Motor Run Capacitor - Air Compressor Replacement Value discrepancy? - Page 1
Forum section: electronics / repair

author: CaptainObvious
author_rank: Contributor
timestamp: May 12, 2016, 07:20:26 am
content: I made one of those "good" Craigslist purchases, bought a 4hp peak 10 gallon compressor that was known to pop breakers for $20 USD. You know how it goes... you fix one with a very similar issue, so you automatically assume the next one you try to fix will be the same issue! Not this time around, damn you Murphy! My previous repair consisted of an old compressor we've had for years.. started popping the outlet breakers after a short time running (Range from 30 seconds to 5 minutes). After about 15 minutes of looking for obvious stuff, I ended up removing the power cable from the outlet and you can visibly see the ground pin was loose. Long story short, spliced a new AC plug on there, viola! Haven't had a popped circuit on that old compressor.So here we are with today's purchase! Here's a link to the manual for this compressor, just a cheap-o harbor freight air compressor:http://www.about-air-compressors.com/support-files/90234.pdfSYMPTOM:Central Pneumatic Model 90234 Air compressor will start without any issues, runs for a short amount of time (anywhere from 30 seconds to 5 minutes) and the "breaker" ("Overload button", per user manual, everyone online calling it Thermal Overload Protection) pops. I'm able to push the "reset" on the "Breaker" ("Overload button", per user manual) and it will immediately start back up, again running for a few minutes, then shutting off.ATTEMPTED:I checked continuity from earth on the motor chassis (and compressor chassis) to the earth pin on the AC plug, it IS showing continuity. I replaced the cable anyways, because the ground pin felt fairly loose, it did appear to extend the amount of time that was running by a small amount, but I'm not sure if that was me assuming, because that was my first fix attemptIt's plugged directly into the wall, no extension cords/power strips, etc, and also tried a breaker with no other items plugged in, same effectI thought maybe a pressure valve was faulty? (I have little to no knowledge on motors/compressors, just my assumption from research!) So I attempted to run the compressor with the valve open, as to not build up internal pressure, same effectChecked the oil, it does look a little dirty, but the oil indicator does show full on oil... so I left it as is, did NOT replace the oil yet.So that brings me up to my next question(s)! After digging online for a bit, I did read that the run capacitors could potentially cause this issue as well, so hell.. I figured maaaybe I'll just buy a replacement (~$15-25) and see if that helps it, don't want to spend much more money on it, more of a "oh I think I can fix this easily, if not... learning experience!". I could not find an exact match for the capacitor uF / Voltage specs in the same configuration.QUESTION:First... do you think I'm going down the right path with the capacitor at this point? Other easy potential fixes? And last, assuming we're past the first two questionsam I safe to use a 60uF 400v to replace a 65uF 250vruncapacitor?I read that you want to keep the capacitance values exact for the motor winding... going too high or too low can cause all kinds of shenanigans being misaligned, etc. but I'm not having much luck on Amazon for an exact match, any insight would be greatly appreciated!Here's dual cap with the exact uF value, could I just use this without the 5uF cap in circuit?:http://www.amazon.com/CAPACITOR-440V-450V-ROUND-Certified/dp/B00NTOFEP0/ref=sr_1_2?ie=UTF8&qid=1463035729&sr=8-2&keywords=65+uf+run+capacitorHere's the round cap (looks closer to the current cap) that is 5uF less:http://www.amazon.com/CBB65-Conditioner-Compressor-Running-Capacitor/dp/B008AGMQ6C/ref=sr_1_1?m=A1THAZDOWP300U&s=merchant-items&ie=UTF8&qid=1463036468&sr=1-1&keywords=65+uf

author: tautech
author_rank: Super Contributor
timestamp: May 12, 2016, 08:23:37 am
content: Start and run caps are IME quite different values, the run cap normally 1/2 or less of the start cap.It could be the motor is damaged but since it's an open frame type it's unlikelycookingodours will still remain to be sniffed.Thermal Overload Protectionbreakers do tire with repeated openings but to be sure that's not the cause you'd want to put a clamp ammeter on the supply and determine just what current the motor is using.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 12, 2016, 03:20:25 pm
content: If it were mine, I'd try the 60 uF 450 volt unit. The tolerance of both the old and the new capacitors is +/- 5 % which is about 3 uF which means that there's a good chance that the new unit will have exactly the same or very close to the capacitance as the old unit had when new."Is it safe?" Almost certainly. "Will it fix the problem?" Less certain, see tautech's answer above.But first I'd try to measure the old capacitor to make sure it's actually bad.

author: kleblanc
author_rank: Unknown
timestamp: May 12, 2016, 03:40:08 pm
content: I would check all of the caps , I would lean towards the start caps causing the issues, locked rotor drawing the surge amps for to long.

author: Kevman
author_rank: Regular Contributor
timestamp: May 12, 2016, 07:01:48 pm
content: An air compressor with a motor like that... Its amazing it runs at all.Anyway, shouldn't there be a centrifugal switch on the motor that takes the start capacitor out of circuit once the motor spins up? If that's stuck then you will get the exact problem you are having. And I've seen them get stuck.

author: CaptainObvious
author_rank: Contributor
timestamp: May 12, 2016, 07:54:48 pm
content: Appreciate the insight so far everyone! I don't have a clamp meter.. I've been meaning to buy one for general around the house shenanigans, so I ordered one today (Paid $4 overnight shipping, thanks Amazon!) so I'll be getting it tomorrow! I'll test the current as it's leading up to popping it's thermal fuse, and see if that provides anymore details.@TautechThe motor doesn't seem to feel too warm to the touch, or smell burned... but to be fair, who knows what the last owner did or did not do! One thing I've read with these compressors, is people bypassing the thermal overload switch all together and just running it, I was tempted to do this to see if it pops my circuit breaker too. But alas, I wasn't thrilled with venturing into the unknown without asking a more knowledgeable bunch.@alsetalokin4017That was my initial hunch, that they would be very close due to the tolerances, I just wasn't sure how far the tolerance should really be off with motors. You know how it is when you're looking for information and not very knowledgeable on the subject, it's easy to get sucked into misinformation/over complicating stuff when you're reading! I'll be yanking the capacitor tonight and testing the capacitance, I only have a low-end ExTech 430 but should be sufficient. (of course, after discharging said capacitor)@kleblancThe only reason I didn't really think it was the start capacitor, the motor seems to start with no issues (no sputtering/bogging down, etc) and run for up to 5 minutes. I can constantly reset the thermal fuse to keep it running. I was under the (limited) impression that the start capacitor leaves the circuit when the motor begins to turn. (Potentially the centrifugal switch on the motor that Kevman mentioned, more below)@KevmanNo comment, good ol Harbor Freight for you! In regards to the centrifugal switch, I did read that as potentially being the issue as well. My thought on that though... if the start capacitor was stuck in the circuit, it would have (visibly?) blown long ago, correct? After reading/watching videos about these capacitors, start capacitors generally have a very low duty cycle and will blow if in circuit for more than a few seconds. (I assumed this after watching this TEMCo video)Thanks again guys, I'll be checking the capacitor tonight and report back, then tomorrow I'll be testing the current!

author: bills
author_rank: Frequent Contributor
timestamp: May 13, 2016, 01:23:33 am
content: Be sure to check the check valve if it is bad the compressor will have start problems.# 57 on parts list.

author: Kevman
author_rank: Regular Contributor
timestamp: May 13, 2016, 01:51:46 pm
content: Quote from: CaptainObvious on May 12, 2016, 07:54:48 pmI know a guy that ran a hot tub pump with a stuck starter switch for months. The only symptom was an insane electricity bill. Its life may be negatively impacted, sure, but there's nothing that will cause it to pop.

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2016, 12:53:42 pm
content: Those motors typically rely on the run capacitor to keep them running, a low value run cap will result in your symptom of high current tripping the overload in a few minutes. First change the run capacitor, preferably with a plastic cased unit, as that has the important M8 screw on the bottom to hold it to the chassis, and which should have the right connector for your use, on compressors typically a 2 wire cable coming out to the control box. Spade version in a metal case is for AC use, where it is in an enclosed part of the unit, as live terminals are exposed on it.First thing to fail is the capacitor, and if that does not cure it then change the thermal overload, as prolonged operation of those will cause them to fail as well from eroded contacts.As well do not switch the compressor off and on from the wall outlet, use the switch on the pressure valve, as that vents the outlet of the compressor when it reaches set pressure or is turned off, so the next start does not have the added load of trying to start against the existing air pressure in the tank, only having to start the unloaded compressor so it is up to speed before it has to start pumping into the tank. Switching at the wall means the pipe stays at full pressure, and this will trip the overload almost immediately as the motor cannot turn over against the load, it has too little power to start up against the pressure pushing down on the piston.

author: johansen
author_rank: Super Contributor
timestamp: May 14, 2016, 02:38:40 pm
content: there's no guarantee the manufacturer designed it correctly.basically you need to increase or decrease the run capacitor until the motor pulls a minimum of current from the line.the problem is, you need to verify that the power lost in the start winding is a safe level. so basically you need three amp meters. one on the line, one on the run winding, and one on the start winding. and you need to measure the resistance of the start winding so you can calculate a safe current. (or just measure its temperature with a thermocouple buried between the start winding and the run winding, at the hottest location you can find.)adjust the run capacitor size to get the lowest current on the run winding, without exceeding a safe level through the start winding.

author: CaptainObvious
author_rank: Contributor
timestamp: May 20, 2016, 02:53:16 am
content: Just an update (a delayed one, at that! Travelling for work lately..) I tested the current draw through the thermal cutoff while it was running and was running on average about 13.5 amps, slowly climbing to 14, and that's when it seemed to cut off, right around 14 amps. Ran it a half a dozen times, each cutting off around 14. Always using the power switch on the compressor to power it off and back on after the reset, of course!Tonight, out of curiosity, I bypassed the cutoff all together by just bridging the contacts and it seems to run just fine! Never climbs above 13.5, maaaybe 13.6 amps, and I'm able to build pressure and run it continuously without the continual shutdowns, requiring me to reset the cutoff.Seeing as this is a "thermal cutoff", and not a "circuit breaker", am I safe to rely on my houses 15 amp breaker forpermanentuse? (any safety issues?) Or does it make (safety) sense to replace this cutoff? Main reason I'm curious, I don't see any temperature rating on the cutoff... wasn't sure exactly what to look for and how the temperature ratings come about!I suppose I could call Harbor Freight, but I have a hunch I'll get "they don't make parts for that" anymore.Thanks again for the insight everyone. Maybe it won't be a semi-wasteful purchase after all!

author: johansen
author_rank: Super Contributor
timestamp: May 20, 2016, 03:59:39 am
content: Quote from: CaptainObvious on May 20, 2016, 02:53:16 amIts a circuit breaker, not a thermal breaker, unless its laced into the windings or senses the temperature of the motor through some other means.I'd give you 50/50 odds the motor will blow before the breaker does in the event something goes wrong and the motor starts pulling more than 13.6 amps.

author: CaptainObvious
author_rank: Contributor
timestamp: May 20, 2016, 04:27:00 am
content: @johansenThanks for the clarification, and that's where some of my confusion came from.. reading those pesky internet forums and misunderstanding or getting the wrong information, haha. Intuition lead me to believe the same thing, I figured well it's only 2 wires... and technically only 1, just in series with the AC, so it had to be a breaker!When you say 50/50 chance the motor will blow before the breaker, do you have an idea what should I be expecting for the average amperage? Does 13.5 seem high, or are you just implying the motor would likely fail from drawing more than 13.5 amps, and fail before the power actually exceeded (and long enough to trip the breaker) the breakers rating?It's "rated for 3 HP" and "peak 4 HP" (I do quotes because marketing?) per the manual, and using Google's Watts to HP calculator, I calculated the 13.5 amps~ * 120V = 1620 watts = 2.172456 HP, so in my eyes the motor is running under the specs? I did see it mentions mechanical watts... but even searching for electrical watts, i get the same results, implying I should be safe around 13.5 amps, and technically even the full 15 from my breaker? But there would have to be a reason they put a 15 amp breaker on it... Sorry for the questions, guess that's why I'm here, to try understand!

author: Kevman
author_rank: Regular Contributor
timestamp: May 20, 2016, 02:02:11 pm
content: Seems high to me, still, given the physical size of the motor. Only real way to be 100% sure is to compare with a working unit of the same.The motor on my air compressor is a 'true' 3hp nameplate unit, and its rating is 26A @120v. This is inline with what 3hp motors are listed online for Baldors and other industrial motors. So there's no way that's a 3hp motor you have there- its just marketing BS.I would also be surprised that they would put a 15A breaker on something that draws 14 in operation. Its just too close.

author: johansen
author_rank: Super Contributor
timestamp: May 20, 2016, 06:07:31 pm
content: Quote from: CaptainObvious on May 20, 2016, 04:27:00 amIt takes more than 15 amps to trip a 15 amp breaker usually... sometimes as much as 20 amps for a few minutes.the question is, does the motor fail once it starts pulling enough current to trip the breaker. copper losses in the motor run with amps squared. if your motor windings are hot now, at 13.6 amps.. they will be very hot by the time the breaker trips. and they will be even hotter years from now when the fan clogs up with dust.the simple fact is these small induction motors are just large enough not to burn out, under moderate usage when they ship them from the factory. when the run cap fails they may burn out later due to the increased amps in the run winding as a result. when the motor clogs up with dust, they fail by overheating.

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2016, 09:03:50 pm
content: Stick with the house breaker for now.Just because the motor is rated to a higher HP that it's using, don't worry.It's been selected for this compressor and it's typical duty rating.....that's not to say that prolonged running won't overheat it. Prolonged dusting, die grinder, air drill etc will be the test to see if your can still place your hand on the motor. (turned off of course)Sounds like you might be good to go....just keep an eye on it.