Field: A term commonly used to describe the stationary (stator) member of a DC motor. The field provides the magnetic field with which the mechanically rotating (armature or rotor) member interacts.
Field Weakening: The introduction of resistance in series with the shunt wound field of DC motor to reduce the voltage and current which weakens the strength of the magnetic field and thereby increases the motor speed.
Flange: Mounting endshield with special rabbets and bolt holes for mounting such equipment as pumps and gear boxes to the motor or for overhanging the motor on the driven machine.
Flux: The magnetic field which is established around an energized conductor or permanent magnet. The field is represented by flux lines creating a flux pattern between opposite poles. The density of the flux lines is a measure of the strength of the magnetic field.
Form Factor: The ratio of RMS current to average current. This number is a measure of the current ripple in a SCR or other switch-mode type of drive. Since motor heating is a function of RMS current while motor torque is a function of average current, a form factor greater than 1.00 means some fraction of motor current is producing heat but not torque.
Form Wound: A type of coil in which each winding is individually formed and placed into the stator slot. A cross sectional view of the winding would be rectangular. Usually form winding is used on high voltage (2300 volts and above) and large motors (449T and above). Form winding allows for better insulation on high voltage than does random (mush) winding.
Four Quadrant: Refers to a motion system which can operate in all four quadrants; i.e., velocity in either direction and torque in either direction. This means that the motor can accelerate, run, and decelerate in either direction.
Fractional-Horsepower Motor (FHP): A motor usually built in a frame smaller than that having a continuous rating of one horsepower, open construction, at 1700-1800 rpm. Within NA frame sizes, FHP encompasses the 42, 48 and 56 frames. (In some cases the motor rating does exceed one horsepower, but the frame size categorizes the motor as a fractional.) The height in inches from the center of the shaft to the bottom of the base can be calculated by dividing the frame size by 16.
Frame: The supporting structure for the stator parts of an AC motor. In a DC motor, the frame usually forms a part of the magnetic coil. The frame also determines mounting dimensions.
Frame Size: Refers to a set of physical dimensions of motors as established by NA. These dimensions include critical mounting dimensions. NA 48 and 56 frame motors are considered fractional horsepower sizes even though they can exceed one horsepower. NA 143T to 449T is considered integral horsepower AC motors and 5000 series and above are called large motors.
Frequency: The rate at which alternating current makes a complete cycle of reversals. It is expressed in cycles per second. In the U.S., 60 cycles (Hz) is the standard while in other countries 50 Hz (cycles) is common. The frequency of the AC current will affect the speed of a motor.
Frequency Response: Frequency response for an incremental encoder is the maximum frequency of the output signal in Hertz.
Friction: A resistance to motion caused by contact with a surface. Friction can be constant with varying speed (Coulomb friction) or proportional to speed (viscous friction).
Front End of A Motor: The front end of a normal motor is the end opposite the coupling or driving pulley (NA). This is sometimes called the opposite pulley end (O.P.E.) or commutator end (C.E.).
Full-Load Current: The current flowing through the line when the motor is operating at full-load torque and full-load speed with rated frequency and voltage applied to the motor terminals.
Full-Load Torque: That torque of a motor necessary to produce its rated horsepower at full-load speed, sometimes referred to as running torque.
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