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Inductance pulse testing for three-phase inductors

Posted: 05 May 2014     Print Version  Bookmark and Share

Keywords:Three phase line inductors  reactors  chokes  AC impedance  inductance 

Three phase line inductors, also called reactors or chokes, help limit and control the load current as well as suppress current spikes and are widely used to protect variable-frequency drives and motors. They also mitigate the interference between multiple drives located next each other and reduce voltage notching.

These inductors are normally built by winding three-legged laminated cores with constant cross-sectional area. The main parameter is the AC impedance at the rated current which is calculated from the inductance L as follows:

Equation 1

Conventional inductance test
The inductance of each leg of the reactor is measured by connecting the inductor to a variable high current 3-phase sinusoidal 50 or 60Hz supply. The current I for all three coils is adjusted individually to be within the rated rms current range. The rms voltage across each coil can then be read. The amplitude inductance of each phase k is given by:

Equation 1

The test setup requires three voltage and three current meters and is normally used only to test the inductance at a single current value. The difference between outer and middle coils (about 4 to 9%) is mostly neglected.

di/dt or pulse inductance test
This test method is widely spread for the testing of single phase power chokes at high currents. It uses a rectangular voltage pulse applied to the component being tested. A current ramp is then created in the test component and its di/dt slew rate used to calculate a complete differential inductance curve up to saturation, as follows:

Equation 1

The test instrument DPG10 by ed-k, allowing currents up to 1500 A is available in the market since 2005 and has been widely adopted as the industry standard for applications such as solar and UPS inverters; commutation, PFC, storage and line chokes for SMPS; rotor/stator inductance; etc. Thanks to its high current capability this testing method has complemented and even replaced conventional LCR sinus wave bridge methods in these applications.

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