Cooker hood repair

[stussy]

i have a cooker hood to repair, it is single phase 230V, the start/run capacitor has melted.
the motor and other components are in full working order this capacitor just needs replacing.
it has a value of 4μF rated at 250V

i dont have the same one to hand but i do have a 4μF 250V capacitor out of a fluorescent fitting.

these capacitors are different physically but do have the same values.
am i right in saying the fluorescent fitting capacitor will be OK to use
as a replacement?

or is the dielectric inside different for the transient current the motor will pull upon start up?
im sure this will only be trivial as the motor is only tiny!
thanks in advance!

 

[Marvo]

I don't think the lighting cap and motor cap are interchangeable. With the motor cap, the 230v label indicates the discharge voltage. With the lighting cap the 250v indicates the supply voltage it's designed to power factor correct.

My advice is to buy a motor cap of the same ratings.

 

[stussy]

a start capacitor is not to power factor correct, its job is the opposite.
to make it out of phase so the motor will start. either way a capacitor with the same spec should be interchangeable?

 

[Marvo]

a start capacitor is not to power factor correct, its job is the opposite.
to make it out of phase so the motor will start.

I think you misunderstood what I was trying to say. A motor cap has a specific discharge voltage, a lighting cap doesn't.

either way a capacitor with the same spec should be interchangeable?

The spec isn't the same. the voltage on the motor cap is it's discharge voltage, the voltage stated on the lighting cap is it's supply voltage.

A lighting capacitor is designed to be connected across the supply whereas a motor capacitor definitely is not

I'll see if I can find an web site article that explains it better than I can.

 

[stussy]

from some brief study on wikipedia i found this...

Run capacitors are designed for continuous duty, and they are energized the entire time the motor is running.[1] Run capacitors are rated in a range of 3–70 microfarads (µF), with voltage classifications of 370 V or 440 V.[1] Single phase electric motors need a capacitor to energize a second-phase winding. If the wrong run capacitor is installed, the motor will not have an even magnetic field, and this will cause the rotor to hesitate at those spots that are uneven. This hesitation can cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat.


while start capcitors...

Start capacitors briefly increase motor starting torque and allow a motor to be cycled on and off rapidly.[1] Start capacitors have ratings above 70 microfarads (µF), with three major voltage classifications: 125 V, 250 V, and 330 V.[1] A start capacitor stays energized long enough to rapidly bring the motor to 3/4 of full speed and is then taken out of the circuit, such as by a centrifugal switch that releases when rotating at or around that speed.
Examples of motor capacitors are: a 35 µF, at 370 V, run capacitor, or an 88–108 µF at 250 V start capacitor.


so im guessing the one i have broken is a run capacitor, would i be ok to replace that?

 

[Marvo]

Some basic info on diferent capacitors and their functions is available here. (This is the site directory, just download the pdf's from there)

 

[stussy]

thank you very much for this info, im still unsure if it will work and if its a good idea lol

 

[Marvo]

thank you very much for this info, im still unsure if it will work and if its a good idea lol

It's not a good idea, It won't work

 

[tonto33]

if you got the make and model of hood will be able to find you correct cap for it

 

[Pinto Baz]

I doubt the lighting capacitor will be robust enough, if it melts again the best you can hope for is the customer expecting you to come back and fix it again properly for free. (I won't bother getting all dramatic about the worst case scenario!) If you are going to design you own bespoke appliances you need to be 100% sure of what you are doing. Just get the right part and tell the customer they will have to wait a while until it arrives. You are not leaving them without electricity, it's only a cooker hood.

 

[stussy]

im not designing my own appliances, im using a part to replace another that has failed.

but im unsure if it will work, hence singing up to a forum and asking advice....

i wouldn't install something without been sure about what i was doing, that is bad practice.

i am not a electronics expert so i don't know everything they is to know about capacitors, i only know the basics so thats why im asking the questions.

so in conclusion a 4μF capacitor 250v out of a fluorescent fitting WILL NOT provide the correct phase angle needed for the motors constant operation?
this is due to the dielectric inside been of a different composition then that of the current one?

current one is a small black rectangle shape rated at 4μF 250v with 2 black flying leads with a small moulded plug the connects to a small PCB for the fans operation.

 

[Nick4u1]

Um yeah...best avoided.