Measured motor performance

Voltage: 24V to 36V

• Rated output: Approximately 10W to 30W

• Rotational speed: several thousand rpm

Measurement result

Outside the motor housing, the magnetic flux density was less than 2 gauss, both when the motor was stopped and when it was operating. Almost no difference was observed depending on the measurement direction (side, axial, or opposite direction).

*The distance (0/1/3mm) is the separation between the tip of the Gauss meter probe and the surface of the motor housing (0mm = contact).

Analysis and Summary

These motors are entirely encased in an iron casing, which itself acts as a magnetic shield. Since leakage flux is a factor that reduces efficiency in motor design, the structure is designed to minimize the leakage of magnetic fields to the outside.

Therefore, the magnetic field around the low-voltage motor is very small, and it is thought that placing an MRAM nearby will not have any effect.

Measurement result

The magnetic flux density outside the motor housing was less than 2 gauss, and no significant magnetic field was observed even with larger sizes and higher rated voltages.

Analysis and Summary

Even in high-voltage motors, the magnetic flux is contained within the casing due to the high magnetic shielding effect of the iron casing.

Therefore, the magnetic field around the high-voltage motor is very small, and it is thought that placing an MRAM nearby will not have any effect.

Measurement result

• Lateral direction: Maximum approximately 50 Gauss

- Axial direction and opposite direction: Approximately 20 Gauss

While the magnet alone exhibited a magnetic field of several hundred gauss or more, its sides were covered in plastic, and even the surface only registered around 50 gauss.

Analysis and Summary

Drones often lack magnetically shielded enclosures to reduce weight, resulting in leakage magnetic flux into the surrounding environment. However, even in such cases, the leakage is only around tens of gauss.

Furthermore, because the motors and control board (equipped with an MCU) are physically separated in the drone, the influence of magnetic fields is reduced due to magnetic field attenuation caused by the distance.

Structurally, leakage flux exists, but its impact on MRAM in actual applications is expected to be limited.

Measurement result

- At low current: Magnetic flux density is below the detection limit.

- During high current: Tens of gauss near the casing

consideration

The magnetic field levels observed in this instance did not affect the operation or data retention of the MRAM. However, it should be noted that materials with low magnetic permeability, such as aluminum, will exhibit increased leakage flux compared to conventional iron-cased motors.

Measurement result

• Leakage flux from motor housing: Less than 2 gauss

• Magnets for magnetic sensors:

• 0mm: Approximately 14,000 gauss
• 2mm: Several hundred gauss
• 5mm: Approximately 200 Gauss

Analysis and Summary

While the motor itself is magnetically shielded, the magnets used for magnetic sensors become a very strong magnetic field source. If the MCU is placed within a few millimeters of the magnets, it may exceed the magnetic field tolerance specifications of the built-in MRAM.

Therefore, careful design is required when combining a motor with a magnetic sensor with an MCU equipped with MRAM.

Measurement result

A magnetic field leakage of up to approximately 10 gauss was observed near the gap, but it was less than a few gauss in other regions.

Analysis and Summary

Although there is some localized magnetic field leakage, it is not thought to be at a level that would affect the MRAM.

Target inductor

• ① Sagami Electric Co., Ltd. Power Inductor CVK2522H-5R4M
  -Magnetic permeability (μ) = 40
  -External dimensions: 20 × 25 × 22.5 mm
  -Inductance value L = 5.4 uH
• ② Sagami Electric ferrite core inductor (same size and L value as CVK2522H-5R4M)
  -Magnetic permeability (μ) = 3000
  -External dimensions: 9 × 16.5 × 22.3 mm​
  -Inductance value L = 5.4 uH

Simulation conditions

・Current conditions

• DC 100A (The current value at which the inductance changes by 30 % from the initial inductance value in ①)
  • Substrate conditions
• Board size: 75 × 75 × 1 mm
  
  
  

  ➀CVK2522H-5R4M

② Ferrite core inductor

Figure 4: Simulation conditions (Image provided by Sagami Elecs Co., Ltd.)

simulation result

①CVK2522H-5R4M

・X–Y plane:

A separation distance of approximately 3.0 mm is sufficient to reduce the magnetic flux density to 200 gauss.

・YZ plane:

A separation distance of approximately 1.5 mm is sufficient to reduce the magnetic flux density to 200 gauss.

② Ferrite core inductor

・X–Y plane:

The magnetic flux density was well below 200 gauss.

・YZ plane:

A separation distance of approximately 0.5 mm is sufficient to reduce the magnetic flux density to 200 gauss.

Figure 5: Magnetic flux density simulation results (Image provided by Sagami Elecs Co., Ltd.)

Summary of evaluation results

- Confirmed that even under conditions where a large current flows, a large portion of the magnetic flux is confined inside the inductor core.

The state of leakage flux differs depending on the permeability μ of the material; materials with higher μ tend to confine the magnetic flux internally.

The magnetic flux leaking around the inductor is limited and does not affect the operation of the MRAM.

This data provides an example of the design guideline that, in a typical power supply circuit, the effects of magnetic fields can be sufficiently suppressed by placing the inductor and the MRAM-integrated MCU at an appropriate distance from each other.

Although the magnetic field is concentrated near the inductor, it attenuates rapidly with distance, and its impact on surrounding circuits is limited.