Just wondering .....

How stable is, and what affects the actual rpm (stability) of an AC motor ?

Very basically. Say, an AC, 110v 60 cycles  motor, nothing on the shaft, plugged directly into a wall socket.
Will the rpm be rock solid and constant ? Dependent on the 60 cycles being steady from the utility ? Quality of the motor construction ?
Or what else would / could cause an unstable rpm ? Absolutely no load. I assume over time due to thermal effects it might change slightly, but I mean fluctuations ... + / -
while running.
I have no real way to test, other than to put a disc on a motor shaft and move my index sensor to it. If I did, should I expect to see 0 wavering of the reading on the Mach screen RPM report ?

As usual ... just curious.
Thanks,
Russ
 

If you are talking an induction motor, one without windings on the armature, then the motor will always run less than synchronous speed. For 60 cycle AC a 2 pole motor will not run at 3600 rpm it will run a little less at no load. The 'slip' is needed so that the armature is moving through the magnetic fields to generate current and magnetize the armature. If it turned at 3600 rpm there would be no relative motion and no magnetization so no torque. This means an AC induction motor will run at a different speed for every load.

There are sychronous motors with wound rotors and slip rings that will run at 3600 rpm on 60 cycle power. However it only pays on really big motors.

Hope this makes sense

Hi Gary, makes perfect sense, thanks.
I was reading about synchronous motors and your explanation makes it even easier to understand.
I guess that's why an induction motor is either 1725 or 3450 instead of 1800 and 3600 respectively.
I understand that part.

What I am meaning to ask is not the actual rpm, but the fluctuations in the speed .... if any and what would affect that. (induction motors)

Thanks again,
Russ

The two things that affect the rotation speed are line frequency and load.  I once installed a large diesel generator for a customer because he was monitoring line frequency and it varied from 59 Hz to 61 Hz all the time.  This was affecting his process, and when he complained to the power company they told him they don't guarantee 60 Hz continuously, they provide an average of exactly 60 Hz over the course of a year! Load is the big variable, because the motor needs the slip to work.

If you need a really constant speed you will need feedback or a stepper motor. Microstepped motors will run at a very constant speed because the step frequency is typically crystal clock controlled in the drive.  Another way to get a very constant speed is with a motor and an encoder.  Then it doesn't matter to much what kind of motor it is, DC, Servo, 3 Phase with Inverter etc. As long as the drive has an encoder input you can get very steady speed. Lots of choices, and modern electronics has made this pretty cheap to do.

If you tell me what is you are trying to do I can make some suggestions.

Not looking for any specific rpm.
Just looking for a steady, stable non wavering NO LOAD rpm from a common AC induction motor.
.....and what might affect that, if anything.

" it varied from 59 Hz to 61 Hz all the time."
What was the frequency of these variations ? Per second ? per minute ? per hour ? week ? ........
 
Thanks,
Russ

So I did a test.
Put a slotted disc directly on the .5 hp motor, nothing else, no load. Mounted the sensor.
Mach reports 1797-1799.  No Index debounce set, Not averaging.
Put 2 precision machined V pulleys on, new v belt plenty tight, remounted the sensor at the spindle disc.
Mach reports 195* - 199*. WAYYYY to goofy to cut good threads.
(pulley ratio is just what I had available for the test, I'd normally reduce about 2:1)
The belt is not perfect, but it's new and shouldn't wander the spindle rpm THAT much, but maybe a little.

Is the motor too small maybe ? It doesn't slow during threading, it's just not stable.
Does it need a timing type belt and pulleys maybe ?

I know, I know .... a servo would be the bees knees, I know. Or a vector type motor and drive with feedback, both a bit pricey.
I'm seeing more and more why commercial units cost so much. 

Thanks,
Russ
 

You got my curiousity Russ.

Don't have any rpm checkers here that are accurate enough to check the motor rpm.
If you get a loan of one make sure you know what the % accuracy since don't think some of cheaper ones will give you +-1 rpm at 1750.

Way back, when we were checking the index on my lathe, we used  a very accurate pulse counter and compared index input to Mach vrs displayed rpm and pulses sent to the axis and it was very close. I know that varying belt tension on say a stepper that it will change steps per unit, but the steps per unit are repeatable per rev.

I do have faith in the DRO readout as being accurate, again based on what Mach is being told.
The problem is one needs to isolate each step for comparison, as you simply did, but that must be compared to a known / calibrated standard. IE. indexer puts out x pulses compared to an electronic device that is more accurate then the index output.

RICH

I pretty much trust the Mach display, my test kinda shows that. (fluctuations)
I've almost come to the conclusion that one really needs to overkill the spindle motor HP (short of feedback).
I might slap a 2hp inverter duty motor on this puppy just to see.
If that's what it takes, so be it.

Thanks Rich,
Russ

Russ,
I think a V-belt drive is problematic too. As you apply a load the belt pulls down into the V and changes the effective diameter.  This might cause a fairly large speed change.  The real problem all the way around is that in thread cutting you load and unload the motor at the beginning and end of the cut.  What you really need is for the X axis (carriage) to follow the A (spindle axis). Think of it as a linear move using the X and A together. I am adding a little lathe attachment to my mill and I am driving the spindle with a stepper so I can use it as a fourth axis and I am thinking threading too.  Don't know how that will work. However I have a large driver on the   A axis and I have 960 in/oz stepper and can get a 1600 oz/in stepper driven at 68 vdc so that should be too bad for a mini lathe.

Russ,
I think a V-belt drive is problematic too. As you apply a load the belt pulls down into the V and changes the effective diameter.  This might cause a fairly large speed change.  

I can certainly vouch for that ! Even at no load, the slight variations in the belt width throughout its periphery WILL introduce a variable rpm at the output shaft/spindle. Also a larger, thicker cross section belt will have an oval shape preset in it from packaging (maybe the mfg process too ?) that WiLL again introduce a sort of "surging" in the output as each end of the preformed oval comes into  alignment with the pulleys. You can easily feel this effect while turning the spindle by hand. An undersized motor will definitely chase this variation up and down .... I recently proved that here with testing. Thin section, 3/8" v belts are better, but still not as good as a flat XL type. As a test, a V belt, running on its back, on XL type pulleys is quite stable, provided the belt does not have a strong memorable oval set into it, but torque transfer is not very good .... this is just a test.

What I'm finding ...... Overkill on motor size (open loop, induction), and flat, flexible belts.

Thanks Gary,
Russ