Delco motor capacitor

[FlatbeltParticipant @flatbelt]

My stepdaughter has caught the engineering bug from me and is starting to collect her own tooling. She couldn’t resist a little Portass lathe despite warnings about its limitations, so we’re getting it up and running as a useful piece of nostalgia, although we have other more serious equipment for more serious jobs.

It’s a lovely post-war outfit on a home-made wooden stand with foot motor/treadle, later modified to take this Delco motor. I have stripped the motor to change the disintegrating cloth-insulated aluminium wiring, and have checked the windings with a Megger.

The original starting capacitor is toast, can anybody offer advice on a suitable capacitance for a replacement? There are no clues on it or the remains of the wiring layout printed on the terminal cover.

Any help gratefully received. Photos of the lathe-and a couple of questions- to follow!

[Flatbelt]

On 24 March 2024 at 06:50 Flatbelt Said:

…can anybody offer advice on a suitable capacitance for a replacement?

I am not sure if a search engine counts as an ‘anybody’ but if you put “motor start capacitor sizing” into one of them, it offers lots of advice.

If the advice you find is unclear, please cite the page(s) you find and ask here.

Another option might be to look on the other machines you have and see what is used there. Perhaps try to establish if there is a correlation between motor size and capacitor value.

[Chuck TaperParticipant @chucktaper]

A direct answer to your question ( & copied from here )

5-1. Formula for calculating the required size of a motor STARTING capactitor when the motor size is given in horsepower HP

C = (7150 x HP) / (V x PF)

C = Capacitance in uF or microfarads – this is the capacitor size that we are calculating

7150 is a constant

HP = the motor’s rated horsepower (such as 0.5 hp or 1 hp etc.)

V = the motor’s rated voltage (such as 120VAC, 220 VAC, etc.)

PF = the motor’s required power factor to start up (we’re calculating a starting capacitor size in this example) expressed in joules or microjoules – which of course you probably don’t know.

Round up the number that you calculate to a typical whole number capacitor size such as 5uF 50 uF or 100 uF. etc.

Remember to choose a capactor whose voltage rating is at least equal to the rated voltage of the motor. It’s perfectly fine to use a capacitor whose voltage rating is greater than the motor’s voltage.

For example if your motor runs at 220V your capacitor’s voltage rating must be 220V or larger. A 330V rated capacitor is fine.

Example:

We have a 2 hp 120VAC electric motor and we will assume its running PF is 80%, so we use the formula above, plugging in those numbers:

C = (7150 x 2) / (120 x 0.80)

C = 14,300 / 96

C = 148 uF

So we’ll round this up to select a 150 uF starting capacitor, choosing one that is rated for 220V or greater.

[Brian WoodParticipant @brianwood45127]

How very useful Chuck Taper, thank you.

Regards   Brian

[noel shelleyParticipant @noelshelley55608]

Since you have a megger can you get your hands on a good multimeter that reads capacity ? If so try reading the value – if it fits any formula you’ve been given then it will prove the working. A power factor of 80% will be in the ball park. A 400v value is what you need and don’t be concered at the tolerance which may be 10% or even 20%. Check the centrifugal switch whilst you have it apart.

[SillyOldDufferModerator @sillyoldduffer]

Proceed with fingers crossed!  There are at least 3 different types of single-phase motor.  Some motors have one capacitor, others two, and there is often a centrifugal switch as well.

Judging by the photo, this example is simple: a single START capacitor (only two wires leading to it), without a RUN capacitor, and probably without a centrifugal switch.   If there is one, and perhaps it is faulty, should be found at the pulley end of the motor.   It connects the RUN capacitor to the windings when the motor is stopped, and disconnects it once the motor is running.  If the switch fails open, the motor won’t start:  if it fails closed, the motor will overheat.   (But I guess this motor is designed with a hefty run winding that’s left in circuit permanently, and there is no switch.)

The capacitor value(s) depend on the windings and other technical characteristics of the motor, known to the designer, but rarely shared with the public.    One’s best hope is that the old capacitor is marked, but they aren’t always, and of course ancient markings tend to become unreadable.  Have a close look, perhaps breathing on the can to see if a little condensation brings any markings out.

Otherwise, Chuck’s answer is a good place to start, even though it requires us to guess the power-factor at start, which isn’t the same as the motor’s power factor when running, and isn’t easy to measure. 0.8 is reasonable.   So applying Chuck’s formula for this 1/6HP HP motor suggests a 5 microfarad capacitor.

The good news is that the value of the capacitor in this application isn’t too critical – only has to be near enough to start the motor.  Too small is worse than too big, so I’d try anything between 5 and 15uF.    If the motor won’t run (and the windings/switch are OK) try bigger.

It’s important to use a Motor Start Capacitor rather than any other type.   They’re common as muck: this page is Mouser,  plenty of other suppliers like Farnell, RS Components, etc etc.   The capacitor must be rated for more than mains supply voltage, so 280Vac in the UK, which is usually well above 220V.

Is fixing an ancient motor is worth the cost and risk?   By modern standards, old insulation has poor resistance to age, heat and wet.  The photo shows the original wires leading to the capacitor are unacceptably dangerous, and they at least have to be replaced, and a grommet fitted.   The interior wiring may not be much better, especially if the motor has ever got damp.

The risk of using a potentially dodgy motor depends on the circumstances:  extremely unwise to leave one running unattended because it might catch fire and burn the building down.  Assuming the owner knows what to look for and the motor doesn’t get hot, smell, or tingle, intermittent use on a small lathe in a dry workshop is probably OK provided the whole is competently earthed.   A professional would insist on a Megger test.

Dave

 

[FlatbeltParticipant @flatbelt]

Thank you both- I did have a scout about online first, but was unsure how confident to be without knowing the power factor of the motor. I haven’t any other similarly tiny motors to compare to- this is our first ‘toy’ lathe, all the other kit is much larger or 3-phase.

A table of starting capacitor values suggested a range of 30-40uF for a 1/6hp motor. Using the formula provided by Chuck Taper above:

 

C= (7150 x 0.16) / (229 x 0.8)

C= 1144 / 183.2

C =6.2 uF

Which differs wildly to the suggestion in the table I found….

The original capacitor is in a bad way, damp and leaking, so no chance of getting a sensible value from it sadly.

[FlatbeltParticipant @flatbelt]

Thank you Dave, your reply was posted while I was putting the last post together.

Yes it’s a single capacitor motor- capacitor start with centrifugal switch which I’ve dismantled and cleaned. The cloth covered aluminium wiring has been replaced back to the windings themselves which have seen a Megger and are sound. The motor had no earth but will be, and will only be used attended.

As well as a plethora of small DC motor armatures I have hand wound new windings for an old induction motor before and would consider giving this one the same treatment- I like to keep things original- but there are of course plenty of other jobs I’d rather be doing!

This little motor has thrown me somewhat as the capacitor values seem tiny compared to other machines.

[JohnFParticipant @johnf59703]

Hi flatbelt, electrics are defiantly “not my thing”  lots of good info on here however if you are in the UK I would suggest contacting Power Capacitors in Birmingham — I have ford them to be most helpful with advice and supply of capacitors when I have needed them

John

[noel shelleyParticipant @noelshelley55608]

Just in passing, I don’t think I have ever seen a motor with the centrifugal switch at the PULLEY end, normally at the Non Drive End ! Noel.

[Flatbelt]

On Flatbelt Said:

Thank you both- I did have a scout about online first, but was unsure how confident to be without knowing the power factor of the motor. I haven’t any other similarly tiny motors to compare to- this is our first ‘toy’ lathe, all the other kit is much larger or 3-phase.

A table of starting capacitor values suggested a range of 30-40uF for a 1/6hp motor. Using the formula provided by Chuck Taper above:

 

C= (7150 x 0.16) / (229 x 0.8)

C= 1144 / 183.2

C =6.2 uF

Which differs wildly to the suggestion in the table I found….

The original capacitor is in a bad way, damp and leaking, so no chance of getting a sensible value from it sadly.

Quite a sensible value.  The start/run cap on the little motor on my Quorn is about 8uF IIRC.

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