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Replacing motor start capacitor(s).

rons

Diamond
Joined
Mar 5, 2009
Location
California, USA
I hate it when a start capacitor (the alum foil type) blows up like a party popper.

Questions:
Have you replaced equivalent rating for capacitance and voltage?
Or do increase voltage rating, say 250VAC to 330VAC?
Who is a good source for them. I don't entirely trust on-line credit card transactions.

In my motor operating book the installation section says that a motor is not to remain in stock for more than 2 years before the start capacitors have to be replaced. So I wonder if all the old start capacitors in surplus stores are any good, or do they just need to be formed.
 
One other factor to consider is case size. Many newer units are smaller in size than ones used on older motors.
That case size can limit what is available in terms of voltage rating. You probably want to check, that it fits the old enclosure snugly.

Grainger has a good selection, if you prefer local.

It's always a good thing to check the starting switch, and the starting load, when one goes pop. Hard starting is a major reason.

SAF Ω
 
Even the best of them WILL fail eventually (I assume you mean the "aluminum electrolytic" type). There is no reason that there must be something wrong for it to happen. But it is sensible to be sure that there is in fact no other problem.

They are actually severely overloaded during the start, and they heat up during the start as a result. That's one reason they may be rated in number of starts per hour. If already hot, and another start occurs, the capacitor will just get even hotter. You don;t want them too hot, because that makes them fail fast.

Over time, that heating degrades the part, and they will fail, one way or another. Might "open up", as in conduct no current, or they might toss their cookies all over. It just depends what specifically fails inside the part.
 
As JST described, heat is the most common failure mode from frequent start/stops with electrolytic capacitors. They do have a shelf life, one reason why power supplies that use electrolytic capacitors that sit on the shelf for years, will often go up in smoke shortly after putting them into service. Usually one wants to power up the capacitors incrementally, reforming (capacitors with high levels of DC leakage) in my experience doesn't work very well for old (like 5+ years) electrolytic capacitors, but is recommended for bringing on equipment that has been powered down for a long time or reconditioning capacitors. Shelf life of electrolytic's various by manufacturer and capacitor specifications, I have seen anywhere from 2-10 years. Going to a higher voltage capacitor would not significantly affect the longevity over the stock voltage, vs. buying a better quality capacitor. There are temperature ratings for capacitors, so buying one with a higher temperature rating say 105C vs. 85C would be a better choice. Less chance of the capacitor swelling and the electrolyte boiling off.

Often AC repair, motor shops and pool repair shops have a wide range of motor capacitors. I have no issues with buying them online either, providing they are not some no-name generic capacitor. On single phase AC motors, such as AC units and equipment, I just plan on replacing the capacitors on a regular bases. I frequently see the start capacitors fail on a lot of Chinese motors in mills and lathes, very poor quality.
 
A higher voltage capacitor should last longer, on the order of square of the increase in voltage.

For example, recently I used a 440uf 110vac motor start capacitor to provided 11 amps or so of reactive current to offset my diy generator welder, open circuit about 60vac, it lasted about half an hour before it got hot enough to push the oil out the vent port. which would not have happened if it wasn't lying on its side. it was also a used capacitor, iirc it was previously used on a 1 hp random orbital floor sander which was thrown out, so it probably had a lot of use. but never enough to actually heat it up, aside from the fact it was mounted directly to the side of a 67% efficient 1 hp motor. (no, really, that's whats on the nameplate. 73% power factor iirc)


Anyhow, the current flowing through the capacitor is what heats it up, and the heat is proportional to the amps squared, and the amps are proportional to the voltage.

then there is the problem of the oxide layer breaking down. you can't apply dc to reform it because the negative plate will get worse.. while the positive plate reforms.


but the equivalent series resistance is what's responsible for them exploding.. and running them at lower voltages reduces the heat generated substantially.
 
Over time, that heating degrades the part, and they will fail, one way or another. Might "open up", as in conduct no current, or they might toss their cookies all over. It just depends what specifically fails inside the part.

The manufactured dated code was Oct 2004. I remember seeing a life span chart listing`number of hours not number of starts.

This is the capacitor. There are two in parallel.

161-193 MFD 25 Volt WEG Start Capacitor
 
Grainger has a good selection, if you prefer local.

I might buy four 90uF oil filled ones at Grainger for the equivalent capacity of 2 x (161-193uF). The motor capacitor enclosure is a huge on. If I can only not mutter "ripoff joint" as I enter the front doors at the Grainger store.
 
A thought if you have lots of space: motor run capacitors may be made to higher standards of quality (but they tend to cost more and take up more space)- unless some of the other people here who have a great depth of knowledge in these matters think that the run capacitors are less-well-suited to handle the short-duration-high-stress that a start cap has to.

I'm interested in knowing because my 7.5hp RPC's start capacitors are of way unknown source and age but are probably well over 10 years old and I want to do something about them before they go kerblang/kersplat within the switchgear housing, which will not be pretty or easy to deal with (and could wreck some of the relays). I have been looking at using a bank of run caps in a separate housing (because I will need a batch of run caps to add up to the same uF as the pair of existing start caps), since I won't be able to get run caps to fit in the main housing. Thanks
 
I might buy four 90uF oil filled ones at Grainger for the equivalent capacity of 2 x (161-193uF). The motor capacitor enclosure is a huge on. If I can only not mutter "ripoff joint" as I enter the front doors at the Grainger store.

I'd not bother with "oil filled" these days at anything under 600 Volt, working.

Better off, money-wise, using a commercial HVAC industry specific source. They consume these goods by the ton.

I've used one in Florida that has an ebay presence. Decent caps, 400+ V, US-made, even. Decent costs. Rapid shipping.

Been some years now since last I bought, so best to search for yerself and be "current".
 
Monarchist, I am going the 3 phase route. The first skein for the start wiring is black. But it does ohm out ok with no shorts. It appears that over time it just got screwed.
 
Monarchist, I am going the 3 phase route. The first skein for the start wiring is black. But it does ohm out ok with no shorts. It appears that over time it just got screwed.

Be sure to check the head pressure unloading system for functionality. If a check valve is leaking or a vent valve is non functional, a 3Φ unit may not fare much better. Bad valving leads to bad motors on piston pumps. No motor will last long, with a non functioning unloader, unless it's oversized for the application, be it 1Φ or 3Φ.

It's a pretty common problem on piston pumps, those valves need some maintenance over time, or the motor suffers.

SAF Ω
 
Be sure to check the head pressure unloading system for functionality. If a check valve is leaking or a vent valve is non functional, a 3Φ unit may not fare much better. Bad valving leads to bad motors on piston pumps. No motor will last long, with a non functioning unloader, unless it's oversized for the application, be it 1Φ or 3Φ.

It's a pretty common problem on piston pumps, those valves need some maintenance over time, or the motor suffers.

SAF Ω

There are two small pipes connected to the heads, one connected to the first stage and one connected to the second stage. They are bent around the pump and connect to a hub on the opposite side of the pump flywheel. There is a third pipe connected to the hub cap that is bent downwards and it appears to be the exhaust pipe. When I turn the pump flywheel by hand air exits the exhaust pipe.

Should I unbolt the three bolts that connect the hub cap and inspect?
 
Ron, there are many types of unloader systems, and they are similar but different.

Made of 2 basic parts.
1st a check valve to hold back tank pressure from the compressor head or pistons. Some check valves are in the tank or line to the tank, other brands may use the valves in the head for the check valve.

2nd a valve of some type to bleed off pressure in the line between the head output and the tank check valve.

It sounds like you have the centrifugal flyweight unloader on the pump crankshaft. A valve, usually a Schroeder type or similar that opens and closes with crank speed, just like a motor start switch. It bleeds off the head pressure when the crank is at zero speed, and closes when the crank attains speed.

The system is best tested when the unit is cycling, but can be checked for dirt, corrosion or worn parts before put into service. With the system at rest the centrifugal valve should open the head output ports to the atmosphere or inlet line.

When the system is running, the valve should close, to retain pump pressure output.

When the system shuts off, the valve should open and hiss for a moment, while bleeding off head pressure.

The check valve should block reverse flow of tank pressure, in the discharge line, back to the pump at all times.

There are other types of systems, and one is a continuous run unloader, where the motor stays running, and a special unloader valve senses tank pressure, and applies a pressure line back to the heads to actuate some unloader diaphram actuated output valves. The diaphram valves hold open the output valves, and the air just recirculates inside the head chamber, until the unloader calls for more output pressure.

To help further, you would need to show some photos of the unit piping, valves and a model type. There are many styles.

A link to a couple champion manuals, they use a system that sounds like the type you have.

[URL="www.morseindustrial.ca/client/R10_R15%20Manual.pdf"]
[/URL]
http://www.liberatedmanuals.com/TM-5-4310-358-14.pdf

https://www.aircompressorsdirect.com/stories/405-How-to-Access-Old-Air-Compressor-Manuals.html

The air compressor unloader valve

Air compressor unloader valve | Air Compressor Guide


And a description from Curtis on various types.
Curtis-Toledo-Compressor-Unloading Manual.jpg

SAF Ω
 
To help further, you would need to show some photos of the unit piping, valves and a model type. There are many styles.

SAF Ω

I will accept your offer. Thank you. I can get some pictures tomorrow. Your help is appreciated.

Ron
 
Is this Woodhead 67W81 outlet suitable for the output of a RPC idler?

View attachment 211584

Up to its 20A rating, seems so.

Sez 4 "poles", 5 wires, eg; good if you've put a Delta-Wye transformer on the output and re-derived a local Neutral.

Ordinary Delta output, no Neutral, one less conductor and contact would be more appropriate.

JM2CW, but what with a "mix" of Delta and Wye here, I'd not want the possible "surprises" of cordsets and receptacles being agnostic.
 
Is this Woodhead 67W81 outlet suitable for the output of a RPC idler?

Possibly, but probably the wrong part for your 240V delta output, for a run of the mill RPC.

That model # is for a 208Y with 5 wires, I think what your looking for is #67W75 Intended for 240V delta and 4 wires @ 20A. Contains a NEMA L15-20 receptacle.

However L15-20 receptacles are only HP rated at 3HP @ 240V, if that rating fits the bill, should be fine.

See this link for a Leviton L15-20R HP rating, Leviton also makes the watertight cover/recpt with identical part #s.

2421

SAF Ω
 
Possibly, but probably the wrong part for your 240V delta output, for a run of the mill RPC.

That model # is for a 208Y with 5 wires, I think what your looking for is #67W75 Intended for 240V delta and 4 wires @ 20A. Contains a NEMA L15-20 receptacle.

However L15-20 receptacles are only HP rated at 3HP @ 240V, if that rating fits the bill, should be fine.

See this link for a Leviton L15-20R HP rating, Leviton also makes the watertight cover/recpt with identical part #s.

2421

SAF Ω

Since my compressor 5Hp motor will cycle on/off the lower rating might be ok?
 








 
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