[galderdi]

I am currently in the early stages of a ground up project. The end result will be a home built road registered EV.

I got thinking about isolation between the drive train and the chassis. Being DC there is expected to be some leakage through the commutator. There are limits to the allowable leakage. So I was hoping to improve my chances of passing the test by maximising the chassis isolation. I am thinking through all the points of contact to understand where I can/can't isolate. Here is my thinking.

• The DC motor and gearbox will be mounted together so conductivity between the two will be high.
• The gearbox and motor mounts will be rubber mounted so the isolation there should be fairly good.
• There will be gear shift with cables. I need to double check but I think they have isolation at both ends.
• The Handbrake cable would be conductive. But maybe I can isolate at the handle end.
• The brake lines are rubber but I suspect the fluid will be somewhat conductive? Maybe I can isolate at the peddle end.
• The drive shafts are metallic and would conduct through to the hubs. But the hubs are isolated with bushings in the control arms.
• The shocks too are isolated with bushings at each end.

Did I miss anything?
Does anyone have any experience/results from undertaking a similar exercise?

 

[John_G]

I am NOT an EE and do not play one on the 'net and I do know know how the test you reference is conducted (pun intended) nor what the limits are.

One thing I do know, is that if you isolate the drivetrain as described:

• A dangerous voltage potential can exist between it and the chassis
• Stray current WILL, at least ultimately, find a way "home"
• I believe the correct approach is to make sure, as much as possible, that:

a) all the "chassis" and drivetrain parts are electrically bonded - NOT isolated
b) the 12v system is completely isolated from the HV system
c) consider using a grounding strap from the chassis to ground (looks like a piece of serpentine belt)

Because, after all, we are trying to avoid situations that can result in an electrical shock. If all the hardware (metal) is at exactly the same potential, placing you hand on the door while actuating the shifter is no problem.

 

[Duncan]

I would agree with John_G

The isolation requirements are not so much "isolation" but safety - the idea is that your car will include something that will measure any "leakage" and shut down if it detects any

With a brushed DC motor that would be all of the time - so here at least if you are running a brushed DC motor you do not need (and cannot use) that type of safety feature

 

[galderdi]

Ah ok. So if the chassis is connected to Ground the leakage through the motor will gradually drain the battery right?

 

[Duncan]

I have my HV positive and negative isolated from the chassis
The 12v negative is connected to the chassis

There is going to be a current path from the motor to the chassis - but it should be a very high resistance

When you are not driving the battery terminals are both isolated by the contactors - so no drain then

There may be a drain through the carbon dust but it should be in milliamps or less so not a measurable problem

My motor is connected through the driveshafts to the chassis - The next time I do a rebuild I think I will do as John suggests and add some grounding straps

 

[galderdi]

Yeah, all good. For road registration they measure the leakage and there is an acceptable limit for DC motors. I am just nervous about passing that test. Hence I was thinking about total isolation so the leakage would not make it to the chassis. But I'll stick with the grounded approach and just trust that it will all work out. I'll make sure to give the motor a good clean just prior to the tests.

Although another thought just crossed my mind. Maybe the two ideas are not incompatible. All the terminals on my motor appear to be isolated from the motor case. So maybe my aim should be to isolate the motor case from the chassis. But then still connect the HV negative to the chassis. Then the leakage should not be detectable on the chassis. Or am I wrong about the negative terminal being isolated from the motor case?

 

[Duncan]

Not a good idea
Both terminals should be isolated and in NZ we have to have a contactor on both sides!

Measure your motor - a decent multimeter will have a megohm setting - the case should be isolated from all four motor terminals - which at that setting will all be connected together

Give it all a good clean before the leakage test and it should be pretty high -

 

[brian_]

Although another thought just crossed my mind. Maybe the two ideas are not incompatible. All the terminals on my motor appear to be isolated from the motor case. So maybe my aim should be to isolate the motor case from the chassis. But then still connect the HV negative to the chassis. Then the leakage should not be detectable on the chassis. Or am I wrong about the negative terminal being isolated from the motor case?

That's the opposite of normal EV practice, in which both sides of the high voltage circuits are isolated from component cases, and component cases are bonded together and to cable shields.

 

[galderdi]

That's the opposite of normal EV practice, in which both sides of the high voltage circuits are isolated from component cases, and component cases are bonded together and to cable shields.

yes yes, that makes much more sense. Ok that finalises the approach then. However it does put me back in line to deal with the leakage test as that approach does make the leakage visible on the chassis. But as Duncan said, hopefully a good clean should be enough to make it pass. If not then I probably have bigger issues to solve.

 

[John_G]

Ah ok. So if the chassis is connected to Ground the leakage through the motor will gradually drain the battery right?

No, the motor terminals are only energized while you are driving it. Furthermore, electrons are funny, they ALWAYS take the path of least resistance and if they cannot return home, they do not leave.