Permanent Magnet Motors
From LeonardoWiki
Q and A interviews on PM motors
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 as 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 smaller units do not need a cooling fan. They have a smooth surface which makes them suitable for applications in the food processing industry.
A promising segment is the machinery industry.
The production of rare earth PM in China (a main supplier of Neodymium) 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 converter using PM and induction coils.)
What about the mechanical strength and construction of the permanent 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.
What materials are used for the permanent magnets?
For high temperatures SmCo is used. Nd2Fe14B is used in most cases. There are calculations that if everybody driving a car replaced it with a hybrid car there would not be 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.
| Magnet material | Price $/lb | Br - Max (Gauss) | Coercivity (oersted) | Typical P | Curie Temp (degrees C) | Beta (1/degrees C) |
|---|---|---|---|---|---|---|
| AlNiCo 5-7 | 40 | 12,500 | 650 | 50-75 | 850 | 0.0001 |
| Ferrite | 7 | 4,000 | 3,600 | 10-20 | 450 | 0.002 |
| NdFeB | 100 | 11,000 | 10,000 | 2-5 | 320 | 0.001 |
| SmCo | 180 | 8,000 | 7,500 | 4-8 | 800 | 0.00045 |
See this link http://www.choruscars.com/Chorus_NEO_WhitePaper.pdf for a description on the situation regarding Neodynium supply.
Another article (in Dutch): http://www.algemene-energieraad.nl/newsitem.asp?pageid=13047
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.
What about the supply of these materials, is there enough and which countries have these materials
See Q3
What about the durability of PM motors compared to asynchronous motors?
The lifetime of an asynchronous motor is higher compared to a 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.
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.
Who are the major actors for promotion of the permanent magnet motors?
The major use of PM motors started when Toyota made the hybrid car. Hitachi is nowadays a big actor for PM.
What are the benefits of using permanent magnet motors?
Higher energy density and better efficiency
A PM motor is a synchronous machine, so when fed with DC, i. e. when the frequency is 0, ints speed is actually 0, so the rotor is held in its present position and can no longer move. This advantage can be used for transports which have a slope.
Is the noise production of PM motors equal to asynchronous motors?
In so far as PM motors can do without a fan, they produce less noise.
If the temperature within the motor changes (overloading, short circuit), what will happen with the permanent magnet?
Demagnetization 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%.
Can any frequency converter control the permanent magnet motor?
No. They need a dedicated inverter, but inverters compatible with either type of motor may be avaiable in the foreseeable future.
What is the motor range of permanent magnet motors?
- Hybrid cars and SUV up to 150 kW
- Traction up to 1000 kW
- Wind turbines 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).
For what applications can permanent magnet motors be used?
See Q1 and Q13
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 that’s not sufficient by PM motors.
Within the automotive sector, PM motors are also used because they need less maintenance.
For what applications is it not economical to use permanent magnet motors?
Where conventional induction motors can be used, since they are cheaper.
There is a growing interest in renewable energy sources, can permanent magnet motors be used within this area (pros and cons)?
PM motors can be used for low speed windturbines in order to avoid the need for a gearbox (gearboxes need frequent maintenance).
For tidal power, PM will not be used.
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.
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 [1].
They can be operated with an inverter only, they do not start up alone. Therefore they will never provide a replacement for the very generic application fields of asynchronous induction motors.
How about other new motor technologies (reluctance motors, superconducting motors)
Within the next 10 years it can be expected that SRM could substitute PM motors within cars. They are more robust compared to PM motors.
Can PM technology be effective for producing micro-generators? If so, in what range and for which applications (cogeneration, wind, ...)?
See Q 17
What are your expectations of the market volume of permanent magnet motors within 5-10 years?
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Notes
PM motors can be used at higher speeds. Compared to lower speeds there will be more losses within the stator and rotor due to eddy currents. This means the conventional material used for other motors cannot be used. Next to new material, high tensile material and new design rules for optimization of the PM motor are needed.
http://dyneo.leroy-somer.com/en/
http://www.sew-eurodrive.de/movigear/englisch/index.html
New permanent magnet motors from ABB eliminate gearboxes http://www.abb-drives.com/StdDrives/RestrictedPages/Marketing/PressRelease/documents/Permanent%20Magnet%20Motors%20Eliminate%20Gearboxes.pdf
ABB Permanent Magnet Motors http://www.abb.com/product/seitp322/24da38e06740a773c1256de2004c5420.aspx
