Capacitor

[tjwoodms]

Hi Chaps
I need a little help please if possible.
I was called out today to look at a non running old single phase sedgwick morticer a 571 i think circa 1985.
Diagosis was no problem she is suffering from a failed capacitor my problem is there were no makings on the capacitor fitted so i dont know what rating i need somewhere in the back of my mind 75uf

i will give sedgwick a call in the morning but im not very hopefull due tothe age of the machine

If any of you chaps who have one of these morticers could have a quick look for me i would be eternaly greatfull

 

[9fingers]

Tim,
In the absence of a firm answer, for conventional single phase motors reckon on 30-40 mFd per horsepower.
A little larger will be better than too small especially on a machine such as a planer that has a heavy cutter to accelerate.
For a morticer it is a lot less critical as there is minimal starting load imposed by the machine

Modern motors can use all sorts of starting schemes so the rule does not hold true. Now that copper is so expensive and capacitors relatively cheap, motor designers are coming up with all sorts of schemes based on cost rather than good engineering.

hth

Bob

 

[tjwoodms]

Thanks Bob

so with a 1.5hp motor as a rule of thumb a 50-60uf should do the job nicely

I havnt come across the 30-40 basic rule before thats usfull information thankyou

 

[9fingers]

Yes that should be fine for a morticer but as I say if it were 1.5hp on a planer (2 pole motor and a speed up pulley drive), I'd choose 60 or more.
I've got a big bank of capacitors all 30mfd that I try out in various series parallel combinations until I get a reliable start. Anything more than a second or so is too slow and likely to cause problems down the line. Then I order up one - usually from ebay suppliers to fit.

If there is enough room, I fit motor run type capacitors even in motor start applications. they only cost a little more and last better - less call backs that way.

Bob

 

[Giff]

Hi 9fingers

I had some trouble earlier in the year when I changed my 250mm blade to a 315mm...it is a Record 315mm saw with option for 250mm with scoring. It continually wouldn't start and cut tripped the circuit ( 16amp motor rated fuse) so I changed the capacitor for a new one of the same rating....it seemed better but still was hesitant with the larger blade. I changed to the 250mm blade and it is ok 90% of the time. I am not sure if it will be ok with the larger blade.The question is is there any problem fitting a higher value capacitor. When I explained my problem to the electrical wholesalers he suggested that but I was unsure if it was safe to do...Thanks Giff

 

[9fingers]

I assume you have a TS315 fitted with a 3hp motor?
You say "a motor rated fuse". Do you mean a fuse (rare) or a miniature circuit breaker (more normal)

If it is a MCB is it marked type B or type C?

I would be reluctant to advise fitting a much larger capacitor without the knowledge of the above.

I would not be surprised if your motor has two capacitors - one for starting and one for running.

What is the value of your starting capacitor please (mFD and voltage)

My saw is also 3hp and pulls 25 amps or more during starting but I have and old fashioned re-wireable 15 amp fuse and it gives no trouble.

Bob

 

[Giff]

Thanks Bob sorry it is a MCB and I will check tomorrow C or B.....I know the electrician who did the installation a year or so ago said it was "motor rated" It's the start that seems to be the problem, it runs fine after that..The capacitor I replaced was labeled. CBB60 450vac 50/60hz 50uf +/- 5% and I took the old one for a direct replacement. Thanks Geoff

 

[9fingers]

Hi Geoff,

If your MCB is a type B then I suggest swapping it for a type C. If however you already have a type C, then maybe try a 75mfd 450v AC capacitor or, in consultation with your electrician, see if the installation is suitable for a 20a type C breaker.

Good Luck

Bob

 

[Giff]

Hi Bob

It is a C16 MCB..thanks for the suggestion I will get a 75mfd and try it when I change to the 315mm blade next. Really helpful advice which is very hard to get !

Thanks again best regards Geoff