TOPIC 3

Simple inductive voltage divider

U

Um

Um-1

Uk

U1

Uout

Simple inductive voltage divider

out i D D1

1 jk

i

D

D k m

U U U

• 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

UU

Uout

Uout = 0.64 ... 74 U

Two-stage divider

U

I

I1 I2E R

Equivalent circuit of the two-stage divider

I1

z1

z2

I2

Z1

Z1

Z1Z2

UN turns

N turns N turns2

1

1

2

1 Z

z

I

I,

Z

UI

Improved two-stage divider

VF

E

R

U

Calibration of IVDs

Calibration points:

i /11 , i = 1, 2, ..., 10

0.090 909 090.181 818 180.272 727 270.363 636 360.454 545 45

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

IDG IDR IDT

1

10/11

1/11

2/11

0

LIA

Calibration of reference divider

LIA

IDG IDR T

1

10/11

1/11

2/11

0

Simple AC current comparator

D

Np

Nd

Ns

Ip

Is

primary winding

secondary winding

detection windingtoroidal magnetic core

detector

Np Ip - Ns Is = 0

AC current comparator with shields

primary winding

secondary winding

copper shieldmagnetic shieldcopper shield

detection windingtoroidal magnetic core

detector

D

Magnetic shield consisting of four toroids, 1 - 4

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

Error of an AC current comparator

ε 1ps n

II

where

n is the turns ratio of the ratio windings and

Is , Ip are the currents in ratio windings measured at their respective marked terminals when these terminals are at ground potential.

Compensated current comparator

N1

N2

N2

I1

I2

Ic

D

A B

Is

compensationwinding

Compensated current comparator

At balance, there is no flux in the toroidal magnetic core, the resultant magnetizing m.m.f. for this core being

N1 I1 - N2 Is - N2 Ic = 0 .

The resultant magnetizing m.m.f. for the shield is Um.s. = N1 I1 - N2 Is = N2 Ic ,

and the corresponding shield flux is

Φm.s. = Um.s. / Rm.s. = N2 Ic / Rm.s.

Compensated current comparator

A voltage induced by Φm.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

CCC CT

C GRb

r

AT

DC current comparator

Np

Ns

Ip

Is

D

modulation winding

detection winding

DC current comparator

D

Ip

Is

peak detector (PD)

Automatically balanced DC current comparator

Ip

Is A

PD

OSCRb