Thread: capacitor" too hi or low enough"

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capacitor" too hi or low enough"

a question was ask not to long ago about capacitor sizing what is the rule of thumbif one does not have the excect capacitor the unit takes,comprasor/blower/fan motor how low ir how hi of a capacitor can i use and still be safe to the unit, for exsample the condanser fan motor it takes a 5.7 uf but i noly have a 7.5 would it be oky, can i go higher but not lower and how hi or low can i use ?

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on start capacitors 10% variation won't be a problem. But on RUN capacitors
the mfd should not vary (can go up on voltage but mfd should be same) Get Copeland's electrical handbook and learn how to combine Capacitors in parralel and series so you can always provide proper mfd:

examples: 2ea 30 mfd 440v
in series 30 mfd
in parralel 15 mfd

a 5 + 10 in parallel = 15 mfd (mfd adds)

I need a 15 mfd at 440 v
all I got are 30 mfd at 330
30 + 30 in parallel = 15 mfd at 660 volts (volts add)

pages A7,A8,A9 of Copeland's handbook give details on this

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sorry
2 each 30 mfd in parralel = 60 mfd (mfd's add - I got bifocals)

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Sorry, my typing didn't follow my brain today

here is an excerpt from Copeland's handbook

Capacitor Voltage
The voltage rating of a capacitor indicates the highest nominal
voltage at which it is designed to operate. Use of a capacitor
at voltages below its rating will do no harm. Run capacitors must
not be subject to voltages exceeding 10 percent of the nominal
rating, and start capacitors must not be subjected to voltages
exceeding 30 percent of the nominal rating. The voltage to which
a capacitor is subjected is not line voltage, but is a much higher
potential (often called back electromotive force or back EMF) which
is generated in the start winding. On a typical 230 volt motor,
the generated voltage may be as high as 400 volts and is determined
by the start winding characteristics, the compressor speed, and
the applied voltage. PARALLEL AND SERIES CAPACITORS Capacitors,
either start or run, can be connected in series or parallel to
provide the desired characteristics if the voltage and MFD are
properly selected. When two capacitors having the same MFD rating
are connected in series, the resulting total capacitance will be
one half the rated capacitance of a single capacitor.

The formula for determining capacitance (MFD) when capacitors are
connected in a series is as follows:
1/MFD = 1/MFD + 1/MFD

For example, if a 20 MFD and a 30 MFD capacitor are connected in series,
the resultant capacitance will be:
1/MFD = 1/20 + 1/30
Get common denominator
1/MFD = 3/60 + 2/60
1/12 = 5/60 = 3/60 + 2/60
MFD = 12

the voltage rating of similar capacitors connected in series is equal
to the sum of the voltage of the two capacitors. However, since the
voltage across individual capacitors in series will vary with the rating
of the capacitor, for emer¬gency field replacements, it is recommended that
only ca¬pacitors of like voltage and capacitance be connected in se¬ries to
avoid the possibility of damage due to voltage be¬yond the capacitor limits.
When capacitors are connected in parallel, their MFD rating is equal to the
sum of the individual capacitances. The voltage rating is equal to the lowest
voltage rating of the in¬dividual capacitors.
MFD + MFD + MFD
20 = 15 + 5

It is possible to use any combination of single, series, or parallel start
capacitors, with single or parallel run capacitors (run capacitors are seldom
used in series).

just try doing a few till you get the hang of it, and very seldom will it be
that you can't provide proper mfd's

5. rocket, is the copeland book something that can be downloaded from their website or does it have to be ordered from copeland? If it can be downloaded, do you have a link?

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you can get at any Copeland Wholesaler, though has \$7.00 price, a good customer should be able to get it free. (I did)
"Electrical Handbook 6400"

7. This is Turtle

Here is the old time rules when all Cap.s was retangle only.

Up or Down 10&#37; of the uf ----- Up or Down to the next size of the uf.

Here is the uf sizes 2.5 , 5 , 7.5 , 10 , 12.5 , 15 , 20 , 25 , 30 , 35 , 40 , 45 , 50 , 55 , 60 , 70 , 80 , 90 .

TURTLE

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Turtles rule is fine for START capacitors

it is wrong for RUN Capacitors, RUN capacitors need to be same mfd
wrong mfd can seriously affect performance and rpm of motor, don't do it!

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questions:
how long is start cap in the circuit while compressor is running (hint less than 5 seconds)

how long is run cap in the circuit while compressor is running (hint all the time)

define power factor (as related to a run capacitor)

add on limits per Tecumseh on substituting run capacitors
10 to 20 mfd (max + 2-1/2 mfd)
20 to 50 mfd (max + 5 mfd)
over 50 mfd (max + 10 mfd)
above from Tecumseh handbook table 3-8

add on limits per Copeland on substituting run capacitors
0 mfd

Answering the first 3 questions indicates why RUN cap mfd should not vary

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Originally Posted by rocket
Sorry, my typing didn't follow my brain today

here is an excerpt from Copeland's handbook

Capacitor Voltage
The voltage rating of a capacitor indicates the highest nominal
voltage at which it is designed to operate. Use of a capacitor
at voltages below its rating will do no harm. Run capacitors must
not be subject to voltages exceeding 10 percent of the nominal
rating, and start capacitors must not be subjected to voltages
exceeding 30 percent of the nominal rating. The voltage to which
a capacitor is subjected is not line voltage, but is a much higher
potential (often called back electromotive force or back EMF) which
is generated in the start winding. On a typical 230 volt motor,
the generated voltage may be as high as 400 volts and is determined
by the start winding characteristics, the compressor speed, and
the applied voltage. PARALLEL AND SERIES CAPACITORS Capacitors,
either start or run, can be connected in series or parallel to
provide the desired characteristics if the voltage and MFD are
properly selected. When two capacitors having the same MFD rating
are connected in series, the resulting total capacitance will be
one half the rated capacitance of a single capacitor.

The formula for determining capacitance (MFD) when capacitors are
connected in a series is as follows:
1/MFD = 1/MFD + 1/MFD

For example, if a 20 MFD and a 30 MFD capacitor are connected in series,
the resultant capacitance will be:
1/MFD = 1/20 + 1/30
Get common denominator
1/MFD = 3/60 + 2/60
1/12 = 5/60 = 3/60 + 2/60
MFD = 12

the voltage rating of similar capacitors connected in series is equal
to the sum of the voltage of the two capacitors. However, since the
voltage across individual capacitors in series will vary with the rating
of the capacitor, for emer&#172;gency field replacements, it is recommended that
only ca&#172;pacitors of like voltage and capacitance be connected in se&#172;ries to
avoid the possibility of damage due to voltage be&#172;yond the capacitor limits.
When capacitors are connected in parallel, their MFD rating is equal to the
sum of the individual capacitances. The voltage rating is equal to the lowest
voltage rating of the in&#172;dividual capacitors.
MFD + MFD + MFD
20 = 15 + 5

It is possible to use any combination of single, series, or parallel start
capacitors, with single or parallel run capacitors (run capacitors are seldom
used in series).

just try doing a few till you get the hang of it, and very seldom will it be
that you can't provide proper mfd's

Hi

Im confused on the example below. Can someone help me to understand this? A 20mfd and a 30mfd equals 12mfd????? Also is there an illustration of how these are connected in series??????

The formula for determining capacitance (MFD) when capacitors are
connected in a series is as follows:
1/MFD = 1/MFD + 1/MFD

For example, if a 20 MFD and a 30 MFD capacitor are connected in series,
the resultant capacitance will be:
1/MFD = 1/20 + 1/30
Get common denominator
1/MFD = 3/60 + 2/60
1/12 = 5/60 = 3/60 + 2/60
MFD = 12
Last edited by AtoZhvac; 09-29-2008 at 12:41 AM. Reason: Added question

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Does voltage make a difference on capacity requirement?

Originally Posted by rocket
questions:
how long is start cap in the circuit while compressor is running (hint less than 5 seconds)

how long is run cap in the circuit while compressor is running (hint all the time)

define power factor (as related to a run capacitor)

add on limits per Tecumseh on substituting run capacitors
10 to 20 mfd (max + 2-1/2 mfd)
20 to 50 mfd (max + 5 mfd)
over 50 mfd (max + 10 mfd)
above from Tecumseh handbook table 3-8

add on limits per Copeland on substituting run capacitors
0 mfd

Answering the first 3 questions indicates why RUN cap mfd should not vary
Let me add this to the confusion; What if L1-L2 = 208VAC, What if L1-L2 =230VAc, What if L1-L2 = 245Vac. Do you need to adjust capacitance?

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Here are some good threads on capacitance in the pro section.

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As someone mentioned above,10% diff. of mf rating on start caps is ok.Run cap mf rating should be the same as original.VAC ratings must be the same or greater on capacitors.For example:a 35mf 370 vac cap can be replaced with a 35mf 440 vac cap.

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