EMC problem for electric tool, mainly from the following two aspects:, restrain disturbance source: the biggest electromagnetic disturbance source is the motor of electric tools. And actually reduce the spark series motor, is to reduce the electromagnetic disturbance produced by series motor. And motor commutation spark is an extremely complex process, there are electric magnetic, mechanical, chemical, three aspects of reasons, in the design and manufacturing process can be eliminated by the following methods and mitigation. ()Change the commutation position, this is often used in the design process, in the case of brush is fixed to the rotor winding along the motor rotation direction commutator hooks. ()Choose the appropriate brush device: brush with graphite and electrochemical storage is appropriate; Brush the bakelite (with brass materialAt present most companies use brass)And brush inside the cavity dimensions with brush should cooperate with appropriate size, not too loose; Spring pressure selection in ~ Newton per square centimeter is appropriate, and in the brush. ()Ensure process requirements: rotor dynamic balance; The rotor radial beat; Commutator surface finish; Cooperate and hardness of shaft and bearing, the oil composition, oil content; Brush installed straightness and so on, should meet the high standards. , reduce the disturbance of the coupling between the source and the sensitive circuit: it is because of non sinusoidal motor current, lead to the harassment by the power cord to the gaiden, so it can be filtering, grounding method isolates harassment or lower. ()Capacitor filter: capacitance through to the noise source of the public to provide a low impedance path to the voltage spike bypass. Capacitor installation where or how to connect mainly depends on the types of the noise. To make the capacitor has good filtering effect, between it and the point of noise source of the public attachment to short. ()Inductance filter: another methods to minimize electromagnetic disturbance is placed directly on the brush a inductors, role is to prevent when brush by reversing chip clearance flow into a sudden change in the current brush. The inductance of the inductance is about ~ H. Series reactor in the circuit can be combined in and bypass capacitors to the ground to form a low pass filter, it can enhance the filtering effect of a single inductor or capacitor. It's good for restraining noise. The difference between a single capacitor and LC filter is very big. LC filter is more than a single capacitor filter bandwidth wide, therefore to brush the broadband noise has a bigger effect. ()Power supply filter: power supply filter can filter out of differential mode and common mode noise of inductance and capacitance, guarantee the power grid from product internal noise pollution. But, as with other filter, using a power supply filter the key point is to make sure a wire connected to noise source public short as far as possible. Power line filter is installed at the entrance of the power cord. In some products, filter installed in the middle of the product, this will make the product inside the radiated interference coupled to the power input, the complete failure of filter. ()Shield: screen can be divided into electric shielding and magnetic shielding, etc. , they are all use of metal material container, in which will need protection circuit package, in order to prevent electric field and magnetic field coupling interference, also shield should be grounded. According to different inhibition spectrum, different structure, different environment applications, different threshold, choose suitable shielding materials and to ensure that the shield combined with electrical continuity. Shielded cable is on the outside of the insulated wire again pack a layer of metal, form a shield. The shielding effectiveness of shielding layer is mainly composed of ground, harassment by short circuit to ground, is not coupled to the core. ()Grounding: a method is very important. Filter connector must be good grounding to the desired filtering effect. For bypass capacitors, if the grounding impedance is too big, not good bypass. For connector directly installed on the front panel of the filter, filter the contact between the connector and the conductive shell if it is not very good, but only in some point contact. Leads to the high frequency of contact resistance is bigger, and degrade the performance of the filter. If you want to satisfy the strict limit, can be installed between the filter connector and chassis panel special electromagnetic sealing gasket. Shielding and grounding are closely linked, in view of the different types of electric and magnetic fields, field size, and so on can adopt different ways of earthing.
