Single phase 2hp capacitor start induction motor start problem.

Discussion in 'General Electronics Chat' started by AndyP, Jan 13, 2015.

  1. AndyP

    Thread Starter New Member

    Jan 13, 2015
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    Hi, first let me say thanks for this great site. the reference section is fantastic.

    I bought a 12" thickness planer and it has a 2hp induction motor that has trouble starting. The motor was probably made in the 50s.

    The motor is an English Brooks Motors Ltd single phase R8-A64505 capacitor start, 230v, 1430rpm 50Hz. The hp rating says 1hp but I think its possibly 2.1hp. On the motors plate I can just about make out what could be a 2 before the 1. In addition the FLA is 6.1 which I think would mean it has to be about 2hp. I am not 100% sure as this is old and the motor is physically large, 12" diameter.

    It looks like this motor , although that's an R9 model.

    If i can get it up to speed (no load) and then pull the belt tight it works fine. But if i try to start it with the planer head attached, it never gets up to speed. The planer head is well lubed.

    Capacitor:
    It takes about 5 seconds to start with no load other that its 10" pulley. It originally had a 30uf start capacitor. I did abit of searching and found that 200uf was about right for a 2hp motor so I ordered one and a 100uf which I tried first, because I thought the kick, might tip the planer over. Well even with 300uf (paralleled capacitors) it still takes 5 seconds to start.

    Wiring:
    So next I checked the wiring, all seems to be ok. All VIR wire, (50s stuff), a bit perished, but the run winding is permanently connected and the start winding is connected to the capacitor and centrifugal switch. Am I right in thinking that the order positive--switch--run-winding---capacitor---negative is irreverent? The wires looked factory anyway.

    Bearings:
    I have greased one bearing through the ?cup? No grease nipple just a brass cap that you unscrew. If you have ever seen a brooks motor you know what I mean. It was filled with grease, so I added new. Perhaps its supposed to be oil. Anyway greased, stated, and the internal fan through the excess grease out and onto my chin. I called it a day.

    If I spin the motor by hand, it doesn't feel stiff, but at the same time its not spinning like a hard drive motor. The rotor is heavy. When its up to speed and you turn it off it takes ages to stop, so the bearing must be fairly good.


    Anyone have any ideas of what else it could be? Is a UK 13A socket too little for the in-rush current?

    Any help appreciated.

    Andrew
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I would strip it down and do a number on it, if it is as old as me it is way past its due preventative maintenance date. New bearings and switch would be in order.
    One reason for weak start on the very old induction motors was the rotor bars were copper, not aluminum as they are now, after many heat ups, the solder or joints on the end of the rotor used to lose connectivity and the motor would lose power.
    The order of the switch/winding/cap does not matter.
    If the original was 30uf I would stick to it.
    Your video mentions no vibration, this is a sign that the motor was very well balanced when manuf.
    Max.
     
  3. AndyP

    Thread Starter New Member

    Jan 13, 2015
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    0
    Thanks for the reply. Just point out the video is not mine, just a similar motor. Total rebuild? I thought it may be something simple:-(

    I'm not sure if the 30uf capacitor is original. According to my research, that's about right for a run cap. So I have worked on the basis its been installed in error. Is there any method for selecting cap sizing, the manufacture must have had some method. Every source I have found is a vague rule of thumb. There must be some theory even if its complex. Anyone have any links?

    On the rotor solder joints, is this something that would need to be baked? Or is it possible with a 100w soldering gun?

    My makeshift solution at the moment is a clutch. Basically an idler pulley.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If your is anything like the one in the clip, the rotor bars are cast in so the flung solder should not be a problem, you would normally do it it with a large gas torch etc, after heating the rotor up.
    The capacitor is sized so that the phase shift in the start/run winding is ~90° so to do it after market, it would either be by experimenting or using a D.B. 'scope.
    Max.
     
  5. tcmtech

    Well-Known Member

    Nov 4, 2013
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    1,626
    Have you confirmed that your start winding is good?

    If it was me testing the motor I would wire one of the new start capacitors up to feed the start winding directly bypassing the centrifugal switch and see if it starts properly.

    If it does the switch is the problem.

    Capacitor size wise I typically go with about 75 uF per HP on a 230 VAC motor. But as other info has shown there is a lot of leeway in sizing a start capacitor for a motor.

    Typically more capacitance gives faster and more torquey start up at the cost of higher in rush currents.
     
  6. AndyP

    Thread Starter New Member

    Jan 13, 2015
    4
    0
    MaxHeadRoom:
    Good to hear the rotor bars are probably cast in so the solder problem may not be the problem.

    I can understand the 90° for a run cap. I am guessing the load would affect it somewhat. But for the start cap, is the intention to get it at 90° when power is first applied then as it comes up to speed, the phase becomes more and more away from 90° until the cap is switched out? I appreciate what I'm asking is probably quite complicated, so if you could point me to some info that would be great.

    tcmtech:
    The centrifugal switch tested fine, it almost looks new.

    I have measured the resistance of the terminals:
    motorwiring.gif
    Notes:
    Shorting the capacitor I get 1.6 ohms between Z2 and A2,
    My cheap MM reads about 1.2 ohms touching the test leads together.

    Also The inrush peaks at about 30A then settles back to 6A

    So 25.4 ohms in the start winding. Could this be the problem?

    I was going to try to take it apart today, but it took nearly 1/2 an hour to remove just the tapered gib key holding the pulley on, and it only 34 degrees here right now.

    Thanks for the help so far.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The switch would be N/C. at rest.
    That current is what I would expect from that size motor, The peak inrush is due to Both winding's essentially being very low/ short circuit at switch on, this coupled with the rotor bars acting as a shorted turn secondary of a transformer.
    Once the frequency generated in the rotor approaches the applied frequency, (within a few cycles ~7hz), the current will drop.
    Max.
     
  8. AndyP

    Thread Starter New Member

    Jan 13, 2015
    4
    0
    Oops, yes sorry error in drawing, it is normally connected. And I can hear and see it disengage when the motor is at near full speed.

    So is the start winding bad? ~25 ohm? Bad solder joint?
    I don't know what gauge the wire is in the start windings, but the wire leading to the winding is roughly half the diameter of the wires leading to the run winding.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That doesn't sound that far off for a motor of that size, All that is needed to start is the initial phase shift between the start and run winding, it may be a question of obtaining 2 or 3 caps and experiment series/parallel etc to get in the ball park.
    Max.
     
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