Re: Starting capacitor sizing

I trust you are correct, Bill. Perhaps a better choice for a word to replace (the) *purpose*, would be (the) advantage
.... of the capacitor in the motor's starting circuit is to increase torque during startup

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"Bill Noble" <nobody@xxxxxxxxxxxxxxx> wrote in message news:h2r4ug$qqv$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
aaah, this is partially correct, but the way to look at it is that the capacitor, along with the start winding provides a phase shift to act "kind of" like a 3 phase motor - so you can get the rotating magnetic field rather than just a 180 deg back/forth magnetic field which will just make the motor buzz -

so, you want whatever value gets you about 60 deg phase shift so you can get the rotation of the field

"Wild_Bill" <wb_wildbill@xxxxxxxxxxxxxx> wrote in message news:Kj74m.226950$op1.122943@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.

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"RoyJ" <spamless@xxxxxxxxxxxxx> wrote in message news:0d2dndt_gJ7gds3XnZ2dnUVZ_h2dnZ2d@xxxxxxxxxxxxxxxx
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%.