What are the essential elements in the basic knowledge of motor winding? Understanding these fundamentals is crucial for anyone involved in electric motor design or maintenance. In this article, we delve into the basic knowledge of motor winding, including key concepts such as mechanical and electrical angles, pole pitch, coil pitch, and wind factors. By exploring these topics, readers will gain a solid foundation in motor winding, enabling them to optimize motor performance and efficiency.
Mechanical Angle vs. Electrical Angle
When arranging motor windings within the lamination core slots, a specific pattern must be followed to generate symmetric sinusoidal AC or a rotating magnetic field. Besides other parameters, the concept of electrical angle is crucial to understanding the relative positions of coils and windings.
In mechanics, a circle is divided into 360 degrees, known as the mechanical angle. In electrical engineering, the angle measuring electromagnetic relationships is called the electrical angle. It divides each cycle of a sinusoidal AC into 360 degrees along the horizontal axis. As a conductor passes through a pair of magnetic poles, it corresponds to a 360-degree electrical angle.
The relationship between electrical and mechanical angles in a motor is given by:
Electrical Angle(α)=Number of Pole Pairs×Mechanical Angle×360°
For a two-pole motor, the electrical angle equals the mechanical angle, while for a four-pole motor, the electrical angle is double the mechanical angle.
Pole Pitch (τ)
Pole pitch refers to the distance along the circumference of the motor’s lamination core that each magnetic pole occupies. Typically, this is measured as the slot pitch between the centers of two adjacent magnetic poles. For the stator, this distance is calculated along the inner circular surface’s air gap, while for the rotor, it is along the outer circular surface’s air gap.
There are two common ways to express pole pitch: in terms of length or the number of slots. The latter is more frequently used. Pole pitch is generally calculated using the formula: 𝜏=𝑍/2𝑝
where 𝑍 is the number of slots, and 𝑝 is the number of pole pairs.
Coil Pitch (y)
Coil pitch is the number of slots spanned by a coil’s two sides. It can be classified into three types:
Full Pitch: When the coil pitch equals the pole pitch (y=τ).
Short Pitch: When the coil pitch is less than the pole pitch (y<τ).
Long Pitch: When the coil pitch is greater than the pole pitch (y>τ).
Short-pitched windings are common due to their advantages like reduced copper usage and improved power factor.
Winding Factors
Winding factors are crucial for optimizing motor performance. They include:
Pitch Factor (Kp): Adjusts for the effect of short-pitched coils.
Distribution Factor (Kd): Accounts for the distribution of coils within the slots.
The overall winding factor is the product of these two factors: Kdp1=Kd1Kp1.
Slot Pitch Angle (α)
Slot pitch angle is the electrical angle between adjacent slots:
α=Total Electrical Angle/ Number of Slots=Zp×360°
Phase Belt
A phase belt refers to the portion of each phase winding that occupies a specific region under each magnetic pole. This is typically represented by the electrical angle or the number of slots. In a three-phase motor, the winding under each pole pair is divided into six regions, with three regions per pole.
Given the slot pitch angle 𝛼=360°×𝑝/𝑍, for a motor with 4 poles and 24 slots, each region of the phase belt is qα=Z/6P*360P/Z=60°. Winding arranged in this manner is known as a 60° phase belt winding. Due to its clear advantages, such as balanced magnetic fields and reduced harmonics, this type of winding is commonly used in three-phase motors.
Slots per Pole per Phase (q)
Slots per pole per phase refer to the number of slots allocated to each phase winding under each magnetic pole. This number determines the number of coils within each phase winding for each pole. It is calculated using the formula: q=Z/2 Pm.
where: Z is the number of slots in the core, 2P is the number of poles, and m is the number of phases. If q is an integer, the winding is called an integer slot winding. If q is a fraction, it is known as a fractional slot winding.
Conductors per Slot
The number of conductors per slot in motor windings must be an integer. For double-layer windings, this number should be an even integer. The number of conductors per slot in a wound rotor winding is determined by its open-circuit voltage. In medium-sized motors with wound rotors, there must be two. The number of conductors per slot in the stator winding can be calculated using the formula: NS1=NΦ1m1a1/Z1.
NS1 is the number of conductors per slot in the stator winding,
NΦ1 is the conductors per slot based on air gap flux density,
m1 is the number of phases in the stator winding,
a1 is the number of parallel paths in the stator winding,
Z1 is the number of slots in the stator.
Conductors per Phase
Conductors per phase refer to the total number of series-connected turns in each phase winding of the motor. This number depends on the parallel paths within each phase winding. If the motor has a single parallel path, all series turns under each pole must be summed to determine the total number of series turns in the phase winding.
For motors with multiple parallel paths (e.g., two or three paths), the number of series turns per phase is based on one parallel path. This is because the series turns in each path are identical, and their parallel connection does not increase the total series turns.
Total Number of Coils
Motor windings are made up of coils of varying sizes and shapes. Each coil has two sides placed into the core slots. In single-layer windings, each slot contains one coil side, so the total number of coils is half the number of slots. In double-layer windings, each slot contains two coil sides, making the total number of coils equal to the number of slots.
FAQS
What are motor windings?
Motor windings are conductive copper wires wound in specific patterns within a motor. They generate a magnetic field when electrical current passes through them, converting electrical energy into mechanical energy to make the motor shaft rotate.
What types of motor windings exist?
Motor windings can be classified into single-phase and three-phase windings. Single-phase windings are found in household appliances and consist of main and auxiliary windings. Three-phase windings, used in industrial motors, consist of three windings spaced 120 degrees apart and can be connected in star or delta configurations.
Conclusion
Understanding motor windings is essential for anyone involved in the design, maintenance, or operation of electric motors. These basic concepts of mechanical and electrical angles, pole pitch, coil pitch, winding factors, slot pitch angle, phase belts, slots per pole per phase, conductors per slot, and the total number of coils form the foundation of motor winding knowledge.