Having previously tried to estimate the pump inductance of the Ulka EP5 Pump, and without really being certain of the accuracy, I decided to try and actually measure it. Unfortunately, this is made difficult by the series diode built into the EP5 pump. Although I had a spare pump, I didn’t want to completely destroy it by opening it up, so I decided to try and reach the terminal by performing “minimally invasive surgery” on the pump…
Having done this, a Sharpie pen was used to mark a point above the metal tab, which was approximately where the “U” is in the ULKA logo as shown by the small black dot below:
Then a pin vice was used to carefully and slowly drill a 1mm hole until it reached the surface of the metal tab, while taking care not to drill too far:
Having done this, we can now directly access both terminals of the solenoid and completely exclude the diode from our measurements. Using a low cost LCR component tester (fish8840 Taobao) it is possible to measure the inductance and series resistance as shown below. Successive readings showed 847.8mH, 856mH and 859.3mH with an average of about 854mH. The series resistance was about 165Ω as measured previously.
As both sides of the diode are now accessible, we can also check the diode forward voltage as shown below. This shows Vf=706mV and C=40pF:
The inductance was measured as 854mH and the series resistance was 165Ω. The diode forward voltage is 706mV. These tests are carried out at a low voltage, and without any hydraulic load on the pump. It is possible that the solenoid plunger position may also affect the measurements. Note that the measured inductance is significantly different to the previous estimate.