Can anyone fill in one of the many gaps in my knowledge? I would like to know of any way that it is possible to establish that a start/run capacitor in-situ has failed using just a multi-meter that does not measure capacitance. Many thanks
Start/run capacitors
- Thread starter jj4091
- Start date
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With the capacitor isolated, you could use an analogue multimeter to measure resistance across the capacitor. Expect the needle to 'kick', and the 'resistance ' to slowly 'increase' as the capacitor is charged by the meter battery, and the charging current drops.
Then reverse the connections, and you should observe a bigger 'kick' as the capacitor discharges through the meter, and re-charges with the reverse polarity.
With experience, you should get an impression of the value of the capacitor by noting the rate the meter needle drops, especially if you have a few exemplars to compare it with.
A good telephone linesman would be able to approximate the distance to a cable fault using this technique to measure the capacitance of cable pairs.
Beware of capacitors that may remain charged to about 400 volts (the peak voltage, not the RMS value of the supply), for some considerable time after they have been disconnected and removed from equipment!
That used to be done to wake up apprentices - Charge a capacitor on a bench power supply, bend the leads back over the case, and leave it sitting on the bench for a poor unsuspecting apprentice to pick up and play with
Then reverse the connections, and you should observe a bigger 'kick' as the capacitor discharges through the meter, and re-charges with the reverse polarity.
With experience, you should get an impression of the value of the capacitor by noting the rate the meter needle drops, especially if you have a few exemplars to compare it with.
A good telephone linesman would be able to approximate the distance to a cable fault using this technique to measure the capacitance of cable pairs.
Beware of capacitors that may remain charged to about 400 volts (the peak voltage, not the RMS value of the supply), for some considerable time after they have been disconnected and removed from equipment! That used to be done to wake up apprentices - Charge a capacitor on a bench power supply, bend the leads back over the case, and leave it sitting on the bench for a poor unsuspecting apprentice to pick up and play with
Thanks for that but I only have a digital multimeter, if I select "peak hold" & try it that way do you think I may get the same effect? The capacitors that I wish to test read open circuit when I use the meter in the normal manner but I presumed this was because they were draining the meter battery.
I'm not sure what you are trying to fix but in my experience with dead power supplies just do the following...
Isolate power supply.
disscharge all capacitors in power supply.
Replace all capacitors in power supply.
Caps are cheap so just do them all, especially those cheeky philips ones!
I once went on a 1 week power supply course which went into great detail the workings of linear and switched mode power supplies. On the last day the lecturer said 'replace all capacitors and if it still doesn't work just buy a new one'!
Isolate power supply.
disscharge all capacitors in power supply.
Replace all capacitors in power supply.
Caps are cheap so just do them all, especially those cheeky philips ones!
I once went on a 1 week power supply course which went into great detail the workings of linear and switched mode power supplies. On the last day the lecturer said 'replace all capacitors and if it still doesn't work just buy a new one'!
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not a power supply chris.. he's trying to see if the capacitor on a single phase motor has gone..
the charge / dischage trick can also be done with a digital insulation resistance meter.. it will charge the cap up to the setting voltage though so be warned..
the charge / dischage trick can also be done with a digital insulation resistance meter.. it will charge the cap up to the setting voltage though so be warned..
Thanks to thornton - chris & col jack, it is a single phase motor, on an old compressor, which looks as though it has a start capacitor that switches out when it gets up to speed & 2 run capacitors in series which stay connected( although it may be the other way round as the control panel is such a birds nest I cannot quite tell, yet). I think because of its age I may struggle to get replacements, which is why I wanted to be sure they were faulty.
RS components and the like list a range of generic motor start /run capacitors with values from 2 to 50 microfarad, so you could make up the values you need if necessary.
Or you could try a firm that re-winds motors. They might be able to check out the capacitors for you.
Another suspect on cap. start motors is the centrifugal switch mounted on the armature. They sometimes get bunged up with dust and fluff, so they don't switch.
I saw a lathe motor that was bunged up with magnesium and aluminium swarf once - It was quite spectacular when powered up, but there was not much left of the motor afterwards.
Or you could try a firm that re-winds motors. They might be able to check out the capacitors for you.
Another suspect on cap. start motors is the centrifugal switch mounted on the armature. They sometimes get bunged up with dust and fluff, so they don't switch.
I saw a lathe motor that was bunged up with magnesium and aluminium swarf once - It was quite spectacular when powered up, but there was not much left of the motor afterwards.
Compressor motors have a hard life. Hopefully your machine will have a non-return valve in the air delivery pipe and a solenoid dump valve (wired to the motor starter) that dumps air whilst the motor is getting up to speed.
As to testing the capacitors with only a basic digital multimeter. You cannot do it, but there are other ways. If you obtain a high value resistor, say 100,000 ohms and connect the capacitor to a battery through the resistor the meter will show a slowly rising voltage. The capacitance can be deduced by the length of time taken to reach half battery voltage. Of course some capacitors have self discharge resistors built in and these may spoil your experiment.
Another way: - After checking that the capacitor measures infinity on the meters ohms range, connect it in series with a lamp bulb holder. Connect to mains (BE CAREFUL AS I CANNOT BE HELD RESPONSIBLE FOR ANY MISTAKES) Try a 100watt light bulb in the holder. If it lights the capacitor is OK. Now measure the voltage across the bulb and the voltage across the capacitor. You will find that the total is MORE than 230volts! This is because vectorially the voltages are at right angles to one another. You could draw a right angled triangle and make these two voltage vectors the opposite and adjacent sides. The long side (hypotenuse) will be 230. As the same current went through both the bulb and the capacitor its REACTANCE can be deduced and from this the capacitance (Xc = 1/square root of 2Pi f C)
These motor capacitors cost several pounds each so they are worth testing.
As to testing the capacitors with only a basic digital multimeter. You cannot do it, but there are other ways. If you obtain a high value resistor, say 100,000 ohms and connect the capacitor to a battery through the resistor the meter will show a slowly rising voltage. The capacitance can be deduced by the length of time taken to reach half battery voltage. Of course some capacitors have self discharge resistors built in and these may spoil your experiment.
Another way: - After checking that the capacitor measures infinity on the meters ohms range, connect it in series with a lamp bulb holder. Connect to mains (BE CAREFUL AS I CANNOT BE HELD RESPONSIBLE FOR ANY MISTAKES) Try a 100watt light bulb in the holder. If it lights the capacitor is OK. Now measure the voltage across the bulb and the voltage across the capacitor. You will find that the total is MORE than 230volts! This is because vectorially the voltages are at right angles to one another. You could draw a right angled triangle and make these two voltage vectors the opposite and adjacent sides. The long side (hypotenuse) will be 230. As the same current went through both the bulb and the capacitor its REACTANCE can be deduced and from this the capacitance (Xc = 1/square root of 2Pi f C)
These motor capacitors cost several pounds each so they are worth testing.
Thanks tickly t & retirednow, the lamp in series with the capacitor sounds like an excellent idea which I had not thought of. I did think of putting my meter in series with each capacitor in the same manner, but was not sure if capacitors charge to a higher than the supply voltage.
You can't tell the exact value without a caacitance meter or bridge.
Capacitors are cheap enough in Maplin.
See here
http://www.maplin.co.uk/Search.aspx?criteria=motor capacitor&source=15&SD=Y
Capacitors are cheap enough in Maplin.
See here
http://www.maplin.co.uk/Search.aspx?criteria=motor capacitor&source=15&SD=Y
Among other ways ou can charge them up to a known voltage and measure the time taken to discharge to half voltage with a known load. From there it is not too hard to calculate the capacitance.
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