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"FREE" Electric Motor

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  • "FREE" Electric Motor

    Yesterday a friend of mine gave me a 1/2 hp 2-speed capacitor start motor. It is a GE motor, made in Canada, old but not ancient, (the end bells are die-cast white metal, either zamak or aluminum.)
    Not only did it not run, it did not even turn! Having a KEEN analytical mind, I surmized, (better than guessing,) that the bearings were shot.
    Today I took it apart. The capacitor was cooked, but the bearings, such as they are, were fine. Since this is a cap-start motor, I expected it to have sealed ball bearings-not so. They were sleeves, STEEL sleeves yet, set in felt. Long ago, the felt was soaked in oil, but today it was as dry as a popcorn fart! There was no scoring, so I am in the process of saturating the felt with #10 oil. I think that it will take a while. I expect that when I reassemble it and install a new cap, it will run fine.
    The capacitor,a Mallory, was rated 161-193 mfd, 110 volts. Can someone please explain the range of capacitance? While I am on the subject, how do you determine the required capacitor size, (when the original is either missing or not legible?) Another point, this motor is 1750/850 rpm. Most single phase two-speed motors that I have seen are 1725/1140 or thereabouts. They were common split phase, fan-duty motors. I am wondering why all two-speed motors do not operate at full speed and half speed, rather than 66% of full speed.
    I am sure that one of you folks will clear all this up for me.
    Incidentally, as near as we can estimate, that motor is about 30 years old, and has been sitting, drying its bearings, for 20 years. Since there is no provision for lubricating the bearings, (short of disassembling the motor,) it would seem that it has a design life of about 25 years, (or LESS!")
    Duffy, Gatineau, Quebec

  • #2
    1750 for 4 pole configuration, 850 for 8 pole.
    The capacitor is calculated in order that the split phase winding is as close to 90° phase shift to the applied voltage as possible.


    • #3
      As I understand it, because of their chemical nature electrolytic capacitors are not precise in their degree of capacitance. Secondly, because part of their ability to act as a capacitor at all is due to the chemical component, their effective capacitance will vary over their life and operating conditions (applied voltage, etc.).

      Since a motor start capacitor is only in operation briefly, and since all it has to do is shift the phase of a set of windings sufficiently for the motor to start turning, there is a lot of wiggle room in the actual value of the capacitor used. While it may not be ideal, any capacitor within a fairly wide range will "get the motor going".

      I think this is more or less correct, but the real EE's can correct me where I'm wrong.
      "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979


      • #4
        Such capacitors generally have only a set minimum that it must meet. It can be a lot over that and will work fine. The values of those types of electrolytics are extremely variable due to variations in foil spacing, oxide thickness and other differences from batch to batch. It isn't important as long as it meets the minimum.

        BTW, hard steel sleeve bearings are very common. The last a very long time. What matters is the difference between metal properties, not so much the difference in actual metal elements. As usual, hard against soft is the best match for a sleeve bearing.
        Last edited by Evan; 04-24-2015, 11:55 PM.
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        • #5
          The important thing is to get motor rated AC capacitor, although the larger start capacitors are electrolytic, and electrolytic are basically DC components, the AC motor type is a bi-polar construction, two caps back to back, in order to operate on AC. A normal electrolytic will not do.


          • #6
            The capacitor is indeed workable over a range of values, and the rating reflects that. There are a number of standard ranges, each corresponding to a type and size of motor.

            You do want a "motor starting" capacitor, which is made to withstand the starting surges, and will last many years of normal use. That type has a suitable combination of microfarad value, voltage rating, ESR, heat tolerance, and current rating for the motor size range each is intended to work with.

            The size in microfarads is determined so as to get a suitable phase shift in combination with the inductance, resistance, and mechanical positioning of the start winding. The electrical shift may not be 90 degrees, but will be set to produce the most effective starting torque with the least current, in that particular model of motor. A capacitor-start motor normally has the least start current for it's size.

            Hard steel was often used for bearings on watchmaker's lathes. In fact, at one time they were considered to be a very superior type of bearing, and the lathes with them were known as "hard" lathes. The "soft" type lathe, with bronze bearings, was looked down on as a "cheap import" type, coming from the "inferior" makers in europe. This was back when the American watchmaker lathe was a world class product.

            The hardened steel spindle ran in the hardened steel bearings, and with suitable oil, was perfectly satisfactory, although I am not sure it was actually so much superior to bronze types.

            (reference: Goodrich: "The Watchmaker's Lathe")
            Last edited by J Tiers; 04-25-2015, 01:29 AM.

            Keep eye on ball.
            Hashim Khan