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General knowledge about working with an AC motor? Answered

Hi Everybody!  I recently salvaged a 1/3 hp single phase AC motor, which I'd like to use for a lathe.  I've been looking at diy lathes on instructables and elsewhere and have found that DC motors are mostly used in these kind of projects.  I understand that DC is generally battery powered and AC requires a power cord and each can be converted to the other.  My question is whether an AC motor is suitable for a lathe and whether any extra safety concerns are involved.  Should I convert it to DC?  Should I buy a DC motor instead? 

Also, any information about whether or not the single phase business is a problem would be helpful too. 

I have found a lot of interesting theory and a few interesting tidbits online, but I could really use some experienced opinions.  And I don't know anywhere better than instructables to ask for those.

Help, please, and thank you!

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steveastrouk

Best Answer 7 years ago

Lathes on here are largely toys. Real ones use AC motors - 1/3hp will be quite nice for a small lathe - we have lathes that can swing 6" work, that runs nicely on a slightly bigger one.

Speed control is difficult with AC motors, in a lathe that's a benefit in some ways. The only way to change speed is with pulleys.

Steve

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gruffalo childsteveastrouk

Answer 7 years ago

'Speed control is difficult with AC motors' This is exactly what I wanted to hear for a week!!!
Can't we somehow adjust the input frequency to change speed?

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steveastroukgruffalo child

Answer 7 years ago

Yes, but with BIG caveats.

1.) YOU CAN'T GET THE SAME POWER OUT AT LOWER SPEEDS - is the first one, both from the basic physics (ie Power = n x torque - and MAXIMUM torque is fixed for the motor.
2.) Even operated in limits, the cooling fans DON'T WORK as efficiently at lower speed.
3.) The equivalent circuit of an AC motor, as seen at its terminals IS A FUNCTION OF SPEED AND LOAD. At rest for example, its nearly a short circuit. When you add frequency into the mix, the maths gets nasty quickly, and that's why 32 bit embedded processors are used in drives......

Steve

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ARJOON

7 years ago

just use a permanent splt capacitor motor from sewing machines. it has speed control. in my mum's factory there are dozens of 3.5hp ones lying on the floor. she got those with a very reasonable price. buy 1get 1 free. you can get it in closed factories for very cheap too cheap. i bought a very powerful thyristor for Rs25

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steveastroukARJOON

Answer 7 years ago

3.5 hp ? That's a heck of a sewing machine ! What did they make ?

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ARJOONsteveastrouk

Answer 7 years ago

yeah belt driven. look for looping machines.all sewing machines connected to one motor on a long table . i will send you a photo tomorrow if you don't believe it.

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steveastroukARJOON

Answer 7 years ago

Ah ! That's what's called a line-shaft drive.

Steve