Rare Earth - Strategic Metals In Hybrid Vehicles

Neodymium costs about $20 to $30 per kilogram See:

What does it matter if you can re-use them?

Rare ear magnet motors are a solution that came out of the servo drive industry.

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.

China is dramatically ramping up hybrid auto production. It wont be available for reuse for years.

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.

Disk drives have two servos, both use rare earth PMs.

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 issues more.

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)

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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.