Series Motor Capacitors which change in Value - Page 1
Forum section: electronics / projects

author: beanhauler
author_rank: Contributor
timestamp: May 04, 2021, 09:44:03 am
content: So here is the classic motor no start syndrome but you can spin it up by spinning the rotor by hand.I thought it must have some capacitance left as it will start intermittently depending on physical position of rotor.I wonder why these die is there a paper on it anywhere? Physical separation of foil layers increases? effective SR changes? Short in layer?I am aware that this motor had been stalled on a few occasions and although it is physically protected by a thermistor wonder if that takes its toll on the caps.Jury is out.Not that it matters as you just have to replace.This one is marked as 16ufd and measures 4.58ufd.replacing restored normal operation.This is from a 240V cap start/cap run config.

author: Gyro
author_rank: Super Contributor
timestamp: May 04, 2021, 10:18:07 am
content: Motor-run Polypropylene film capacitors (MKP) self heal in the same way that Class X capacitors do. The metallisation on the film is quite thin and is designed to evaporate away if there is a weak spot, transient damage, or excessive dissipation, in order to avoid the capacitor going short.Unfortunately this process leads to slowly decreasing capacitance over its life - faster if it has a rough life (lots of mains transients, hot running etc). It is better that it does this though than going short circuit and burning out the motor winding.These capacitors are built to a price too, some name brands are better than others, but they all ultimately have a service life. Luckily they are normally easy and cheap to replace.Having said all of the above, a drop from 16uF to 4.6uF probably means that the end metallisation to a large proportion of the wound film has suddenly failed, maybe a duff one.P.S. Motor-start capacitors (bipolar electrolytic) will getveryupset in motor stall situations as they are intermittently rated and will quickly cook!

author: beanhauler
author_rank: Contributor
timestamp: May 04, 2021, 01:09:30 pm
content: Examining the outer container revealed some minute cracks.Out with the hacksaw and slowly peeled the layers to inspect them.Appears that the film layer has a lot of crazing on the surface which looks like moisture which has dried and probably affects the surface area of the foil.The patterning of residue probably accounts for 75% of the surface area which would make sense for a degradation from 16 to 4 ish.No burns were apparent or shorts.2 pics. one showing a wide of the foil and another on the microscope showing the residue.nb Reference to cheap is relative to income but definitely cheap reference to motor replacement.£15.00 is still a lack of three beers ish.

author: Gyro
author_rank: Super Contributor
timestamp: May 04, 2021, 04:41:10 pm
content: Yes, if it has cracks then moisture ingress is always possible, the epoxy seal doesn't look great around the leads either. That second magnified photo (nice photo) looks like self-healing on a grand scale though (film punctures).Ducati (yes, them) make good motor capacitors at a sensible price, LCR too. Epcos are very long lived, but expensive. Better capacitors have automatic pressure disconnect mechanisms built in, in case of over-pressure (swelling).

author: beanhauler
author_rank: Contributor
timestamp: May 05, 2021, 08:38:38 pm
content: Thank you for your comments.This motor actually runs my koi pond filtration and its not until we have a jolly power cut that you find it wont start again.Given the cheap price of pic micros these days you would have thought that someone would have made a current /voltage phase sniffer which learns the phase offset when new and monitors when it changes or the effective starting torque drops off.