17-05-2012, 10:38 AM
Motor Construction
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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.
Image source
Long, Schofield, Howe, Piron & McClelland
"Design of a Switched Reluctance Machine for Extended Speed Operation"
IMEDC June 2003
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.
History
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.
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.
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.
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.