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3 Phase motor running slow

Discussion in 'Electrics UK' started by fuzzytrifle, 28 May 2020.

  1. fuzzytrifle

    fuzzytrifle

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    Hi, I wasn't sure where to ask for help and thought I'd try here - I need some advice on a 3 Phase motor that's running slow - I've got an old woodworking combination machine that's fitted with three small 3 phase 2.2kW motors. I tried to use the table saw earlier today but the motor's running slow and has practically no power/torque - the blade stopped when I tried to rip some 2"x2" softwood - it used to rip 9"x4" oak. The machine's been standing unused for about five years and everything was working fine the last time it was used, although I do remember working it quite hard on a dull blade - but just had to get the job done. All wiring connections are okay, I've tried swapping motors over, but it still ran slow. I'm guessing I need to find a replacement, or get it rewound - I'd just like to know what has caused it lose power so I can avoid it happening again. Many thanks in advance.
     
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  3. oldbutnotdead

    oldbutnotdead

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    Are you on a real 3 phase supply or using one of those converters from a single phase supply? Have you metered from phase to phase and each phase to neutral and phase to earth?
     
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  4. Simon35

    Simon35

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    Is it running slow without any load? Does it drive through a gearbox? Is any gearbox and bearings in working condition?

    If you've swapped the motor and it's still running slow, then as above, what's supplying it? Phase converter, VSD? VSD will have lost its settings after 5 years off.
     
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  5. fuzzytrifle

    fuzzytrifle

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    Thank you for your responses - yes it's a 3 phase supply, the machine is connected to the 3 phases and earth, there's no N connection, and there's no gear box. The three motors are identical with varying sized drive pulleys. I don't know how to test from phase to phase - swapping motors, changing the wires from like to like is about all I'm capable of/willing to do on a 3ph supply. I've swapped the supply, which comes from the machine's function selector switch, to the other two motors in turn, and both run at full speed on that supply. The faulty motor runs slowly (at about half speed) on all three supplies from the function selector switch.
    I had something similar happen to a planing machine some time ago, the machine ran at about half speed for a minute or so before stopping completely. The electrician found that the thermal overload switch had failed, I sold it for parts as the cost of a replacement motor and switch was more than the price of a used machine - this was before the availability of cheap invertors and nobody wanted small used 3ph machines.
     
  6. conny

    conny

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    Sounds like the motor may have developed shorts in the windings if the other motors run up to speed on the same supply.

    What size and speed motor is it? Anything below 11kw is usually binned and replaced by rewinding companies as it is the cheapest option unless it is a specialist motor.
    Do you have a rewinding company near you? If so they can test and best advise. They may also be willing to source you a replacement unit much cheaper than going direct to a supplier yourself.
     
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  7. fuzzytrifle

    fuzzytrifle

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    It's only a 2.2kw, 3HP motor, I've found the same spec motor on ebay for less than £150. So, considering the amount of use it's going to get I'll probably go for that, although I'm not sure what the quality will be like at that price.
     
  8. conny

    conny

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    It's possibly a used item but a lot of people strip down machinery and sell as spares because they can generate more money that way.
    Does the seller give any guarantees? Second hand is usually about 3 months whereas brand new should be at least 12 months.
     
  9. bernardgreen

    bernardgreen

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  11. fuzzytrifle

    fuzzytrifle

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  12. bernardgreen

    bernardgreen

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    Seller appears to be
    LEVER & WOOD LIMITED

    19 Stirlin Business Park,
    Sadler Road,
    Lincoln,
    Lincolnshire,
    England, LN6 3RS
     
  13. JohnW2

    JohnW2

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    Indeed. Given the preceding post from the OP, are you intending to imply that it is (or is not) a "reputable company" - and, if so, which?

    Kind Regards, John
     
  14. bernardgreen

    bernardgreen

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    The company may sell direct and give a better price than if bought through ebay.
     
  15. Lower

    Lower

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    All 3 phase, 2 pole, 2.2kw motors will run at the same speed. The speed is determined by the number of poles and the frequency of the electrical supply.

    Most 2.2kw motors will have the same frame size and the relative dimensions from the feet to the motor output shaft are standardised.

    We buy quite a few motors (around 50 a month) and pay about that price (inc VAT) for a decent quality, 4 pole, chinese manufactured, 2.2kw motor.
     
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  16. mattylad

    mattylad

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    So you have tried swapping motors and its still slow - then this to me sounds like you need to be learning how to check the supply voltages etc.
    Not just replace the motor as it may well do the same if the supply is low.
     
  17. JohnW2

    JohnW2

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    Has the OP not "covered all those bases" with ....
    :?:

    Kind Regards, John
     
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