Permanent magnet motors

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Permanent magnet motors

• Electricity Energy team session
• August 7, 2009

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What is the application field of the permanent
magnet motor?

• The main benefit of the PM motor is a 3 times
  higher torque to weight ratio compared to
  induction motors. Therefore any motor application
  requiring intermittent operation could be well
  served by a PM motor. However, PM motors are
  currently 3 times as expensive then induction
  motors, and there are some issues with (rare
  earth) material supply.
• PM motors are used within traction and for hybrid
  cars or electrical vehicles. They are also used
  for servo motors in industry.
• PM motors are compact, efficient and do not need
  a cooling fan. They have a smooth surface which
  makes them suitable for applications in the
  foodprocessing industry.
• A promising segment is the machinery industry.
• The production of rare earth PM in China (a main
  supplier of Neodynium) was a mere 10 MW in 2007
  (on a market of 208,980 MW). Still it is
  recognized as a market with large (export)
  potential. Applications are for elevators,
  textile machines and oil pumping fields.
• (Ocean Power Technologies has United States
  Patent 7,323,790 for a Wave energy converters
  using PM and induction coils.)

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What about the mechanical strength of the magnets?

• Pressure is not a problem for PM, however tensile
  forces are not allowed. This means that within
  the design, magnetic forces should be calculated
  in such a manner that tensile forces will not
  happen.
• Corrosion can be a problem for PM. Care should be
  taken when PM are used e.g. within offshore wind
  turbines.

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What materials are used for the permanent magnets?

• For high temperatures SmCo is used. Nd2Fe14B is
  used in most cases. There are calculation that if
  everybody driving a car would replace it with a
  hybrid car there is not enough Nd by 2020. There
  are only mines open in China and Russia at other
  locations Nd is hard to get.
• Other alternatives to SmCo and NdFeB are Fe3O4 or
  AlNiCo. Cobalt is one of the most costly
  components.

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Is there any alternative for the material?

• Alternative materials have lower energy
  intensity. For some applications, ceramic magnets
  are being used / considering, but their
  performance is significantly lower than rare
  earth PM.
• See http//www.stanfordmagnets.com/magnet.html
  for an overview of magnet types and their
  performance.

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What about the durability of PM motors compared
to asynchronous motors?

• The lifetime of a asynchronous motor is higher
  compared to PM motor. The first one is more
  robust. However, since PM motors can provide high
  torque over a wide speed range, they can replace
  an induction motor gearbox, reducing
  maintenance cost and downtime.
• A PM motor can not be used at high temperatures.
  Temperatures well below the Curie temperature can
  irreversibly damage the PM. The Curie temperature
  for ndFeB PM is 320 degrees. For SmCo, it is 800.
  AlNico 850 and Ferrite PM 450 degrees.
• Also current in the motors conductors can damage.
  Therefore, a PM always has a thermal current
  rating as well as an absolute peak current
  rating.

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What about the repairability of PM motors
compared to asynchronous motors?

• Asynchrounous motors are easier to repair after a
  short-circuit failure. Within PM motors it is not
  easy due to the glue that is used for the magnets
  in the rotor. Moreover, the magnetic rotor
  complicates repairs.

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What are the benefits of using permanent magnet
motors?

• Higher energy density and better efficiency
• Once the 3 phases are short-circuited, the rotor
  can no longer move and is blocked. This advantage
  can be used for transports which have a slope.
• PM motors have a constant torque at every speed
  and can be nominally loaded at each speed while
  asynchronous motors can not be operated at
  nominal load at low speed due to insufficient
  cooling (at low speed, the fans will not produce
  enough cooling).

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Is the noise production of PM motors equal to
asynchronous motors?

• Since PM motors do not have a fan, they produce
  less noise.

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If the temperature within the motor changes
(overloading, short circuit), what will happen
with the permanent magnet?

• Demagnitization can occur when the magnet is
  exposed to temperature or mechanical shock. It
  appears that this problem is being resolved
  nowadays (through advanced controllers?).
• In any case, overloading of the motor should be
  limited. ABB recommends to limit short-time
  overloadability to 120-150.

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What is the motor range of permanent magnet
motors?

• Hybrid cars up to 15 kW
• Larger SUV up to 150 kW
• Traction up to 1000 kW
• Windturbines at least up to 500 kW probably 1000
  kW
• Small motors in heat pumps
• Servo motors up to 12 kW
• About 10 of the market for induction motors up
  to 2 kW can be replaced with PM motors within 5
  up to 10 years. For larger motors in stationary
  applications, the replacement rate will be lower
  because of the cost barrier (more magnets needed
  in larger motors).

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Will permanent magnet motors replace induction
motors used within the industry? If yes what
industries or processes are in favour for
replacements?

• PM motors will be used at locations where space
  is limited. However the first step will probably
  be that asynchrounous motors will be replaced
  with synchronous motors and when thats not
  sufficient by PM motors.
• Within the automotive sector, PM motors are also
  used because they need less maintenance.

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For what applications is it not economical to use
permanent magnet motors?

• Where conventional induction motors can be used,
  since they are cheaper.

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Why are PM motors currently popular in hybrid or
electrical cars (pros and cons)?

• Space is a limitation within these cars. Also PM
  motors have higher efficiency and a higher
  torque/weight ratio.
• To minimize cooling needs in vehicles, permanent
  magnets must be able to operate at 180 - 200
  degrees.

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What is the major threat for permanent magnet
motors?

• Price and shortage of material.
• On cost, reduction efforts are ongoing to improve
  alloy design and processing, particularly by
  powder processing

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How about other new motor technologies
(reluctance motors, superconducting motors)

• Within the next 10 year it can be expected that
  SRM could substitute PM motors within cars. They
  are more robust compared to PM motors.

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Conclusions

• A lot of uncertainty
• PM is a value-added business opportunity to motor
  manufacturers, not unlike inverter drives
• PM tends to apply to new application fields for
  motors
• There is (almost) no doubt that the squirrel cage
  induction motor will loose some market share in
  the near future, but the induction motor remains
  the default choice
• PM motors mainly compete with CMR motors?
• Should / can we stop progress?

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Copper content

• Superconducting motor
• PM motor
• SR motor
• Induction motor
• Induction motor with copper rotor

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Actions

• Speed up capability for market investigation
• Promote PM motors in new applications
• PM versus CMR motor investigate through OEM
• Copper content of various motor types MIDM