Electric Motor

An electric motor is a device that transforms electrical energy into mechanical energy. It operates by utilizing the interaction between the motor’s magnetic field and the electric current flowing through a wire winding, producing force in the form of torque applied to the motor’s shaft. While an electric motor generates mechanical energy from electricity, an electric generator functions in the opposite way, converting mechanical energy into electrical energy.

Electric motors can be powered by direct current (DC) sources, such as batteries or rectifiers, or alternating current (AC) sources, including power grids, inverters, or electrical generators.

They are classified based on factors such as power source, construction, application, and type of motion output. Motors can be brushed or brushless, operate on single-phase, two-phase, or three-phase power, and may use axial or radial flux. Additionally, they can be air-cooled or liquid-cooled, depending on the application.

Standardized motors are widely used in industrial applications. The largest electric motors, with outputs exceeding 100 megawatts, are employed in ship propulsion, pipeline compression, and pumped-storage power plants.

Electric motors are found in various applications, including industrial fans, blowers, pumps, machine tools, household appliances, power tools, vehicles, and disk drives. Small motors are even used in electric watches. In some cases, such as regenerative braking in traction motors, electric motors can function in reverse as generators, capturing and reusing energy that would otherwise be lost as heat or friction.

These motors generate either rotary or linear force (torque) to drive external mechanisms, classifying them as actuators. They are typically designed for continuous rotation or for linear movement over a significant distance relative to their size. While solenoids also convert electrical energy into mechanical motion, their movement is limited to a short range.

Components of Electric Motor

An electric motor consists of two primary mechanical components: the rotor, which is the moving part, and the stator, which remains stationary. Electrically, it comprises two main elements: the field magnets and the armature. One of these components is connected to the rotor, while the other is attached to the stator, forming a complete magnetic circuit.

The field magnets generate a magnetic field that interacts with the armature. These magnets can be either permanent magnets or electromagnets. In most designs, the field magnet is located on the stator, while the armature is mounted on the rotor. However, in some motor configurations, their positions may be reversed.

Rotor

The rotor is the rotating component of an electric motor responsible for delivering mechanical power. It typically contains conductors that carry electric currents, which interact with the magnetic field generated by the stator. This interaction produces a force that causes the rotor to spin, ultimately driving the motor’s shaft and enabling mechanical motion.

Stator

The stator is the stationary part of an electric motor that surrounds the rotor and typically contains field magnets. These magnets can be either electromagnets—formed by wire windings around a ferromagnetic iron core—or permanent magnets. The stator generates a magnetic field that interacts with the rotor’s armature, exerting force on its windings to produce rotation.

To minimize energy losses, the stator core is constructed from multiple thin, insulated metal sheets called laminations. These laminations, made from electrical steel, are designed to optimize magnetic properties such as permeability, hysteresis, and saturation while reducing losses caused by eddy currents.

In AC motors powered by mains electricity, the windings are often impregnated with varnish in a vacuum to prevent wire vibration. This process helps protect the insulation from wear and ensures long-term reliability. In some applications, such as deep well submersible pumps, washing machines, and air conditioners, the stator is encapsulated in plastic resin to prevent corrosion and reduce noise transmission.

Gap

The air gap between the stator and rotor enables the rotor to rotate freely. The size of this gap plays a crucial role in determining the motor’s electrical performance. A smaller gap enhances efficiency by strengthening the magnetic field interaction, while a larger gap weakens the motor’s performance. However, if the gap is too narrow, it may lead to unwanted friction and noise, potentially affecting the motor’s durability and operation. Therefore, the air gap is carefully optimized to balance performance and mechanical stability.

Armature

The armature is composed of wire windings wrapped around a ferromagnetic core. When an electric current flows through these windings, the resulting magnetic field generates a force (Lorentz force) that causes the rotor to spin. The windings are coiled around a laminated soft iron core, which forms magnetic poles when energized.

Electric machines can have either salient-pole or nonsalient-pole configurations. In a salient-pole motor, the stator and rotor cores feature protruding poles that face each other. Each pole is wrapped with wire, and when current flows, it creates alternating north and south poles. In a nonsalient-pole (round-rotor) motor, the rotor core is cylindrical with windings evenly distributed in slots around its circumference. When alternating current is supplied, rotating magnetic poles are generated.

A shaded-pole electric motor has an additional winding around a portion of the pole, which delays the magnetic field phase, ensuring smooth rotation.

Commutator

A commutator is a rotary electrical switch that delivers current to the rotor and periodically reverses its direction to maintain continuous rotation. It consists of a cylindrical structure made up of multiple metal contact segments mounted on the armature.

The commutator works in conjunction with brushes, which are made of soft conductive materials like carbon. These brushes press against the rotating commutator segments, ensuring a continuous electrical connection. As the rotor spins, the brushes maintain contact with different segments, reversing the current flow in the rotor windings with each half-turn (180°). This reversal ensures that the torque acting on the rotor remains in the same direction, allowing smooth and continuous motion.

Without the commutator’s function, the torque direction would alternate with each half-turn, causing the rotor to stop instead of rotating consistently. Though widely used in traditional motors, commutated motors have largely been replaced by brushless motors, permanent magnet motors, and induction motors, which offer greater efficiency and durability.

Shaft

The shaft is the rotating component of the motor that extends outside the motor housing to transfer mechanical power to an external load. It is directly connected to the rotor and rotates with it. Since the forces from the connected load act beyond the outermost bearing, this type of load arrangement is referred to as an overhung load. The shaft must be precisely aligned and robust enough to withstand torsional stresses and mechanical wear.

Bearings

Bearings support the rotor and allow it to rotate smoothly around its axis. They help transfer axial and radial forces from the shaft to the motor housing while minimizing friction. Bearings play a critical role in maintaining motor efficiency and longevity, as improper lubrication or misalignment can lead to excessive wear and mechanical failure.

Inputs of Electric Motor

Power Supply

A DC motor is typically powered through a split-ring commutator, which periodically reverses the current direction in the rotor windings to maintain continuous rotation.

For AC electric motors, commutation is managed using either a slip-ring commutator or external electronic commutation. AC motors can be classified based on speed control into fixed-speed or variable-speed types. They can also be categorized as synchronous (where the rotor speed matches the supply frequency) or asynchronous (where the rotor speed is slightly less than the synchronous speed due to slip).

A universal motor is a special type of electric motor that can operate on both AC and DC power sources, making it versatile for various applications such as power tools and household appliances.

Control

DC motors can achieve variable speed control by adjusting the applied voltage or using pulse-width modulation (PWM), which regulates the power supplied to the motor efficiently.

For AC motors operating at a fixed speed, they are typically powered directly from the grid or through motor soft starters, which help reduce the inrush current during startup.

AC motors that require variable speed control are powered using power inverters, variable-frequency drives (VFDs), or electronic commutation technologies to precisely regulate motor speed and torque.

The term electronic commutator is commonly associated with self-commutated brushless DC motors (BLDC) and switched reluctance motors (SRMs), which use electronic circuits instead of mechanical brushes to control motor operation.

Types of Electric Motors

Electric motors operate based on three fundamental principles: magnetism, electrostatics, and piezoelectricity.

1. Magnetic Motors

In most motors, magnetic fields are created in both the rotor and the stator. The interaction between these fields generates torque, causing the rotor to turn. The strength and position of these magnetic fields are controlled by either:

• Switching poles on and off at the right moment.
• Adjusting the strength of the magnetic field.

Electric motors can be powered by either direct current (DC) or alternating current (AC), with some motors designed to work with both.

2. AC Motors

AC motors are categorized into two main types:

• Synchronous Motors – The rotor spins at the same speed as the stator’s magnetic field.
• Asynchronous (Induction) Motors – The rotor speed is slightly less than the stator’s field speed (due to slip).

3. Fractional-Horsepower Motors

These motors are rated below 1 horsepower (0.746 kW) or have a frame size smaller than standard 1 HP motors. They are commonly found in household appliances and small industrial applications.

Types of Motor Commutation

Self-Commutated	Externally Commutated
Mechanical Commutation	Electronic Commutation
AC Motors	Asynchronous Motors
– Universal (AC/DC)	– Squirrel-Cage Induction Motor (SCIM)
– Repulsion Motor	– Single-Phase Motors (Shaded Pole, Split-Phase)
– PM Motors (Brushless DC, BLDC)	– Stepper Motors
– Switched Reluctance Motor (SRM)	– Hysteresis Motor

Key Motor Abbreviations

• BLDC – Brushless DC Motor
• BLAC – Brushless AC Motor
• PMSM – Permanent Magnet Synchronous Motor
• SCIM – Squirrel-Cage Induction Motor
• WRIM – Wound-Rotor Induction Motor
• SyRM – Synchronous Reluctance Motor
• VFD – Variable Frequency Drive

Self-Commutated Motor: Brushed DC Motor

A brushed DC motor is a self-commutated electric motor that relies on a mechanical commutator and carbon brushes to reverse the current in the motor windings, ensuring continuous rotation. These motors are commonly used in applications requiring simple control, affordability, and high starting torque.

Construction and Working Principle

A brushed DC motor consists of the following key components:

• Stator (Field Magnet): Provides a stationary magnetic field, which can be created using permanent magnets (PM) or field windings.
• Rotor (Armature): A rotating coil of wire that carries current and interacts with the stator’s magnetic field.
• Commutator: A segmented cylindrical component attached to the rotor shaft that periodically reverses current direction.
• Brushes: Conductive carbon or graphite contacts that transfer power to the rotating commutator.

When power is supplied, current flows through the brushes to the commutator and into the armature windings. The interaction between the armature’s magnetic field and the stator’s field generates a force, causing rotation. The commutator ensures that the current direction is reversed at the right time, maintaining continuous motion.

Types of Brushed DC Motors

Brushed DC motors can be classified into different types based on their field winding configuration:

Separately Excited DC Motor – The field winding is powered separately from the armature, allowing independent speed control.

Shunt-Wound DC Motor – Field winding is connected in parallel with the armature. Provides a nearly constant speed under varying loads.

Series-Wound DC Motor – Field winding is connected in series with the armature, offering high starting torque but speed variation with load changes.

Compound-Wound DC Motor – A combination of shunt and series windings for better speed regulation and torque control.

Cumulative Compound: Enhances torque performance.

Differential Compound: Provides precise speed control but is less commonly used.

Permanent Magnet DC Motor (PMDC) – Uses permanent magnets instead of field windings, offering compact size, high efficiency, and maintenance-free operation.

Permanent Magnet DC Motor (PMDC)

A Permanent Magnet (PM) motor generates its magnetic field using permanent magnets instead of field windings on the stator, eliminating the need for external excitation. This design reduces power consumption and simplifies the motor’s structure. The stator contains permanent magnets that provide a constant magnetic field, while the rotor (armature) has windings that receive electrical current through brushes and a commutator. As current flows through the armature windings, it creates an electromagnetic force that interacts with the stator’s magnetic field, causing the rotor to spin. However, since the stator’s field is fixed, speed control through field strength adjustment is not possible, unlike in wound-field DC motors.

Permanent magnet motors offer several advantages, including higher energy efficiency, as they eliminate power losses associated with field windings. They also have a compact design, making them ideal for miniature motors where reduced weight and size are critical. Additionally, they provide a strong, stable magnetic field without requiring an external power source for excitation. However, PM motors have some limitations. Their speed control is restricted, as the fixed field prevents adjustments through field weakening. Moreover, large PMs are costly and challenging to manufacture, and their strong magnetic forces make assembly difficult and sometimes hazardous. Historically, permanent magnets struggled to retain their flux after disassembly, making wound-field motors a more practical choice for larger applications.

To improve performance, miniature PM motors often use high-energy neodymium magnets (NdFeB alloy), which offer a higher flux density. This makes PM motors competitive with synchronous and induction machines. In small PM motors, at least three rotor poles are used to ensure startup from any position, and the outer housing is typically a steel tube, which links the curved field magnets and enhances magnetic efficiency. While PM motors are widely used in small-scale applications, larger wound-field motors remain preferred for high-power industrial uses due to their better speed control and cost-effectiveness.

Electronic commutator (EC)

Brushless DC (BLDC) motor

A Brushless DC (BLDC) motor, also known as an electronically commutated (EC) motor, eliminates many of the issues associated with brushed DC motors. Instead of a mechanical commutator and brushes, BLDC motors use an electronic switch that synchronizes with the rotor’s position. This design increases efficiency, with BLDC motors typically achieving 85% to 96.5% efficiency, compared to 75-80% for brushed DC motors.

The BLDC motor produces a trapezoidal counter-electromotive force (CEMF) waveform, influenced by both stator winding distribution and the placement of permanent magnets in the rotor. These motors may have single-phase, two-phase, or three-phase stator windings and often use Hall effect sensors for rotor position detection and commutation control. The precise control and efficiency of BLDC motors make them ideal for applications requiring accurate speed regulation, such as in computer disk drives, video cassette recorders, CD-ROM spindles, laser printers, and cooling fans.

BLDC motors offer several advantages over conventional motors. They operate more efficiently and generate less heat than AC motors with shaded-pole designs, leading to longer-lasting components like fan bearings. Since they lack a commutator, they experience less wear and tear, resulting in a significantly longer lifespan compared to brushed DC motors. Additionally, the absence of brushes eliminates electrical noise and radio frequency (RF) interference, making them suitable for sensitive electronics such as audio equipment and computers. The built-in Hall effect sensors can also serve as a tachometer signal, enabling precise closed-loop speed control in servo applications.

Another key advantage of BLDC motors is their spark-free operation, making them safer for use in hazardous environments where volatile chemicals or fuels are present. This also prevents ozone generation, which can be a concern in poorly ventilated areas. Furthermore, BLDC motors are generally quiet, reducing vibration-related issues in sensitive equipment.

BLDC motors are commonly used in small electronic devices like computer cooling fans, but they also scale up to larger applications, including electric vehicles and electric model aircraft. Some high-power BLDC motors can reach ratings of up to 100 kW, demonstrating their versatility across a wide range of industries.

Switched Reluctance Motor (SRM)

A Switched Reluctance Motor (SRM) is a type of electric motor that operates without brushes or permanent magnets, and its rotor carries no electric currents. Instead, torque is generated through the misalignment of rotor and stator poles. As the rotor aligns itself with the stator’s magnetic field, the stator’s field windings are sequentially energized, causing continuous rotation.

The motor works by guiding magnetic flux through the path of least resistance, directing it through the nearest rotor poles to the energized stator poles. This interaction magnetizes the rotor poles, generating torque. As the rotor moves, different stator windings are energized in sequence, ensuring sustained rotation.

Due to its simple and rugged construction, the SRM is commonly used in appliances and vehicles, offering a reliable and efficient alternative to traditional electric motors.

Universal AC/DC motor

A universal motor is a commutated, electrically excited motor with either series or parallel winding, capable of operating on both AC and DC power. This versatility is due to the fact that, on AC power, the current in both the field and armature coils reverses polarity simultaneously, ensuring that the resulting mechanical force always maintains a constant direction of rotation.

Universal motors are commonly used in sub-kilowatt applications at standard power line frequencies. Historically, they served as the traction motors in electric railways, but their efficiency suffered when running on AC due to eddy current heating in the solid iron motor field pole-pieces, which were originally designed for DC operation. As a result, they have become less common in modern railway systems.

One of the key advantages of universal motors is their high starting torque and compact design, particularly when operated at high running speeds. However, they require more maintenance and have a shorter lifespan compared to other motor types. They are typically found in devices with high starting torque demands but infrequent use, such as household blenders.

These motors often feature multiple field coil taps for stepped speed control, and some models integrate diodes for half-wave rectified AC operation. Additionally, universal motors are well-suited for electronic speed control, making them a preferred choice for washing machines, where they enable both forward and reverse drum movement by switching the field winding relative to the armature.

Unlike squirrel cage induction motors (SCIMs), which are limited by the power line frequency, universal motors can operate at much higher speeds. This makes them ideal for appliances like vacuum cleaners, hair dryers, and blenders, where both high speed and lightweight design are essential.

They are also widely used in portable power tools such as drills, sanders, and saws, as their characteristics align well with the demands of these tools. Many vacuum cleaner and weed trimmer motors run at over 10,000 RPM, while some miniature grinders can exceed 30,000 RPM.

Externally Commutated AC Machine

An externally commutated AC machine refers to AC induction and synchronous motors designed for operation on single-phase or polyphase sinusoidal or quasi-sinusoidal waveform power. These motors are optimized for use in fixed-speed applications, where they receive power directly from the AC power grid, or in variable-speed applications, where they are controlled by variable-frequency drive (VFD) controllers.

Induction Motor

An induction motor is an asynchronous AC motor in which power is transferred to the rotor through electromagnetic induction, similar to a transformer. The stator functions as the primary winding, while the rotor acts as the secondary winding. Due to their efficiency and durability, polyphase induction motors are widely used in industrial applications.

Types of Induction Motors

Induction motors are categorized into two main types:

1. Squirrel Cage Induction Motor (SCIM) – These motors have a solid-bar rotor winding (usually made of aluminum or copper), electrically connected by rings at both ends. The entire structure resembles a rotating exercise cage, giving it the name squirrel cage motor. The rotor’s magnetic field is created by currents induced from the stator. The shape and structure of the rotor bars influence the speed-torque characteristics. At low speeds, the induced current is at line frequency and remains in the outer cage, while at higher speeds, the slip frequency decreases, and current flows deeper into the rotor. This effect allows some variation in rotor resistance, which impacts the motor’s performance. Most SCIMs use uniform bars to simplify design and manufacturing.
2. Wound Rotor Induction Motor (WRIM) – Unlike SCIMs, WRIMs have insulated wire windings on the rotor, which are connected to slip rings on the motor shaft. These slip rings allow external resistors or other control devices to be inserted into the rotor circuit. By adjusting these resistors, the motor’s speed and torque characteristics can be controlled. However, using resistors results in power dissipation and lower efficiency. Alternatively, a converter-inverter system can be used to return slip-frequency power to the system, improving efficiency.

Operation and Speed Control

• WRIMs are ideal for high-inertia loads or applications that require high starting torque across the full speed range. By adjusting the resistance in the rotor circuit, the motor can achieve maximum torque at a lower supply current, making it efficient for heavy-duty operations.
• The motor speed can be adjusted by modifying the rotor circuit resistance, shifting the torque-speed characteristics. Increasing resistance lowers the speed at which maximum torque occurs, but excessive resistance reduces the torque output.
• When connected to a variable load, the motor operates at the speed where its torque matches the load torque. Reducing the load increases the speed, while increasing the load slows the motor down. However, if slip losses are dissipated through external resistors, efficiency and speed regulation are poor.

Torque Motor

A torque motor is designed to operate indefinitely in a stalled condition, meaning the rotor can be blocked from turning without overheating or sustaining damage. In this state, the motor continuously applies a steady torque to the load, making it ideal for applications that require constant force or tension. One of its common applications is in tape drives and reel systems, where it is used in supply and take-up reels. When operated at low voltage, the motor maintains a light and steady tension on the tape, ensuring smooth operation regardless of whether the capstan is feeding the tape. At higher voltages, the motor provides increased torque, enabling fast-forward and rewind functions without requiring additional gears or clutches.

Another significant use of torque motors is in force-feedback systems, particularly in computer gaming. They are integrated into steering wheels to simulate real driving conditions by providing resistance that mimics road surfaces, collisions, and other driving dynamics. Additionally, torque motors play a crucial role in electronic throttle control systems for internal combustion engines. In this application, they work against a return spring to adjust the throttle based on the governor’s output. The governor monitors engine speed through electrical pulses and modulates the current to the motor accordingly. If the engine slows down, the governor increases the current, producing more torque to open the throttle. Conversely, if the engine speeds up, the current decreases, allowing the return spring to close the throttle and reduce speed.

Torque motors offer several advantages, including continuous stall operation without overheating, smooth and precise control of force and tension, and direct drive capability, which eliminates the need for additional mechanical components such as gears and clutches. These characteristics make them highly effective for applications that demand reliable, controlled movement and consistent performance in mechanical and electronic systems.

Synchronous Motor

A synchronous motor is an AC electric motor that operates at a speed directly proportional to the frequency of the AC supply, meaning it has zero slip under typical conditions. Unlike induction motors, which require some slip to generate torque, synchronous motors maintain a constant speed. The rotor spins with coils passing magnets at the same frequency as the AC power, generating a magnetic field that drives the motor.

One common type of synchronous motor is similar to an induction motor but has a DC-excited rotor field. It uses slip rings and brushes to conduct current to the rotor, ensuring that the rotor poles remain synchronized with the stator field. Another variation, designed for low-load torque applications, features flats ground onto a conventional squirrel-cage rotor, creating discrete poles. A historical example is the synchronous motors used by Hammond in its pre-World War II clocks and older Hammond organs. These motors had no rotor windings and relied on discrete poles, making them not self-starting. The clocks required manual starting via a small knob, while the organs used an auxiliary starting motor activated by a manually operated switch.

A specialized type of synchronous motor is the hysteresis synchronous motor, which operates as a two-phase motor with a phase-shifting capacitor for one phase. It starts similarly to an induction motor, but once the slip decreases, the rotor (a smooth cylinder) becomes temporarily magnetized, behaving like a permanent magnet synchronous motor. The rotor material, which retains magnetization like a common nail, allows the poles to remain fixed in position without drifting.

Low-power synchronous timing motors, such as those used in traditional electric clocks, often feature multi-pole permanent magnet external cup rotors. To provide starting torque, they use shading coils. A well-known example is the Telechron clock motor, which incorporates shaded poles for starting torque and a two-spoke ring rotor, functioning like a discrete two-pole rotor to maintain precise timing. These motors are widely used in applications requiring constant speed and precise synchronization, such as clocks, timers, and some industrial machinery.

Doubly-fed Electric Motor

A doubly-fed electric machine is a type of motor or generator that contains two independent multiphase winding sets, both of which contribute active power to the energy conversion process. At least one of these winding sets is electronically controlled, enabling variable-speed operation. The presence of two independent winding sets, often referred to as a dual armature design, represents the maximum configuration possible in a single machine without duplicating the overall topology.

One key advantage of a doubly-fed motor is its expanded constant torque speed range, which extends up to twice the synchronous speed for a given excitation frequency. This provides twice the constant torque range compared to singly-fed electric machines, which have only a single active winding set. This extended range makes doubly-fed machines particularly useful in applications that require variable-speed operation with efficient energy conversion, such as wind turbines, high-performance industrial drives, and some electric propulsion systems.

A major benefit of a doubly-fed motor is that it allows for the use of a smaller electronic converter, reducing the cost and complexity of power electronics. However, this advantage may be offset by the additional cost of rotor windings and slip rings, which introduce maintenance challenges. Furthermore, controlling the motor speed near synchronous speed can be difficult, presenting challenges in applications that require precise speed regulation. Despite these drawbacks, doubly-fed electric machines remain an attractive option for applications demanding high efficiency, wide speed range, and reduced converter sizing.

Comparison by Major Electric Motor Categories

Here’s the comparison of motor types in a tabular format:

Type	Advantages	Disadvantages	Typical Applications	Typical Drive/Output
Brushed DC Motor	Simple speed control, low initial cost	Requires maintenance (brushes), medium lifespan, costly commutator and brushes	Steel mills, paper machines, treadmills, automotive accessories	Rectifier, linear transistor(s), or DC chopper controller
Brushless DC Motor (BLDC)	Long lifespan, low maintenance, high efficiency	Higher initial cost, requires electronic controller (EC)	Hard disk drives, CD/DVD players, electric vehicles, UAVs	Synchronous; single-phase or three-phase PM rotor, trapezoidal stator winding, PWM inverter
Switched Reluctance Motor (SRM)	Long lifespan, low maintenance, high efficiency, no permanent magnets, simple construction	Mechanical resonance, high iron losses, requires electronic controller (EC)	Appliances, electric vehicles, textile mills, aircraft	PWM and other specialized drive types
Universal Motor	High starting torque, compact, high-speed	Requires maintenance (brushes), shorter lifespan, noisy, only economical for small ratings	Power tools, blenders, vacuum cleaners	Variable single-phase AC, phase-angle control with triac(s)
Squirrel-Cage/Wound-Rotor Induction Motor (SCIM/WRIM)	Self-starting, low cost, robust, reliable	High starting current, lower efficiency due to magnetization needs	Fixed-speed applications (fans, blowers, compressors), variable-speed high-performance loads	Fixed-speed: General use, Variable-speed: Vector-controlled VSDs
Capacitor-Start SCIM	High power, high starting torque	Speed slightly below synchronous, requires a starting switch or relay	Appliances, stationary power tools	Fixed or variable single-phase AC, phase-angle control with triac(s)
Capacitor-Run SCIM	Moderate power, high starting torque, long lifespan	Speed slightly below synchronous, slightly higher cost	Industrial blowers, industrial machinery	Fixed or variable-speed AC
Auxiliary Start-Winding SCIM	Moderate power, low starting torque	Speed slightly below synchronous, requires starting switch/relay	Appliances, stationary power tools	Fixed or variable-speed AC
Shaded-Pole Induction Motor	Low cost, long lifespan	Low starting torque, low efficiency, small power ratings	Fans, appliances, record players	Fixed-speed AC
Wound-Rotor Synchronous Motor (WRSM)	Synchronous speed, more efficient than induction motors	Higher cost	Industrial motors	Fixed or variable-speed three-phase, six-step CS load-commutated inverter or PWM inverter
Hysteresis Motor	Accurate speed control, low noise, no vibration, high starting torque	Very low efficiency	Clocks, timers, sound equipment, hard drives	Single-phase AC, two-phase capacitor-start, capacitor-run motor
Synchronous Reluctance Motor (SyRM)	More robust and efficient than SCIM, runs cooler, avoids PM demagnetization	Requires a controller, not widely available, high cost	Appliances, electric vehicles, textile mills, aircraft	VFD, DTC type or PWM inverter
Pancake (Axial Rotor) Motor	Compact design, simple speed control	Medium cost, medium lifespan	Office equipment, fans, pumps, industrial/military servos	Brushed or brushless DC
Stepper Motor	Precision positioning, high holding torque	Some models costly, requires a controller	Printers, floppy disk drives, industrial machine tools	Not a VFD, position determined by pulse counting

This structured table makes it easy to compare different motor types based on their advantages, disadvantages, applications, and control methods.

Operating Principles

An electric motor converts electrical energy into mechanical energy by utilizing the force between two opposing magnetic fields. At least one of these fields must be generated by an electromagnet through the magnetic field created by an electrical current.

Force and Torque

The force between a current (I) in a conductor of length (ℓ) perpendicular to a magnetic field (B) can be calculated using the Lorentz force law:

F = Iℓ × B

where × represents the vector cross product.

The most general approach to calculating forces in motors involves tensor notation.

Power

The output power of an electric motor is given by:

P_em = Tω = Fv

where:

• ω: shaft angular speed (radians per second)
• T: torque (Newton-meters)
• F: force (Newtons)
• v: velocity (meters per second)

In Imperial units, the mechanical power output is given by:

P_em = (ω_rpm T) / 5252 (horsepower)

where:

• ω_rpm: shaft angular speed (rpm)
• T: torque (foot-pounds)

In an induction motor, the relationship between motor speed and air gap power is given by:

P_airgap = (R_r / s) * I²_r

where:

• R_r: rotor resistance
• I²_r: square of current induced in the rotor
• s: motor slip (the difference between synchronous speed and slip speed)

Back EMF

When armature windings in a DC motor move through a magnetic field, a voltage is induced. This voltage opposes the supply voltage and is known as back electromotive force (EMF). The back EMF is proportional to the running speed of the motor and plays a crucial role in speed regulation.

In AC machines, back EMF is considered particularly important for precise speed control in variable frequency drives (VFDs).

Losses

Motor losses occur due to several factors, including:

• Resistive losses in windings
• Core losses
• Mechanical losses in bearings
• Aerodynamic losses (especially in motors with cooling fans)
• Commutation losses (sparking in mechanical commutators or heat dissipation in electronic commutators)

Efficiency

The efficiency (η) of a motor is calculated as:

η = Pm / Pe

where:

• η: efficiency
• Pe: electrical input power
• Pm: mechanical output power

The electrical input power is given by Pe = IV, and the mechanical output power is given by Pm = Tω.

Motor efficiency varies by type:

• Shaded pole motors: 15%-20%
• Permanent magnet motors: Up to 98%

Efficiency is typically highest at about 75% of the rated load. Additionally, larger motors tend to be more efficient than smaller ones.

Many national regulatory bodies enforce efficiency standards to promote the use of high-efficiency motors.

Goodness Factor

Eric Laithwaite introduced a metric to assess motor efficiency, known as the Goodness Factor (G):

G = (ω) / (Resistance × Reluctance) = (ωμσ A_m A_e) / (l_m l_e)

where:

• G: Goodness factor (values >1 indicate higher efficiency)
• A_m, A_e: Cross-sectional areas of the magnetic and electric circuit
• l_m, l_e: Lengths of the magnetic and electric circuits
• μ: Permeability of the core
• ω: Angular frequency of the motor

According to this formula, the most efficient motors have relatively large magnetic poles. However, this equation applies primarily to non-permanent magnet (PM) motors.

Performance parameters

Torque

Electromagnetic motors generate torque through the vector product of interacting magnetic fields. To calculate torque, it is essential to understand the fields in the air gap. Once these fields are established, torque is determined by integrating all force vectors multiplied by their respective radii. The current flowing through the windings produces these fields. However, in motors that use magnetic materials, the field is not always directly proportional to the current.

A key factor in electric motor selection is the relationship between current and torque. The maximum torque a motor can produce depends on the peak current, assuming no thermal limitations.

When designed optimally—considering factors such as core saturation constraints, active current (torque current), voltage, pole-pair number, excitation frequency (synchronous speed), and air-gap flux density—all electric motors and generators exhibit similar maximum continuous shaft torque within a given air-gap area. This air-gap area, along with winding slots and back-iron depth, defines the physical size of the electromagnetic core.

Some applications, such as accelerating an electric vehicle from a standstill, require short bursts of torque that exceed the maximum continuous operating torque. The ability to generate such bursts is constrained by magnetic core saturation, temperature rise, and voltage limits, which vary across different motor types.

Motors without a transformer circuit topology, such as wound rotor synchronous machines (WRSMs) and permanent magnet synchronous machines (PMSMs), cannot exceed their torque limits without saturating the magnetic core. Once saturation occurs, additional current does not increase torque. Additionally, excessive current in PMSMs can damage the permanent magnet assembly.

Motors with a transformer circuit topology, such as induction machines, induction doubly-fed machines, and wound-rotor doubly-fed (WRDF) machines, can produce torque bursts. In these machines, the electromotive force (EMF)-induced active current on either side of the transformer opposes each other, preventing core saturation. As a result, these machines can temporarily exceed their maximum design torque by two to three times.

Among electric machines, the brushless wound-rotor synchronous doubly-fed (BWRSDF) machine is unique in having a fully dual-ported transformer circuit topology. Unlike other machines, both ports of its transformer circuit are independently excited without a short-circuited port. While dual-ported transformer circuits tend to be unstable, requiring a multiphase slip-ring-brush assembly to transfer power to the rotor windings, they offer a significant advantage.

If torque angle and slip could be precisely controlled while ensuring brushless power delivery to the rotor, BWRSDF machines could produce torque bursts far beyond any other motor type. Calculations suggest they could generate torque bursts more than eight times their normal operating torque.

Continuous Torque Density

The continuous torque density of conventional electric motor is determined by the air-gap area and the depth of the back-iron. These factors depend on the power rating of the armature winding set, the machine’s operating speed, and the maximum achievable air-gap flux density before core saturation. Even with high-coercivity materials like neodymium or samarium-cobalt permanent magnets, the continuous torque density remains nearly the same across different electric machines when their armature winding sets are optimally designed.

A key factor affecting continuous torque density is the cooling method used. The ability to dissipate heat directly impacts the permissible operating duration before overheating damages the windings or degrades the permanent magnets.

Variation in Torque Density Across Motor Types

Some sources suggest that different electric machine topologies exhibit varying torque densities. One study provides the following specific torque density values for different motor types:

• Surface Permanent Magnet (SPM) Brushless AC, 180° conduction – 1.0 Nm/kg
• SPM Brushless AC, 120° conduction – 0.9–1.15 Nm/kg
• Induction Machine (IM, Asynchronous Machine) – 0.7–1.0 Nm/kg
• Interior Permanent Magnet (IPM) Machine – 0.6–0.8 Nm/kg
• Doubly Salient Variable Reluctance Machine (VRM) – 0.7–1.0 Nm/kg

For reference, the torque density values are normalized to 1.0 for the SPM Brushless AC motor with 180° conduction.

Liquid cooling significantly enhances torque density, offering approximately four times greater performance than air-cooled motors.

Comparison of Torque and Power Density

A comparison of different electric motor types—DC motors, Induction Motors (IM), Permanent Magnet Synchronous Motors (PMSM), and Switched Reluctance Motors (SRM)—shows the following:

• Torque Density:
  • DC: 3
  • IM: 3.5
  • PMSM: 5
  • SRM: 4
• Power Density:
  • DC: 3
  • IM: 4
  • PMSM: 5
  • SRM: 3.5

Additionally, sources indicate that PMSMs with power ratings up to 1 MW have considerably higher torque density compared to induction machines.

Continuous Power Density

Continuous power density is defined as the product of continuous torque density and the constant torque speed range. It represents the sustained power output an electric motor can deliver relative to its mass.

High-performance electric motors can achieve power densities of up to 20 kW/kg, meaning they can generate 20 kilowatts of output power per kilogram of motor weight. This metric is crucial in applications requiring compact and lightweight yet powerful motors, such as electric vehicles, aerospace systems, and industrial automation.

Acoustic Noise and Vibrations

Acoustic noise and vibrations in electric motors typically originate from three primary sources:

1. Mechanical Sources – These arise from components like bearings and structural imbalances.
2. Aerodynamic Sources – These result from airflow disturbances, such as those caused by shaft-mounted cooling fans.
3. Magnetic Sources – These are caused by electromagnetic forces, including Maxwell forces and magnetostriction, which act on the stator and rotor structures.

Among these, electromagnetically induced acoustic noise is often responsible for the characteristic “whining” sound of electric motors. This noise is generated due to fluctuations in the magnetic forces within the motor, impacting overall performance and user experience.

Standards

• American Petroleum Institute: API 541 Form-Wound Squirrel Cage Induction Motors – 375 kW (500 Horsepower) and Larger
• American Petroleum Institute: API 546 Brushless Synchronous Machines – 500 kVA and Larger
• American Petroleum Institute: API 547 General-purpose Form-Wound Squirrel Cage Induction Motors – 250 Hp and Larger
• Institute of Electrical and Electronics Engineers: IEEE Std 112 Standard Test Procedure for Polyphase Induction Motors and Generators
• Institute of Electrical and Electronics Engineers: IEEE Std 115 Guide for Test Procedures for Synchronous Machines
• Institute of Electrical and Electronics Engineers: IEEE Std 841 Standard for Petroleum and Chemical Industry – Premium Efficiency Severe Duty Totally Enclosed Fan-Cooled (TEFC) Squirrel Cage Induction Motors – Up to and Including 370 kW (500 Hp)
• International Electrotechnical Commission: IEC 60034 Rotating Electrical Machines
• International Electrotechnical Commission: IEC 60072 Dimensions and output series for rotating electrical machines
• National Electrical Manufacturers Association: MG-1 Motors and Generators
• Underwriters Laboratories: UL 1004 – Standard for Electric Motors
• Indian Standard: IS:12615-2018 – Line Operated Three Phase a.c. Motors (IE CODE) “Efficiency Classes and Performance Specification” (Third Revision)