Is anything special about motor start capacitors?

[mikey9]

My dishwasher was failing, and I quickly found the problem was a failing capacitor on the circulation pump.
Marked as 2.5uF 400V. No problem, except not easily found locally. So I rummage around and find an old fluorescent lamp with a power-factor correction capacitor of the right value. Well, labelled as 3.5uF which would probably be near enough, but tests as 2.5. Normal to degrade with age? So I hook it up and Bob's your uncle. My question: is this a long-term solution? Why is "motor start capacitor" a special category? Does it matter if the case is metal vs plastic? If the temperature range and voltage spec is good, what potential problem is there?
The only thing I can think of is waterproofing, but the rest of the electrical/electronic parts in there are not waterfproofed.

 

[Julie.]

In terms of start capacitor, no nothing special, a run capacitor may be used in its place, however the reverse is not true.

Start capacitors, because they are only used for starting and are disconnected above ~75% normal speed are often only rated for intermittent use - due to losses (often the dielectric, electrolytic capacitors for example are generally only used as start).

Since the capacitor you found was for continous use, it should be acceptable.

However as it is from an aged unit, it may be near its end of life anyway, motor starting is quite a heavy duty for a capacitor so it may not last long.

A 2.5uF 400v capacitor for electrical appliances is easily available, try amazon it will be around £5 or A$22

For example in Australia

https://www.amazon.com.au/Start-up-Capacitor-Motor-25x51mm-Miflex/dp/B01CSXOMZA

 

[Julie.]

Sorry, should add

Is is definitely a start capacitor, and not a run capacitor?

If you are buying one, it is best to pick up a run capacitor just in case.

 

[mikey9]

In terms of start capacitor, no nothing special, a run capacitor may be used in its place, however the reverse is not true.

Start capacitors, because they are only used for starting and are disconnected above ~75% normal speed are often only rated for intermittent use - due to losses (often the dielectric, electrolytic capacitors for example are generally only used as start).

Since the capacitor you found was for continous use, it should be acceptable.

However as it is from an aged unit, it may be near its end of life anyway, motor starting is quite a heavy duty for a capacitor so it may not last long.

A 2.5uF 400v capacitor for electrical appliances is easily available, try amazon it will be around £5 or A$22

For example in Australia

https://www.amazon.com.au/Start-up-Capacitor-Motor-25x51mm-Miflex/dp/B01CSXOMZA

Thanks for your helpful reply.
I'd already ordered this: https://www.kogan.com/au/buy/agencyspareparts-capacitor-3uf-round-450v-metal-61mm-x-39mm-cap003m

For amusement, official part is over $100:

ASKO DISHWASHER D5434 capacitor |

ASKO DISHWASHER D5434 capacitor Australian Distributor | Appliance Spare Parts | Free Delivery | Same Day Dispatch Guaranteed | Parts List | Installation Tips | Exploded Diagrams | Trade Pricing |

spares.bigwarehouse.com.au

Actually, not so much amusing as disgusting, should be criminal.

Though it does appear that the metal capacitors used in dishwashers cost many times as much as other similar-specc'ed caps.
Looking at aliexpress, plastic motor capacitors are around a dollar, so something must be different.

Puzzles de course à moteur, ordinateurs, onglets de pelle, CBB60, 450V, 3UF, 4UF, 5UF, 6UF, 8UF, 10UF, 12UF, 15UF, 18UF, 20UF, 25UF, 30UF, 35UF, 40UF, 45UF, 50UF 450ylique - AliExpress 502

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

Puzzles de démarrage Itor, 450V0.8uF 1uF 1.2uF 1.5uF 1.8uF 2uF 2.2uF 2.5uF 2.8uF 3uF 3.5uF 4uF 4.5uF 5uF 450V CBB61 - AliExpress 502

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

I had to google "run capacitor", and see they do the same as a start capacitor in the sense of providing a phase shift to the auxiliary winding. It is not clear for what purpose. But this is just a simple small water pump, with one capacitor, so I assume only for start torque.
I can use a screwdriver to start the motor if the capacitor is not connected.

 

[Julie.]

Thanks for your helpful reply.
I'd already ordered this: https://www.kogan.com/au/buy/agencyspareparts-capacitor-3uf-round-450v-metal-61mm-x-39mm-cap003m

For amusement, official part is over $100:

ASKO DISHWASHER D5434 capacitor |

ASKO DISHWASHER D5434 capacitor Australian Distributor | Appliance Spare Parts | Free Delivery | Same Day Dispatch Guaranteed | Parts List | Installation Tips | Exploded Diagrams | Trade Pricing |

spares.bigwarehouse.com.au

Actually, not so much amusing as disgusting, should be criminal.

Though it does appear that the metal capacitors used in dishwashers cost many times as much as other similar-specc'ed caps.
Looking at aliexpress, plastic motor capacitors are around a dollar, so something must be different.

Puzzles de course à moteur, ordinateurs, onglets de pelle, CBB60, 450V, 3UF, 4UF, 5UF, 6UF, 8UF, 10UF, 12UF, 15UF, 18UF, 20UF, 25UF, 30UF, 35UF, 40UF, 45UF, 50UF 450ylique - AliExpress 502

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

Puzzles de démarrage Itor, 450V0.8uF 1uF 1.2uF 1.5uF 1.8uF 2uF 2.2uF 2.5uF 2.8uF 3uF 3.5uF 4uF 4.5uF 5uF 450V CBB61 - AliExpress 502

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

I had to google "run capacitor", and see they do the same as a start capacitor in the sense of providing a phase shift to the auxiliary winding. It is not clear for what purpose. But this is just a simple small water pump, with one capacitor, so I assume only for start torque.
I can use a screwdriver to start the motor if the capacitor is not connected.

The motor size doesn't matter, it depends upon whether the motor has a centrifugal switch or not (opens when the motor is up to speed).

If there isn't a speed switch, it is likely to be a run capacitor, not start.

Can't tell about the one you ordered, it doesn't give any details as to being continously rated, or the dielectric.

As for the aliexpress stuff, some of it is fine, other stuff is appallingly low quality, hence the price difference.

The "correct" capacitor you linked to is a ducati - these are very good quality, Metallized polypropylene film, and are run capacitors.

They are about £11 over here, I would have thought you could get the exact same thing over there without that markup.

I think the ducati part number is 416.33.64

Other physical sizes, such as 416.33.95 will be ok

 

[brianmoooore]

Unless this is a very large commercial dishwasher, I very much doubt that the motor has a centrifugal switch, and the capacitor will almost certainly be constantly be in circuit.

 

[Julie.]

...
Though it does appear that the metal capacitors used in dishwashers cost many times as much as other similar-specc'ed caps.
...

I missed this point, sorry.

No they are not similar spec'ed.

The correct capacitor is protection class S2, that means when it fails is goes open circuit, most of the cheaper ones are class S0, or S1, that means there is no way to know how it will fail, or it will fail short circuit - these types should not be used on a home appliance!!!

For use in electrical power applications a capacitor it is not just its capacitance and voltage that matters. The dielectric is important, as is its protection class. (There are also a nunber of other aspects - generally applicable to capacitors used in electronics.)

 

[mikey9]

I very much doubt that the motor has a centrifugal switch, and the capacitor will almost certainly be constantly be in circuit.

Ah, correct. I checked a similar pump and it was kind enough to have a schematic, see attached.
Is it still true that the sole (main?) purpose of the auxiliary winding is to provide starting torque?

The correct capacitor is protection class S2, that means when it fails is goes open circuit,

Ah, I think this is the sort of answer I was looking for in my original question. Thanks!
The motor capacitors I see here are marked "P2", which google says means explosion-proof, and corresponds to S3.
The one I ordered is S2, so I will install that when it arrives. Meanwhile the cap from the fluoro lamp will do.

 

[Lucien Nunes]

Agree with the above, what you are seeing in the differing price and construction is the difference between protection classes and constructional qualities. Fundamentally, they are all non-polarised film caps that do the same job, just that some do it more robustly and safer than others.

Is it still true that the sole (main?) purpose of the auxiliary winding is to provide starting torque?

In a permanent-split-capacitor motor, the auxiliary winding connected to the capacitor increases the torque when running, not just when starting. Without it, the motor would not develop any torque at all when at a standstill, so it is definitely required for starting. Once started, it might carry on running without the run cap but its performance would probably be reduced, with lower stall torque, more vibration and perhaps more heat generated in the main winding.

Normal to degrade with age?

Some types, yes. They are the 'self healing' capacitors that use a metallised film dielectric. When hit with a transient voltage spike e.g. from the power grid that causes an internal arc or damages the film, instead of failing short-circuit it is designed to 'seal off' the damaged areas and restore the insulation resistance, but with some of the capacitance lost. Other types do not do this, the capacitance does not fall but eventually the insulation resistance does, or it goes solidly short-circuit.

If there isn't a speed switch, it is likely to be a run capacitor, not start

A centrifugal switch or alternatively a relay, that disconnects the start winding when the motor current drops below a threshold as it reaches speed.