DIYbanter - View Single Post - Motor start capacitor

Bob Minchin[_4_]

wrote:
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??

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