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Induction Motor - Rotor and Iron Lamina

Source: learnenginering.com

Induction Motor - Current, Magnetic Field, Lorentz's Force

Source: learnenginering.com

Induction Motor - Cutaway

Source: Wikipedia.org

Induction Motor - Stator and Rotor of a fan

Source: Own photo.

Here are some parts of an asynchronous induction motor aka squirrel cage type rotor. As clearly shown by the pictures, the squirrel cage is firmly mounted to the shaft, or vice versa depend on how we see it. As we know that in an induction motor, the powered coils in the stator will induce current in the squirrel cage of the rotor, then the induced current will generate a magnetic field and the Lorentz's force makes the rotor turn. As current is now flowing in the squirrel cage, the shaft should be electrified (there is current too.)

If so, then why does it not cause an electric shock when we touch it?

The most frequently I touched is water pump and fan, which at least through the bearing, the shaft is connected to the cover/body.

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    \$\begingroup\$ You feel an electric shock when current flows through a part of your body - your body becomes part of the circuit. How would touching the shaft of the motor make you a part of the circuit? \$\endgroup\$
    – brhans
    Commented Nov 2, 2020 at 4:02
  • \$\begingroup\$ Then, why the current not follow to my body, just like when we touch other electric equipment, or electric line? \$\endgroup\$
    – Sitorus
    Commented Nov 2, 2020 at 4:08
  • \$\begingroup\$ The shaft is far from the airgap. The squirrel cage is what is near the airgap and the shaft is not electrically connected to it. (I think). \$\endgroup\$
    – DKNguyen
    Commented Nov 2, 2020 at 4:09
  • 2
    \$\begingroup\$ Sometimes current does flow through the shaft. I am not sure if it is leakage from the stator (somehow) or induced current. \$\endgroup\$
    – user57037
    Commented Nov 2, 2020 at 5:48
  • 1
    \$\begingroup\$ @mkeith, you are correct. On a VFD information course I took I learned that this is a particular problem with VFD harmonics and long cables to the motors. The combination causes high frequency circulation currents along the shaft and the bearings arc and can get chewed up in a week. The solutions are (1) Shorter cables. (2) Better cables - screened, but I can't remember the details. (3) Filters. (4) Isolated bearing at one end of the motor. \$\endgroup\$
    – Transistor
    Commented Nov 2, 2020 at 9:30

4 Answers 4

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If so, then why does it not cause an electric shock when we touch it?

An induction motor is basically a rotating transformer and transformers provide primary to secondary galvanic isolation. In addition, the secondary (the rotor) is shorted out and hence only produces a volt or so of induced voltage (even if you could measure it). But you can't really measure it without getting inside the machine. Finally, for most induction motors (not all I might add), the case is earthed and that earths the rotor.

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  • \$\begingroup\$ The secondary (the rotor) is shorted out and hence only produces a volt or so of induced voltage (even if you could measure it). This is the strange for me. There is voltage but will not make any electric shock. I appreciate if any further explanation regarding this. \$\endgroup\$
    – Sitorus
    Commented Nov 5, 2020 at 13:09
  • \$\begingroup\$ I am unsure what it is about that statement that is strange to you (I can't read your mind). 1 volt or so is not regarded as a shock hazard. If you want to know why only 1 volt is produced then look up squirrel cage induction motors. \$\endgroup\$
    – Andy aka
    Commented Nov 5, 2020 at 13:13
  • \$\begingroup\$ One more thing, as we may see in the picture 3 (the cutaway), the rotor is actually connected though the shaft - to the bearing - to the body - to the stator as the stator is mounted to the body. So, it is clearly like transformer. Unfortunately, as far as I could understand especially form the source of the picture 1 and picture 2 learningengineering.com, the induction is not through the bearing. Quite bizarre. \$\endgroup\$
    – Sitorus
    Commented Nov 5, 2020 at 13:13
  • \$\begingroup\$ Why should it be through the bearing. The magnetic fields are targeted to the rotor. I can't account for someone else's diagrams being misleading and also note that this site is not a forum; it's a Q and A site. You make a Q and I make an A. \$\endgroup\$
    – Andy aka
    Commented Nov 5, 2020 at 13:15
  • \$\begingroup\$ It is not a small voltage or current as it is produce a big force (Lorentz's force). For the motor I put above, yes, it is small. But the same method applied to a very big motor. The bigger the motor, the bigger the voltage, which it should make electric shock, or probably electrocute. \$\endgroup\$
    – Sitorus
    Commented Nov 5, 2020 at 13:16
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From depiction in picture 2 above, we can see that the bars are ended at both side with caps made of metal which a very good conductor. Hence, the bars and the caps make a closed loop with an almost neglectable resistance. So, it is shorted at both ends, there is a closed path for the flow of the current. The current will be swirling at that closed path bars-caps. The more detail is here.

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  • \$\begingroup\$ This answer (including the video link) have no relevance to your question \$\endgroup\$
    – vu2nan
    Commented Nov 5, 2020 at 19:44
  • \$\begingroup\$ Key answer is: SHORTED. Means, short circuit. Current will not flow to another if there is a short circuit. You can find it in 3:18 here. \$\endgroup\$
    – Sitorus
    Commented Nov 6, 2020 at 1:10
  • \$\begingroup\$ I repeat, that's a given. That's the reason why I likened the motor to a transformer, with the stator being equivalent to the primary and the rotor the secondary. \$\endgroup\$
    – vu2nan
    Commented Nov 6, 2020 at 2:51
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Because it's a closed circuit, and it has no potential in regards to ground.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I think the rotor is not like that. It is like antenna, which that at a time, the current flow to one side (end) and 20ms later (in the case of 50Hz) it flow to another side (end). Touching the end of the shaft should make path to the ground. And the shaft is connected to the body through the bearing. But however, that what we need to know. \$\endgroup\$
    – Sitorus
    Commented Nov 2, 2020 at 10:58
  • \$\begingroup\$ Well yes, the rotor is connected to ground. And that's the whole magic. It's grounded. You touch the same ground with your hand and with your feet. And if you don't ground the rotor, it's not much different. You touch the axle with your hand and no other part of that circuit. \$\endgroup\$
    – Janka
    Commented Nov 2, 2020 at 11:11
  • \$\begingroup\$ From the picture two above, the bars are ended with a metal which is called caps. The caps act as short circuit to create a closed loop between the bars and the caps. The more detail is available [here] (youtu.be/jNWlWzFzHi4). \$\endgroup\$
    – Sitorus
    Commented Nov 5, 2020 at 15:13
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A one word answer would be 'Isolation'.

Isolation is a means of coupling one electrical circuit to another, without making a direct wire connection between the two.

Electrocution occurs when the body comes in contact with a live wire and ground, with the neutral wire being earthed.

An isolation transformer is an example of two electrical circuits being coupled without a direct wire connection. Electrocution would not be caused by contact with either secondary terminal of the isolation transformer and ground, with neither terminal being earthed.

Likewise, it would be safe to make contact with the shaft/body of an induction motor, with its stator and rotor being isolated and the rotor earthed through its body.

However, electrocution due to insulation failure cannot be ruled out in the absence of earth-fault tripping mechanisms.

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  • \$\begingroup\$ An isolation transformer is an example of two electrical circuits being coupled without a direct wire connection. If I have transformer ratio 1:1, it just to isolate from 220VAC to 220VAC in the secondary side, so it will not cause electrocution if I touch the output cable of the transformer? Indeed, I have never try for sure. But however, we know there are voltage and current there. \$\endgroup\$
    – Sitorus
    Commented Nov 2, 2020 at 10:52
  • \$\begingroup\$ Certainly not, when you touch both the secondary terminals! \$\endgroup\$
    – vu2nan
    Commented Nov 2, 2020 at 10:57
  • \$\begingroup\$ I have to test this. If it is true, then your answer I will accept. \$\endgroup\$
    – Sitorus
    Commented Nov 2, 2020 at 10:59
  • \$\begingroup\$ Caution! Take care to avoid contact between the 2 secondary terminals. You'll also need to ensure that the transformer body / the earth terminal provided is connected to a good earth ground. \$\endgroup\$
    – vu2nan
    Commented Nov 2, 2020 at 11:09
  • \$\begingroup\$ From the explanation in here, it is not the answer. The answer is because the the bars are ended with caps which acts as short circuit to make the bars and the caps a closed loop. The detail can be found here. \$\endgroup\$
    – Sitorus
    Commented Nov 5, 2020 at 15:09

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