Shielding on motor cable stops motor

T Chap

I am hoping someone can make sense of this. A brushless DC motor which uses hall sensors and encoder, and a Prop driven main controller board that sends direction, PWM to control speed.

I used a 3conductor + shield (3 x 16 guage pvc wrapped conductors + outer foil wrap/4th unshielded wire). The shield is connected on one side only to GND, the other end floats. There is a common ground between the controller and motor driver. The shield is absolutely not connected to any of the 3 motor wires. When left floating, the motor runs, but when I connect the shield to GND, the motor stops, while the PWM goes to max to try to correct the problem, so it is not coming from the controller side.

I wanted to try to prevent any motor noise from affecting the hall sensors and encoder, although so far it hasn't been needed.

Any thoughts appreciated on this phenomenon.

Bruce Bates

Originator -

One connects wires for continuity, but never shields. The point is that you don't want "conduction", otherwise you'd be defeating the purpose of the shield; foil or otherwise. One always leaves one side of a shield unconnected. The last thing you want to do is "spread" the noise.

Does that make sense?

Regards,

Bruce Bates

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When all else fails, try inserting a new battery.

T Chap

Thanks Bruce, but I'm not really following you. I only have one end of the shield connected to GND, just as in audio applications where the shield goes to GND. So, there is no conduction with the shield, just a single ended grounded fold wrapped around the 3 main conductors.

Bruce Bates

Originator -

Perhaps it was my misunderstanding. When you said "When left floating, the motor runs, but when I connect the shield to GND ..." I thought you meant connected at both ends. Since that's NOT the case, then I don't have an answer for you except to check for continuity between the shield and each of the conductors with the POWER OFF. If there is no continuity, then I'd buy what you say about the shield not touching anything. It's just a way to be sure.

Regards,

Bruce Bates

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
When all else fails, try inserting a new battery.

Post Edited (Bruce Bates) : 10/31/2008 5:34:33 PM GMT

Phil Pilgrim (PhiPi)

What is the frequency of the PWM? How long is your cable? What kind of motor driver are you using? Is it rated for capacitive loads? This might be a capacitance issue: when the shield is grounded, there's more capacitance between the wires and ground. I've seen op-amp outputs choke on too much capacitance. I'm not sure about motor drives, though.

-Phil

T Chap

The cycle length of PWM is 1600 clocks at 80_000_000, but then again the driver has it's own current limiting PWM factors that I am not sure of. The motor cable is 6'. It is a BLDC driver (moto BLDC driver IC MC33035 into an Intersil 3 phase Mosfet Driver, 6 x IRF540's. The shield meters in the hundreds of megohms between any of the 3 motor phase wires. It must be a capacitance issue, although the motor cable is coiled up at the moment, so maybe some electromagnet effect or inductance issue of sorts.

No big deal really, I was just trying to understand why I can't shield it. The Hall sense and encoder lines are shielded which solves that end of things, but I was hoping for added noise immunity for the motherboard and Qprox sensors by shielding the motor. My guess is that I can use separately shielded motor wire rather than 3 conductor + shield.