The purpose of the capacitor in the motor's starting circuit is to increase
torque during startup (thus reaching the run switch speed sooner), so a
manufacturer could utilize a more common or cheaper capacitor if the OEM is
specific about the motor application.

There are a lot of split-phase induction motors that aren't built with a
starting capacitor, and they work fine in applications where there is none,
or a very low load at startup.

When I've used general purpose utility motors for applications where the
starting load torque was more significant than say, a fan application for
example, I've added a starting capacitor to increase the starting torque.

When dealing with used motors, if I see signs of previous repairs (stripped
screw slots, etc), I'll review my motor notes to see if the start capacitor
value is within reason for similar motors, or comparable to the 500uf/HP
guideline.

In my experience, not many motors are marked with start or run capacitor
values, with the exception of PSC permanent split capacitor types, and not
many motors can be looked up since most manufacturers usually don't provide
capacitor values for split-phase models.

In the HVAC servicing trade/industry, there are add-on booster capacitors
for increasing a motor's starting torque which may be reduced by voltage
drops or other issues.
These capacitor-booster products typically utilize thermistors to limit the
application time duration of the boost.

--
WB
..........
metalworking projects
www.kwagmire.com/metal_proj.html


"RoyJ" wrote in message
m...
I headed to the shop, pulled apart 3 motors:

-2hp cap start/cap run TEFC running on 240 volts: 189-227mfd/165 volts
-1hp cap start compressor running on 120 volts: 534mfd/125 volts
-1hp cap start saw duty running on 120 volts: 216-259mfd/110 volts

Now I'm really confused!

Wild_Bill wrote:
When I asked about starting capacitor values at the local motor repair
shop, I was told that 500uF per HP is the correct size/value.

Many of the fractional HP motors that I've serviced all seem to follow
this same guideline.
The marked values of about 110uF for 1/4 HP, 140uF for 1/3 HP, etc.

FWIW, the actual measured value will vary considerably from the value
marked on the capacitor case, as the capacitance value tolerance can be
20%.