# 101.28940 original motor specs?



## Czimm (Nov 23, 2013)

My Craftsman 101.28940 (which I got a week ago) does not have it's original motor.  So I'm wondering what the RPM of the original motor was.  I'm thinking that if it is drastically different than the motor that is currently on my lathe, the RPM's will all be different than those listed on the speed chart.  The replacement motor has a RPM of 1725 at 115V.  Your help and thoughts are appreciated.


----------



## Privateer (Nov 23, 2013)

The Atlas manual for the early lathes calls for a 1740 RPM 1/3 or 1/2 HP motor. That's directly from the text. Most I've seen are 1725 RPMs off the shelf. I purchased a HF motor 1/2hp 1725 rpm and its been serving me quite well. 

Terry


----------



## pipehack (Nov 23, 2013)

I bought a Cman lathe not too long ago. I had the original motor which crapped out on me shortly after purchasing it. I got a free motor from my buddy who owns a HVAC shop. 1725 rpm with a 5/8" shaft. Works well and came with the mounting bracket. A little wiring and it was done. My point is see if you can find one of those motors. I really don't think that 15 rpm (1725 or 1740) is going to make a bit of difference.  IMHO.


----------



## wa5cab (Nov 24, 2013)

*Re: 101.28940 original motor specs?  And motor standards.*

Czimm,

Synchronous (maximum) speed or RPM for an AC induction (as opposed to series) motor depends upon the number of magnetic poles in the motor and the frequency of the AC mains.  In the US, the numbers are 3600 RPM for 2-pole, 1800 for 4-pole, 1200 for 6-pole, etc.  

The most common type of motor used on home and small industrial machines in the US since about the mid 1930's is the single-phase 4-pole capacitor start type.  In a vacuum with no load, these will all run at very close to 1800 RPM.  But as you actually load one of these motors, the slip factor increases (it slows down) slightly.  Methods for rating (i.e., what's printed on the nameplate) these motors for horse-power and RPM have changed slightly over that period.  For the most part, a motor rated 1/2 HP/1740 RPM in 1940 would today be rated 1/2 HP 1725 RPM.  And under the same actual load would produce about the same actual spindle RPM.  

So don't worry about the difference between how your lathe will run with a 1950's vintage original rated 1740 or 1750 RPM or with a new one bought last week rated 1725 RPM.  Unless the new one is an overrated cheap Chinese model, which wasn't a problem that one had to worry about 60 years ago...  

Robert D.


----------

