Thruster wire showing continuity when tested with multimeter

We’ve been using T100 thruster for our ROV for a while now. We are currently using 8 x T100 thrusters for our rov. All other thrusters seem to be working fine but when everything is connected, there is a delay in thruster signaling. Additionally, we’ve been facing an issue with a thruster which when provided PWM runs for some time then stops then again rotates, and so on. There is no continuous rotation for that particular thruster. While checking for any shorted wires of our thruster, we found that there is no physical damage to the wire which can cause such a problem, but two of the three wires show continuity when tested with a multimeter? Can anyone help figure out what the problem is?

Hi @KRUNAL, welcome to the forum :slight_smile:

In a slightly simplified form, this is how a brushless DC motor works:


Basically, the current needs to go in one wire and out another, and the active pairing alternates to make the rotor actually spin. All three phase wires should be connected to each other, so if I’m understanding you correctly one of your wire pairings didn’t show continuity, which likely means there’s been a wire breakage or similar.

This sounds like an intermittent issue, so it’s possible the breakage is very slight, and depending on how the thruster vibrates when it moves it may at times jostle and disconnect, and then with some slight wiggling from the other two phases being activated ends up reconnecting and can start spinning again.

If you’re able to identify the location of the breakage you may be able to solder it back together and pot it, but it’s likely that will be quite difficult to do, particularly given the motors are designed with all the wires potted to start with.

It’s also worth noting that if the breakage is from something small and sharp getting inside the thruster and breaking through the epoxy potting, then even if you’re able to restore the electrical connection it may not last very long due to corrosion.