U.L. (Underwriter's Laboratory): An independent testing organization, which examines and tests devices, systems and materials with particular reference to life, fire and casualty hazards. It develops standards for motors and controls used in hazardous locations through cooperation with manufacturers. U.L. has standards and tests for explosion-proof and dust ignition-proof motors, which must be met and passed before application of the U.L. label.

U Frame: A previously used NA designation indicating frame size and dimension (prior to 1965 the standard frame sizes per horsepower rating).

Unipolar Driver: A step motor driver configuration that uses a unipolar power supply and is capable of driving phase current in only one direction. The motor phase winding must be center tapped (6 or 8 lead) to operate with a unipolar driver. The center tap is used instead of providing the current reversal of a bipolar driver.


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