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quanchai 07-31-2012 09:45 PM

old capacitor
Hi guys

does an old capacitor cause electrical equipment to consume more power ?

jbfan 07-31-2012 09:57 PM


quanchai 07-31-2012 10:00 PM

then why is my ac split unit drawing 16 amps when it should only draw 12.8 amps as it says in its manual ???

Doc Holliday 07-31-2012 10:08 PM

Yes, a weak or even wrong size capacitor will cause a motor to increase it's energy consumption. Replace the cap and see if there's a difference.

If no difference with a new capacitor, your motor is struggling and becoming weak.

quanchai 07-31-2012 10:34 PM

well i just tested the cap and i got a reading of 33 uF on the cap it says 40 uF do you thing this is why there is 3 amp extra current ?

Doc Holliday 07-31-2012 11:14 PM


Originally Posted by quanchai (Post 978774)
well i just tested the cap and i got a reading of 33 uF on the cap it says 40 uF do you thing this is why there is 3 amp extra current ?

Most likely. There should be a +/- percentage number stated on the cap as well, right next to the 40uf. It would look something like +/- 6%. That number could be different than 6, I'm just using that number as an example.

At 10% (just a max percentage on average run caps but usually much bigger ones) less than 40 which is 36 uf the cap would be at it's weakest to assist the motor in it's normal operating range. You are well below 10% at 33 so your motor is struggling.

6% less then 40 (which is 2.4) would be 37.6 so anything lower than that would cause the motor to struggle to run. You get the idea.

quanchai 07-31-2012 11:33 PM

Thanks Doc

but what does the precentage mean ?

Doc Holliday 08-01-2012 06:43 AM

It's the percentage above or less than 40 (in your case) that the run cap is still good. You multiply 40 by whatever the percentage number is and then either add or subtract that percentage total to (again, in your case) the 40. Outside of that plus or minus and the cap is no good for the motor.

Once more and using 6% +/- of 40 as an example:

40 x 6% = 2.4

40 + 2.4 = 42.4 (so if your 40mfd cap read above 42.4mfd it would not be good for the motor it's running, the motor would spin too fast)

the opposite for 40mfd - 2.4 which would = 37.6 (and in this case if lower than 37.6, when the motor is running too slowly, it compensates by pulling more amps)

What you have is a run capacitor which is always in circuit of the motor and needs to be correctly sized and within that size range (mfd) to the motor to keep it running at it's design rpm.

A start capacitor drops out of circuit as soon as the motor is up to speed, within a fraction of a second usually, and then the motor is on it's own.

ddawg16 08-01-2012 09:51 AM

How are you measuring the current? Do you know if your meter is accurate?

Have you considered using the meter on your load center? Turn off everything else in the the AC for 15 min while seeing how many KW your using...x 4 and you will have your Kwh...from the normal math....

quanchai 08-01-2012 06:46 PM

I am using a clamp meter to measure the current of the AC unit

quanchai 08-01-2012 06:49 PM

the split unit started to get over 13 amps after i got it recharged with gas

can it be that the unit is over charged ?

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