F - Terms

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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 a 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
A figure of merit which indicates how much rectified current departs from pure (non-pulsating) DC. A large departure from unity form factor (pure DC, expressed as 1.0) increases the heating effect of the motor and reduces brush life. Mathematically, form factor is the ratio of the root-mean square (rms) value of the current to the average (av) current or I^rms/lav.

 
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.

 
FRACTIONAL-HORSEPOWER MOTOR
A motor usually built in a frame smaller than that having a continuous rating of one horsepower, open construction, at 1700 -1800 rpm. Within NEMA frame sizes FHP encompasses the 42, 48 and 56 frames. (In some cases the motor rating does exceed 1 HP, 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 (see frame size).

 
FRAME SIZE
Refers to a set of physical dimensions of motors as established by NEMA. These dimensions include critical mounting dimensions. 48 and 56 frame motors are considered fractional horsepower sizes even though they can exceed 1 horsepower, 143T to 449T are considered integral horsepower AC motors and 5000 series and above are called large motors. (For definition of letters following frame number, see Suffixes.)

 
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 more common. The frequency of the AC will affect the speed of a motor (see Speed).

 
FRONT END OF A MOTOR
The front end of a normal motor is the end opposite the coupling or driving pulley. (NEMA) 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.