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Electric Drives - Special Purpose Motors

(Description and Applications)


Motor Construction

Special purpose designs have been developed to solve a wide range of drive problems. Some common examples are included here.


Integrated Starter Generator (ISG)

The electronically controlled integrated starter generator used in mild hybrid electric vehicles (HEVs) combines the automotive starter and alternator into a single machine. The conventional starter is a low speed, high current DC machine, while the alternator is a variable speed 3 phase AC machine.

The ISG has four important functions in a hybrid vehicle application

  • It enables the "start-stop" function, turning off the engine when the vehicle is stationary saving fuel.
  • It generates the electrical energy to power all the electrical ancillaries.
  • It provides a power boost to assist the engine when required, permitting smaller engines for similar performance.
  • In some configurations it recuperates energy from regenerative braking.


In a typical implementation (below), the ISG is a short axis, large diameter "pancake" shaped switched reluctance machine mounted directly on the end of the engine crankshaft between the engine and the clutch in the gearbox bell housing.

Integrated Starter Generator Components

The ISG is a bi-directional energy converter acting as a motor when powered by the battery or a generator when driven by the engine.

The system voltage in a mild HEV is 42 Volts which means that, for the same cranking power as a 12 Volt machine, the starter current can be reduced. Typical power throughput is between 5kW and 15 kW with a possible peak power of 70 kW for cold cranking..

The brushless ISG design eliminates one rotating machine completely as well as the associated commutator and brushes from the DC machine and the sliprings and brushes from the AC machine. The starter solenoid, the Bendix ring (starter gear) and the pulley or gear drive to the alternator are also no longer needed and because of the higher system voltage, the diameter and weight of the copper cabling is also reduced substantially.

The savings however come at a cost. The system must be integrated with several subsystems as follows

  • An AC/DC converter to rectify the generator output voltage.
  • A DC/DC converter to supply the vehicle's electrical power system voltages.
  • Power electronics and software to control the ISG current, voltage, speed, torque and temperature as appropriate.
  • An overall energy management system integrated with the vehicle's engine, battery and brakes.


Larger versions of this construction are also used in full hybrid electric vehicles.


The switched reluctance machine with its simple rotor of inert iron is very robust, able to operate at high speed and to withstand the harsh operating conditions in the engine compartment.




Outer Rotor Motors

There are many designs using this construction, mostly for small sizes. Two examples of low power motors are shown below. High power versions are used for "in wheel" automotive applications.

  • Inside Out Motor
  • These are permanent magnet motors with the moving magnets arranged around the periphery of a multi pole fixed stator carrying the field windings.


    Diagram of Outer Rotor Motor


    Low power versions are used in small cooling fans and direct drive record player turntables. Higher power versions are used for in-wheel motors in automotive and eBike drive systems. Because of their construction however they are vulnerable to damage from the high lateral forces and shock loads found in these higher power applications.


  • Toroidal Coil Motor
  • This is an "inside out" brushless permanent magnet motor with a toroidal wound stator covered by a cup shaped permanent magnet outer rotor.


    Diagram of Torroidal Coil Motor


    Because of the low inertia and friction free rotor, the toroidal motor is capable of speeds up to 25,000 RPM. Suitable for low power applications it is used for example to drive the polygonal rotating mirrors which are mounted directly on the rotor in laser printers.


Linear Motors

In most cases the linear motor can be considered as a conventional rotary motor with both the stator and the rotor split and rolled out flat. The same electromagnetic forces apply and these have been employed in similar classes of AC and DC machines. Except for traction motors the travel of the motor armature is usually quite short.

  • Linear Stepping Motors
  • The most common application is the stepping motor. Stator poles are laid out along the track and excited by windings fed from a pulsed DC source. Permanent magnets forming the armature are held in the carriage. The carriage moves along the track in response to pulses sent to the stator windings in much the same way as the rotor turns in a brushless DC motor. Closed loop control is possible by mounting a position sensor on the carriage.

    Despite the elegance of the linear motor, linear motion is more often provided by the less expensive and more mundane method of using a rotary stepping motor driving a lead screw.

  • Maglev Traction Motors
  • The principle of the linear induction motor is used to propel high speed Maglev (Magnetic Levitation) trains which float on a magnetic field created by electromagnets in the trackbed under the train . A separate set of trackside guidance magnets is used to control the lateral position of the train relative to the track. Thus the maglev train uses electromagnetic forces for three different tasks, to suspend, to guide and to propel the train.

    Maglev trains have been developed in several countries of the world using a variety of configurations. Examples of the essential features are described below.


    • Propulsion
    • The train has no onboard motor. Electromagnets in the trackbed are excited in sequence creating a linear rather than a rotating field. By transformer action, the trackbed coils induce currents in coils on board the train which are used to energise powerful electromagnets. The Lorentz force between the trackbed currents and the onboard electromagnets causes the magnets to be propelled along by the moving field.

      The principles involved are very similar to those of the induction motor but with the static and moving parts interchanged. See diagram below.


      Diagram of Linear Motor and Magnetic Levitation

      For illustrative purposes the track can be likened to a ladder formed by the unrolled squirrel cage rotor of the induction motor. In this case however it is fixed and it supplies the moving field. Currents are induced in the train's electromagnets which are equivalent to the stator poles of the induction motor but in this case the magnets are free to move. In practical designs the trackbed currents are actually provided in a series of individual coils laid along the track.

    • Levitation
    • Various levitation schemes are used. The force holding the train aloft can be created by the magnetic repulsion between the same electromagnets on the track and the onboard electromagnets in the train which are used for propulsion. The train's levitating magnets are powered by direct current supplied by a battery which is kept charged by an induction generator taking its power from the currents induced by the trackbed coils in the onboard generator coils.

      In the diagram above, when the magnet is directly above the current carrying conductor as shown, the magnetic forces (north and south poles) from the two adjacent current loops cancel out and there is no lift. If however the magnet is moving very quickly over the coils, it will reach a position over like, repulsive, poles (north poles in the diagram) which are displaced from the attractive south poles so that the net effect is a force repelling the magnet away from the track. This is only possible because the current in the trackbed magnets lags the voltage due to the inductance of the windings, creating a delay in the build up of the balanced field by which time the magnet has moved into the adjacent region where there is a net repulsive force. This effect only happens when the magnet on the train is moving at high speed across the trackbed magnets. Thus the train needs to be in motion for this system to work and the train needs wheels for support as it accelerates from rest and when it is slowing to a halt.

      Alternatively levitation can be provided by separate windings. The train's levitation magnets protrude from the side of the train and run between pairs of vertically separated electromagnets in guideways at each side of the train, rather than in the trackbed. This arrangement creates an attractive force above the train's magnets combined with a repulsive force beneath the train's magnets to provide the levitating force.

    • Guidance
    • For guidance the train uses magnetic fields provided by a separate set of weaker magnets along each side of the train. Similar in principle to the levitation magnets they are used to control the lateral position of the train relative to the track.


    Excitation of the trackside magnets is arranged such that only the section under the train is active. As the train moves along the track between sections the current to the previous section is switched off and the current to the next section is switched on pulling the train along. This serves the dual purpose of avoiding losses by energising only the section of track directly under the train and at the same time, since the power to the rest of the track is switched off, it provides security against electric shock to anybody near to the track and avoids the possibility of accidentally short circuiting the system by dropping rubbish onto live conductors.

    Very high armature currents of thousands of amps or more are involved and some designs use high temperature superconductors ( HTS ) in the onboard magnets, cooled with liquid nitrogen or helium to minimise the resistive losses.
    As might be expected some sophisticated control systems are needed to keep everything on track.




Axial Field Motors

Axial field motors have been developed for applications which require short, flat, "pancake" construction.


Printed Circuit (PCB) or "Pancake" Motor

The printed circuit motor is an example of an ironless or coreless motor with several unique features. The pancake construction uses an axial magnetic field to achieve the short flat construction. Radial field PCB motors are also possible.

  • Construction
  • The rotor windings are printed, stamped or welded onto a thin, disc shaped glass fibre circuit board which rotates in the air gap between pairs of permanent magnets arranged around the periphery of the disk. The windings fan out in a series of radial loops around the surface of the disk. The magnets are arranged alternatively north and south so that the magnetic fields in the air gaps of adjacent magnet pairs are in opposite directions. The magnets are held in place by two iron end caps in a compact "pancake" shaped block to complete the magnetic circuit. Current is fed to the rotor windings via brushes through precious metal commutator segments printed on the disc.

Diagram of Printed Circuit, Pancake Motor

  • Operating Principle
  • Traditional electric motors have a radial magnetic field or flux with the rotor current flowing axially along the length of the rotor. In typical printed circuit motors the construction is reversed. The magnetic field is axial (oriented along the axis of the machine) and the current flows radially from the axis to the edge of the disc and back again. A tangential force on the disk is created by the current passing through the magnetic fields in the air gaps between the pole pairs of the permanent magnets. So that the return current does not cancel out the effect of the outgoing current, the return wire is physically separated or displaced to one side from the outgoing wire by the width of the magnet. In this way it interacts with the magnetic field of the adjacent magnet which is in the opposite direction and thus reinforces the tangential force on the disk.

    In many ways it is similar to Faraday's 1831disk or homopolar motor which used a single magnet and was driven by a unidirectional current fed by brushes at the centre and on the periphery of the disk.


  • Applications
  • The printed circuit motor is a very compact and light weight design making it useful in confined spaces. Since the rotor does not have drag a lump of iron around, it has very low inertia and can run up to speed very quickly. Because of the many commutator segments and the low current capability of the windings, the PCB motor is only suitable for low power applications and is not suitable for continuous operation. It is however ideal for servo systems and industrial controls and automotive applications such as electric window winders.


Micro-motors (Micro-ElectroMechanical Systems - MEMS)

  • Electrostatic Motor
  • The motor shown below is an example of semiconductor manufacturing technology used to fabricate very small mechanical components. It measures 100 microns across, or about the width of a human hair. Similar in principle to a reluctance motor, it depends on electrostatic attraction, rather than magnetic attraction, between the stator and rotor poles. Because the dimensions are so tiny, very high electric fields can be built up with only a few volts between the motor poles.


    Diagram of Electrostatic Micro Motor

    Photo of Integrated Circuit Electrostatic Micro Motor

    Fan Long-Shen, Tai Yu-Chong and Richard S. Muller 1989
    IC-processed electrostatic micromotors
    Sensors Actuators 20 41-7


    The motor is not assembled from individual components. Instead the components are built up on a semiconductor substrate by masking and etching and a mask-less post-processing release step is performed to etch away sacrificial layers, allowing the structural layers to move and rotate.

    Micromachined micromotors can be monolithically integrated together with the necessary CMOS drive circuits, containing oscillators, frequency dividers and counters, and transistors for the drive circuit all on one silicon chip.

    Common uses include defense/munitions applications, computer hard drives, optics, sensors and actuators.




Nano-motors (Nano-ElectroMechanical Systems - NEMS)

  • Electrostatic Motor
  • Even smaller motors have been made using nanotechnology. An example is shown below. It consists of a tiny gold slab rotor, about 100 nm square, mounted on concentric carbon nanotubes. The outer tube carries the rotor, driven by electrostatic electrodes, rotating around an inner tube which acts as a supporting shaft. By applying voltage pulses of up to 5 Volts between the rotor plate and stators, the position, speed and direction of rotation of the rotor can be controlled. It measures about 500 nanometers across, 300 times smaller than the diameter of a human hair.


    Electrostatic Nano Motor Credit: Zettl Research Group
    LBNL, University of California, Berkley


    The motor was built from multiwalled nanotubes created in an electric arc and deposited on the flat silicon oxide surface of a silicon wafer. A rotor, nanotube anchors and opposing stators were then simultaneously patterned in gold around the selected nanotubes using electron beam lithography. A third stator was already buried under the silicon oxide surface. The silicon was then etched to create a trough beneath the rotor with sufficient clearance for the rotor to rotate.

    Possible applications are moveable mirrors for optical switches or paddles for moving fluids.




Electric Machine Fundamentals







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