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Electric Motors Explained

electric motor

The function of an electric motor is to turn electric energy into mechanical energy. Most electric engines work on the principle of magnetic fields interacting with electric currents in a wire winding, which generates torque in the shaft of the motor, allowing the energy to be used by any number of machines attached to the other end of the shaft.

Electric motors can be powered by either direct current (DC) sources like batteries or rectifiers; or by alternating current (AC) power sources, such as a power grid, generators, or inverters. In fact, an electricity generator is mechanically identical to an electric motor, the only difference being that it converts mechanical power generated by internal combustion (or other non-electrical sources such as wind power) to electrical power, whereas electric motors use electricity as the source, and put out mechanical power.

Electric motors are usually classified according to power source type, application, motion output, or internal construction. A wide division of category types is between AC and DC, but motors may also be classified as brushed or brushless; single-phase, two-phase, or three-phase; air-cooled or liquid-cooled.

General-purpose motors, most commonly designed for industrial use, have standard dimensions and characteristics to provide predictable power outputs and usages across many similar purposes. Other uses, however, require much larger versions of these engines, and are made for ship propulsion, pipeline compression, and pumped-storage applications. These engines have ratings as high as 100 megawatts.

Electric motors are also found in many household and office devices as well, from fans and blowers, to pumps, machine tools, disk drives, power tools, and mixers of various sizes and uses. Even watches can have small electric motors in them. Most electric cars use electric motors not only for locomotion, but also for generating power when slowing the vehicle – using the resistance of the engine rather than breaks whenever feasible. That allows energy that would otherwise be lost to heat and friction, to instead be added back into the battery.

Electric motors use torque (rotary force) to attach the power generated by the motor to some other mechanism. The particular mechanism doesn’t matter and can be anything from an elevator to a drill bit or a fan rotor. In most cases, the motor is designed to run continuously at a more or less steady pace, though there are exceptions, such as with hand-held drills and saws. Magnetic solenoids are transducers that produce mechanical energy from electrical power, but this is effective only over limited distances, whereas regular electric motors can move many times the length of the engine itself – electric cars are an obvious example.

Read next: Drill Bits Guide

One of the primary benefits of electric motors is their high efficiency. The internal combustion engine (ICE) for example, is below 50% efficient, compared to the 95% efficiency of electric motors. As well as this advantage, electric motors are also significantly smaller than ICE engines, much cheaper to build, and they do not exhaust carbon or greenhouse gasses into the atmosphere. They also provide even, consistent, and instant torque at any speed which reduces the complexity of attached mechanisms in many cases, such as removing the need for multiple gearing.

It is for reasons like these that the popularity of the electric engine continues to eat away at the internal combustion engine market, slowed mainly by the need for charging infrastructure and social comfort levels to catch up to the technology of the engines and batteries themselves. Battery technology has reached a functional level but is also predicted to improve significantly as the market puts more value on – and therefore money into – the electric motor and battery technology sector.

Components of Electric Motors

An electric motor consists of two main components – field magnets and armatures – that move relative to each other. The two components combine to create a magnetic circuit.

The field magnet is usually a set of electromagnets that surround the rotor and create a magnetic field that passes through the armature. The electromagnets consist of wire wound around a ferromagnetic iron core, which guides the field. Permanent magnets may also be used, though this is less common.

The armature is the part of the motor that actually develops the force. It consists of wire windings around a ferromagnetic iron core, like the field magnet. When electric current is passed through the wire, it creates a magnetic field upon which the field magnet exerts force, pushing it away and around in a rotary motion. This torque is called the Lorentz Force.

One of the two components (field magnet or armature) is mounted onto the stationary part of the motor which is attached to the frame. This stationary part is called the stator. The other component is mounted on the rotor, which is the part that turns. Either part may be mounted on either mount, though it is most common to have the field magnet on the stator and the armature on the rotor.

More specifically, a motor consists of these basic mechanical components: The rotor, the bearings, the stator, the windings, and the commutator.

Bearings

Bearings are used to support the motor while allowing it to turn on its axis. The motor housing supports the bearings. The motor shaft passes through the midst of the bearings and extends out to the point at which load is applied when attached to other machines or components. Since the load forces extend beyond the outer bearings, we say that the load is overhung.

Rotor

In an electric motor, the part that moves (rotates) is the rotor. It is the rotor that delivers the mechanical power out to attached components and machines. It usually has conductors laid into it to carry currents that accept the force from the magnetic field of the stator. In some cases, the rotors will carry permanent magnets and the stators hold the conductors, but this is less common.

It is necessary to have a gap between the stator and rotor in order for the rotor to move. This gap is usually as small as possible, as the more space there is, the less effective the motor will be, losing power and performance with each increase. Very small gaps, however, can increase noise and may pose mechanical problems – though the noise levels are usually still much lower than with internal combustion alternatives.

Stator

The stationary part of the electromagnetic circuit that surrounds the rotor is called the stator. It is usually comprised of field magnets. These can be electromagnets made up of wire wound around a ferromagnetic iron core (the most common form), or they can be permanent magnets. This setup generates a magnetic field which passes through the rotor armature and its windings, exerting a force that is in turn applied to the shaft.

The stator core consists of laminations – numerous thin, metal, sheets that are insulated from each other. These reduce energy loss that would otherwise occur in a solid core. Washing machines and air conditioners use resin-packed motors. In these motors, the stators are completely encapsulated in plastic (resin) to reduce the noise and vibration otherwise generated during use.

Windings

Windings are made up of coils of wire wrapped around a laminated magnetic core of soft iron. When current is run through these coils, they form magnetic poles. Electric motors come in either salient- or non-salient-pole configurations.

Salient-pole machines use ferromagnetic cores on the rotor. Their stators have ‘poles’ (projections with wire windings below each pole face) that face each other and activate magnetically when current is run through them.

Non-salient-pole configurations (also called round-rotor and distributed field) does not have projecting poles, but rather uses a smooth cylinder with evenly distributed windings running in slots around the circumference. The resulting poles rotate continuously. A shaded-pole version of this includes a delay in the phase of the magnetic field, caused by a winding around part of the pole.

There are motors that use conductors made of thicker metal (usually bars or sheets of aluminium or, more commonly, copper) and are powered by electromagnetic induction.

Commutator

Some motors have a rotary electrical switch that supplies current to the rotor. This switch is called a commutator. A commutator is made up of metal contact segments affixed to the armature of the machine. Electrical contact is made via ‘brushes’ make contact with the rotating armature. The brushes are made of soft conductive material, often carbon, and make sliding contact with one segment after another as the armature rotates, all the while supplying current to the rotor. The commutator segments are connected with the windings, and the commutator reverses the current direction periodically to keep the magnetic field between the stator and rotor always exerting force in the same direction. This prevents the rotor from being stopped by opposing magnetic force every 180 degrees of rotation.

Commutators are not very efficient motors and have therefore been overtaken in popularity by brushless direct current motors, induction motors, and permanent magnet motors.

Brushes will wear over time, so you may sometimes need to buy new carbon brushes for your tool.

You can see if your carbon brushes need replacing here, and learn more about them here.

Power supply and control

Motor supply

The power for a DC motor is usually provided via a split ring commutator, whereas an AC motor’s commutation is usually achieved either via a slip ring commutator (as with the DC) or via external commutation. Control types for AC motors can be variable-speed, synchronous, or asynchronous. Universal motors can run either on AC or DC power.

Motor control

By adjusting the DC voltage, the speed of a DC motor can be adjusted up or down. This can also be achieved by using pulse-width modulation (PWM).

AC motors, by contrast, are usually powered directly from the grid, or via motor soft starters, and operate at fixed speeds. They can, however, be operated at variable speeds using technologies such as power inversion, or the use of electronic commutator technologies or frequency drives.

‘Electronic commutator’ technology usually applies to self-communicated brushless DC motors and switched reluctance motor applications.

Types of Motors

There are three general types of electric motors, each operating around a specific principle of design or technology: magnetism, electrostatics, and piezoelectricity.

In magnetic motors, both the rotor and the stator produce magnetic fields which interact to produce a force. This force is directed into torque, which powers a shaft, which can then be used to power attached machinery. In magnetic motors, one or both of the magnetic fields must change during the rotation, to prevent opposing forces from stopping the rotation. This is achieved either by switching the polarity on and off in time with the rotation, or by varying the strength of the pole(s) at the right time during each rotation.

Of the two main types of motor, the older DC form is being replaced by (asynchronous or synchronous) AC electric motors.

Synchronous motors, once they have started, require synchrony of speed with the moving magnetic fields for normal torque conditions. The field must be provided by some form other than induction. Separately excited windings or permanent magnets are often used.

Most household and industrial motors do not require a full horsepower (0.746 kW) or more of output; these are therefor called fractional-horsepower motors, and include all motors exerting less than 1 HP, and all motors mounted on a standard-frame size smaller than that for a standard 1 HP motor.

Notes:

  • Rotation is independent of the frequency of the AC voltage.
  • Rotation is equal to synchronous speed (motor-stator-field speed).
  • Fixed-speed operation rotation in SCIM is equal to the synchronous speed, minus the slip speed.
  • The WRIM in non-slip energy-recovery systems is usually used only for starting the motor, but it can also be used to vary load speed.
  • BLDC-motor drives are usually with trapezoidal-current waveform, unlike induction- and synchronous-motor drives with are typically with six-step or sinusoidal-waveform output. The behaviour of sinusoidal and trapezoidal PM machines is the same with regard to fundamental aspects.
  • WRIM is used in slip-recovery and double-fed induction-machine applications for variable-speed operation.
  • Whereas a wound winding is connected externally through slip rings, a cage winding is a short-circuited squirrel cage rotor.

Abbreviations:

  • BLAC – Brushless AC
  • BLDC – Brushless DC
  • BLDM – Brushless DC motor
  • EC – Electronic commutator
  • PM – Permanent magnet
  • IPMSM – Interior permanent-magnet synchronous motor
  • PMSM – Permanent magnet synchronous motor
  • SPMSM – Surface permanent magnet synchronous motor
  • SCIM – Squirrel-cage induction motor
  • SRM – Switched reluctance motor
  • SyRM – Synchronous reluctance motor
  • VFD – Variable-frequency drive
  • WRIM – Wound-rotor induction motor
  • WRSM – Wound-rotor synchronous motor
  • LRA – Locked-Rotor Amps: This is the expected current during starting conditions with full voltage. It occurs instantly.
  • RLA – Rated-Load Amps: This is the maximum current draw for a motor under any/all operating conditions. Because RLA is sometimes mistakenly called ‘running-load amps,’ some mistakenly believe that the motor should always pull these amps. Prior to 1976, this was called ‘FLA’ or ‘Full Load Amps.’

Self-commutated motors

Brushed DC motors

All self-commuted DC motors run on DC power and most are of the small permanent magnet (PM) type. These employ brushes to reverse motor windings’ current in synchronism with rotation.

Electrically excited DC motors

Components of a brushed electric motor with PM stator and two-pole rotor. The polarities of the inside faces of the magnets are indicated by N and S (the outside polarities are therefore the opposite).

Commutated DC motors each use a set of windings wound around a rotating armature mounted on a shaft. The commutator (the rotary electrical switch that reverses the flow of current to enable continuous rotation) is located in the shaft. DC motors therefore have AC current in the rotating windings. One or more pairs of brushes carry current to the commutator from the external electric power source.

The armature is made up of one or more wires coiled around a laminated core of ‘soft’ ferromagnetic material. When the current passes through the brushes to the commutator, it follows through into one of the windings of the armature, creating a temporary electromagnet. Near this is the stationary magnetic field of the PMs or field coil, which are attached to the motor frame. Since the motor frame is stationary and the armature can rotate, the resulting magnetic charge turns the armature and the motor shaft to which it is attached. As the rotor turns, the commutator switches power from one coil (in the armature) to another. This prevents the pole from settling into a static distance from (or proximity to) another pole, creating a constant power of rotation on the armature and shaft. If this were not the case, the motor would flip to a certain position and stop, much like a compass needle does when turning in its housing.

The DC motor does have some limitations, mainly resulting from the need for the rapidly turning brushes to maintain contact with the commutator and the friction and sparks that results from it. Sparking can make or break circuits through the rotor coils, jumping over insulating gaps between commutator sections. This can, in some designs, cause a shorting together of adjacent sections (coil ends) while jumping the gaps. In addition to this, the inductance of the rotor coils increases when each circuit is opened, increasing the sparking even further. This dampens the speed of the machine and can cause other problems, like overheating, or even catastrophic melting of the commutator. The output of the motor is limited by the current density per unit area of the brushes, along with their particular resistivity. Sparking also generates RFI (radio frequency interference), electrical noise, and premature wearing of the brushes and commutator. Replacing a commutator carries a significant cost, especially on larger motors; on smaller motors the commutator is often an integrated part and cannot be replaced without replacing the whole rotor.

Most commutators are cylindrical (which makes sense as they are often located in the shaft), though some are flat discs made up of three or more segments mounted on an insulator.

More speed is achieved by using smaller brushes (with lower mass and lower contact area), whereas larger brushes will maximise motor output. Larger brushes do have increased risk of excessive sparking and bouncing, however, so a balance must be reached depending on the purpose of the motor. Likewise, stiffness is a factor. Stiffer brush springs allow a given size of brush to work at a higher speed, but with lower efficiency and higher wear due to increased friction. DC motors are designed with compromises in mind, balancing output power, speed, efficiency, and wear.

Definition of a DC engine:

  • Armature circuit has a winding that can be either stationary or rotating because the load current is carried there.
  • Field circuit is a set of windings that produces a magnetic field enabling electromagnetic induction.
  • Commutation occurs, which enables rectification or the derivation of DC in DC machines.

The five types of brushed DC motors include the DC shunt-wound motor; the DC series-wound motor; the DC compound motor (both the cumulative compound configuration and the differently compounded configuration); the PM DC motor; and the separately excited DC motor.

Permanent magnet DC motors

Main article: Permanent-magnet electric motor

A permanent magnet (PM) motor relies on PMs to create the magnetic field that interacts with the rotor field to produce torque – in other words, the stator frame does not have a field winding.

In large motors, compensating windings may be used to improve commutation when the motor is under load. Large PMs are also very costly and both difficult and dangerous to assemble, and so they are seldom used in field machinery. The fixed field of a PM also means it cannot be adjusted for speed control.

PM fields are most useful in tiny motors because they do not require additional power to generate the magnetic field.

PM motors may use high energy magnets (such as neodymium, or neodymium-iron-boron alloy) in an effort to minimise the weight and size of the overall machine. High-energy PM machines have a higher flux density, which makes them competitive to optimally designed singly-fed synchronous and induction electric machines.

Electronic commutator (EC) motors

Brushless DC motors

The BLDC design is an effort to eliminate or minimise the problems inerrant in the standard DC motor. An external electronic switch, synchronised with the rotor’s position, is used to replace the mechanical rotating switch. BLDC motors achieve an efficiency of 85-90% (sometimes more, achieving up to a reported 96.5% at the time of writing). By contrast, DC motors that employ brushes typically reach efficiency ratings of 75-80%.

BLDC motors are recognisable first by their trapezoidal counter-electromotive force (CEMF) waveform. This is produced by the even distribution of the stator windings and by the placement of the permanent magnets in the rotor. The stator windings if trapezoidal BLDC motors (also called ‘electronically commutated DC motors’ or ‘inside out DC motors’) can be with single-phase, two-phase, or three-phase and have Hall effect sensors on their windings to sense the position of the rotors. This also allows for low coast closed-loop control of the electronic commutator.

BLDC motors are especially useful when precise speed control is desired, such as with disc drives, office fans, laser printers and other copying machines. Their advantages over conventional motors include:

  • They are more efficient and cooler-running than shaded-pole motors.
  • They are longer-wearing than shaded-pole motors due to cooler running and lack of some parts that can wear out, such as a commutator.
  • Lack of a commutator also means lower RF noise.
  • The Hall effect sensors can double as tachometer signallers for closed-loop control (servo-controlled) applications.
  • Precise speed control can be achieved by synchronising the motor to an internal or external clock.
  • There is no chance of sparking (or the resulting ozone, lack of efficiency, and wear created by sparking).
  • Excellent for use in small devices to get rid of heat.
  • Very quiet running.
  • Low vibration.

BLDC motors can range from the very small to much larger motors with ratings of 100 kW or more, such as those used in electric vehicles and high-performance model aircraft.

Switched reluctance motors

The SRM has no electric currents, in the rotor, and no brushes or permanent magnets at all. The torque is produced via a slight misalignment between the poles on the rotor and the poles on the stator. The rotor aligns with the stator field, while the windings on the stator field are sequentially energised to create rotary motion.

The field windings create a magnetic flux along the path of least magnetic resistance, which in this case is though the poles of the rotor that lie closest to the energised stator poles. The stator poles are magnetised in this way, creating torque. It is the sequential energising of the windings that creates ongoing torque and rotary motion.

SRMs are popular for use in some appliances and vehicles.

Universal AC/DC motors

A universal motor is one that can operate on AC or DC power. These are a commutated electrically excited series or parallel wound motor. The current in the field and armature coils alternate (reverse polarity) in synchronism, allowing AC operation with a constant direction of rotation.

The universal motors that operate on normal power line frequencies are often in a range less than 1000 watts. These formed the basis of traditional railway traction motors for the electric railway system. The eddy current heating of magnetic components in DC-designed motors, being used with AC power, caused a loss of efficiency. The unlaminated (solid iron) motor field pole pieces used in those older systems are now rarely used.

The advantage of using a universal motor is that AC power supplies can be used with motors more closely resembling DC motors – for example motors with high starting torque (under high running speeds) and compact designs. The disadvantages include higher maintenance needs and a shorter lifespan for the commutator. Food mixers and power tools often use this type of design, as they are used intermittently (rather than for extended periods) and have high demands for starting torque. Some speed control is attained using multiple taps in the field coil, though this is imprecise. Household blenders, for example, often couple a field coil (with multiple taps) with a diode that is inserted in series with the motor. This causes the motor to run on half-wave rectified AC. Electronic speed control is common in universal motors they are therefore popular in appliances such as domestic washing machines. The ability to rotate the drum forwards and backwards (during agitation) is achieved by switching the field winding with respect to the armature.

Universal motors ca run at much higher speeds than SCIMs, as the latter are limited to the power line frequency. This means that universal motors work better for some household appliances, like blenders, hair dryers, and vacuum cleaners; and for portable power tools such as drills, sanders, and hand-held power saws – any device, really, for which high speeds and light weight are both necessary. As a frame of reference, rpms for many weed trimmers and vacuums exceed 10,000; and miniature grinders can exceed 30,000 rpm.

Externally commutated AC machines

AC induction and synchronous motors are designed to optimise operation on single-phase or polyphase sinusoidal or quasi-sinusoidal waveform power. In other words, they work on the AC power grid or for VFD controllers with variable speeds. There are two parts to the AC motor: the stator and the rotor.

The stator is stationary and has coils that produce a rotating magnetic field using AC power. The rotor is given a torque by the rotating field, and is attached to the output shaft.

Induction motors

Cage and wound rotor induction motors

An induction motor may be defined as an asynchronous AC motor in which power is transferred, via electromagnetic induction, to the rotor. A simple way to picture this is to think of it as a rotating transformer, with the primary side represented by the stator and the secondary side represented by the rotor. Induction motors of the polyphase type are widely used in industry.

The category of ‘induction motors’ can be divided into the subgroups of Squirrel Cage Induction Motors (SCIMs) and Wound Rotor Induction Motors (WRIMs).

SCIMS are characterised by a heavy winding of solid aluminium or copper bars (sometimes other metals) which are electrically connected by rings on the other end of the rotor. The entire structure of the bars and rings, taken together, resemble a squirrel cage like those used for pet rodents, and it is from that resemblance that we take the name.

Currents are induced into the winding to produce a magnetic field for the rotor. The shape of the rotor bars determines the characteristics with regard to speed and torque. The current induced in the squirrel cage is nearly at line frequency at low speeds, and tends to be in the outer parts of the cage. With acceleration, the slip frequency drops and more current is present in the interior of the winding. The resistance of the winding portions in the interior and outer parts of the cage can be changed by shaping the bars. The effect is to cause variable resistance in the rotor circuit. Most squirrel cage motors, however, have uniform bars in the cage.

In WRIMs, the rotor winding is comprised of multiple turns of insulated wire, connected to slip rings on the motor shaft. A control device (such as an external resistor) can be connected in the rotor circuit. These allow for some control of the motor speed, at the cost of significant power loss due to dissipation of power in external resistance. Some slip-frequency power can be recovered by feeding a converter from the rotor circuit and adding an inverter or separate motor-generator.

WRIMs are used mostly for high inertia loads requiring very high starting torque over the whole of the speed range. These motors can produce maximum torque at a relatively low supply current, through the whole range of speed, through correct selection of resistors used in the secondary resistance or slip ring starter.

The torque curve of the motor is modified by the amount of resistance connected to the rotor circuit and can thus control the motor speed. Increase the value of resistance and the speed of maximum torque drops. The torque can be even further reduced by increasing connected resistance beyond the point that maximum torque occurs at zero speed.

In situations in which the load has a torque curve that increases with speed, the motor will run at a speed where the torque developed by the motor and the load torque are equal. Reducing load will cause an increase in the motor speed, just as increasing load will cause a slow down until load and motor torque are equal. When operated in this way, the slip losses are dissipated in the secondary resistors, and are significant. Overall efficiency and speed regulation are also poor.

Torque motors

A torque motor is designed to operate indefinitely while stalled. In other words, the rotor can be blocked from turning, torque remains steady and continuous, and no damage occurs to the motor.

Common uses of torque motors include supply- and take-up reel motors for tape drives. Driven from a low voltage, the result is a constant light tension while the tape is moving and also while it is at rest. At higher voltages (and thus higher torque) the motors can produce higher speed fast forward or rewind functions without the need for any additional gears, clutches, or other mechanics. They are also used to produce resistance in simulators and computer games, such as force feedback steering wheels.

Torque motors are also used to control the throttle in internal combustion engines. Using an electronic governor, the motor works against a return spring, moving the throttle in accordance with the governor’s output. The governor counts electrical pulses from the ignition system (or from a magnetic pickup) and makes small adjustments to the current supply to the motor. The current is increased if the engine drops lower than the desired speed, developing more torque, which pulls against the return spring and opens the throttle. The current is decreased if the engine runs too fast, by reducing current to allow the return spring to pull back and close the throttle.

Synchronous motors

A synchronous electric motor runs on AC and consists of a rotor spinning with coils passing magnets at the same rate as the AC, which produces a magnetic field and subsequence torque. Under usual operating conditions, it therefore has zero slip. Induction motors, on the other hand, require slip to produce torque. (There is a type of synchronous motor that is much like an induction motor, but that is excited by a DC field.)  In the synchronous motor, slip rings and brushes conduct current to the rotor, and the rotor poles connect to each other and therefore move at the same speed. It is from this synchronisation of the rotor pole speeds that we get the name. For low load torque uses, there is a type of synchronous motor that creates discrete poles by using flats ground onto a conventional squirrel cage rotor. Another has no rotor windings and discrete poles, but is not self-starting. It was made by Hammond for clocks prior to WWII, and for organs. The clocks require use of a small knob in the back to start them, while the organs have an auxiliary starting motor connected to a manually operated switch.

Lastly, hysteresis synchronous motors are (essentially) two-phase motors, but with a phase-shifting capacitor on one phase. Though they start much like induction motors, they differ in that a sufficient decrease in the slip rate will temporarily magnetise the rotor (which in this case is a smooth cylinder`). Because the poles are distributed, it acts much like a permanent magnet synchronous motor (PMSM). The rotor will stay magnetised, but can be easily demagnetised, due to the material used for its construction. The rotor poles do not drift once they are running, reliably staying in place.

Traditional electric clocks employ low-power synchronous motors to keep time. These may have multi-pole permanent magnet external cup rotors and shading coils to produce the starting torque. Telechron clock motors, for example, have a two-spoke ring rotor and shaded poles for starting torque which perform much like a discrete two-pole rotor.

Doubly-fed electric machines

A doubly fed electric machine is called ‘doubly fed’ because it has two independent multiphase winding sets. These contribute active power to the energy conversion process. One or both of the winding sets is electronically controlled for variable speed operation. The use of more than two independent multiphase winding sets requires topology duplication. Doubly fed electric motors have an effective constant torque speed range that is twice synchronous speed for a given excitation frequency. Singly fed electric machines, by contrast, have only one active winding set and therefore half of the constant torque speed range of doubly fed machines.

One advantage of the doubly fed motor is that it allows for a smaller electronic converter. This comes with a disadvantage, however, in that the cost of the rotor winding and slip rings may offset any savings in power electronic components. It is also difficult to control speeds near the synchronous speed, which limits applications.

Special magnetic motors

Rotary

Ironless or coreless rotor motors

In principle, none of the motor designs described above require that the iron (steel) parts of the rotor actually rotate. If the soft magnetic material of the rotor is in cylindrical form, then torque is only exerted on the windings of the electromagnets (the only exception to this statement is the effect of hysteresis). Because of this, it is possible to design a coreless (sometimes called ‘ironless’) DC motor. This kind of permanent magnet DC motor is optimised for very rapid acceleration. The rotor has no iron core, sometimes consisting of only a winding-filled cylinder or a self-supporting structure of magnet wire and bonding material. The rotor itself fits within the stator magnets. The return path for the stator magnetic flux is provided via a soft stationary cylinder inside the rotor. Another kind of design for coreless rotor motors has the stator magnets surrounded by the rotor winding basket. The rotor sits inside a magnetically soft housing cylinder which also provides the return path for the flux.

The lighter weight of the coreless rotor allows for the rapid acceleration as compared to other designs, often achieving a mechanical time constant in less than a millisecond, especially if the windings are aluminium as opposed to the heavier copper option. A disadvantage, however, is that the lack of metal mass means there is no effective heat sink. Even small coreless motors must be actively cooled using forced air. Software such as Motor-CAD has helped to increase the thermal efficiency of these motors by minimising design flaws.

Mobile phones often use a type of these motors for the vibrating feature. Tiny cylindrical permanent magnet field types, or disc-shaped types with thin, multipolar disc fields, attach to an intentionally unbalanced rotor structure of moulded plastic with two bonded coreless coils. The rotor coils are powered by metal brushes and a flat commutator switch, and when activated the imbalance of the rotor structure causes the vibration.

Fast head positioners for rigid-disk (sometimes called ‘hard disc’) drives use limited-travel actuators with no core and a bonded coil between the poles. The magnets are permanent, thin, and high-flux. The foundational technology, used in loud older speakers, gave rise to the casual use of the term ‘voice coil’ structure, but the earlier rigid-disk-drive heads that moved in straight lines and had a similar structure to those loud speakers, have since been replaced.

Pancake or axial rotor motors

The pancake motor or printed armature has arrays of high-flux magnets with a disc-shaped set of windings running between them. The space between the circle of magnets and the rotor are slightly spaced to form an axial air gap. The overall profile of the motor is quite flat, giving rise to the common name of the design. Among brand names using this design is the fairly well-known ServoDisc.

The printed armature is made from punched copper sheets, laminated together using thin composites. The result is a rigid disk with a thin profile. The lack of a separate ring commutator makes this design unique among brushed motor designs; in this design the brushes run directly on the surface of the armature.

Another way to manufacture this is to wind copper wire and lay it flat, with the commutator, in a flower and petal shape. Epoxy potting systems are commonly used to stabilise the windings due to their moderate, mixed viscosity and long gel time. This results in low shrinkage and low exotherm in working conditions. They are typically UL 1446 recognized as a potting compound insulated with 180 °C (356 °F), Class H rating.

Ironless DC motors have the unique advantage of not ‘cogging’ – in other words not having torque variations resulting from changing attraction between the magnets and the iron. Parasitic eddy currents cannot form in an ironless rotor, which substantially improved efficiency but requires a higher switching rate (>40kHz) or DC for variable-speed control, due to the lower electromagnetic induction.

Ironless DC motors were first used to drive the capstan(s) of magnetic tape drives, to achieve the necessary minimal acceleration to operating speed and minimal stopping distance. Pancake motors are popular in high-performance servo-controlled systems, robotics, and medical devices. There is a wide variety of constructions currently available, for a broad range of uses, from low-cost pumps and basic servos to high-temperature military use.

A different approach to the design is the Magnax motor. These employ a single stator pressed between two rotors. Sample performance is peak power at 15 kW/kg, and sustained power of approximately 7.5 kW/kg. The yokeless axial flux motor has a shorter flux path, so the magnets are farther from the axis. This allows for zero winding overhang because 100 percent of the windings are active. Construction using rectangular-section copper wire can enhance this effect further. The motors can be stacked to work together in parallel. Any instabilities are mitigated by making sure to place of equal and opposing forces from the rotor disks, and connecting the rotors directly, using a shaft ring, which cancels out the magnetic forces.

Sizes of Magnax motors range from 15cm to 5.4m in diameter.

Servo motors

A servomotor is used for position-control or speed-control feedback control systems. Often sold as a complete unit, they are used with machine tools, pen plotters, and other process systems. The motors used in a servomotor system require well-documented performance assessments, as speed vs torque curves and must be high ratio. Winding inductance and rotor inertia may limit the overall performance of the servomechanism loop, and so they must be known and considered prior to assessing suitability of components. If the servo loop is slow responding, it may use conventional AC or DC motors and drive systems with position or speed feedback on the motor. With increases in dynamic response, however, the motor designs must be more specialised. An example of a specialised design is the coreless motor. AC motors favour permanent magnet BLDC, synchronous, induction, and SRM drive applications, due to their superior power density and acceleration abilities as compared to DC types.

Servo systems employ continuous feedback while the motor is running, as opposed to the stepper motor which uses open-loop and relies on the motor not to miss steps. Any feedback (i.e. a ‘home’ switch) would be external. The controller in dot matrix computer printers, for example, makes the print head stepper motor move to the left hand extreme of its track upon start-up, at which point it senses a home position and stops stepping. A bidirectional counter in the microprocessor keeps track of the print-head position from that point in time until the power is shut off.

Stepper motors

Stepper motors are used mostly for situations in which precise rotations are necessary. An internal rotor is controlled by electronically switched, external magnets which interact with the permanent magnet of the rotor or, in alternative designs, the magnetically soft rotor with salient poles. The stepper motor is therefore situated somewhere between a DC electric motor design and a rotary solenoid design. Each coil is alternately energised, causing the rotor to align itself with the magnetic field of the energised field winding. The stepper motor starts and stops in quick successions, rather than running continuously, and can do so forwards or backwards. It can start, stop, accelerate or decelerate, or change direction at any time.

In the simpler forms of the design, the motor drivers entirely energise or de-energise the field windings, making the rotor ‘cog’ to a set number of positions. As designs get more sophisticated, proportional control is possible, allowing the rotors to stop between cog points and to achieve a much smoother rotation. This ability to control ‘steps’ to such a fine degree is called ‘microstepping.’ When used as part of a digital servo-controlled system, computer-controlled stepper motors are a key component among the most versatile of positioning systems.

Stepper motors are able to stop at a specific angle, in discrete steps, and are therefore used in read/write head positioning for floppy disc drives, and in other older types of computer drive systems. The precision necessary for pre-gigabyte era computer systems was primitive enough for this type of motor to suit the purpose well, correctly positioning the read/write head of the disc drive to an adequate degree of accuracy and with adequate speed. With the advancement of drive technology, however, the precision and speed necessary quickly outpaced the abilities of the stepper motor and they became uncompetitive and obsolete for that purpose. Newer hard disc drives use voice-coil-based head actuator systems. So-called ‘voice coil’ system is named after the structure of a typical, cone-shaped loudspeaker, which historically used this technology to position the heads. Modern versions (actuator coil conductors) use a pivoted coil mount which allows the coil to swing back and forth, perpendicular to the magnetic lines of force, but otherwise similar to the blades of a rotating fan.

Stepper motors are still common in many computing-related applications, including optical scanners, digital photocopiers, printers, etc. They are suitable to move scanning elements, print head carriages, and platen or feed rollers. Many computer plotters, which were industry standard until around 1990’s, have been replaced by large-format ink jet and laser printers which employ rotary stepper motors to move the pen and platen. These systems use either linear stepper motors, or servomotors with closed loop analogue control systems.

The tiniest of commonly used stepping motors are found in quartz analogue watches. These have only one coil, one permanent magnet rotor, and draw very little power, making them almost ideal to wristwatch requirements. In fact, some more complex watches, such as chronographs, may contain more than one stepping motor in a single watch.

Stepper motors and SRMs are classified as variable reluctance motor type. Their designs are closely related to those of the three-phase AC synchronous motors. Common uses for stepper motors include running print heads and optical scanners, and computer numerical control (CNC) machines including routers, CNC lathes and plasma cutters.

Linear motors

The term ‘linear motor’ refers to any electric motor that has been ‘unrolled’ – in other words, instead of producing torque, it produces force in a straight line along its length. Induction motors and stepper motors are the most common type of linear motor.

Two common places to find linear motors include roller coasters (where the rail controls the rapid motion of the motorless cars) and in maglev trains. Back in 1978, the HP 7225A pen plotter used two of these linear stepper motors to move the pen along the X and Y axes.


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