Electromagnetic brake is an electric device that relies on the electromagnetic attraction generated by the electromagnetic system to make the armature work externally. Due to the characteristics of convenient loading and unloading, good response performance, high reliability, and green environmental protection, electromagnetic brakes are widely used in construction machinery.
1. Fault mechanism
Inductive coils are the main component of electromagnetic brakes and the root cause of most faults. The important feature of an inductive coil is that it generates a strong induced electromotive force at the moment of circuit on and off, especially at the moment of circuit off. This electromotive force is usually several to several hundred times the normal working voltage. Such high impulse voltage causes great damage to the electromagnetic brake itself and also has a significant impact on subsequent equipment.
An inductive coil, in addition to having a certain amount of inductance L, also has parameters such as wire resistance R, core loss, and capacitance between turns and layers of the coil. The equivalent circuit of the actual inductance coil is connected in series with R and L, and all losses of the actual inductance coil are represented by the losses on R; Using an equivalent capacitor C in parallel at both ends of the inductance coil, it represents the inter turn and inter layer capacitance of the coil, as well as other distributed capacitors, to form the equivalent circuit of the actual inductance coil.
When the contact disconnects the inductance circuit, theoretically, the current in the inductance is suddenly interrupted, and Counter-electromotive force will be generated at both ends of the inductance. Because the current change rate is very large at this time, the opposite voltage will be generated at both ends of the inductance, which tends to be infinite (in fact, it cannot be infinite). Assuming that the magnetic field energy stored in the inductor coil is W in steady-state, the magnetic field in the inductor needs to continue to maintain the continuity of current I when the contacts just separate. At this point, I charges C, and an arc is generated when the breakdown voltage is exceeded, which keeps the current in a conductive state. When the arc is pulled apart to a certain distance and extinguished, the contact opens. At this point, the self induced potential generated by the inductance coil will continue to maintain current conduction, forming an RLC series oscillation circuit. If this voltage is less than the breakdown voltage of the contact gap, capacitor C will continue to charge, and an increasing peak voltage will be established at both ends of the capacitor, i.e. at both ends of the coil. Until it is higher than the breakdown voltage of the breaking contact gap, the contact gap will be broken again, and the originally charged capacitor C will be charged in reverse to the DC bus through an arc.
