topic 3. simple inductive voltage divider tape-wound toroidal core made from thin strip of a soft...
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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.
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
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).
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
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.