Every modern vehicle is equipped with an electric generator, which produces current to operate the on-board electrical system and all its devices. One of the main parts of a generator is the fixed stator.
Generator Stator Function
Modern automobiles and other vehicles use synchronous three-phase alternators with self-excitation. A typical generator consists of a fixed stator mounted in a housing, a rotor with field winding, a brush assembly (which supplies current to the field winding), and a rectifier unit. All parts are assembled in a relatively compact design that is mounted on the engine and belt driven from the crankshaft.
The stator is the fixed part of an automobile alternator that carries the operating winding. During generator operation, it is in the stator windings that electric current is generated, which is converted (rectified) and fed into the on-board network.
The stator of an alternator has several functions:
- Carries the operating winding in which the electric current is generated;
- Acts as a housing part to house the operating winding;
- Plays the role of a magnet core to increase the inductance of the working winding and the correct distribution of the magnetic field lines of force;
- Acts as a heat sink - dissipates excessive heat from heated windings.
All stators have essentially the same design and are not distinguished by a variety of types.
Stators are essential in industrial machines such as pumps, compressors, and conveyor systems. They keep these systems running smoothly and efficiently by converting electrical energy into the precise mechanical motion needed to perform heavy-duty tasks.
Generator Stator Design
Structurally, the stator consists of three main parts:
- Ring Core;
- Working winding(s);
- Winding insulation.
The core is assembled from iron ring plates with grooves on the inside of the core. The plates form a package, the rigidity and monolithicity of the structure is given by welding or riveting. Slots are made in the core for laying the windings, and each tab is a yoke (core) for winding turns. The core is assembled from 0.8-1 mm thick plates made of special grades of iron or ferroalloys with a certain magnetic permeability. The outside of the stator may have fins to improve heat dissipation, and may have various grooves or recesses for mating with the generator housing.
Three-phase generators use three windings, one per phase. Each winding is made of copper insulated wire of large cross-section (diameter from 0.9 to 2 mm or more), which is laid in a certain order in the grooves of the core. The windings have leads from which the alternating current is drawn, usually the number of leads is three or four, but there are stators with six leads (each of the three windings has its own leads to make connections of one type or another).
The core grooves contain insulating material that protects the wire insulation from damage. Also in some types of stators insulating wedges can be inserted into the grooves, which additionally act as a fixing device for winding turns. The stator assembly may additionally be impregnated with epoxy resins or lacquers, which ensures the integrity of the structure (prevents the windings from shifting) and improves its electrical insulating properties.
The stator is rigidly mounted in the generator housing, with the most commonly used design today being one in which the stator core acts as the housing component. It is realized simply: the stator is clamped between two covers of the generator housing, which are fastened by studs - such "sandwich" allows to create compact designs with efficient cooling and easy maintenance. A popular design is the one in which the stator is combined with the front cover of the generator, and the back cover is removable and provides access to the rotor, stator and other parts.
Types and Characteristics of Stators
Generator stators differ in the number and shape of slots, winding arrangement in the slots, winding connection scheme, and electrical characteristics.
There are two types of stators according to the number of slots for winding turns:
- With 18 grooves;
- With 36 grooves.
Today, the 36-groove design is most commonly used because it provides the best electrical performance. Generators with stators with 18 slots can be found today on some early domestic cars.
There are three types of stators according to the shape of the grooves:
- With open grooves - grooves of rectangular cross-section, in them additional fixation of winding turns is required;
- With semi-closed (wedge-shaped) grooves - the grooves are narrowed to the top, so the winding turns are fixed by inserting insulating wedges or sleeves (PVC tubes);
- With semi-closed slots for windings with single coils - the slots have a complex cross-section for laying one or two turns of large diameter wire or a wide strip of wire.
There are three types of stators according to the winding layout:
- With loop (loop distributed) scheme - the wire of each winding is stacked in the grooves of the core in loops (usually one winding is stacked with a step in two grooves, in these grooves are stacked turns of the second and third windings - so the windings acquire the shift necessary for the generation of three-phase alternating current);
- With a wave concentrated circuit - the wire of each winding is laid in the grooves in waves, bypassing them from one side or the other, and in each groove lies two turns of one winding, directed in one direction;
- With wave distributed scheme - the wire is also laid in waves, but the turns of one winding in the slots are directed in different directions.
With any type of stacking, each winding has six turns distributed over the core.
As far as characteristics are concerned, for stators, the rated voltage, power, and rated current in the windings are of greatest importance. Stators (and generators) are divided into two groups according to their rated voltage:
- With 14 V winding voltage - for vehicles with 12 V on-board voltage;
- With 28 V winding voltage - for vehicles with 24 V on-board voltage.
The generator produces a higher voltage, as there is an inevitable voltage drop in the rectifier and stabilizer, and the input to the on-board power supply is already a normal voltage of 12 or 24 V.
Most generators for cars, tractors, buses, and other equipment have a rated current from 20 to 60 A, for cars 30-35 A is enough, for trucks - 50-60 A, for heavy equipment generators with current up to 150 and more A are produced. At the same time, the power of generators ranges from 400 to 2500 watts.
Principle of Operation of the Generator Stator
The operation of the stator and the entire generator is based on the phenomenon of electromagnetic induction - the generation of current in a conductor that moves in a magnetic field or rests in an alternating magnetic field. Automotive generators use the second principle - the conductor in which the current occurs is at rest, while the magnetic field is constantly changing (rotating).
When the engine is started, the alternator rotor starts to rotate and at the same time the battery voltage is supplied to the alternator excitation winding. The rotor has a multipole steel core which becomes an electromagnet when current is applied to the winding, accordingly, the rotating rotor creates an alternating magnetic field. The force lines of this field intersect the stator around the rotor. The stator core distributes the magnetic field in a certain way, its lines of force cross the turns of the working windings - in them, due to electromagnetic induction, current is generated, which is removed from the winding terminals, fed to the rectifier, stabilizer, and on-board network.
When the engine speed increases, part of the current from the stator operating winding is fed to the rotor excitation winding - so the generator enters the self-excitation mode and no longer needs an external current source.
During operation, the generator stator is subjected to heat and electrical stresses, and is also subjected to negative environmental influences. This over time can lead to deterioration of the insulation between the windings and electrical breakdown. In this case, the stator needs to be repaired or completely replaced. With regular maintenance and timely replacement of the stator, the generator will serve reliably, consistently providing the vehicle with electrical power.
