Does current have to return to its source?

Question

I am not the most knowledgeable when it comes to electricity, so apologies if my questions lacks necessary information that is needed to get a good answer.

I work with 3 phase electric motors, and I understand the basic principals of how they work. When testing these motors, I make it a point to discharge the cables after the motor has been spun during testing by touching the phase cables to a steel sheet of metal, what I consider to be ground, that is apart of the structure of the motor tester. I had a coworker tell me that I should be discharging the cables by touching the cables to the metal chassis of the motor itself. So I suppose my question is: What is best to do in this scenario? Does current HAVE to return to its source, through the chassis? Or is it just as viable to touch the cables to the steel sheet (ground). Does it matter? Keeping in mind the goal is to simply discharge the leftover voltage in the system as a safety precaution.

EDIT: I would like to clarify that I have been electrocuted when handling these cables after motor testing multiple times, hence why I ask the question. To me, this implies that somehow there is current still in the system but I do not understand how if copper stator windings aren't capable of storing current.

I just want to understand if it's a law that current MUST return to its source.

Answer 1

I think what you are describing, is residual electric charge in the motor. This could be possible if:

1. The shock only lasts a very brief amount of time (a fraction of a second.)
2. The motor is rather big (the bigger the motor, the more likely this would be possible.)
3. The motor was spinning, but is now stopped, and nothing at all is connected to the motor phases - no VFD's, no reactors, no capacitors anywhere - just the motor wires.

So to reiterate: I'm assuming the motor is connected, ran, and disconnected via a switch/contactor/breaker. When disconnected, nothing at all is connected to the motor.

The windings inside the motor have a small parasitic capacitance to the chassis. This is a physical property (the windings are coated in a very thin enamel insulation, thus the proximity of a winding to the chassis equates to two plates of a capacitor.) Which means that a tiny amount of electric charge could remain between one or more phases and the chassis if suddenly disconnected.

For small, low-voltage motors, this would likely not even be detectable or dissipates too quickly to measure. But on large, medium and high-voltage motors, residual charge might be able to give a noticeable "poke" for a short time.

Does current have to return to its source?

Current implies that electricity is actively flowing, but what we're talking about here is really static electricity. The motor is disconnected, and one or more windings retains a portion of it's voltage (charge) with respect to the chassis. This charge doesn't go anywhere immediately, because the windings are insulated. The electric and magnetic fields have stopped moving, but there is still a small charge present, waiting for an unsuspecting person to complete the circuit (between a winding and the chassis.) So in this respect, it could be looked at like a small capacitance between each winding and the chassis.

When that person completes the circuit, the static charge very rapidly flows through the person, giving them a shock. Only during the entirety of the shock, is any current actually flowing. When the shock is over, the winding voltage and chassis voltage are now the same - charge has been balanced - so no more current flows.

The solution to such a phenomena would be to simply touch (safely - insulated alligator clips) a "bleeder resistor" from chassis to each phase, directly at the motor's terminals. A 5-Watt resistor, value of 100 kΩ (a hundred thousand ohms) for 240 V, 220 kΩ for 480 V, 470 kΩ for 690 V etc, would dissipate that charge in under a second for all motors except the very largest ones. Do not run the motor with any such resistors connected, as it would tend to "electrify" the chassis. Do this only for discharging stored charge.

Curiosity got the better of me, so I just ran an LCR Meter (inductance, capacitance, resistance) test between chassis and a phase of a 300HP induction motor. Test reported 20 nF (0.02 µF) with a dissipation factor of 0.2, meaning it can store enough charge, long enough, to get a shock. For bigger and higher quality motors, I imagine this would be even more of a problem.

Answer 2

Electric motors don't, as a general rule, store electricity. Not in the sense of what you're talking about anyway (e.g., something that could come back and discharge "later" and be a safety issue, etc). The main components that do that, that you have to worry about, are capacitors. So unless you have one or more capacitors as part of your system, there's no "stored current" to discharge.

If you did have a capacitor to discharge, you'd generally want to discharge it by placing a suitable load directly across the terminals of the cap. By "a suitable load" I mean something with the correct amount of resistance to discharge the cap in a controlled way. Using a near 0 ohm load like, say, a screwdriver or a jumper wire, can cause problems if the cap is storing a large amount of energy. Wires and screwdrivers have been melted by people doing this. You don't necessarily want a dead short for this!

And whatever load you connect has to be able to handle the amount of current that will be discharged (which will be a function of the voltage of the cap and the resistance of the load).

Note that connecting something to "ground" does not necessarily discharge it. If the "thing" under consideration isn't itself connected to ground in a suitable way, you may not be completing a circuit at all, and nothing will discharge. Note that "ground" is relative - there's "Earth Ground" which is literally what its name says, but devices can have "ground" buses that aren't tied to an earth ground at all. And random pieces of metal laying around your shop (including the legs of your workbench, random pieces of conduit, etc.) may well not be connected to earth ground (or anything else). In the end, whatever you're doing, you need to fully understand exactly how a circuit is completed or you're probably not going to discharge the device in question.