 Previous Page 151.04
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Introduction
to AC Motor Rewind
Background The first step in preparing a stator for rewinding is to determine the
type of winding it has, or should have, and develop the appropriate winding diagram. In
order to do this, you must understand: Once all the necessary information has been assembled, development of the winding diagram can begin.
Basic rewind terminology, formulas, SYNCHRONOUS SPEED: This term refers to the speed of the rotating magnetic field in the stator. The motor rotor will never turn at synchronous speed due to various mechanical friction losses, windage, and built-in slip. The difference between synchronous speed and actual rotor speed is called slip and is expressed in a percent of synchronous speed. The usual amount of slip in a 60-Hertz, three-phase motor is two or three percent. Synchronous speed must be known to determine the number of poles and pole-phase groups (PPGs) in the stator winding and can be easily determined by use of the following formula:
Example: Synchronous speed of a four-pole motor N = 120 x 60 ÷ 4 PHASES: As the name implies, three-phase motor stator windings will consist of three separate groups of coils called phases. Phases must be displaced from each other by 120 electrical degrees. Phases must be electrically balanced (contain the same number of coils) and connected for the same number of poles. Phases will always be designated as A, B, and C. |
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| POLES: Poles refer to a coil or group of coils wound and connected to produce a unit of magnetic polarity. Poles are referred to as either north or south. The number of poles a stator is wound for will always be an even number and refers to the total number of north and south poles. For example, a four-pole motor will have two north and two south poles alternately spaced around the stator. (See figure at right.) The number of poles in the stator must be known to determine the number of PPGs in the motor and can be easily determined by the formula below. P = 120 x F ÷ N Example: P = 120 x 60 ÷ 1800 ACTIVE COILS PER PHASE: An active coil is a coil that is electrically connected in the stator circuit. (It is sometimes necessary to install, but not connect, certain coils in the stator winding. Coils that are not connected are called dummy coils and will be discussed next in this lesson.) Each phase must contain an equal number of active coils to ensure electrical balance (phase current). In almost all cases, the total number of coils in the stator will be equal to the number of slots. Active coils per phase can always be determined by dividing the total number of coils by three (number of phases). Example: In this example the answer is a whole number, meaning all coils in the winding will be active. In this case the winding is known as an integral slot winding. If the answer is a mixed number, the winding is called a fractional slot winding. Example: In this example, the winding will require the use of two dummy coils to compensate for the fractional number. DUMMY COILS: As discussed previously, dummy coils are used to fill slots but are not electrically connected in the stator circuit. If only one dummy coil is required, it can be installed in any slot. If two dummy coils are required, they can be installed in any slots 180 mechanical degrees apart. Dummy coils will always be left open circuited. POLE-PHASE GROUPS (PPGs): This term refers to the number of coils connected in series to form the magnetic pole of a phase. Each phase must contain an equal number of PPGs. Total PPGs can be determined by multiplying the number of motor poles by three (number of phases). Example: 4 poles x 3 phases = 12 PPG |
Click on image
to enlarge. Alternate Pole Polarities for One Phase of a Four-Pole, Three-Phase AC Motor
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More ac motor rewind terminology  |
For purposes of this course, motors are considered to be
four-pole, three-phase, 60 Hz. 