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Is there a permanent magnet ac motor ?

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  1. #1
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    Default Is there a permanent magnet ac motor ?

    I have an automatic coffee machine which recently went out with a flash and the safety switch tripped. Inside is a brush type motor with a capacitor from each brush to ground, one capacitor didn't look too healthy so I have replaced it. The motor is a permanent magnet motor marked 230v 50Hz, the terminals are marked + and - .Connected to the mains through a series lamp current limiter, the shaft just oscillates back and forward about 1/10 turn with the lamp lighting up to a good level. The same setup but with a bridge rectifier in the circuit runs the motor as you would expect, reverse the connections and the motor runs in the other direction, lamp does not come on at all.
    What gives with 50Hz and + and - marked on the terminals, I always thought that a universal ac/dc motor had feild coils not permanent magnets, does anyone know of a permanent magnet motor that runs on ac, if there is such a thing then I guess that the motor is stuffed as it won't run on ac.

  2. #2
    The Master's Apprentice Bedford's Avatar
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    Quote Originally Posted by familyguy View Post
    does anyone know of a permanent magnet motor that runs on ac
    I'm not totally sure, but I installed an exhaust fan today that had the smallest motor I've ever seen (on an exhaust fan), about 50mm in diameter. When I spun the blades before installing, they appeared to have intermittent tension, indicating permanent magnets, but didn't look any further.
    Posted by John2b, And no, BEVs are not going to save the planet, which doesn't need saving anyway.

  3. #3
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    Quote Originally Posted by familyguy View Post
    does anyone know of a permanent magnet motor that runs on ac.
    Many 'stepper motors' are PM AC.
    Also, syncronous PM AC motors are relatively common (aka 'brushless DC motors).

    If you are certain that the magnets are permanent & not wire wound, then what you have is a DC machine (because it has a commutator). In this case, it can only be driven by some kind of DC supply. A zener diode is quite common. Is there a zener diode or rectifier inside the coffee machine?

  4. #4
    2K Club Member chrisp's Avatar
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    Some pictures would be good. Also, what does this motor do in the machine?

  5. #5
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    Thanks for the prompt replies.
    Buried deep inside the machine I found a printed circuit board with two bridge rectifiers, one of them feeds the motor via a change over relay that reverses the motor rotation when needed. I believe that the motor is in fact a dc motor and is working ok as evidenced by my earlier test with a bridge rectifier. The motor is the main power source for the coffee maker, an automatic type that grinds the beans, and then brews the coffee, the motor loads the ground coffee into the brew unit and then packs the ground beans down. The motor is without a doubt a permanent magnet motor. The motor housing may be a common part used in a variety of motors hence the 230v 50Hz printed on it.
    I re-assembled the machine it turns on for about 15 sec. and then shuts down, no error message on the lcd display. The original problem where the machine went out with a flash and a bang was caused by moisture in the works, as it looks like the part that supplies water under pressure has a leak on the underside allowing moisture into the section housing the electrics. There are flash marks around one of the motor terminals and also on the adjacent damaged capacitor where the short occurred.

    On tracing the circuit board I can see a surface mount triac supplying the bridge rectifier for the motor, there could be a fault on this board or possibly on the cpu board. I haven't looked yet but I'm guessing that the cpu board has some sort of one chip computer similar to a picaxe or basic stamp. Not having access to spare boards I would need to buy them and unless they are reasonably priced (less than $75 for both) it's not worth repairing as it probably needs a new brew unit to replace the leaking one.

    The machine in question is a Sunbeam Intuitive, mine is about 1.5 - 2 yrs old and looking at online reviews, it seems I've had a good run.

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