246 – Analysis of Electromagnetic Force Acting Between a Coil and a Magnet

Application Note / Model Data


The widely used three-phase synchronous motor can be driven efficiently and stably but costs tends to increase due to using an inverter. Whereas since single-phase synchronous motors do not require inverters, they can reduce costs. Furthermore, because they are brushless, maintenance costs can also be lowered.
In a single-phase synchronous motor, the back emf and the current flowing through the coils change in accordance with the rotation speed of the rotor. At low rotation speed, since the back emf is small, a large current flows through the coils, and magnetic saturation occurs in the magnetic circuit.
On the other hand, the back emf increases at high speed, so the current flowing through the coils and torque decrease. Because of this when designing a motor it is important to understand characteristics for a wide range of operating points. Obtaining N-T-I characteristics from performing analysis in JMAG at the design study stage contributes greatly to the speed of motor development.
In this example, a use case in which the N-T-I characteristics for a single-phase synchronous motor are obtained is presented.

Attractive Force Acting Between Coil and Magnet

The current flows in a direction perpendicular to the attractive force acting between the coil and the magnet. During such a time, the forces acting between the coil and the magnet are balanced. Fig. 1 shows the attractive and repulsive forces acting between the coil and the magnet.

Forces Acting on the Magnet

Upon confirming the electromagnetic force distribution generated on the magnet shown in Fig. 1, both magnetic forces, one acting on the lower face and the other acting on the upper face of the magnet, differs by 4 order of magnitude from the net force on the entire magnet. The forces acting on the two magnet faces are obtained from the nodal forces.
Since the net force on the entire magnet is obtained from the cancellation of the electromagnetic forces distributed in this way, the electromagnetic force must be computed with very high accuracy. The forces acting on the magnet are shown in Fig. 2.

Magnetic Field Lines Extending in Air

Since magnetic flux lines giving rise to electromagnetic force extend far in the air, accurate computation is required over a wide region. Fig. 3 shows the lines of magnetic force extending in the air.

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