EMI Solution of Baolong Motor
Posted in 2019-07-20 17:12:21 in Tech Blog, 2 Comments

Background analysis:

When the motor is running, the radio frequency of different types of mobile phones will be interfered with：

a. The interference of coreless motor has 3~5dB in the frequency band around 900HZ .

b.The coreless motor has different interference to different frequency band：800MHz:-12dB,900MHz:-12dB,1800MHz:-19dB,2000MHz:-22dB.

c.The coin motor has 3 ~ 20dB interference in all frequency bands.

d.The core motor has no interference at present.

Basic theory of electromagnetic compatibility Electromagnetic compatibility

In the same electromagnetic environment, the equipment can work properly under the interference of other equipment, and at the same time, it does not interfere with other equipment.

Electro Magnetic Compatibility (EMC)。

Three Elements of EMC: Interference Source, Sensitive Equipment and Coupling Path.

Coupling Path: Space Coupling, Wire Coupling (Power Line or Signal Line)

Reasons for DC motor interference:

a.Dc motor in the moment of commutation, due to the sudden change of current arc caused by the loop or radiation interference;

b.Dc motor in the moment of commutation, the direction of coil current changes, and the coil inductance causes loop interference;

c.Interference caused by coil inductance and radiated outward through external wires or FPCB;

d.Magnetic field interference caused by leakage of motor magnet steel and alternating magnetic field of coil;

e.The high frequency on and off state of the PWM power supply circuit of the motor produces a wide frequency range and more intense interference.

The causes of serious interference of coin motor are as follows:

As shown in the figure, the coin motor has only two coils, the working resistance can be divided into single-phase and two-phase, cycle switching, and the current changes greatly. There is elasticity at the contact point between the brush and the HPCB commutator surface. When the motor rotates, the contact elasticity and the contact surface state will change, which will lead to the change of the contact resistance, which is easy to produce high frequency harmonics due to the change of the contact resistance. The harmonic interference is coupled to the mobile phone signal by conduction or radiation.

Solutions:

a. Circuit or radiation interference caused by sudden change of current in DC motor at the moment of commutation ☆Increase capacitance, bead or inductance at the end of the power supply circuit

☆ Increase varistors, capacitors, beads or inductors inside and outside the motor

b. High frequency harmonic interference, radiation interference outward via external conductors or FPCB

☆Wire strands with the shortest length; ☆ FPCB coated with silver paste ☆Puting shield or RF antenna is far away.

c.Magnetic field interference caused by leakage of motor magnet steel and alternating magnetic field of coil.

☆ Puting shield or RF antenna is far away.

d.The high frequency on-off state of motor PWM power supply circuit produces wider frequency range and stronger interference.

☆The filter of capacitance, magnetic bead or inductance is added in the power supply circuit.

Baolong Successful Solution to Disturbance (Increase Cost):

☆ Add capacitance, beads or inductance to the motor.

☆ FPCB coated with silver paste

Baolong Successfully Reduces Disturbance Solutions (No Cost Increase)

☆The arc of motor commutation is reduced by reducing carbon film resistance.

☆Adjust the elasticity of motor brush to reduce the fluctuation of contact resistance at the moment of commutation.