Measuring Ulka EP5 Pump Inductance

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…

To start with, shining a torch through the plastic pump body reveals the location of the metal tab terminal on the solenoid, and the series diode:2016-05-04_ulka_exam1

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 165Ī© as measured previously.2016-05-04_ulka_coil_LR

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:

2016-05-04_ulka_diode_VFCWhen finished, the hole will be filled with epoxy to seal it back up again and make it safe.


The inductance was measured as 854mH and the series resistance was 165Ī©. 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.

20 thoughts on “Measuring Ulka EP5 Pump Inductance”

  1. James, did you ever manage to calculate the max back-EMF voltage spike generated by the pump when chopping it with your IGBT?

    1. Hi Philippe. The short answer is no, I haven’t calculated this. Although I did create a SPICE model (in LTSpice) treating the pump as an inductor, the model didn’t seem to agree very well with real world measurements taken with my oscilloscope or multi-meter, so I wouldn’t put too much stock in the results for estimating back-EMF.
      I think a more complex model would be needed, and the best way would be to physically measure this on a real pump under various loads, then add a decent safety margin.
      I don’t have a differential ‘scope probe, so the way I’ve done this in the past is to use a battery powered ‘scope which isn’t referenced to mains ground. I also used a couple of resistors to make a potential divider to reduce the voltage to a measurable level.

    2. Initial test shows it’s over 500V on my 120V Ulka (I’ll need more attenuation on the probe before I measure again). Unfortunately before I managed to add more resistors I made a mistake and smoke… I now need to fix my circuits before I take new measurements to get the actual value!

      1. Was this with a resistor and diode? (I guess the worst case back EMF would be with the original manual switch and no snubber)
        Sorry to hear you let the magic smoke out!
        I made a mistake with mine once, which resulted in this

        1. Yes this is with a resistor and diode in parallel + the switch (IGBT) between pump and neutral/GND.

          I’m still not quite sure I understand this whole thing. My understanding is that the worst EMF is actually based on the timing of the switching off – the worst will be when you switch off when the current is at it’s highest point (there is a phase shift in the pump so I don’t know when that point actually is). back-EMF V = -L * di/dt.

          Having a snubber or not shouldn’t affect the EMF voltage spike value should it? It does however change how the current get split out?

          1. Hi Philippe, assuming your mains voltage is 110V (which I’ve read is more like 120V nominal), the peak should be about 120 x 1.414 = 170V and from that plot the spike looks like about 241 pixels versus the AC peak at 149 pixels, so at a rough estimate it could be 170V * 241px / 149px = about 275V.

          2. James, I did the same calculation indeed – I just wish I could have gotten the correct values on the graph directly since I set the proper attenuation (19Mohm probe resistor + 1Mohm internal scope resistor should be 20x attenuation). No idea why it shows this way.

            I will do a few more tests today – including overlapping the IGBT signal with the different power settings.

            If there are other parts of the circuit you would like me to measure I am more than happy to provide those while I got everything setup.

  2. I got a differential probe scope – I’ll calculate some good resistance values and give it a shot, very curious to see what comes up šŸ™‚

      1. Hi Philippe, I’d completely forgotten about this old post which has some ‘scope traces from my pump:
        (it also discusses the resistor values and using the subtraction function on the ‘scope to “fake” differential measurements – which I’d completely forgotten about)
        You can’t see any back EMF spikes there, presumably because this was using a diode snubber, with no series resistor at the time.

          1. Hi Philippe, as I understand it, the issue with just using a diode is that the back EMF through the diode keeps the solenoid coil energised (i.e. after you switch the IGBT off, the solenoid stays on for a while). Using a series resistor with the diode helps, but the disadvantage of the resistor is that the voltage across the inductor (and IGBT) will be higher, and also dissipates a lot of power in the resistor (instead of dissipating it in the solenoid coils).
            I think a more sophisticated snubber may be possible, but I haven’t studied this in enough depth to be in a position to design one yet. I’ve got a good text book on snubber design that I need to make time to read šŸ˜‰

      1. Current of pump and especially collectector – emiter voltage of IBGT. While using triac there are no voltage spikes and there is no current lag. Iā€™m trying to use IGBT in PWM mode as thyristor drive is not very predictable for lower outputs.

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