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Old 25th June 2008, 06:05 AM   #1
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Default motor run capacitor

i do a search about them but return to much to screen over.

whats is the motor run capacitor? which do they usually being used? whats is the different between motor run cap and normal caps we uses in audio application?

for audio component tweak. which part do this motor run caps fit best? power supply? thanks in advance
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Old 25th June 2008, 06:39 PM   #2
gareth is offline gareth  Wales
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Motor run capacitors are, as the name says, for the 'starting' of larger AC motors. They are primarily for single-phase motors and 'off-set' the phase components to get the motor running.
As such they have to be a pretty tough device, to handle the motor inrush current, so they are pretty hard wearing too.
Try a search in Farnell's or RS....

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Old 27th June 2008, 01:16 PM   #3
star882 is offline star882  United States
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Quote:
Originally posted by gareth
Motor run capacitors are, as the name says, for the 'starting' of larger AC motors. They are primarily for single-phase motors and 'off-set' the phase components to get the motor running.
As such they have to be a pretty tough device, to handle the motor inrush current, so they are pretty hard wearing too.
Try a search in Farnell's or RS....

Gareth
No, a motor run capacitor is used just to provide a phase shift for a single phase motor. A motor start capacitor is used to start a single phase motor, and there is a relay or electronic circuit to disconnect the capacitor after starting.
Probably the best source for those are HVAC supply stores.
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Old 27th June 2008, 01:56 PM   #4
oshifis is offline oshifis  Hungary
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Quote:
Originally posted by star882
a motor run capacitor is used just to provide a phase shift for a single phase motor
Or rather to provide 90 phase shift in order to run a 2-phase motor from a single phase.
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Old 28th June 2008, 06:12 AM   #5
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thanks for all the info. then whats the different between motor run caps with normal audio cap? why some people prefer motor run cap for their power supply? thx
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Old 28th June 2008, 07:08 AM   #6
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Quote:
Originally posted by milen007
why some people prefer motor run cap for their power supply?

For polyprop they are cheap.
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Old 28th June 2008, 08:52 AM   #7
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Usually a motor run cap is the only way to get a high value (>10uF) poly cap.
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Old 28th June 2008, 12:24 PM   #8
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I thought motor run capacitors are paper and oil?
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Old 28th June 2008, 01:07 PM   #9
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They can be all sorts of construction. The 40uF ones I have are polypropylene.

In my post above I should have said non-electrolytic cap really.
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Old 28th June 2008, 01:20 PM   #10
AndrewT is offline AndrewT  Scotland
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Motor run and motor start capacitors come in both electrolytic and metalised film.
They are usually rated for 250Vac or 400Vac.
The metalised film are usually much cheaper and probably just as good as many of the mid priced "audio" film caps.
The film type 400Vac motor run caps would make excellent <=550Vdc PSU caps for tube/valve amps.

The motor run and motor start spec is determined by the continuous vs short term Line voltage capability.
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