Re: [ATM] DC Motor for grinding machine question

["Don Clement" <don_clement@charter.net>]

In addition to the ability to control motor speed, DC motors and three-phase
(polyphase) motors (with VFD) share a common advantage over the AC
single-phase motor. That advantage is the superior smoothness of DC motors
and three-phase motors. I have experienced  firsthand the superior
turbine-like smoothness of three-motors and DC motors using my lathes. The
effects of the pulsating single phase AC motor show up as a periodic or
harmonic pattern in the surface finish of a turned part. The same must hold
true with a polishing machine, which the effects of using a single-phase
motor will show up in the surface finish of a polishing machine. I also
think the surface finish of a polishing machine would benefit from an added
random "white noise" signal either into the motor or through a inline
solenoid (voice coil actuators).



Here is what E.A. Loew said  about the smoothness of three-phase motors in
"Direct and Alternating Currents Theory and Machinery" 1946:



"At a given load, polyphase generators and motors develop a constant torque,
like a steam turbine, while the torque of a single-phase motor is pulsating,
like the torque of a

reciprocating engine. It was shown in Chap. XXIII  that the power in a
single-phase circuit is pulsating. Since the power output of a generator or
motor is proportional to the product of its torque and speed, it follows
that for a given speed the instantaneous torque is proportional to the
instantaneous power and pulsates in the same manner as the power. When the
power factor is less than unity, the torque is actually negative twice
during a cycle. The single-phase motor must depend upon its flywheel effect
to carry it over the periods when its developed torque is less than that
required by the load. The constancy of the torque developed in polyphase
motors is demonstrated in the chapter on motors."





Don Clement

Running Springs, California

_______________________________________________
ATM mailing list http://www.atmlist.net/