cap start cap run motors

Discussion in 'General Electronics Chat' started by gwalker, Aug 2, 2011.

  1. gwalker

    Thread Starter New Member

    Aug 2, 2011
    I followed some older threads but didn't help my problem . I am an electrician working on a greenhouse vent opening system , and have built a control panel for manual and automatic control of 3 single phase 230v motors with reversing starters .The original installation was in a state of corrosion and decrepitude and had several haywire modifications to function .My rebuild could not rely on any part of that installation to guide me .These are cap start cap run motors with an external start relay and drive a reduction gearbox , which in turn drives a series of shafts and levers to open the vents .the problem I am having now after replacing start capacitors and start relays is this .The motor will start from fully closed and run fully open no problem and the reverse , but in Auto mode when the vents are partially open the motor , when started to open more , hums and won't move , the current in the run winding has a breif initial spike (one second or so ) then continues to build to 5 times the full load current and trips the overload .On manual closing I get a breif spike in current then it quickly drops to normal levels and the vents close .This is the same when opening from fully closed .The windings were not identified so I assumed the one with the highest resistance to be the start winding and have the run capacitor in series with it and the start relay/start cap connected in parralell to the run cap .So on starting both capacitors are in series with the start winding then the start relay operates leaving only the run capacitor in series with the winding .I have suggested that the mechanical load may be the issue but the owner assures me that they used to operate automatically .Would this motor run with a weak or faulty run capacitor ( does a capacitor lose effectivness over time ?)but at lower torque ? If I had my parralell start/run capacitor assembly connected in series with the wrong winding could this cause a lower output torque in the motor ? Any help or suggestions would be appreciated .
  2. creakndale

    Active Member

    Mar 13, 2009
    I can't comment on your specific circumstance but I recently had a similar one. All of a sudden my 2 HP air compressor began struggling to start up. It would hum for a few seconds before it would eventually start.

    I noticed it had 2 capacitors hanging off the motor. They are labeled: 161-193uF 110VAC and 20Uf 370VAC. After doing some research, I concluded that the 161-193uF was the motor Start capacitor and the 20uF was the motor Run capacitor. My thinking was the 161-193uF motor Start capacitor was bad because it was having trouble starting but once it got going seemed to run OK. However, the motor Start capacitor tested good. The 20uF motor Run capacitor tested open circuit. I replaced the 20uF motor Run capacitor and now the compressor starts and runs like new.