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2 speed motor tripping

Discussion in 'Electrics UK' started by flyingsparks, 24 Nov 2018.

  1. flyingsparks

    flyingsparks

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    Customer has a saw which has a small motor which from looks of it has two sets of windings/poles for two speeds. It can be operated manually or as part of a program using the PLC. However it is operated it is the same thermal overload and contactors which operate. It is tripping the thermal overload occasionally when manually operated. Tried changing the overload but its the same. Can't fully test the motor as the windings are connected inside the motor somewhere (not at the terminals).
    Anyone come across a motor like this before with two speeds?
    The motor plate is below:
    motor.jpg
     
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  3. conny

    conny

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    There are 2 separate windings inside as the speed difference is too great for a single winding. Does it trip on both speeds or just one speed?
    Is the saw blade sharp as a blunt blade will force the motor to work harder.
     
  4. flyingsparks

    flyingsparks

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    I believe it trips on both speeds. Its a motor which lifts the blade up and down, I believe on a worm screw type of set up. There is no way to test the windings, as the actual winding star point is inside the motor.
     
  5. conny

    conny

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    How many leads are there in the cable, 3 or 6? Because the speed range is so different I believe there should be 6. If ths is the case you need to find which 3 leads run the high speed and which 3 run the low speed. It's possible that if you can't get a separation of 3 and 3 leads there is a short across the windings. Maybe cheaper to buy a new motor rather than have it rewound. The short may only show up when it starts getting hot.
     
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  7. flameport

    flameport

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    If it's only tripping on manual, possible that the switch or whatever is worn out and is intermittently connecting both windings at the same time.
     
  8. flyingsparks

    flyingsparks

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    This is something I will look into for sure. Thanks.
     
  9. flyingsparks

    flyingsparks

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    There is 6. Makes it difficult to check the motor windings properly as the connections are within the motor. But yes, I could check there isn't a short between the two sets of three.
     
  10. conny

    conny

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    That will be your first step. Once you have separated the 2 sets of leads you need to megger between them, just checking with an ohmmeter is not sufficient as you need to pass a working voltage between them. If there is a circuit between them then it's a rewind/new motor. If they are open circuited then the next step is to test the resistance of each winding. There may be a very slight difference between the leads of the same winding, e.g. On the fast winding you may get for example 5.24, 5.25 and 5.27 This is acceptable and due to the method of winding. The same applies to the slow winding though the figures will be different.

    I've just had a thought. If you energise the brake separately does the shaft turn freely? It may be the brake is binding and causing the motor to run under strain. (Only just had a close look at the N/P)
     
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