Reactor Current Interruption

Sudden interruption of current through a reactor may cause high voltages to develop in the winding. This may be seen by considering the voltage across a reactor with linear inductance L and varying current i in the winding. Let current i be substituted for I_M in equation 37; it may be transposed to give

[37a]

where L is in henrys and i in amperes. If this expression for φ be substituted in equation 1, we obtain

[40]

Equation 40 states that the magnitude of voltage across a reactor is equal to the inductance multiplied by the rate of current change with time. The sense or direction of this voltage is always such as to oppose the current change. Therefore, if current interruption takes place instantaneously, inductive voltage is infinitely large. In an actual reactor, losses and capacitance are always present; hence interruption of reactor current forces the reactor voltage to discharge into its own capacitance and loss resistance. The curves of Fig. 73 show how the reactor voltage e rises when steady current I flowing in the reactor is suddenly interrupted. The maximum value to which voltage e could rise under any condition is IR₂, where R₂ is the equivalent loss resistance. R₂ depends mostly on the reactor iron loss at the resonance frequency determined by reactor inductance L and capacitance C. This frequency is 1/T, where T is Conditions for high voltage across the reactor occur with high values of k, the ratio of to 2R₂. If subject to sudden current interruptions, reactors must be insulated to withstand this voltage, or must be protected by spark gaps or other means. The curves of Fig. 73 are based on equation 41:

Fig. 73. Reactor voltage rise.

[41]

where

If there is appreciable circuit or wiring capacitance shunting the reactor after it is disconnected, this contributes to the total reactor capacitance C.