In recent years permanent magnet DC motors (brushed and brushless types) have become more commonplace. Why are they used and when should we consider using one in place of a more traditional iron core motor?
Iron cored DC motors produce the magnetic flux by the electro magnetism effect of the coil on the ferrous laminated core. The core is not permanently magnetised and work has to be done by the current in the electro-magnet inducing conductor.
The 'energy expenditure' in magnetising the atomic dipoles of the iron core, thus aligning the lines of magnetic flux to create a magnet, is one of the primary things that separates the iron core type motors from the permanent magnet type. This spent energy magnetising and demagnetising the core is referred to as the magnetic hysteresis.
To begin with the process of magnetisation is fast, but as it reaches saturation it slows down. In a DC motor this is happening very quickly as the core is magnetised and de magnetised in the opposite direction as the motor rotates.
At a point the core will not be able to magnetise and demagnetise fast enough to utilise the full magnetic potential of the iron core and is forced to de magnetise before it can reach full magnetisation. This results in a loss of thrust on the core so the torque delivery is no longer linear and begins to tail off. With an iron core motor the higher the current the stronger the electromagnet becomes and temperature does not have such a profound effect on the magnet, only the resistivity heating effect as the conductor increases resistance with heat and the current drops.
Rare earth permanent magnets as the name would suggest are always magnetic and are expensive. This naturally occurring rare earth magnetic material is not magnetised and de magnetised by the winding as the iron core so there is little work to be done by the electricity and thus are more efficient than iron cored motors.
The atomic dipoles are disrupted by temperature. This disrupts the alignment of the atomic dipoles and their natural magnetism decreases. Different magnet types will react differently when exposed to heat. Common rare earth magnets include AlNiCo (Aluminium Al, Nickel Ni and Cobalt C), SmCo (Samarium Cobolt) and NdFeB (Neodymium Nd, Iron Fe and Boron B) and they all have different magnetic depreciation rates with increasing temperature.
Like the iron core magnet the conductors themselves produce a magnetic field which is not always contributing to the field of the permanent magnet and superimpose themselves on the existing magnetic field. This weakens the field of the permanent magnet and is avoided by increasing the air gap between the magnets and the coil. Increasing the air gap however means the flux density is reduced which in turn effects the amount of torque the motor can produce.
So it is a fine line between being too close and too far. The magnetic strength of the core is not increased with the current in the coil like the iron core motor, it is the inherent natural field of the magnet minus any counter effects of the electromagnetism of the current carrying conductor.
In short; for applications that need to be high efficiency or higher speeds (e.g. battery operated, aero / space applications, continuous operation applications or where power density is required) rare earth permanent magnet DC motors are the way forward. If your application is not pushed for space, requires a budget option and high speed/efficiency is not required then perhaps an iron core motor is for you.
Mark Gibbons is a technical engineer with maxon motor uk.