I'm working on a submerged pump application, running on 415V three-phase. I don't know for sure, but I believe the pumps are only a few KW.

The customer is saying that the pumps run fine when the site is on mains power; but when it switches to generator, my motor protection unit trips the pump on Low Power Factor. I’ve asked for the motor protection unit logs but haven’t seen them yet, so that’s all I have to go on. The Low Power Factor threshold should be set to 0.5, but again I can't be sure.

If the generator rotation was incorrect, then the phase fail relay and the motor protection unit should both have tripped, preventing the pumps from running.

If the generator voltage was higher / lower than the mains, then the current would be lower/higher, but the inductance of the motor hasn’t changed so why would the phase shift be different?

I’ve asked a few experienced engineers here, but no-one has any suggestions as to what is going on.

Any suggestions? Thanks for your help!


Edit per request from @Transistor - I'm not sure that the schematics will add anything more than what's described above, it's a simple system. The schematic editor looks Ok for electronics, but not for electrical so I've pulled some wiring diagrams from other sites. These are example schematics from a typical site: System Single Line Drawing

Simple DOL Starters. Ignore the reversing contactors, they're not present at this site (and the Low PF fault is ignored when reversing anyway).

Starter Schematic

  • 1
    \$\begingroup\$ I think a sketch of the layout of the components including the low power factor monitoring and various trips would be of benefit. You can use the CircuitLab button on the editor toolbar to create a schematic. \$\endgroup\$
    – Transistor
    Oct 2, 2019 at 13:03
  • \$\begingroup\$ @Transistor, schematics added, but there's not much to it. The CTs could be above the starter, but that shouldn't make any difference to the readings, nor should the source of the power (mains / Gen). \$\endgroup\$
    – Scott
    Oct 2, 2019 at 14:07
  • \$\begingroup\$ Do you have the characteristics of the generator? Manufacturer, model. kW. \$\endgroup\$ Oct 2, 2019 at 15:06
  • \$\begingroup\$ Forget how the motor trips. It is a symptom. You have to trouble-shoot your problem. What are possible causes of the problem? You have 3 phase, 4 wire coming from DNO. What is input from generator? 4-wires? Is there any current on neutral wire from DNO? Is it both pumps, or just one? Disconnect Pump 1 breaker. Go from mains to generator, does Pump 2 cause problem. Repeat for Pump 1. \$\endgroup\$ Oct 3, 2019 at 23:09

2 Answers 2


Ah, love it when the customer only gives you half the information.... Site visit yesterday, 2Hr drive to get there, problem fixed in 1/2Hr.

  • My motor protection unit was tripping on Low Power Factor...
  • Because the power factor was 0.0...
  • Because the Soft Starter wasn't starting...
  • Because the voltage was too high (above 110%).

The Soft Starter trip was indicated on my Motor Protection unit (via digital input), but the customer had never mentioned that before.

Nominal voltage was 415V; the generator was running at 438V, my meter caught a peak of 493V when the pump was started. Interestingly, when the pumps started on mains power, the voltage dropped down to 405V during the startup; when starting on generator, the voltage went up. There is a mains transformer behind the building, I think it is feeding just this plant.

Fortunately, the customer had a colleague who knew the generator, and we were able to open it up and adjust a tiny pot on the alternator regulator board and bring the voltage down. Took two adjustments, but we have the station running fine on generator now.

Thanks for everyone's suggestions though!


This question on Stackexchange seems to indicate that you'd need a huge voltage change to get much of a PF change. So that's probably not it.

Some possibilities:

  • The generator output isn't sinusoidal, and the power factor trip circuit doesn't know how to handle that. Depending on how the power factor is being measured, a circuit that expects sinusoidal power could have a disproportionate response to harmonics.
  • The phases out of the generator are unbalanced -- even if there's already a breaker for this, I'd check it, or ask the customer to.
  • \$\begingroup\$ Thanks for the hints. I'm on site on Friday to investigate. I can check generator voltage, and I now have a meter that can measure PF and THD, so I'll check the mains quality vs the generator. I don't have a CRO to be able to check how sinusoidal the waveform is, but I would have thought that a diesel motor with a direct drive to a generator should be smooth - there's no inverters. \$\endgroup\$
    – Scott
    Oct 2, 2019 at 18:01
  • \$\begingroup\$ "but I would have thought..." Well, me, too, but the older I get the more I see equipment doing really strange and unexpected things. \$\endgroup\$
    – TimWescott
    Oct 2, 2019 at 19:39
  • \$\begingroup\$ saturating iron will upset the waveforms \$\endgroup\$ Oct 3, 2019 at 2:41
  • \$\begingroup\$ If the voltage is low the induction motors will draw lots of reactive. I would check that first. \$\endgroup\$ Oct 3, 2019 at 18:26
  • \$\begingroup\$ What is the time delay on the “Low Power Factor” trip? It may need to be relaxed when not grid connected. \$\endgroup\$ Oct 3, 2019 at 22:23

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