Overview
|
Demagnetization may occur in an SPM motor during rotation because of the
rising temperature caused by the eddy currents, or the reverse magnetic
field produced by the coil in the permanent magnet.The need to evaluate
the demagnetization using a magnet field analysis is becoming more important
because demagnetization reduces the performance of an SPM motor.The mechanical
characteristics need to be evaluated accounting for demagnetization caused
by rising temperatures while the motor is driven, because the demagnetization
is irreversible even after the temperature decreases once the operating
point of the magnet exceeds the knee point.
This example presents the use of a magnetic field analysis to evaluate
the torque waveform and demagnetization of an SPM motor while changing
the temperature of the permanent magnet. |
Torque Waveform
Closeup |
The torque waveform when the temperature of the magnet is varied from 60
degrees Celsius to 140 degrees Celsius and then back to 60 degrees Celsius
for one electrical period (180 degrees of mechanical angle) is indicated
in Fig. 1.
The average torque is reduced by the thermal demagnetization when the magnet
is at a temperature of 140 degrees Celsius, as indicated in Fig. 1. The
irreversible demagnetization that occurred at 140 degrees Celsius is apparent
when the temperature of the SPM motor is returned to 60 degrees Celsius
with an average torque that has decreased 16%. |
Magnetic Flux Density Distribution
The magnetic flux density at each temperature is indicated in Fig. 2.
The magnetic flux density is weaker all around compared to the magnetic
flux density before the magnet is heated to 140 degrees Celsius as indicated
by Fig. 2. Furthermore, the magnetic flux does not return to its original
intensity after the temperature decreases to 60 degrees Celsius. |
Closeup |
Demagnetizing Ratio Distribution
Closeup |
The demagnetization ratio* distribution after the temperature is returned
to 60 degrees Celsius using the magnetization before heating as the base
is indicated in Fig. 3. The demagnetization indicated in Fig. 3 is the
irreversible demagnetization after exceeding the knee point and shows the
degradation of magnet performance.
There is a wide range of demagnetization when compared to the magnetization
before heating, even after the temperature returns to 60 degrees Celsius,
as indicated in Fig. 3.
*The demagnetizing ratio is the amount of emagnetizationwith reference
to magnetization that is specified. |
Operating Point
| The point selected to display the operating point is indicated in Fig.
4, and the operating point for each temperature is indicated in Fig. 5.
The irreversible demagnetization is apparent because the operating point
has exceeded the knee point for Point 1 after the temperature rises. The
operating point does not return to the original B-H curve after the temperature
is reduced to 60 degrees Celsius. The size of the reverse magnetic field
imposed upon each point is also illustrated on the horizontal axis. |
Closeup |
Closeup |
|
