Inductance test

On 30.11.2000 we bought a new, fully automatic inductance test system. It is not only tested pneumatically, but also every measurement data is saved on the computer with date and time. Like this, quality is guaranteed. The new system tests inductance, capacity, resistance, phase angle, goodness, translation favtor, voltage and high voltage (galvanic separation). A series test with DC premagnetization is also possible. The measurement frequency reaches from 20 Hz up to 500 kHz.

Our inductance test device, type DPG10 from the manufacturer ed-k (www.ed-k.de) has been developed specifically for inductance tests of power chokes. The measurement is done by putting a constant direct current to the inductance, that hast to be tested (magnitude of the realistic operation). The test item will have a current course of which the rising speed di/dt depends on the inductance. With the course evaluation of the rising speed di/dt of the current, a complete inductance curve can be created with only one measurement. The curve shows the inductance course in dependance of the current. The maximum current is 1500 A.

As long as it is no air choke, all power inductances have a certain saturation characteristic, which means, the inductance decreases with rising current. This is because of the core materials, which loose more or less permeability at a certain inductance B, and act like air in an extreme case. The saturation characteristics of a choke can be influenced

- by the core material,
- by the core geometrics
- by the number of windings and
- by the air gap.

However, there are often differences of the calculated inductance and the real inductance once the current reaches a certain value (e.g. rated current).

- due to scattering of the cores
- due to inaccurate or incomplete data sheets of the core
- the choke geometrics cause an inhomogenous field distribution
- due to production tolerances
- due to temperature influences
 

PFC-choke with amorphous core

This example shows a choke for a three-phase power factor correction with a rated inductance of 500 µH and a rated current of 50 Aeff (peak value in this example is approximately 85 A at rated current). The cut core consists of a nanocrystalline alloy with high saturation inductance. The inductance of this choke falls moderatley up to 80A (ca. 20%), which is suggestive for the economic construction of the choke and has no bad influences on the operation. Afterwards the inductance falls rapidly. At 100A the choke only has an inductance of 138 µH. For an operation while overload, the circuit planer has to consider this, even if a short overload operation wouldn’t be any thermal problem for the choke. In this example, we would get frequencies that are too high and would cause an overload for the transistors.

Filter choke with ferrite core E32

The test item in this example is a filter choke for a switch mode power supply with an output voltage of 5V and a power of 100W. The E-core (E32) consists of default ferrite material N27 and has an air gap. This example shows the temperature dependance of the saturation very nicely. The inductance falls at 25°C and 24A down to 20µH. At 125°C and 17,5A, this limit is already reached.