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  • Another humming motor question

    I removed the capacitor; looks alright, but there is no little "I am dead!" flag. This is a Tiawanese motor, (according to the label on the saw,) 3 hp 220volt. The capacitor is a 600mfd 125 volt "motor starting" unit. I understand that the cap is across a 120V leg of the motor circuit, but I thought that 125 volts would be a bit marginal. Out of curiosity, I pulled the capacitor off of a Leeson 2 hp 220 volt motor. It too was rated at only 125 volts, but was 840-900 mfd! What gives?
    I will pick up a replacement cap on Monday and see if that is the problem. While "substitution repair" is not my favorite method, for $10.00, if it solves the problem, I wont have to remove the motor from the saw-NOT a nice job.
    Can someone explain the differences in capacitor sizes and why the voltage?
    Thanks in advance.
    Duffy, Gatineau, Quebec

  • #2
    i dont understand your electrics ..over here the cap usually has more than the running voltage on it

    so if my motor was 240 it would have 300 volt cap upwards on it.

    all the best.markj

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    • #3
      Originally posted by Duffy
      I understand that the cap is across a 120V leg of the motor circuit, but I thought that 125 volts would be a bit marginal. Out of curiosity, I pulled the capacitor off of a Leeson 2 hp 220 volt motor. It too was rated at only 125 volts, but was 840-900 mfd! What gives?
      .
      If this a 220v 1ph motor there is no '120v' leg' , after the initial starting winding inrush, there will be less than 120v across it but it should still be rated for alot higher that the applied voltage which it sees initially.
      I would replace with a higher rating.
      Max.

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      • #4
        Chances are that when the cap is in circuit while the motor is starting, it doesn't see more than that voltage across it. It's in series with a winding and provides a phase shift to give the rotor a kick in one direction.

        If the value of the cap is higher, the shift will be different, and may cause slower starting or higher than required starting current. The optimum capacitance value would be what gives the best balance between run-up speed, current draw, and the electromagnetic/mechanical efficiency of the starting circuit. Other values will work, but the optimum value has to do with the number of turns in the start winding, the resistance of the winding, and the mechanical arrangement of the poles, the voltage applied, etc.

        The voltage across the cap doesn't have to rise to the level of the supply voltage because it's in series with the winding. This is a simplification, but in effect they split the voltage.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          The value of the capacitor is far from critical. If you look at the specs on the cap it may sometimes give something like 600 mfd -10% +150%. As long as it is enough to kick the motor into rotating more isn't a big deal unless it is a LOT more.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Run of the mill starting capacitors are rated at 370 volts. Grainger has a good selection and fair prices. It is hard to find a made in the USA start capacitor.
            Byron Boucher
            Burnet, TX

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            • #7
              Originally posted by darryl
              The voltage across the cap doesn't have to rise to the level of the supply voltage because it's in series with the winding. This is a simplification, but in effect they split the voltage.
              Although at switch on there will be the full voltage across the capacitor due to inrush for up to at least 1 cycle which will be limited by the resistance of the winding only, until the inductance takes effect.
              Max.

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