inductive voltage divider

Inductive voltage divider

An autotransformer that has its winding divided into a number of equal-turn sections (usually 10) so that when an alternating voltage V is applied to the whole winding, the voltage across each section is nominally V/10. The progressive voltages from one end to the section junctions are thus V/10, 2V/10, 3V/10, . . . , and 9V/10. This voltage division can be realized with errors considerably less than 1 part per million of V, and such units therefore find wide use as standards of ac voltage ratio in the discipline of electrical measurements.

The division of voltage will be in error if there are differences of resistance and leakage inductance from section to section, and these errors will be significant if the differences are significant in relation to the input impedance of the winding. Leakage inductance is caused by that very small fraction of the flux from one section's winding which fails to thread the rest of the windings. The most commonly used constructional technique for minimizing such errors is to take 10 equal lengths of insulated copper wire and twist them into a “rope.” The rope is wound onto a toroidal core made of thin, high-permeability, low-iron-loss magnetic material. The strands of the winding are then connected in series so that each strand forms the winding of one section of the 10-section divider (see illustration). The resistances of the sections are very nearly equal since the strands are the same length, and the leakage inductances are also closely equal and small because of the close flux coupling of this type of winding. The low-reluctance magnetic path of the core ensures a very high value of input impedance. Thus, voltage division at low audio frequencies can be accurate to a few parts in 10⁸ of V.

Rope winding on toroidal core making single-decade inductive voltage divider

Inductive voltage dividers operate most accurately in the frequency range 20–1592 Hz but can be constructed to operate at frequencies up to 1 MHz. They are usually designed to accept an input voltage of up to about 0.25 times the frequency in hertz. See Voltage measurement

inductive voltage divider