Design Ideas: July 6, 1995
In some applications, magnetic components are difficult to characterize when steady-state current is flowing. Bridges having the necessary dc capability are expensive. If you don't need extreme precision (for example, 2% or better), other measurement methods exist. The method in Fig 1 is universally applicable over a range of inductances and frequencies. The basis for the inductance measurement is to determine the ac impedance at the frequency (f), ideally the inductor's working frequency of interest. In Fig 1, the ac voltages across the inductor and the reference resistor RR are VLAC and VRAC, respectively, and the circulating alternating current is IAC.
You need the following two components for proper operation: 1. The dc isolation transformer with a gap that prevents saturation at the maximum rated dc. In this example, for 25A current, use an EI-configured core of 0.014-in. grain-oriented silicon steel. The primary winding uses 160 turns of AWG 30 double-covered magnet wire. The secondary winding consists of 32 turns of AWG 14 magnet wire wound in two steps: 16 turns, followed by the primary winding, followed by the final 16 turns. 2. The reference resistor, which must have four terminals (two for forcing current, two for sensing voltage) if its value is lower than 3ê. It also must have negligible inductance and skin effect at the frequency of interest. One way to make a resistor with such characteristics is to connect several (maybe five to 10) metal-film resistors in parallel.
To keep errors low, ensure that 3VRAC is less than VLAC. Check oscilloscope photos to be sure the waveforms are reasonably sinusoidal, denoting that the flux density is below the saturation level. You can also measure the dc IDC by using the same DVM or the oscilloscope in dc mode. To determine the impedance, ZL, use the following equations:
The error in VLAC from VRAC is negligible because these two quantities are 90ø out of phase. Measure both VLAC and VRAC the same way--by using the oscilloscope's p-p value, the DVM's average value, or the DVM's rms value. The power supply should have 3 to 5% maximum ripple and a constant-current setting for ease of making repeated measurements. The zener diodes across the primary winding protect the frequency generator against spikes that occur when you instantaneously apply or cut off the dc.
An example uses a 50-µH inductor operating at 100 kHz, with a 100-mê reference resistor, RR. Measurements yield VRDC=1V (10A), VLAC=1.2V, and VRAC=38 mV.
(DI ##1729)