|Glossary Intro and Glossary Annexes|
Glossary U 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.