Universal Motor:
[may combine this with Inductive
Electric Motor] A
universal motor is a series-wound or compensated
series-wound motor which may be operated either on Direct
Current or on single-phase Alternating Current at
approximately the same speed and output. The term
"Universal" comes from the from the fact that this motor can
be powered by either Alternating Current or Direct Current.
In operation, the universal motor is "series connected",
meaning that the same current passes through both the
armature (rotor) and field (stator). Most universal motors
are designed for operation at speeds of 3500 revolutions per
minute (rpm) and higher. Motors operating at a load speed of
8000 to 24,000 rpms are common.
The tradeoff for Speed is a lowering of “Torque”.
Torque, or Speed features of motors are a function of amount
of copper wire in armature. “The more copper wire, the more
speed in the motor”.
The speed of a universal motor can be adjusted by connecting
a resistance of proper value in series with the motor. The
advantage of this characteristic is obvious in an
woodworking power tool, such as a Router, where -- using
different types and sizes of Router Bits -- for safety, it
is necessary to operate the motor over a wide range of
speeds. Under these conditons, adjustable resistances are
used to vary the tool's speed.
A universal motor on a "portable" benchtop Jointer is
a mixed blessing. (In my
syllabus on the Jointer/Planer I discuss what
authorities say about universal motors on jointers.) On the
one hand, with variable speed control, it allows you to
control the speed of the cutterhead, which ranges from 8,000
to 16,000 rpm's -- simply by dialing a knob -- where the
high speed allows fine jointing of hardwoods like bird's eye
maple. The trade-off is high-pitched noise, but the
glass-smooth surface it produces can't be beat.
Sources: For background on this entry, sources
consulted include Edwin P. Anderson and Rex Miller,
Electric
Motors, Audel, 1968; Anonymous
article on Jointers,
Shopnotes no 48; John English Motors in the
Shop-- "The Differences Between Universal and Induction
Motors," WOODezine - Volume II - Issue III - MARCH 2004
http://www.woodezine.com/03_2004/54025_motors.html
For more info, the online Wikipedia encyclopedia has a
decent entry on
electric motor, including details about universal
motors.
