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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.

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  • \$\begingroup\$ Continuous current - yes. Instantaneous - no. \$\endgroup\$
    – Eugene Sh.
    Commented Jun 28, 2022 at 17:03
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    \$\begingroup\$ Does this answer your question? Why does current have to return to its source in a circuit? \$\endgroup\$
    – JYelton
    Commented Jun 28, 2022 at 17:04
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    \$\begingroup\$ Motors don't store charge in their windings after being powered off - there is no "leftover voltage" to discharge. But even if there was, you'd need to "discharge" them by shorting the pair of winding terminals, not to some unconnected piece of metal. \$\endgroup\$
    – brhans
    Commented Jun 28, 2022 at 17:05
  • \$\begingroup\$ @brhans Hi and thank you for your answer. I have been electrocuted on multiple occasions when handling these cables. At those times, the motor was not connected to any energy storing circuits. In all instances, the motor was spinning, I stopped the motor, I went to remove the cables from their contactors and was electrocuted because I was careless and had my other hand on the motor chassis. To me this implies there is somehow stored current in the motor but perhaps I just do not know enough. \$\endgroup\$
    – Blynx
    Commented Jun 28, 2022 at 18:01
  • \$\begingroup\$ @Blynx Has this happened in multiple plants? Or always the same plant? What were the motor ratings? Was a VFD present? How was the motor shut down? The motor was on one end of the cable but what was on the other end? I assume the motor had come to a complete stop already? \$\endgroup\$
    – DKNguyen
    Commented Jun 28, 2022 at 18:26

2 Answers 2

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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.

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  • \$\begingroup\$ I think this is the best explanation. I will be sure to measure the capacitance (safely) next time I am testing one of these devices. They are very large, and run up to voltages of 750. The rotors in these devices are permanent magnets so they are used as generators when something else spins the motor shaft. I'm sure that has something to do with, as that is apart of the testing procedure. Regardless, thank you for your helpful insight. Appreciated. \$\endgroup\$
    – Blynx
    Commented Jun 28, 2022 at 20:59
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    \$\begingroup\$ Just discovered that this parameter is called "capacitance to ground" and is checked sometimes. Additional static voltage rise can be caused by fly-back voltage during switch-off. \$\endgroup\$
    – Vladimir
    Commented Jun 28, 2022 at 22:38
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    \$\begingroup\$ An "insulation tester" or "megger" is commonly used to check for unwanted conduction between phase windings and/or ground. These work by injecting a high voltage (some up to thousands of volts) and measuring the resulting current flow. Care needs to be taken with these because the same condition (shock hazard) exists if the meter is suddenly disconnected while the high voltage is being emitted. \$\endgroup\$
    – rdtsc
    Commented Jun 29, 2022 at 12:11
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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.

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    \$\begingroup\$ Thank you for your thoughtful answer. I have been electrocuted on multiple occasions during handling of the phase cables immediately after spinning the motor. Granted, I was being careless at the time and have since learned. To me, this implies that somehow there is stored current in the motor. How is this possible? \$\endgroup\$
    – Blynx
    Commented Jun 28, 2022 at 18:06
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    \$\begingroup\$ It's hard to say without more information. I don't know any specifics of the motors you are using, but some kinds of motors do use capacitors to help provide startup current, so there could well be a big ole cap in your system somewhere. There could also be another connection to the mains system that is leaking current somewhere that you didn't anticipate, etc. \$\endgroup\$
    – mindcrime
    Commented Jun 28, 2022 at 18:26
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    \$\begingroup\$ Let me also add that a permanent magnet based motor is effectively also a generator if energy is input into the system by spinning the motor shaft. I don't think that's what you're dealing with here (your mention of 3 phase leads me to think we're talking about AC induction motors), but it's something to keep in mind. \$\endgroup\$
    – mindcrime
    Commented Jun 28, 2022 at 18:28

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