Re: A more accurate approach to machine induction motor rotors to become synchronous

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Kurt Schraner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <k.schraner-at-datacomm.ch>

Hi RMC, all

for my 40Watt 2-pole benchgrinder motor(=3000RPM), with a rotor diameter
of 54mm the 40% surface rule would lead to a 31.7mm-secant length of 
the  flats. My realized 25.5mm flats are slightly over optimum of ~24.5mm. 
This optimum corresponds to 30% surface, rather than 40%. No experience 
from my side with 4-ploe motors, but I rely on Finn Hammer's posting.

BTW: for coiler purpose, where only 2- or 4-pole motors are used, the 
"%surface rules" can be converted to more directly applyable 
"secant/diameter" rules:

2-pole motors: 30%surface ---> secant=0.454*diameter of motor-rotor
4-pole motors: 40%surface ---> secant=0.309*diameter of motor-rotor

It would be of interest, what other coilers have recorded as successful or 
not so succesful flat sizes, in relation to their motors. Then, a more 
reliable base for a "flat-formula" could be gained. I'm ready to collect 
e-mailed data and try converting them to a formula and posting the results, 
if it's desired. In case, you are interested, please report (off-list, to 
k.schraner-at-datacomm.ch) at least:

- number of poles (2 or 4)
- rotor diameter
- flat length (=secant)
- successful sync?

and, if possible, some or all of the following:

- length of rotor
- name-plate power and -voltage
- run capacitor value
- voltage needed for falling in sync
- voltage for falling again out of sync
- size (dia.) and weight of rotary-disk + electrodes
   (ideally: moment of inertia)
- startup time, till sync

I'm not quite shure, if just a geometric relation between flat secant 
length and diameter is all we need for practice. Corrections welcome!


Cheers, Kurt




Tesla list schrieb:
>Original poster: "RMC by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><RMC-at-richardcraven.plus-dot-com>
>Various rules-of-thumb have been written, regarding the amount of rotor
>material to be machined off to make flats to enable synchronous operation.
>This has usually been in the form of "remove a 1 inch depth of material" or
>" make the flats 2 inches across" etc. etc.
>These rules don't take into account the rotor size in the first place,
>whereas a 1939 text by Veinott, published by McGraw-Hill, discusses the
>method a little more precisely.
>Chapter 15, titled "Synchronous Motors", describes the process of
>calculating the size of the flats thus:
>"... it is recommended that the easiest way [to make a non-synchronous motor
>become synchronous] will probably be to mill flats on the rotor surface,
>equally spaced, there being as many flats as there are poles. The total area
>of these flats should be approximately 40 per cent of the total cylindrical
>area of the rotor before the flats are milled ... The motor should be able
>to carry roughly about one-third its nameplate rating".
>So, if people want to get their rotors to lock up *and* avoid removing more
>material than necessary (torque suffers as a result), this is the approach
>to use rather than just assuming a set depth independent of rotor size.
>
>Cheers
>RMC, England
>