Voltech AT3600, Руководство пользователя

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V
OLTECH
AC I
NTERFACE
U
SER
M
ANUAL
98-072
ISS
7
PAGE
43
Selecting Other Current Transformers
A CT (current transformer) is a transformer designed not to accurately transform voltage levels between
windings, as most transformers, but to transform currents.
CTs are designed to present a small series impedance on the primary winding, so the current flow in the main
signal path is disturbed as little as possible.
A CT has a proportion of the primary current flowing in the secondary that is defined by the CT turns ratio.
The Voltech CT10-20 has a ratio of 1:20, so whatever current flows in the primary, 1/20 of that current must
flow in the secondary.
When designing a CT, there are several important parameters that must be taken into consideration. Some of
these are as follows:
1. The minimum and maximum currents to be measured.
2. The required accuracy of the measurements.
3. The burden resistance (resistance on the secondary) of the measurement device.
When choosing a CT for operation on the AC Interface Fixture, the first important parameter to make note of is
the minimum current that the AT3600/AT5600 can measure accurately and the burden resistance that is
presented to the system.
The AT3600/AT5600 can accurately measure about 1/20mA (to about 1%). As currents get smaller than this,
noise in the system and other effects due to large currents flowing in the AC Interface start to swamp the true
signal that is being measured.
The burden resistance in the AT3600/AT5600 changes depending on the level of the current being measured,
because the internal shunt is changed automatically depending on the signal level.
1. Between 0 and 12mA peak, the burden is 12 Ohms.
2. Greater currents than this result in a burden of 2.5 Ohms.
This burden resistance is reflected to the primary of the CT by the square of the turn’s ratio.
Zp = Zs x Np
2
/ Ns
2
Therefore, for the Voltech CT10-20, the primary impedance Zp = 12 x 1
2
/ 20
2
= 0.03 Ohms (for a 12 Ohm
burden) and Zp = 2.5 x 1
2
/ 20
2
= 0.00625 Ohms for a 2.5 Ohm burden. This ensures that the effect on the
main current measurement is small.