interference in motors 1 Causes of electromagnetic interference

 (1) Interference or aging due to sudden changes in the magnetic field in the windings If the current path through the motor coil windings is cut off, the magnetic field in the coils suddenly disappears, and hundreds of volts will be generated on the coils. , even transient overvoltage of thousands of volts. This voltage produces a huge electrical energy impact on other electronic devices in the system, eventually leading to basic loss of control of equipment and systems, errors in logical judgment, and even breakdown or burning of other electromechanical components in the system. Transient overvoltage is related to the size of the load and the impedance of the line. 

(2) Spark discharge between commutator and brush. For brush motors, spark discharge occurs between the brush and the commutator, which also causes noise with an extremely wide spectrum (continuously distributed in the medium wave to very high frequency band), which is harmful to wireless radio, television and various electronic devices that cause interference over a wide range.

 (3) Others. The motors in many electronic products use DC power rectified by bridge rectifier and capacitor filter circuits. Because the conduction angle of the rectifier diode is very small, high peak input current can only flow near the peak value of the input AC voltage. This kind of distorted current waveform is basically very low, but the high-order harmonics are very rich. The pulse width is about 5ms (1∕4T). This high-peak current pulse not only causes serious pollution to the power supply grid, but also causes various other problems. Electrical equipment causes interference. 

2. Generation of electromagnetic interference and suppression measures during circuit design. The electromagnetic compatibility in electromagnetic circuits is largely determined by the components of circuit storage and interconnection: the signal returned from the antenna can radiate electromagnetic energy. It is mainly determined by the current amplitude, frequency and geometric area of the current coil. Generally, there are 3 main sources of electromagnetic interference: power supply, high frequency signals, and oscillator circuits. The causes and preventive measures are analyzed below. First, when a CMOS flyback converter changes its output state, both thyristors will be turned on at the same time for a short period of time. This can cause the current to increase very quickly, causing electrical currents to appear on the power lines.

A current spike that causes a short circuit in a long or short power line. This was confirmed to produce electromagnetic interference an important reason.

It is very effective to attenuate the supply voltage fluctuations to close to a 100nF bypass capacitor.

However, due to the parasitic components of the circuit, such as the impedance of the integrated and power supply lines, the bypass capacitor cannot Effectively reducing current peaks, it cannot reduce radiated interference. To suppress these current spikes

(at least on the power supply line) so that it does not extend to other parts, coupling capacitors and power supplies between poles An induction coil is added between the lines to facilitate interference suppression.

Secondly, in digital systems, the highest continuous frequency is usually generated by the clock generator.

The C24xDSP uses 1 crystal in its internal oscillator, which helps reduce high frequency current while

The area enclosed by the current path reduces electromagnetic interference. Since the crystal has a resonant frequency of several hundred k Due to the high impedance, the current at the frequency that causes the crystal to resonate is very small. However, the CMOS reverse converter

The output voltage is a square wave signal containing harmonics, so the crystal no longer represents a high impedance,

This can cause a lot of current. The solution is to add a series resistor to reduce these current components

few.

Electromagnetic interference suppression Although the phenomenon of electromagnetic interference is widespread, electromagnetic interference cannot be completely eliminated. Measures can only be taken to minimize the electromagnetic interference caused by the motor to the equipment and system work. The following measures can be adopted.

(1) Damping. Damping can effectively reduce and reduce the interference of transient overvoltage to the motor by surge voltage in the system loop. Generally, damping wires can be used, such as conductive rubber wires, carbon fiber impregnated wires, variable pitch resistor wires, magnetic body windings, double resistance wire windings, and shielded wires. Use the above types of wires

As the power lead of the motor. The damping wire can also reduce and suppress the spark discharge interference between the brush and commutator

(2) Filtering. There are two types of noise generated by motor brushes: common mode and differential mode. The solutions include capacitor, inductor and grounding. For common mode noise reduction, a capacitor can be connected between each lead of the motor and the ground. For differential mode noise, most of the differential mode noise is generated by the contacts of the brushes and the commutator.

(3) Grounding. The way in which ground noise suppression operates is important. If the ground impedance is too large, it will not function as a good noise bypass. If the motor housing is used as a ground terminal, the paint on the housing must

Remove it so that the wire can make good contact with the ground. It cannot be connected solely by the 4 to 5 threads of the connecting screw.

(4) Shielding. Shielding is very effective in suppressing radiated noise. The metal casing of the motor acts as a shield. Its shielding effect depends on the properties of the material, radiation frequency, and the presence of

The shape and quantity of various discontinuities are related, such as copper and lead; electromagnetic field waves have great reflection loss, so they are suitable as one of the brush gas commutators of electric motors: -4y people spend thousands of interference shields. Iron and special

Magnetic materials such as iron-silver-diamond alloy with high magnetic permeability have large attraction losses due to the removal of ~m field waves, so they can be used as shielding casings for the motor assembly (or permanent magnet pole).

Due to the presence of thick wire entry and exit holes, motor shaft holes, and electromagnetic shielding of the motor housing, there are discontinuities. Conductive rubber or metal pads can be used on the window holes. The power outlet wires should pass through the through holes on the shielding cover.

Capacitors are connected to the motor to eliminate electromagnetic interference through the south hole.

(5) Other measures. Interference suppression can also start from the motor itself, such as design and processing technology. Because some parts have poor contact and the brush is not clean

Causes interference several times greater than during normal operation

Condition. In order to suppress interference, it is necessary to keep the contact part white, the fork r reliable, the opening and closing movements normal, and the pressure of the contacts kept uniform; it is necessary to keep the brush and commutator clean, to ensure the quality of the brush itself, that is, to reduce surface roughness degree; the commutator is made of hard pure copper, molybdenum copper or silver-silver copper, and the matching brushes are made of pure graphite, copper graphite or silver graphite; the surface of the commutator and the rotor must be coaxially ground, and the brushes It should be processed in advance; during assembly, in order to make the center line of the rotor axis of the gas motor intersect vertically in the center of the brush holder, adjust the pressure of the brush spring to be uniform and symmetrical; during design, a transient suppression device such as a silicon diode should be connected to the circuit to prevent the inductance effect. The voltage surge peak; the armature installation axis is symmetrical in the center to avoid uneven air gaps and asymmetric windings; the base must be fixed reliably to avoid unstable operation of the motor during mechanical operation.