TOPIC 3

Simple inductive voltage divider

Tape-wound toroidal core made from thin strip of a soft magnetic material (e.g. of Supermalloy). A rope of m strands wound a sufficient number of times around the core; the ends are soldered together to form a continuous ratio winding.

Main causes of voltage division errors Low-frequency errors are mainly due to unequal voltage drops which are produced by the magnetizing current when it passes through unequal resistances and leakage inductances of divider sections. High-frequency errors are mainly due to loading by various parasitic capacitances.

Multi-decade Kelvin-Varley divider U U U out U out = U

Two-stage divider

Equivalent circuit of the two-stage divider

Improved two-stage divider

Calibration of IVDs Calibration points: i /11, i = 1, 2,..., etc.

Calibration of IVDs Calibration procedure: Comparison of the divider under test with an 11 section reference divider. Calibration of the reference divider based on employment of an auxiliary 11:1 transformer (it is not necessary to know the exact value of the transformer ratio before the experiment).

Comparison of two-stage IVDs

Calibration of reference divider

Simple AC current comparator D N p N d N s I p I s primary winding secondary winding detection winding toroidal magnetic core detector N p I p - N s I s = 0

AC current comparator with shields primary winding secondary winding copper shield magnetic shield copper shield detection winding toroidal magnetic core detector D

Magnetic shield consisting of four toroids, By means of this shield, leakage magnetic fluxes are kept from reaching the detector winding.

Error of an AC current comparator where n is the turns ratio of the ratio windings and I s, I p are the currents in ratio windings measured at their respective marked terminals when these terminals are at ground potential.

Compensated current comparator N1N1 N2N2 N2N2 I1I1 I2I2 IcIc D AB IsIs compensation winding

Compensated current comparator At balance, there is no flux in the toroidal magnetic core, the resultant magnetizing m.m.f. for this core being N 1 I 1 - N 2 I s - N 2 I c = 0. The resultant magnetizing m.m.f. for the shield is U m.s. = N 1 I 1 - N 2 I s = N 2 I c, and the corresponding shield flux is Φ m.s. = U m.s. / R m.s. = N 2 I c / R m.s.

Compensated current comparator A voltage induced by Φ m.s. into the secondary winding nearly cancels all voltage drops between the points A and B, namely the drop on resistance of the secondary winding, the drop on leakage inductance of the secondary winding, the drop on possible burden impedance in series with the secondary winding.

Calibration of a current transformer D CCCCT CG RbRb r AT

DC current comparator NpNp NsNs IpIp IsIs D modulation winding detection winding

DC current comparator D IpIp IsIs peak detector (PD)

Automatically balanced DC current comparator IpIp IsIs A PD OSC RbRb