From Forge to Factory: How Motors are Made

Electric Motors Overview

Electric motors are electrical machines that convert electrical energy into mechanical motion. They operate through the basic principles of electromagnetism. When the electric current flowing through the motor’s windings is introduced into the motor’s magnetic field, a mechanical force is generated in the form of applied torque on the rotor shaft. The applied torque rotates the motor’s output shaft, which drives the connected load.

These motors are powered by alternating current (AC) power sources like electrical generators, power grids, and inverters or by direct current (DC) power sources like rectifiers or batteries. Hence, electric motors are broadly classified into AC (Alternating Current) motors and DC (Direct Current) motors. The AC motors are categorized as Asynchronous (Induction) AC motors and Synchronous AC motors. DC motors are classified into two types: Brushed DC motors and Brushless motors.

Electric motors (DC and AC) are used extensively, from small home appliances to heavy-duty industrial equipment. Some of their typical applications include vacuum cleaners, fans, water pumps, washing machines, paper mills, power tools, rolling mills, turbines, elevators & escalators, robots, conveyors, and compressors. They are also crucial in refrigeration and cooling systems, high-voltage AC HVAC (Heating, Ventilation, and Air Conditioning) equipment, heavy-duty machinery, ships, electric vehicles, and automated manufacturing systems.

Basic Components of Electric Motors and Their Functions

Depending on the type of current flowing through an electric motor and its application, each electric motor includes different components that enable it to effectively and efficiently convert electrical energy into mechanical motion. Discussed below are some of the critical elements of electric motors.

A) Rotor (Armature)

The rotor, or armature, is the moving part of any electric motor. It comprises electrical conductors, a magnetic part, and a rotating shaft. The magnetic part is made of silicon steel laminations that facilitate the stamping and forming conductor slots of the desired size and shape. The electrical conductors interact with the stator-generated magnetic field to produce a mechanical force for turning the rotor shaft. The rotating shaft (motor’s output shaft) is connected to the machine/equipment driven by the motor.

In a typical electric motor configuration, the rotor consists of electrical conductors that carry the supplied current, which then interacts with the stator’s magnetic field to generate the mechanical force that turns the rotor shaft. Alternatively, the rotor comprises a permanent magnet in other electric motor configurations, while the stator holds the electrical conductors (windings).

B) Stator (Stator Core)

The stator is a stationary component of the electric motor’s electromagnetic circuit and is mainly composed of permanent magnets or windings (electrical conductors). Its function is to produce a magnetic field of the desired strength and shape. The stator core (stator frame) comprises several thin sheets of metal or laminations of silicon steel material, normally with a thickness of approximately 0.5mm (millimeters).

The stator core laminations are generally insulated from each other using a varnish layer. This breaks up their current conducting path and reduces the energy losses (eddy current losses) that would arise if a solid stator core were used.

C) Stator and Armature Windings

Windings are copper or aluminum wires that are placed in coils in the stator and rotor cores. In most motors, the windings are wound around the laminations of the soft-iron ferromagnetic core to generate magnetic poles in the presence of an energizing electric current. Electric motors have two basic magnetic field-pole configurations: salient magnetic poles and non-salient magnetic poles. In motors with the salient-pole configuration, the pole’s magnetic field is generated by a copper wire winding wrapped around the pole just below the pole face. While in the non-salient pole configuration, the windings are distributed in magnetic pole face slots.

The stator windings/coils are usually made of copper wire; rectangular bars of fewer copper wire turns are used for larger motors. While for small motors, round electrical conductors with many turns of copper wire per coil are used. The stator coils are electrically insulated.

D) The Air Gap

This is not a physical component; rather, it’s the distance between the rotor and stator iron cores that allows the rotor shaft to spin. The breadth of the air gap affects the electrical properties of electric motors, as it’s the primary source of the low power factor with which such motors operate efficiently. It should therefore be of the recommended size, given that its breadth is influenced by the motor’s power factor and the magnetizing or energizing current.

E) Bearings

Electric Motor Quality (EMQ) bearings are used explicitly in electric motors as they offer better precision, quiet performance, higher efficiency, and a greater capacity for high motor speeds. Examples of EMQ bearings are single-row radial ball bearings, insulated ball bearings, and ceramic hybrid bearings.

F) Carbon Brushes

These are electrical contacts made of a soft electrical-conductor material, usually carbon. They are connected to the terminals of the motor’s power source and pressed onto the commutator, thereby allowing electric current to flow from stationary wire conductors into the commutator. You’ll often find carbon brushes in DC motors.

G) Commutator

This is an electric rotary switch used to supply direct current (in most DC motors) or alternating current (in certain AC motors) to the rotor. It comprises slip-ring segments that are insulated from the rotor shaft and from each other. This construction allows the commutator to periodically reverse current flow in the rotor (armature) windings, enabling the rotor shaft to spin continuously from pole to pole.

The commutator is a key component in DC motors because, in such motors, rotor current is supplied through stationary carbon brushes that are in contact with a revolving slip-ring commutator. This necessitates the need for a current reversal to optimally apply torque to the connected load while the rotor shaft is rotating from pole to pole. If this current reversal is not provided, your DC motor is likely to brake to a stop.

H) Motor Housing (Frame)

The motor housing protects all the internal components of the electric motor from physical damage and external contaminants like dust or corrosive chemicals. Conventional electric motor housings are typically made of corrosion-resistant alloys or steel. They can be formed from forged metal or fabricated cast.

How are Motor Components Made?

Metal Forging

Some components of electric motors, such as the rotor shaft, bearing holders, end brackets, spindles, yokes (motor frames), and a variety of gears, are made through metal forging. Metal forging is a metal manufacturing process that involves forming and shaping of metals by the application of compressive forces through pressing, hammering, or rolling. The compressive forces are delivered with a die or hammer. Forging processes are often classified according to the temperature of the metal being worked on. Hence, we can have hot, cold, or warm forging. Common metal forging methods include cold forging, roll forging, extrusion, press forging, open-die forging, and closed-die forging.

There are various types of metals that can be forged, including carbon steel, copper, alloy steel, titanium, aluminum, hard tool steels, brass, stainless steel, and high-temperature alloys comprising nickel, cobalt, or molybdenum. The forging process is known to produce motor components with excellent mechanical properties, such as:

• Higher Fatigue Strength: The basic concept of forging is that an original metallic material is plastically deformed into a new shape–the internal structure of a solid metal block is permanently altered into the desired geometric shape. The new shape carries with it all the internal stresses involved in the forging process, as the sub-structure of the forged motor component is not properly aligned. This contributes to a higher level of fatigue strength (fatigue resistance) and rigidity that no metal casting method can attain.
• Reduced Overall Weight: A forged motor component can be made using lesser volumes of material and in stronger geometric shapes that reduce the overall weight of the end product. This is due to the higher relative strength achieved by different forging methods. Also, suppose materials with high strength-to-weight ratios like aluminum are used. In that case, the forging process can have compounding effects on the final weight of the assembled motor without compromising performance.
• Improved Reliability: Forged motor components exhibit greater reliability than cast or machined components due to plastic deformation. The rigidity of these components means that they do not deform significantly over time, making them reliable for applications where such type of wear is intolerable.

Generally, compared to other metal manufacturing processes, metal forging produces some of the strongest motor components in custom sizes and with specific mechanical properties as well as critical performance specifications. For example, conventional electric motor housings are made by press forming, whereby a composite material is press-formed on a forged metal cylinder (made of corrosion-resistant alloys or steel), and the passages in the press-formed material are machined. This reduces the final weight and vibration of the resulting motor housing while increasing mechanical damping and improving the overall performance and corrosion resistance of the assembled electric motor.

Other Motor Manufacturing Methods

In addition to metal forging, other motor components like the motor core (stator and rotor core), motor bearings, and motor casings require different metal manufacturing methods. Some of these methods are discussed below.

Stator and Rotor Core Manufacturing

The core of an electric motor is a ferromagnetic structure normally made of silicon steel (a ferritic alloy of silicon and iron). It is divided into two parts– the stator core and rotor core–with a magnetic circuit being generated between the two core parts. The core is a vital component of the electric motor, and its performance is profoundly affected by its fabrication method. Stamping and core pressing are the common manufacturing methods used to produce the stator and rotor cores.

A typical motor core is often formed of soft magnetic materials due to their high permeability, low hysteresis loss, and ease of fabrication. Common examples of soft magnetic materials that can be used in motor core manufacturing are pure electrical iron, magnetic conductive alloys, silicon steel sheets, and regular carbon structural steel. Compared to carbon structural steel plates, silicon steel sheets are the most popular materials for stamping the stator and rotor cores of DC motors and synchronous AC motors; pure iron, soft magnetic composite (SMC) materials, and magnetic conductive alloys are also widely used in manufacturing the cores of some electric motors.

Manufacturing of Motor Bearings

Bearings used in electric motors are manufactured using conventional bearing manufacturing processes such as molding, stamping and forming, or forging and machining. Most EMQ (Electric Motor Quality) bearings are manufactured from chrome steel. Other materials may be added depending on the required level of bearing performance. The components of the EMQ bearings – two rings (inner and outer rings), rollers or balls, and the bearing cage – are manufactured separately and then assembled. The inner and outer bearing rings start out as tube stock, where cutting machines are used to cut the stock into basic ring shapes. The rings are then hardened—the hardening process involves heating to high temperatures, quenching in oil, and tempering. The rings are then ground and subjected to honing, where the inner and outer races are honed for proper surface finish and geometry.

The bearing balls start out as a rod or wire slug that undergoes a cold heading (cold forming) process to obtain a spherical shape. The resulting shapes are then filled and ground to reduce the size and achieve uniformity of the bearing balls. They are then subjected to a hardening process. After which, they’re re-grinded to achieve the desired size and spherical shape. Finally, the formed bearing balls are subjected to a lapping process for tolerance verification. Depending on the type of material used, the bearing cage can be manufactured either by forging, metal sintering, machining, injection molding of specific plastic materials, or stamping and forming.

After each bearing component is made, it is measured, matched, and assembled into a complete motor bearing. The assembly process relies heavily on matching the bearing components. To create a finished EMQ bearing within the allowed tolerances for bearing run-out, the form, and radius of the ball grooves in the inner and outer rings must be measured and matched with balls of the correct diameter.

Motor Casing Manufacturing

Aluminum electric motor casings are usually formed from fabricated castings. In most cases, the aluminum die-casting process is used to mass-produce motor frames (casings), end plates, and heat sinks made of lightweight aluminum. The process results in reduced motor vibration and lower operating temperatures, consequently increasing the lifespan of the electric motor being manufactured. The “guts” of such motor housings usually contain a variety of gears, a primary shaft, and an electric motor. Custom spot-facing dowel pins and jack screws can be used with dual-mount foot holes for easy installation.

The aluminum die-casting method includes spectrology equipment used to further analyze the aluminum alloy’s chemical composition after visual evaluations. The alloy is then heated to a melting point of about 680°C, after which it’s injected into a mold using a die-casting machine. Once the mold shell cools down, it is removed, and the edges are smoothed using pneumatic equipment. Shot blasting, milling, and hand finishing methods are used to produce a flawless motor casing of uniform surface free of burrs.

The motor casing’s diameter is enlarged via induction heating, and the rotor & stator windings are inserted before the casing cools down to secure them. The casing’s inner diameter is inspected to ensure that it is exactly as specified. An electric motor cannot function effectively if its housing is larger than recommended. Some motor manufacturers normally use mounting holes between 8.39 mm to 8.63 mm as a reference to determine the required casing diameter. Once the casing’s inner diameter is verified, the formed motor housing is cleaned using reducing agents, ultrasonic cleaners, and acid-based cleaners. Rinsing is done in between every cleaning cycle. Lastly, after the QC (Quality Control) team conducts its final tests, the formed motor casing unit is oven dried, making it ready for use.

Motor Assembling in the Factory

Once all the components of an electric motor are manufactured, they are measured and machined to meet the specified dimensions. They are then matched and assembled to make a complete AC or DC electric motor. In some motor manufacturing factories, the key motor components, such as the rotor and stator core, motor housing, permanent magnets, rotor shaft, and windings (coils), are usually purchased from different suppliers who specialize in the manufacturing of such parts. In comparison, the motor winding and final assembly processes are carried out in-house (within the factory).

In summary, a typical motor manufacturing process can be broken down into several steps, namely: (i) Rotor and Stator ferromagnetic core processing, (ii) Motor Axes(Motor Shafts) processing, (iii) Stator Winding manufacturing, (iv) Rotor Squirrel-Cage (the Rotor Winding) manufacturing, (v) Final Motor Assembly, and (vi) Varnishing & Testing. The diagram below shows a schematic overview of the various processes involved in motor manufacturing.

Note: The production of an AC motor involves similar steps as the production of a DC motor. In the initial stage, silicon steel laminations that constitute the stator and rotor iron cores are made. Two metal forging methods–stamping and core pressing– are often used to form and shape the stator and rotor core laminations. Stator windings/coils are then made by wrapping several turns of copper and/or aluminum wire around the stator core. The windings are then inserted in a fabricated outer housing to form a wound stator assembly.

Next, the rotor winding, which is usually of the squirrel-cage type, is manufactured. This rotor cage normally comprises an aluminum casting that incorporates solid electrical conductors, a cooling fan, and end rings at each end of the rotor core. For larger AC induction motors, the rotor squirrel cage is usually made of brass, aluminum, or rectangular copper bars brazed or welded to end rings of similar material. The rotor shaft is produced separately through forging and machining to the specified dimensions. Once the required motor shaft is manufactured, it’s then incorporated into the rotor cage to complete the rotor assembly.

The completed wound stator and rotor assemblies are then passed over to the motor assembly factory. Here, they’re matched and assembled in two identical end frames (motor housings) to form a complete electric motor. The motor is then painted and prepared for shipment.

This entry was posted on June 12th, 2023 and is filed under Uncategorized. Both comments and pings are currently closed.