Motor start capacitor - value ?

[Andrew Gabriel]

Just wondering if the OP knows for sure it's a capacitor-start motor, rather than a capacitor-run motor? The second also gives a higher running torque, but uses higher grade capacitors.

[The Natural Philosopher]

He did say something about a centrifugal switch, which suggests start alone..

[Tim Lamb]

Umm.. A centrifugal switch could also indicate a simple split phase motor: where the LR ratio of a secondary winding gives a small phase change and does not require a capacitor.

Long while since I actually anything about this:-)

[Tim Lamb]

In message , Tim Lamb writes

Oops! ^^knew

[koos.bermuda]

I have an ancient table saw with a British Houston Thompson single phase 1.0 hp motor V200-220V. Amp 6.9, RPM 1425, BS170 APPD - the cap is gone and I cant read any values. Any ideas on the cap size??

[Bob Minchin]

This dates from an era when high value capacitors were expensive and copper windings were cheap. The capacitors were so called ac- electrolytics and a bit specialised for the application and rarely marked with a value.

I would not use values from a modern motor as copper is now expensive and capacitors cheap so the ratios are very different. Use continuously rated motor run types as these will last longer If there is any way you can experiment with values, build up from say

10mfd in steps until you get adequate on load starting torque. Be aware that almost anything will start an unloaded motor. The belts speeding up from 1425 to respectable saw speeds will impose quite a starting load

If you have pick a value to purchase then maybe 15-20mfd would be a good guess.

A good source of suitable capacitors to experiment with are the power factor correction types in old fluorescent lamps which are gradually being outlawed for the more efficient electric ballast units and so often found in the scrap.

The other thing to check is if the capacitor has really failed. Other culprits can be a the centrifugal switches and those early motors are not usually sealed and dust/dirt can prevent the contacts closing.

Good luck

Bob