How to diagnose a faulty motor start capacitor?

I got it out. It's a 9uf 450v (I assume it's 9+9uf). My capacitance meter measures the capacitance at 109+113pf (checked the meter against a couple of other caps). (I don't have a multimeter that measures capacitance, only a multipurpose testing unit ex-China - it tests resistors, diodes, transistors (and identifies the pinouts), caps, and inductors.)

My wife doesn't want me to replace it as she want a new dryer - she says this one is too slow compared to the old Simpson (which was about 20-25 years old when replaced). I don't want to buy a new dryer. This one is 10 years old and has had very, very little use. New dryers are not cheap but I am.

So sneak off and grab a 10mf,run cap,a 9 is probably more difficult to find.
Or an 8mf will probably do.
And a similar voltage.

Yes, I’ll try the local electrical wholesalers tomorrow. I assume it’s a 9+9 uF for start and run.

I’ve probably got some 450v non-polarised polyester or similar caps in my parts drawers but nothing higher than 1uF. Not something I needed in my electronics work.

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Originally Posted by p38arover

Yes, I’ll try the local electrical wholesalers tomorrow. I assume it’s a 9+9 uF for start and run.

I’ve probably got some 450v non-polarised polyester or similar caps in my parts drawers but nothing higher than 1uF. Not something I needed in my electronics work.

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Yeah, don’t use those. NPOs are not self healing so the first couple of surges will set fire to it, then you’ll really be up for a new dryer.

^^ My wife would like that.

Well, it seems it was just a run cap, not a dual start/run cap. I bought one this morning from a local electrical wholesaler (he told me it was a run cap only). Installed and the drier is operational and back up on the wall (I had to get the apprentice concreter neighbour to help me put it back up on the wall mount.)

I stumbled across this while looking for something else. The following may be of interest, even though it turned out to be just a run capacitor.

Cheap split phase single phase motors:
All single phase motors need some form of starting assistance compared to a 3 phase motor.
A start winding consists of a similar number of turns to the run winding but is of thin wire and therefore high resistance. It is placed in slots 90 degrees apart from the run winding but will only give about half of that displacement electrically, resulting in very poor starting torque.

Capacitor start motor: In an attempt to improve starting torque a capacitor is placed in series with the start winding, which because of its electrical properties will give a much better electrical displacement resulting in improved starting torque. The windings are usually the same as previously. The important thing to note here is that a start capacitor is an electrolytic capacitor, and as such cannot be permanently connected. As it is connected in series with the start winding it is disconnected along with the winding, which occurs as before, at around 75% of the motors rated running speed.

Capacitor start, capacitor run: In this case another capacitor is connected in parallel to the starting capacitor and the start winding. This capacitor is of the "paper" variety and can be permanently connected. The starting capacitor ONLY is then disconnected as before, leaving both the run capacitor and the start winding permanently connected. This arrangement has several advantages in addition to improved torque. The vectors and the math's involved are not of concern here!

Permanent split capacitor motor: PSC. This type of motor still has two windings, but it does not have any switch as both the start winding and run capacitor are permanently connected, the capacitor again being of the "paper" oil filled type. It also has a few advantages, such as higher pull out torque, efficiency, and power factor, but it may have lower starting torque.

In my day this last type was very unusual, maybe not even in existence. I used to rewind all types of both single and three phase motors, and don't recall them. I served my time in a farming area where at the time, the rural distribution grid to farms was single phase, 500volts. In some states this system used a single wire earth return (SWER), but where I lived the system did have a return conductor.

The main point here is that we used to get a lot of repulsion induction motors in for service from dairy farms. These 500 volt motors (mostly 5 hp) have a rotor that looks just like a DC generator or motor armature, even being fitted with carbon brushes, albeit for a different function.
Skimming and under cutting the commutators, being a time consuming job, became my function as a form of cheap labor! Always a rush job as well, as the dairy farms needed them back the same day!

The point here being that those motors had good starting torque.

And to answer the question: We used a 500 volt megger to charge up a capacitor as a test. No big fat discharge spark meant a failure!

I studied all this when I did training for the Broadcast Operator's Certificate of Proficiency 20+ years ago (to work as a tech at a radio station). I've forgotten most of it.

I never thought to use my Megger as described.

500v distribution in the country. I'd never heard of that!