DC Motor Fault

[mvermeer]

Question for any of you electrical guys out there: I have a Vetus bow thruster, positive connection on one side, negative back to battery on the on the other side, and a green wire connected to the web of wiring all connected to the anodes on the transom. To make a long story short, with the negative side disconnected, and power applied to the motor on the positive side (thruster not turned on, just positive side connected), I get about 50A dc running through the green wire, which has melted in places. My sump pump also turns on, even though the fuse is blown and the float switch is disconnected.

Thoughts?

 

[High Wire]

You have a serious ground. Disconnect the positive cable to the motor or trip the breaker today. You are going to need professional electrical help. The bilge pump is wired incorrectly.
Question: how are you measuring the 50a thru the green wire?
My guess from 3500 miles away is one of the brush leads is contacting the motor case.

 

[mvermeer]

I am using a clamp on ammeter to measure the current. The motor is disconnected completely and all is good regarding the safety of the vessel. I’ll remove the motor and send it in for servicing. Thanks for the advice ?

 

[High Wire]

Woohoo! Someone has a DC ammeter on board!

 

[mvermeer]

LOL! Yeah well I come from years of commercial crab fishing on 60’ steel vessels, so I’m a little prepared. But very new to the pleasure craft world. There’s quite a difference

 

[High Wire]

You might be able to pull the brush cover plates and take a peak before sending it out.

 

[fgarriso]

Your GREEN bonding wire is also DC grounded, but will not take the current your bow thruster pulls when turned on. Just replace the green bonding wire, and reinstall your battery ground. Then do not do that again.

 

[diver dave]

Is the NEG terminal insulated from the case? OR, more like a starter motor, where the case/mount is expected to carry the current.

I recall a rule somewhere that says that bonding conductors shall not be less than the power conductors. Manytimes ignored on large invertors and motors.

 

[mvermeer]

Your GREEN bonding wire is also DC grounded, but will not take the current your bow thruster pulls when turned on. Just replace the green bonding wire, and reinstall your battery ground. Then do not do that again.

But here’s the thing…the thruster doesn’t need to be turned on for the current to appear in the bonding wire. All it needs is the positive cable to be applied to the motor, and, without the switch being turned on (ie solenoid closed), and with the negative cable removed, current flows thru the bonding wire. Are you saying that if I were to reconnect the negative able, that would stop any current flowing thru the bonding wire?

 

[High Wire]

Guess I read this first post wrong.
1 - with the thruster turned off there should not be current flowing anywhere.Off is not off. So that’s an issue in the direction control solenoid box.
2- With the negative removed the current from problem #1 will return to the battery negative through the green bonding wire if the casing is not insulated from the negative. If the negative cable is connected at the motor is there current flow? The same or more current should go through the negative cable. I don’t think the thruster is set up like a starter motor with the casing negative.
Current follows the path of least resistance but some will still flow through the higher resistance path.

 

[twistedtree]

But here’s the thing…the thruster doesn’t need to be turned on for the current to appear in the bonding wire. All it needs is the positive cable to be applied to the motor, and, without the switch being turned on (ie solenoid closed), and with the negative cable removed, current flows thru the bonding wire. Are you saying that if I were to reconnect the negative able, that would stop any current flowing thru the bonding wire?

Right, you've got a short inside the motor. Good thing you found it because your underwater metals are probably fizzing away every time that positive wire is connected.

 

[High Wire]

Thinking further, the motor HAS to have isolated negative because it reverses direction by reversing polarity at the motor leads. I also have a VETUS thruster and the drawings show two +/- insulated terminal lugs

So:
Problem 1 - Off is not off. Something is bridging the contacts to supply current when the cables are connected but no direction is selected.
Problem 2 - There is an internal ground directing current to the motor case.

 

[mvermeer]

Ok thanks everyone for the advice and info, I will use them both and see what I find. But first, I have to jar some salmon ?