How to measure the magnetic properties of magnet?

Almost all magnet manufacturers use B-H curve testing equipment to get the demagnetization curve of magnet. This method is a closed circuit measurement, and an uniform cross-section sample is clamped between the faces of electromagnet. The testing sample is usually cut from the magnet.

The magnetic properties of permanent magnet

Residual Induction Br:

The residual induction Br is the remaining magnetization in a magnetic material at field strength H=0 kA/m, after it has been magnetized to saturation in a closed path.

Coercivity Hcb:

The coercivity Hcb is the magnetic field strength required to bring the magnetic flux density back to 0 in a ferromagnetic material which has been magnetized to saturation.

Intrinsic Coercivity Hcj:

The product of H and B at the point (H,B) on the B-H demagnetization curve is called the energy product and the maximum value of this products is the maximum energy product.

The influence of coercivity on permanent magnet (PM) motor

According to magnetic flux continuity principle, also based on magnetic circuit design formula and full current formula:

𝑓𝐵𝑔𝐴𝑔=𝐵m𝐴𝑚 And 𝑟𝐻𝑔𝐿𝑔=𝐻𝑚𝐿𝑚

Where f is leakage flux coefficient; Bg is air gap magnetic flux density; Ag is air gap area;

Bm is magnet’s working air gap magnetic flux density; Am is magnet’s area; r is magnetic reluctivity; Hg is air gap magnetic field strength; Lg is air gap length; Hm is magnet’s working magnetic field strength; Lm is magnet’s length.

Then magnet’s operating points P:

𝐵𝑚/𝐻𝑚=𝑓𝐵𝑔𝐴𝑔𝐿𝑚/𝑟𝐻𝑔𝐴𝑚𝐿𝑔

For the same permanent magnet (PM) motor, the operating point is also same. If we choose two kinds of magnet with same Br and different Hcb, the magnetic flux and magnetic potential of magnet with higher Hcb will better than lower one, also on its performance in vibration and electromagnetic interference. The higher Hcb will provide enough electromotive force to ensure outputs of permanent motor, and make operating point closer to magnet’s Max. Energy Products. The higher Hcj can be seen on the abilities of its anti-demagnetization and anti-aging.

The influence of residual induction on permanent magnet (PM) motor

The magnet with high Br will ensure the motors have high speed, high output torque and high power. If we choose two kinds of magnet with same coercivity and different Br, the magnetic flux density and magnetic field strength of magnet with higher Br will stronger than lower one.

The influence of Max. Energy Products on permanent magnet (PM) motor

The higher Max. Energy Products, the better service factor during motor’s actual operation process.

How to choose the right magnet for permanent magnet (PM) motor?

Selection principle:

1. The air gap magnetic flux density should strong enough.
2. The magnetics properties should stable enough under the specified condition, working temperature and operating situations.
3. The magnets should have good mechanical properties to ensure easy machining and assembly.
4. Cost-effective.

Other description:

1. As the magnetic properties perfected gradually, and the price gradually decline, NdFeB magnets are more and more widely used in motor applications.
2. Sm₂Co₁₇magnet often applied to application with high performance and high reliability. SmCo₅ usually used in application with high temperature and high demagnetization field.
3. If the user don’t care about magnet’s volume, weight and magnetic properties, we suggest them choose the low-cost ferrite magnet.
4. If the magnets require high temperature stability, and its working temperature above 300 degrees, we should choose the AlNiCo magnets. These low temperature coefficient magnets often used in the motor for measuring apparatus.
5. For complex shape, multi-poles and large quantities magnet, we’d better choose the bonded magnet.

SDM Magnetics Co., Ltd. relevant experience in motor magnets