Online shopping: how do I find 'run capacitors' for induction motors?

[Whales]

I'm having trouble working out which capacitor to buy as a replacement 'run capacitor' for an AC induction motor.  The datasheets and descriptions for many online items seem ambigous as to what the capacitors can actually handle.

Questions:

• Is the capacitor I am trying to replace a 'run capacitor'?
• How do I determine whether or not a capacitor suits this duty cycle via its datasheet (if it does not directly state that it can be used as a 'run' capacitor)?
• Should I only be looking at round/oval capacitors or are some of the box-shaped ones also capable of 'run' duty?

Background

A big-arse oil-filled cap in the front of my grandmother's industrial sewing machine recently blew up spectacularly, complete with flames.  Luckily it looks like the flames were not sustained:



Here it is mounted near the power switch (switch contacts and spades are clean, no evidence of damage to them):



Note the leaked oil.  I gave my hands a good wash after touching it -- it's probably from the days of PCBs.  Next time I'll wear gloves.

Top wiring going to motor (4 wires): 2 x active, neutral direct, neutral through capacitor.  Mains enters through the bottom and a double-pole switch (partially shown) bridges L and N from top to bottom.

It looks to be an 11uF cap, given the '11' on it and the '11uf' written on the motor:



Based from what I can read from the internet: the size of the capacitor and its capacitance rating suggest it is a 'run capacitor'.  Please correct me if I'm wrong.

As I understand: for single-phase induction motors we generally use up to two types of cap:

• 'run' caps, which are wired in series with a second winding and must survive full duty cycle
• 'start' caps, which are only used for a short time on startup of some systems

For something nice to look at, here is the whole sewing machine:



Looking online

I have been looking on Element14 and RS-online a replacement capacitor.  Although many capacitors look to be physically the right size, details on whether or not they can survive full-duty as a run capacitor are often shady.

Examples:

KEMET  -- Datasheet suggests you use it "in series with a start winding" but later says "Type of Service: Continuous" and gives lifetime in hours of operation.  Misleading?
LCR Components -- Datasheet calls it a "motor run capacitor", but is only a single page long and gives no stats on duty cycle, expected lifetime, etc.
RS Pro -- Have a look at the datasheet for this item if you want a laugh  (I have reported it to RS).
Ducati -- No hint of whether they can survive full duty cycle or not from the datasheet.
EPCOS -- Clearly states that it's a run capacitor.  Direct and to the point.  Most expensive of the options shown here.

I don't want to default to the most expensive item in this list, because:

• That's giving into FUD
• I want to buy atleast a couple of different caps to try out (1) how they physically fit and (2) whether or not a 10uF or a 12uF works better.

Regards, Hales

[tautech]

Couple more bits of info if you may.

How many wires run from the cap/junction box to the motor?
Is there any indication of the telltale click of electromechanical contacts when the motor starts, indication of which would indicate it's a start cap.



For a 400W (~1/2 hp) motor an 11 uF cap IMO will be a run cap.
These are always more expensive due to the 100% duty cycle they must endure.

[Gyro]

All the modern metalized polypropylene run capacitors are rated for continuous run duty. The very old one that failed is probably a paper-in-oil, so the equivalent new one will probably be physically smaller. It's only motor-start capacitors that have short duty because they are bipolar electrolyics and so small for their capacity.

Even if the capacitor that you are replacing was being used as a motor start one, an equivalent value motor-run capacitor will still happily work in its place.

Hope this helps.

P.S. You ought to be able to get an 11uF, but if not, probably go with 12uF as they tend to lose capacity slightly as they self-heal from spikes.

[Whales]

How many wires run from the cap/junction box to the motor?

See original post, just before the picture of the motor.

Is there any indication of the telltale click of electromechanical contacts when the motor starts, indication of which would indicate it's a start cap.

Apologies, I didn't try turning it on without the run cap.  I don't think it would have spun without this to do phase-shifting anyway.

P.S. You ought to be able to get an 11uF, but if not, probably go with 12uF as they tend to lose capacity slightly as they self-heal from spikes.

Why would the original designers have not thought of the same thing and gone for a slightly larger cap than necessary? 

Neither RS or E14 stock 11uF caps.  I'm not so sure that 1uF will make a big difference to the current draw/torque characterics. As it is I'm feel like I'm being over-conservative by considering buying both a 10uF and a 12uF.

All the modern metalized polypropylene run capacitors are rated for continuous run duty.

Many of what I have linked above do not call themselves 'run' capacitors.  Do they have to specifically state it, or am I being overly-cautious?

[SeanB]

12uF 440VAC motor run capacitor will fit in there, and all you will need is some spade connectors for the terminals. Go to the local electrical wholesalers or industrial supplier ( or phone them) and ask for a 12uF motor capacitor. They will have them in stock off the shelf.

I keep a few 50uF, 2uF, 4uF and some 2.2uF capacitors in stock always, as they commonly fail after around 5 years in aircon use. With that one you could even use a lighting power factor correction capacitor, which for a double 4ft fitting is either 8, 10 or 12uF, and which has wire leads.

[Gyro]

Yes, you may be a little overly-cautious, but that's no bad thing. All the caps you linked look suitable for motor-run application (the threaded stud at the bottom is a giveaway too), they're all metalized PP with 450VAC rating. The white ones in particular include more details of approvals and working life vs temperature.

Why would the original designers have not thought of the same thing and gone for a slightly larger cap than necessary? 

Because 11uF is the optimum value. What I'm saying is that self-healing metalized PP capacitors slowly lose capacitance, so going slightly over is probably better than already starting under. The original capacitor was clearly not a self-healing type (possibly oil-paper-foil). Loss of capacitance is the usual reason for replacement, modern PP ones don't usually fail in any dramatic way.

You can find 11uF motor-run capacitors on ebay. I know you want to be cautious, but as long as you pick a seller who specialises in industrial type parts (check their other items) and avoid 'no name' ones then you shouldn't go far wrong. These are commodity parts for repair of industrial and white goods motors. Buyer beware as always of course.

[uncle_bob]

Hi

I'd second the "go to the local store and get one" recommendation. Also consider that the motor may have issues other than the cap. Put another way, a short in the motor may have caused the cap to blow up ....

Bob

[SeanB]

Never had a motor short blow up the cap, but plenty the other way where a shorted cap cooks the motor. Low value cap will eventually get to a resonant state with the start coils and make a very large ( I measured over 800VAC on the one compressor motor with a failing cap, which was still running but not doing a good job of cooling and tripping out on thermal overload, changed cap and cooler again) voltage across the cap. Not great for the winding if not protected, but the motor having poor performance would have been noticed in a sewing machine.

[CatalinaWOW]

Local sourcing is good.  If no local sources, electronics distributors are not the place to go.  Try plumbing supply, air conditioning supply and industrial supply houses instead.  (Don't forget to check if those kinds of places are available locally.)  I have also had good luck with Ebay and Amazon for these things.

By the way, nice machine your mother has.  Those solid things of that era will outlast our civilization.

[SeanB]

Locally I know of 6 places I can buy industrial capacitors off the shelf.  Should not be difficult to call and find them in your area.

[ChunkyPastaSauce]

Go to a local HVAC supply house, they'll probably have an equivalent

[bills]

Motor run caps are  + or -20% so 10 or 12 will be just fine.
try amazon  http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=10+uf+motor+capacitor&rh=i%3Aaps% 

or ebay  http://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=10+uf+motor+capacitor&_sacat=0

[Whales]

Thankyou to everyone.

Because 11uF is the optimum value. [...]  original capacitor was clearly not a self-healing type

Ah, I follow now.

By the way, nice machine your mother has.  Those solid things of that era will outlast our civilization.

It sews at an amazing speed and sits atop a bed of oil.  Anything that tilts up to reveal a pond of liquefied dinosaur must (at least subtly) be hinting about its age and prowess.

Edit:


I'm no mineral oil expert, but I have a suspicion it's supposed to be a little more transparent.

@everyone suggesting I buy from a local supplier: I'll look some up. 

[Gyro]

Is that an oil strainer and oil pump I see? It's really a wet sump sewing machine??? 

[rch]

Is that an oil strainer and oil pump I see? It's really a wet sump sewing machine??? 

It looks as though it might be due a 1,000,000 stitch oil change.

[Whales]

Yes.  Pump and all.  It probably needs a bi-millennial cleanout.