Priniciples of Electromechanical Relay Operation


3.20 Heating Considerations for AC relays

The principle covered in Paragraph 3.12 are generally applicable to ac actuated relays as well, however, additional sources of heat must be considered as a result of eddy current and hysterisis power losses in the magnetic iron parts as well as the losses in the shading ring due to the current induced in it.

It is also worth noting that ac actuated relay coils are generally less tolerant to coil cover voltages than are dc relays. In the case of dc coils, an increase in applied voltage causes a lower rate of increase in current due to the increase in coil resistance caused by coil temperature rise. In ac relays, however, an increase in applied voltage with a resultant increase in coil current also causes an increase in coil resistance, due to heating. More important however, the increased voltage will cause an increase in the magnetic saturation of the magnetic iron components. Increases saturation of the magnetic circuit results in a significant decrease in reactance and, therefore, decreases impedance. As a result, the coil current will increase at a more rapid rate than the applied voltage increase; consequently, overheating due to abnormally high applied voltages is more apt to occur on ac coils than on dc coils.

Another important characteristic of ac coils relates to the effect of shorted turns within the coil. In dc coils a few shorted turns will probably go unnoticed as the only effect is to decrease the coil resistance slightly and, if the applied voltage is constant, the increase in current will make up for the flux loss caused by the loss in turns. On the other hand, a few shorted turns in an ac operated coil can have disastrous results due to the current induced in the shorted turns and the rapid heating that follows. This induced current flows in a direction that opposes the magnetic flux in the main coil winding. This causes an increase in coil power out of proportion to the number of tuns shorted.