Rare ear magnet motors are a solution that came out of the servo drive
There are other options to these motors that don't required rare
earths; The induction motor is one and most interestingly the
switched reluctance motor. The latter is complicated by the need to
fit a position encoder to the motor to inform the controller of rotor
position which generates a few fairly minor cost and reliabillity
issues and the need for carefull control algorthim design.
Personally I feel toyotas Hybrid technology is a bit of a gimmik on
the global scaled simply due to the rare earth metals required for the
batteries and motors and the rediculous costs. They need to go back to
the drawing board.
Stepper motors use permanent magnets. Servos use
coils operating against other coils.
The first large-scale use of rare earth permanent
magents was in Sony's Walkman, which used samarium-cobalt
magnets for both the motor of the tape drive and the
earphones in the headset. The product was a success
because of the dramatic reduction in size and weight.
Stepper motors can use either permanent magnets or simply the
reluctance of a metal rotor. I would say the reluctance types is more
common but I suppose it depends on what use. They are also sometimes
'hybridised' to use both effects. Steppers usually have an unmatched
number of rotor and stator poles to get self synchronising alignment.
I won=92t explain it as there are plenty on the internet.
The term 'servo' is a generic term for a motor and controller designed
to not only control speed precisely but usually also acceleration,
torque, position and fairly rapidly at that. Used for such tasks as
an articulated robot, positioning the cutting head of a machine tool,
cutting table, stacking machines or pointing guns.
Servos can use any technology: bigger ones (above 20KW) are
specially designed low inertial squirel cage induction motors. Below
that size they are usually permanent magnet motors with the magnets
(Sumerian cobalt or Neodymium based magnets) in the rotor. These
permanent magnet versions are usually incorrectly referred to as
'brusheless DC' though they are not really DC, this merely emphasises
that the motors were as controllable as DC servos when only DC motors
could do a good job . The problem with the bigger permanent magnet
motors, apart from expense, is that its hard to insert the rotor
without it 'sticking' to the sides of the stator(ie motor housing).
I suppose a stepper motor could be referred to as a 'servo' as well as
they are very good at positioning due to the precise incremental
control of the steps but they are usually used to position something
such as a print head, plotting pen etc.
The switched reluctance motor works on the same principal as the
stepper motor except that the number of rotor poles and stator poles
are matched and that the stator uses complimentary pole pairs, this
usually means you need a rotor position sensor to get a reasonable
result though the rotor will obviously align with the magnetic
field. The Synchronous Reluctance Motor uses a variable frequency
sinusoidal waveform created by PWM (pulse width modulation) and is
smoother than the switched reluctance.
An electronic controller (eg Siemens Master drive) can control all
three types of motors: squirrel cage inductance, permanent magnet
(servo) and synchronous reluctance. Only the control code of the
'inverter' is varied. Traditional Synchronous motors use a rotor that
is excited by a DC current, you can obviously get rid of that
electromagnet based rotor and replace it with permanent magnets
thereby saving yourself the trouble of slip rings and internal
cooling. Reluctance motors rely on the permeably of the specially
shaped rotor poles. The are very efficient.
Big inductance motors (above 130kW) are easily 99% efficient but their
efficiency drops slightly as they are operated on partial load due or
as the motor is scaled to down.
One can put either a squirrel cage inductance rotor, reluctance rotor
or permanent magnet rotor in the same housing. The inductance motors
is the only one which can be started direct on line and that fact and
the fact it is extremely cheap is why it is the most common motor in
the world, the others need a controller. The reluctance motor is
simply not widely used because it needed a controller. The permanent
magnet motor is similar to the rotor field excited synchronous motor
which is well understood.
Sumarium and Cobolt metals are rare. The neodynium based materials
are cheaper, stronger magnetically however can't handle as high a
temperature and a little more brittle so you need to look at cooling
There is 6.5 billion people on the planet, 4.5 billion want the life
style of the 1.5-2 billion that have more. As the planet heads to 10
billion I'm dubious about any technology relying on rare materials
being able to meet global needs. Maybe just for portable Compact
Cassette or CD players but probably not for 1 let alone 4 billion
cars. (Unless we start mining the moon or asteroids)
The seek head can use lots of kinds of technolgy as can the spindle.
Air core voice coils were the main technology untill recently, dunno
if they still are. Permanent magnet motors are very good but not
irrelplaceable with devices that are potentially much cheaper and
almost as good.
A switched reluctance motor might be turned into an integral motor/
starter/generator that 'is' also the flywheel in an internal
combustion engine. In that way the slight weight disadvantage due to
a heavier rotor becomes an advantage. These motors are however
compact and light and due to their efficiency can be improved.