Measuring Capacitance

[neptune]

I know that you can buy multimeters now with a capacitance scale, but I generally prefer to buy cheap tools because i tend to loose them. I went out to an old Zanussi , It would spin with a light load, but not with a heavy load. I changed the motor capacitor, which cured the fault. I took the old cap home and decided to measure it. First I gave it the usual leakage and low- to-earth tests which it passed. Then using suitable leads, I connected it accross the mains in series with my multimeter on AC amps. The reading was 0.6 amps. [Safety, use insulated leads, set the job up and retire to a safe distance before switching on mains at socket.] Note the reading, disconnect plug from mains, and remember to discharge the cap with an insulated screwdriver before touching it. Now devide 14.4 by the current in amps in this case, 0.6 . The answer is 8.6 . so this nominally 16 mfd cap had deteriorated to 8.6 . Hope this might help somebody. Note that the plug on the test rig should have a 5 amp fuse.OBSERVE ABOVE SAFETY PRECAUTIONS, or buy a dearer multimeter.

[admin]

Shorting capacitor will damaged himself…your measurement is for schort circuit current not for nominal current…
Capacitor role in motor circuit is to dephase current between two coils (depanding of coil resistance and coupling needed you choose capacitor).
In series (accross) you don’t read (measure) anything else that current (power) consumption of user (resistive).

[eastlmark]

Re: Measuring Capacitance

neptune wrote:I know that you can buy multimeters now with a capacitance scale, but I generally prefer to buy cheap tools because i tend to loose them. I went out to an old Zanussi , It would spin with a light load, but not with a heavy load. I changed the motor capacitor, which cured the fault. I took the old cap home and decided to measure it. First I gave it the usual leakage and low- to-earth tests which it passed. Then using suitable leads, I connected it accross the mains in series with my multimeter on AC amps. The reading was 0.6 amps. [Safety, use insulated leads, set the job up and retire to a safe distance before switching on mains at socket.] Note the reading, disconnect plug from mains, and remember to discharge the cap with an insulated screwdriver before touching it. Now devide 14.4 by the current in amps in this case, 0.6 . The answer is 8.6 . so this nominally 16 mfd cap had deteriorated to 8.6 . Hope this might help somebody. Note that the plug on the test rig should have a 5 amp fuse.OBSERVE ABOVE SAFETY PRECAUTIONS, or buy a dearer multimeter.

If it cured the problem, why bother?

[neptune]

Re: Measuring Capacitance

Eastlmark asks why bother. Just curiosity I guess. I still find it interesting to be able to actually measure the capacitance with just an amp meter and a pen and paper. Having read Barons posts before I get the impression he is a very well educated man. However I am not sure I fully understand his reply. It is true that I am measuring the short circuit current of the cap as oposed to the current It would carry in the motor circuit. But I am doing this jut as a way to calculate its capacitance.Baron, are you also saying that short circuiting a charged cao will damage it?

[Martin]

Re: Measuring Capacitance

neptune wrote:I still find it interesting to be able to actually measure the capacitance with just an amp meter and a pen and paper.

Fascinating stuff I’m sure and also not within the abilities of your average multimeter to measure AC capacitance so perhaps an amp meter and pen & paper is the only option 😕

I have to say though that I’m with Mark on this, and would always implement the ‘if in doubt, whip it out’ philosophy with capacitors. But beyond that I can offer this bit of info I gooogled for you. 😉

Measuring AC Capacitance wrote:Consider now a circuit which has only a capacitor and an AC power source (such as a wall outlet). A capacitor is a device for storing charging. It turns out that there is a 90° phase difference between the current and voltage, with the current reaching its peak 90° (1/4 cycle) before the voltage reaches its peak. Put another way, the current leads the voltage by 90° in a purely capacitive circuit.

To understand why this is, we should review some of the relevant equations, including:

relationship between voltage and charge for a capacitor: CV = Q

The AC power supply produces an oscillating voltage. We should follow the circuit through one cycle of the voltage to figure out what happens to the current.

Step 1 – At point a the voltage is zero and the capacitor is uncharged. Initially, the voltage increases quickly. The voltage across the capacitor matches the power supply voltage, so the current is large to build up charge on the capacitor plates. The closer the voltage gets to its peak, the slower it changes, meaning less current has to flow. When the voltage reaches a peak at point b, the capacitor is fully charged and the current is momentarily zero.

Step 2 – After reaching a peak, the voltage starts dropping. The capacitor must discharge now, so the current reverses direction. When the voltage passes through zero at point c, it’s changing quite rapidly; to match this voltage the current must be large and negative.

Step 3 – Between points c and d, the voltage is negative. Charge builds up again on the capacitor plates, but the polarity is opposite to what it was in step one. Again the current is negative, and as the voltage reaches its negative peak at point d the current drops to zero.

Step 4 – After point d, the voltage heads toward zero and the capacitor must discharge. When the voltage reaches zero it’s gone through a full cycle so it’s back to point a again to repeat the cycle.

The larger the capacitance of the capacitor, the more charge has to flow to build up a particular voltage on the plates, and the higher the current will be. The higher the frequency of the voltage, the shorter the time available to change the voltage, so the larger the current has to be. The current, then, increases as the capacitance increases and as the frequency increases.

Usually this is thought of in terms of the effective resistance of the capacitor, which is known as the capacitive reactance, measured in ohms. There is an inverse relationship between current and resistance, so the capacitive reactance is inversely proportional to the capacitance and the frequency:

A capacitor in an AC circuit exhibits a kind of resistance called capacitive reactance, measured in ohms. This depends on the frequency of the AC voltage, and is given by:

We can use this like a resistance (because, really, it is a resistance) in an equation of the form V = IR to get the voltage across the capacitor:

Note that V and I are generally the rms values of the voltage and current.

😀

[eastlmark]

Re: Measuring Capacitance

I gues its a matter of time on your hands to probe into these reasons things have failed. I used to bring every module back with me to do the same but never get the chance to do anything with them. I guess when I was younger I too may have been as curious about that cap as Neptune was but these days I really couldnt give a damn.

[neptune]

Re: Measuring Capacitance

well at least I provoked a bit of lively debate, which was one of my intentions. The mathematical formula I use is Capacitance in mfd is equal to 1,000,000 xI all over2x Pi x fxV where Pi is 3.142 ,f is the frequency[50 hertz] and v is the voltage {220]. For 220 volts this reduces to C= 14.4 x I where I is th current in amps. Iguess I was bored that day.

[admin]

Re: Measuring Capacitance

neptune wrote: But I am doing this jut as a way to calculate its capacitance.Baron, are you also saying that short circuiting a charged cao will damage it?

For the second question… (SCC discharge) YES surely
For the first problem I only can explain that is more difficult to calculate capacitance: is involving also source frequency; induction coil (Henry) and absorbed current (Imax).
In order to understand what happened between two coils supplied with alternative voltage:
– imagine one sinusoidal wave (which is voltage value at coils pole). paste exactly over one identical…. so will look also current diagram thru inductive coil=> magnetic field generated have same frequency and phase => rotor (will be kept still but will hummmm like one transformer).
– If in one coil will be inserted one conder depending on his capacitance will diphase (grate cap.=> grater diphase): take one sinusoidal described earlier and moves her horizontal a bit. So you will see one wave but double (?) => In excitement coil (that with cap.) will be generated one magnetic field in the same direction but little bit late… which will help to turn motor.

Like one supplemental explanation one motor has two or four poles: in order to get one of rotor point to next pole it must be help with that exciting coil and capacitor. Therefore is difference between synchrony motor (this case) and asynchrony motor (with brushes): with you try to stop him it will keep rotation until he will stop suddenly when the other motor will reduce steeply rotation regarding force.

[admin]

Re: Measuring Capacitance

I forgot one example when your motor needs one 18uF capacitor:
– if you put one 12uF he cannot turn;
– if you put one 20uF or higher he will turn but will hear hummm (50Hz sounds) rezonance on motor and his coil and will heat him self very hard. (because exciting coil will generate magnetic field over next magnetic coil field => brake== resistor)

[bazza500]

Re: Measuring Capacitance

baron wrote:

neptune wrote:
But I am doing this jut as a way to calculate its capacitance.Baron, are you also saying that short circuiting a charged cao will damage it?

For the second question… (SCC discharge) YES surely
For the first problem I only can explain that is more difficult to calculate capacitance: is involving also source frequency; induction coil (Henry) and absorbed current (Imax).
In order to understand what happened between two coils supplied with alternative voltage:
– imagine one sinusoidal wave (which is voltage value at coils pole). paste exactly over one identical…. so will look also current diagram thru inductive coil=> magnetic field generated have same frequency and phase => rotor (will be kept still but will hummmm like one transformer).
– If in one coil will be inserted one conder depending on his capacitance will diphase (grate cap.=> grater diphase): take one sinusoidal described earlier and moves her horizontal a bit. So you will see one wave but double (?) => In excitement coil (that with cap.) will be generated one magnetic field in the same direction but little bit late… which will help to turn motor.

Like one supplemental explanation one motor has two or four poles: in order to get one of rotor point to next pole it must be help with that exciting coil and capacitor. Therefore is difference between synchrony motor (this case) and asynchrony motor (with brushes): with you try to stop him it will keep rotation until he will stop suddenly when the other motor will reduce steeply rotation regarding force.

I was just about to say that but you beat me to it 😆

[wilf]

Re: Measuring Capacitance

14.4 what? where do you get that figure from?

xc = 1 / 2 Pi f c

wilf

[neptune]

Re: Measuring Capacitance

Hi Wilf, Iam not trying to calculate XC which I asume is capacitive reactance, I am trying to calculate the actual value of the cap in microfarads. Thus the formula I use is C= 1,000,000xI/ 2pifc. for 220 volts at 50 Hertz, this simplifies to C=14.4 xI. Thus 14.4 is just a constant and as such has no units.
Hi Baron, and thanks for the explanation of the workings of a single phase motor with the capacitor, which i understand. You pointed out that if a motor is designed to work witha 18 mf cap[or in my case a 16mf] it will not work with a 12 or a 20mf. This is why I wished to measure the cap in the first place.
If dichargeing a cap will damage it, a bleed resistor could be used to limit the current Thanks for all interest and replies.

[dpm]

We’ve talked about cap testing a few times; as a multimeter with the relevant uf ranges costs about fifteen quid I know what I’d do…

[Author]

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