Motor run capacitor reads 9.25uF instead of the rated 10uF. Is that too low? - Page 1
Forum section: electronics / beginners

author: Jay112
author_rank: Regular Contributor
timestamp: December 21, 2015, 02:06:56 pm
content: I searched online for the past 20 minutes about this, but couldn't find any answers about how low is too low for a motor run capacitor.My furnace blower stopped working last night. The heating works fine, but the blower never turns on to blow the heat through the ducts.Testing the motor capacitor with my multimeter shows 9.25uF, whereas the capacitor itself says it's rated at 10uF(-5~+10%).Is this low enough to consider it a significant problem, and a likely cause of the blower failure? There are no bulges in the capacitor, and physically it looks normal.

author: Jay112
author_rank: Regular Contributor
timestamp: December 21, 2015, 02:21:17 pm
content: I finally found at least a little info on this:"As someone mentioned above,10% diff. of mf rating on start caps is ok.Run cap mf rating should be the same as original."http://hvac-talk.com/vbb/showthread.php?144742-capacitor-quot-too-hi-or-low-enough-quotSo it looks like the cap probably is too weak.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2015, 02:36:45 pm
content: The value is slighly out of spec from the reading of the cap. However the motor should still run with 7% lower capacitance.So I would expect something else to be bad. However it would be still a good idea to replace the capacitor once the motor is working.My experiance from Compressors is that they run until the capacitance is down to about half the nominal value. This is starting with a load - the fan is more like running free at low speed.Once the capacitance is reduced significantly the voltage at the capacitor can increase - so chances are the step from somthing like 20% capacitance lost to breakdown is rather short, especially if the voltage rating is not very high.

author: Seekonk
author_rank: Super Contributor
timestamp: December 21, 2015, 05:28:21 pm
content: You are definitely barking up the wrong tree. This sounds like a control issue. Most thermostats will have a fan run switch. Try that first. Then try looking at the color codes of the motor. Choose appropriate hot lead, disconnect it from controller and power it directly as a test. Some furnaces will have a plenum temp sensor that powers the motor when the chamber is up to temperature. That may be open.

author: Jay112
author_rank: Regular Contributor
timestamp: December 21, 2015, 05:36:18 pm
content: Thanks for the info and the help, guys!After hours of diagnosing, I found out that the motor itself was blown out. Before the problem started there was a terrible smell of burning plastic. All of the wires coming out of the motor have continuity between them, including the hot and neutral.I ended up buying a new motor that has the same voltage, amps, and RPMs, and it required a different capacitor rating, so I needed to get a new capacitor anyway. The unit is over 20 years old.Only one thing was strange though: Is it possible for a motor to burn out like that, but not show any visible signs of being burnt? I even opened it up all the way, but still didn't see any char marks. But the thing definitely smells like it was burned. The wires on the inside were taped together with a strange yellow tape, so I was guessing maybe the burn marks are hidden underneath the tape?BTW, the motor itself said that it requires a 7.5 uF capacitor, but the capacitor connected to the motor was 10 uF. Is that strange? Could that have caused any problems? I wonder why it was able to run like that for 2 decades.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2015, 08:53:16 pm
content: With a larger capacitor like the 10µF instead of 7.5 µF the motor will run a little hotter than usual. Though this will not be that much extra power so it's possible to work for quite some time, but hotter than designed for and thus not as long as it could be with the right cap.The burned part of the motor must not be visisble - it could be just some burned isolation somewhere inside where you can't see it. Just a few closed loops can cause quite some trouble and even more heat. Using Ohms readings its sometimes hard to impossible to detect such a defect.

author: Jay112
author_rank: Regular Contributor
timestamp: December 21, 2015, 09:09:59 pm
content: Quote from: Kleinstein on December 21, 2015, 08:53:16 pmThanks for the help and the information!

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2015, 06:00:46 pm
content: Your old motor might have bad bushes, which would make it run slow and create extra heat in the windings. They would probably turn easily at low speed but would be noisy under load. But if you smell the crisp smell then it is dead.

author: retrolefty
author_rank: Super Contributor
timestamp: December 22, 2015, 06:08:05 pm
content: Check the motor windings resistance to see if you have an open winding.

author: Jay112
author_rank: Regular Contributor
timestamp: December 22, 2015, 06:21:32 pm
content: Quote from: retrolefty on December 22, 2015, 06:08:05 pmI tested the resistance following these instructions here:http://contractingbusiness.com/service/service-clinic-why-and-how-test-hvac-blower-motors, testing between the white wire and the 3 speed wires, and 2 of the 3 speed wires show open windings.The new motor and capacitor are supposed to arrive tomorrow. I'm hoping this was the only problem with the system.A 20-25 year lifespan for a motor that runs almost every day of the year is pretty good, isn't it? Especially when it had a 10uF cap but was supposed to have a 7.5uF cap?

author: Jay112
author_rank: Regular Contributor
timestamp: December 23, 2015, 10:14:21 pm
content: I tested the new motor separately, and it's working fine. I put it in the furnace, and it wasn't turning on. I saw that there's no power going to the motor.After further examination I discovered a burnt out relay. What do you guys think of these pictures? Does it look recoverable?I'm planning on testing the nearby components now, to see if those were blown too. If it's just the relay then I could probably replace it pretty easily. Maybe I'll have to solder on some connections for the blown traces on the PCB.

author: Jay112
author_rank: Regular Contributor
timestamp: December 23, 2015, 10:52:54 pm
content: I was able to desolder and remove the relay, but found that everything is covered with an annoying enamel coating. I wonder if that coating protected other parts from getting burned out too though.I ordered a couple of replacement relays from Digikey, as well as a used PCB from eBay. I might try to bypass the relay on my own, but otherwise I'll just have to wait for the parts to arrive.

author: Jay112
author_rank: Regular Contributor
timestamp: December 23, 2015, 11:34:26 pm
content: I ended up bypassing the broken relay with a 12 gauge wire, and now everything is working. The only problem is that the fan has to be on constantly, but at least I have heat now, so now there's no more urgency. When I get the new relay in next week I'll try to pop that in, but if that doesn't work then I'll have the used board from eBay as a backup. The used board was only $40, whereas a new board would've costed $140.

author: Seekonk
author_rank: Super Contributor
timestamp: December 31, 2015, 06:45:49 pm
content: I'll take my salad with a low calorie capacitor......I have a friend that manages a pipe weld fab shop. Another part of the companydoes commercial HVAC work. I drop in now and then and pull parts out of HVAC units.I don't have much use for them, but how can you pass up oil filled capacitors. Anice 55uF unit had on the label VEGETABLE OIL. I would have thought that wouldalways have some moisture in it making it unsuitable for this use.Anyway, most of the capacitors were 10uF and right on the money as far as capacitance.There was a GE that measured 8.2uF and a DiverseTech that measured 6.3uF even thoughthe spec was 6%. Even though these were scrapped units it is likely they still worked.It is hard to imagine a failure mode that would cause this reduction in capacitance.Likely this was an error in manufacturing.

author: SeanB
author_rank: Super Contributor
timestamp: January 01, 2016, 12:28:46 pm
content: Vegetable oil would be coconut oil, which can be purified to very low moisture insulating oil.http://www.elect.mrt.ac.lk/Coconut_oil_eru_2001.pdfPretty common in motor capacitors made in the far east these days, as the oil is plentiful and not subject to crude pricing too much.

author: johansen
author_rank: Super Contributor
timestamp: January 01, 2016, 08:32:45 pm
content: Epoxidized soybean oil is used in large transformers, castor oil is also used in capacitors.Sent from my SM-G530T using Tapatalk