Common faults and troubleshooting methods of electromagnetic brakes
1. Fault mechanism
Inductive coils are the main components of electromagnetic brakes, and are also the source of most failures. The important characteristic of an inductive coil is that it generates a strong induced electromotive force at the moment of circuit switching on and off, especially at the moment of circuit switching off. This electromotive force is typically several to several hundred times the normal operating voltage. Such a high impulse voltage will greatly damage the electromagnetic brake itself and have a significant impact on subsequent equipment.
In addition to having a certain amount of inductance L, an inductive coil also has parameters such as conductor resistance R, core loss, and inter turn and inter layer capacitance of the coil. The equivalent circuit of an actual inductive coil is connected in series with R and L, and the losses on R represent all losses of the actual inductive coil; An equivalent capacitance C is connected in parallel at both ends of the inductor coil to represent the coil inter turn and inter layer capacitance and other distributed capacitance, thus forming the equivalent circuit of the actual inductor coil.
When a contact disconnects an inductive circuit, theoretically speaking, the current in the inductor suddenly interrupts, and a back electromotive force will be generated at both ends of the inductor. Due to the extremely high rate of current change at this time, a reverse voltage tending to infinity will be generated at both ends of the inductor (in fact, it is impossible to infinity). Assuming that the magnetic field energy stored in the inductor coil is W in the steady-state state, the magnetic field in the inductor will continue to maintain the conduction of current I when the contacts just separate, and then I will charge to C. When the breakdown voltage is exceeded, an arc will be generated, and the arc will keep the current in a conduction 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 inductive coil will continue to maintain current conduction, forming a RLC series oscillation circuit. If this voltage is less than the breakdown voltage of the contact gap, the capacitor C will continue to charge, and an increasingly high peak voltage will be established at both ends of the capacitor, that is, at both ends of the coil. Until it is higher than the breaking contact gap breakdown voltage, the contact gap will be broken again, and the originally charged capacitor C will be reversely charged to the DC bus through an arc.
As the contact gap continues to increase, the arc is broken again and the above charging and discharging process is repeated again. The discharge voltage gradually increases, and the voltage of capacitor C can reach tens of thousands of volts. Its pulse power is sufficient to damage semiconductor devices, and due to its rich harmonic components, it can interfere with the control system and cause misoperation.
The external environment is also an important factor in the failure of electromagnetic brakes. For inductive coils, the selection of insulating materials and the prevention of short circuits are key, and short circuits are usually the result of insulation damage. The insulation life test of the inductive coil shows that vibration has little impact on the life of the electromagnetic brake, and humidity is not the main factor (humidity will slowly change the resistivity between windings, thereby shortening the life of the electromagnetic brake), while thermal cycling is the main reason for reducing life expectancy.