tripping."
  #5   Report Post  
Jerry Martes
 
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"gtslabs" wrote in message
oups.com...
I finally got my 15 hp phase converter working today using capacitors
and a momentary push button. I plan on using it with a 10 hp 3 phase
air compressor. However just at idle it is pulling 20 amps. I figure
I can use an additional pressure switch to have the converter turn on
before the compressor does. But what do I need to have it turn off
after the compressor cycles off?

Thanks
Steve



Steve

Is there any reason you wouldnt want to spin-up the idler with the
seperate pressure switch and turn it off with the compressor's high pressure
cut off switch??
I dont know what contacts are available on the two pressure switches you
are using. But, if I knew what you had I am confident that even I could
make something with cheap relays to control your motor contactors

Jerry




  #6   Report Post  
gtslabs
 
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Thanks I see the difference now. I have a case of ~130 uF 240V
electrolytic caps. I know these are not for continious use but only
start-up. However I would like to use these to find the correct
capacitance needed then buy the oil run type caps. How long is safe to
run these caps in a continious mode for testing?

  #7   Report Post  
gtslabs
 
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Yes, this is what I was thinking would work. My pressure switches are
double pole or L1, L2, M1, M2. I had trouble figuring how to wire
these with 3 phase wires. I have one leg going directly to the motor
and the other 2 going thru the switch. Perhaps this is not the correct
wiring but I could not easily find a 3 pole pressure switch at
graingers.

  #8   Report Post  
Robert Swinney
 
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Electrolytic "start" caps are not suitable for use as running caps. They
will blow up if left in circuit form more than a few seconds. You can't use
them for run "test caps" as you suggest - they will blow! With a 15 HP
idler motor you will need around 450 microfarads of run capacitance
permanently connected from one side of the input line to the "manufactured
phase". This will result in an "unbalanced" but satisfactory rotary phase
converter. Start by putting in 350 microfarads, or so, and add capacitance
while metering the manufactured phase. Continue to add capacitance until
the manuf. leg voltage rises to approximately 10% greater than the input
(240 ?) volt input line voltage. Be very careful - you are working with
lethal voltages!

Bob Swinney
S
"gtslabs" wrote in message
ups.com...
Thanks I see the difference now. I have a case of ~130 uF 240V
electrolytic caps. I know these are not for continious use but only
start-up. However I would like to use these to find the correct
capacitance needed then buy the oil run type caps. How long is safe to
run these caps in a continious mode for testing?



  #9   Report Post  
DoN. Nichols
 
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In article . com,
gtslabs wrote:
Thanks I see the difference now. I have a case of ~130 uF 240V
electrolytic caps. I know these are not for continious use but only
start-up. However I would like to use these to find the correct
capacitance needed then buy the oil run type caps. How long is safe to
run these caps in a continious mode for testing?


Hmm ... perhaps two seconds *once*, if you are lucky. Be sure
to wear a face shield, as they are very likely to explode in your face.

You really want AC rated oil filled caps for the task.

Remember -- you don't need to buy all the sizes of caps. You
can switch in combinations in parallel to cover a lot of values. For
example, a set consisting of:

1 ea 10 uf
2 ea 20 uf
1 ea 40 uf

can give you every value from 0 to 100 uf in steps of 10 uf.

Add in a 5 uf and you have more resolution to fine tune things,
if you need that, but I suspect that the 10uf steps would get you close
enough.

Add a 100 uf cap, and you can double your range.

If you were going to be doing this frequently, I would put them
all in a box with a toggle switch in series with each cap, so you could
change values while things are running.

And remember -- measure the current between the breaker box and
the caps, not between the caps and the idler motor.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
  #10   Report Post  
DoN. Nichols
 
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In article .com,
gtslabs wrote:
Yes, this is what I was thinking would work. My pressure switches are
double pole or L1, L2, M1, M2. I had trouble figuring how to wire
these with 3 phase wires. I have one leg going directly to the motor
and the other 2 going thru the switch. Perhaps this is not the correct
wiring but I could not easily find a 3 pole pressure switch at
graingers.


The three phase should go through a motor starter contactor,
with heaters to detect over-current when one of the three phases is
lost. So -- your pressure switch controls the coil of the contactor,
and the contactor switches the motor.

Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---


  #11   Report Post  
Jerry Martes
 
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"gtslabs" wrote in message
oups.com...
Yes, this is what I was thinking would work. My pressure switches are
double pole or L1, L2, M1, M2. I had trouble figuring how to wire
these with 3 phase wires. I have one leg going directly to the motor
and the other 2 going thru the switch. Perhaps this is not the correct
wiring but I could not easily find a 3 pole pressure switch at
graingers.


Steve

I would have the idler switched thru a contactor and the compressor
switched thru a "Motor Stater", which has heaters in it to shut it down
when/if damaging current flows to the motor. With the contactors
carrying the load current, the pressure switches need carry only single
phase control current.

If you agree about using contactors but dont have the contactors, I could
be talked into sending you some 3 phase contactors. You'll be lucky if I
have a motor start contactor with the correct heaters. But, I do have some
contactors I'd send, if you want to use them.

I also have an overload supply of Alan Bradley industrial ON-OFF buttons
if you'd like to be able to enable/disable this system with push buttons.

Jerry


  #12   Report Post  
Jerry Martes
 
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Bob

My experience indicates that the improvement in HP delivering capacity of
the idler that results from "tuning" is so small that it probably isnt worth
the expense for powering an air compressor. But, what do I know?? How
much do you suppose the capacity would cost for balancing that 15 HP idler?
How much more performance would be realized as result of tuning?

Jerry



"Robert Swinney" wrote in message
...
Electrolytic "start" caps are not suitable for use as running caps. They
will blow up if left in circuit form more than a few seconds. You can't
use them for run "test caps" as you suggest - they will blow! With a 15
HP idler motor you will need around 450 microfarads of run capacitance
permanently connected from one side of the input line to the "manufactured
phase". This will result in an "unbalanced" but satisfactory rotary phase
converter. Start by putting in 350 microfarads, or so, and add
capacitance while metering the manufactured phase. Continue to add
capacitance until the manuf. leg voltage rises to approximately 10%
greater than the input (240 ?) volt input line voltage. Be very careful -
you are working with lethal voltages!

Bob Swinney
S
"gtslabs" wrote in message
ups.com...
Thanks I see the difference now. I have a case of ~130 uF 240V
electrolytic caps. I know these are not for continious use but only
start-up. However I would like to use these to find the correct
capacitance needed then buy the oil run type caps. How long is safe to
run these caps in a continious mode for testing?





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Robert Swinney
 
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Jerry sez:

" My experience indicates that the improvement in HP delivering capacity of
the idler that results from "tuning" is so small that it probably isnt
worth the expense for powering an air compressor. But, what do I know??"


Hey, Jerry! You da Man with the dyno .... ! If Steve is concerned with the
starting load of an air compressor he may want (need ? ) the load extra
starting capability of a voltage balanced RPC. He did mention that he
wanted to balance for whatever reason. OTOH, "balancing" may not be that
attractive considering the cost of oil-filled caps.

Bob Swinney












  #14   Report Post  
gtslabs
 
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I checked the voltage on the 3 legs without any run capacitors and I
got 220,220,240 so this is within 10% and I may not need any balancing
capacitors after all at idle. I have seven 250V 108-130uF start
capacitors in parallel. How do I determine the required resistance /
watts to put a bleed resistor across them? If Watts=VI then 240*37 =
8.88KW and the resistance of R=V/I gives me 6.5 ohms. Does this sound
about right?

  #15   Report Post  
Robert Swinney
 
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The purpose of a bleed resistor is to .. well, errrr, "bleed" of any
residual charge from a capacitor bank thus eliminating possible shock hazard
to the operator. Sufficient protection would be afforded if the charge
could be bled off in, say 30 seconds. The universal time constant is given
as: T = RxC; transposing gives R = T/C. A cap is said to be fully charged
or discharged after 5 time constants. Therefore the desired RC time constant
of each cap and a bleeder resistor is 6 seconds. Assuming the value of each
of the 7 caps is 120 microfarad, the resistor value is figured as R = T/C,
or R = (6 / 0.000120) = 50000 ohms. The nearest size standard value is 56
K. Figure the power dissipated by each resistor as (E squared) / R; or P =
(240 squared) / 56K = aprox. 1 watt. You would need a 56K, 2 Watt bleeder
resistor across each of the 7 caps. Another way to do it would be place a
single 56K, 10 Watt resistor across the cap bank. All this represents a
power loss of around 7 watts if the bleeder resistor(s) is in circuit
constantly. The bleeder could be applied via a N.C. set of relay contacts
only after shut down, if desired, rather than leaving it in circuit
constantly.

Bob Swinney




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Robert Swinney
 
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Correction: The resistor placed across the entire cap bank would have to be
aprox. 7K ohms, not 56K ohms. Power dissipation of the resistor would be
(240 squared) / 7K = 8.3 watts. It would have to be a 10 Watt resistor.

Sorry,

Bob Swinney


"Robert Swinney" wrote in message
...
The purpose of a bleed resistor is to .. well, errrr, "bleed" of any
residual charge from a capacitor bank thus eliminating possible shock
hazard to the operator. Sufficient protection would be afforded if the
charge could be bled off in, say 30 seconds. The universal time constant
is given as: T = RxC; transposing gives R = T/C. A cap is said to be
fully charged or discharged after 5 time constants. Therefore the desired
RC time constant of each cap and a bleeder resistor is 6 seconds.
Assuming the value of each of the 7 caps is 120 microfarad, the resistor
value is figured as R = T/C, or R = (6 / 0.000120) = 50000 ohms. The
nearest size standard value is 56 K. Figure the power dissipated by each
resistor as (E squared) / R; or P = (240 squared) / 56K = aprox. 1 watt.
You would need a 56K, 2 Watt bleeder resistor across each of the 7 caps.
Another way to do it would be place a single 56K, 10 Watt resistor across
the cap bank. All this represents a power loss of around 7 watts if the
bleeder resistor(s) is in circuit constantly. The bleeder could be
applied via a N.C. set of relay contacts only after shut down, if desired,
rather than leaving it in circuit constantly.

Bob Swinney



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Chuck Sherwood
 
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Correction: The resistor placed across the entire cap bank would have to be
aprox. 7K ohms, not 56K ohms. Power dissipation of the resistor would be
(240 squared) / 7K = 8.3 watts. It would have to be a 10 Watt resistor.


It would probably be much easier to find a 10k 10 watt resister that
is close enough for this job.

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