# RPM for a southbend 9 motor?



## pineyfolks (Sep 19, 2013)

I had to replace the motor on my 12 speed 9inch sb, the original motor was 3450 rpm. The new one is also but it seems to be running faster at top speed than the old worn out one. My other 9inch sb has a 1725 motor. Did they make them with both motor rpm's? I'm sure the 3450 motor was original as it had locating pins on the base. It has a 2 step motor pulley 2" and 3 5/8" dia. It's running fine and nothing is even warm to the touch when running. What rpm motors do you have on yours?


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## Halligan142 (Sep 19, 2013)

I always thought they were all 1725 rpm just in different HP, brands, and types (I.E. Westinghouse vs GE  I have a GE on the lathe and a Westinghouse on the SB drillpress, Instant reverse cap. start vs the old repulsion-induction motors).  I could be wrong though.  I do know that the 12 speed lathes had a speed range of 50-~1200 rpm at the spindle per the catalogs.  If your much above that I'd say you have the wrong rpm.


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## schor (Sep 19, 2013)

pineyfolks said:


> I had to replace the motor on my 12 speed 9inch sb, the original motor was 3450 rpm. The new one is also but it seems to be running faster at top speed than the old worn out one. My other 9inch sb has a 1725 motor. Did they make them with both motor rpm's? I'm sure the 3450 motor was original as it had locating pins on the base. It has a 2 step motor pulley 2" and 3 5/8" dia. It's running fine and nothing is even warm to the touch when running. What rpm motors do you have on yours?



If you have a speed chart and an rpm gauge you can find out if your too fast. Just setup for whatever rpm and measure it.


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## wa5cab (Sep 20, 2013)

It is possible to build a single-phase AC motor that runs at RPM's between 3400 and 3500 off of 60 CPS mains.  These are 2-pole motors and would typically be a little cheaper to build than the more common 4-pole units that are spec'd for RPM's between 1700 and 1750.  But they have lower starting torque and higher pulsing or vibration per revolution than the 4-pole motors.  Anything is possible but I'd be surprised if SB ever spec'd a machine with a 2-pole motor.

If you don't have the manual for your 9", get it.  Although it likely won't list pulley diameters, it should have a chart giving spindle RPM for each location of the belt.  Pick one of the locations, measure the pulley diameters, and calculate the motor RPM required to get the stated spindle RPM.  That would be the motor RPM that SB intended the lathe to be operated from.

One further comment - single phase non-synchronous AC motors run at a slip angle to the rotating AC supply.  The theoretical synchronous RPM for a 4-pole motor running on a 60 CPS supply would be 1800 RPM.  Capacitor start 4-pole AC motors typically run at a loaded RPM between 1750 and 1700 RPM.  At no load, they will approach 1800 RPM.  Different manufacturers will state different RPM's on the nameplate, depending upon their mind-set.  Anything within the range may be taken as equivalent.  Or to put it another way, if your old motor nameplate said 1740 RPM and the new one says 1725, don't worry about it.  The faster RPM was probably measured at a lower percentage load.

Robert D.


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## martik777 (Sep 20, 2013)

Original motor was definitely 1725 rpm. Mine is still running great - love the almost instant reverse!

The countershaft will probably vibrate and noise level will go way up with a 3450


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## GK1918 (Sep 20, 2013)

Myself I always compair things as though, you say a straight 9 so my thinking says its pre 9A so being a car the motor is a 85hp V8
flathead.  So running a say 1938 Ford at 3450 rpms or any make,  aint going to take long.


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## pineyfolks (Sep 20, 2013)

I'll have to check the rpm with a tach, I just thought it was odd that 2 motors listed with the same rpm and same pulleys seem to make different spindle speeds. It seems like I'm 1 speed higher than I had before.


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## pineyfolks (Sep 20, 2013)

This is my old motor,  I was reading it as 3450. I checked my top rpm and I'm getting 1255. I guess the old motor was getting a little slow after 76yrs.


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## ScrapMetal (Sep 20, 2013)

Just curious, were you running the old motor on single phase 440 volts?

-Ron


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## pineyfolks (Sep 20, 2013)

ScrapMetal said:


> Just curious, were you running the old motor on single phase 440 volts?
> 
> -Ron


It was run as I bought it on 110. I couldn't make out the tag where the number 440 is so I have no idea what it refers to. Maybe someone can tell


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## the gentleman (Sep 20, 2013)

1725 RPM motor . . Do not get a 3450 RPM


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## pineyfolks (Sep 20, 2013)

I jumped this over to the electrical issues thread to try to find more info on my old motor. Thanks for your help.


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## ScrapMetal (Sep 20, 2013)

pineyfolks said:


> It was run as I bought it on 110. I couldn't make out the tag where the number 440 is so I have no idea what it refers to. Maybe someone can tell



I'm just wondering (don't remember my power electronics too well any more) what the result would be if that motor was built for single phase 440v and is being run on 120v.  Could be that you wouldn't be getting the 3250 rpm that it is rated at and may be closer to the 1750 rpm that the lathe is rated for.  That could explain the difference in speed you are noticing between it and the new motor.

Pure conjecture,

-Ron


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## Ulma Doctor (Sep 21, 2013)

1725 rpm is the correct motor speed regardless of voltage.


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## wa5cab (Sep 21, 2013)

Ron,

I think that you must be correct about the voltage rating on the old motor being 440.  The nameplate would certainly include the voltage rating, and it does not appear anywhere else.  As to what RPM it "should" run at on 1/4 voltage, who knows?  That far out of spec, all bets would be off!

Robert D.


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## pineyfolks (Sep 21, 2013)

The motor ran fine for the 20+ yrs I've owned it and was being used at the shop I worked at for who knows how long before that. I'm starting to think the end plate had to have been switched as others have said. It was a war production lathe and they may of had to get it running with what they had.


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## OldMachinist (Sep 21, 2013)




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## wa5cab (Sep 22, 2013)

OK.  That makes more sense.  Running a 440 motor on 120 would be about like running a 120 motor on 30.  Probably do nothing but hum.  However, unless the 1155 RPM is a recent failure indicator, it would have had to have been a 6-pole 120 motor.

Robert D.



pineyfolks said:


> The motor ran fine for the 20+ yrs I've owned it and was being used at the shop I worked at for who knows how long before that. I'm starting to think the end plate had to have been switched as others have said. It was a war production lathe and they may of had to get it running with what they had.


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