Reply To: Motors and thermal protection


By the way- no, no “bias” on the mains. In the US, power is transmitted and distributed with 3 phases. By the time you reach the pole outside a residence, you are dealing with just one of those phases. It is brought into a transformer with proper winding ratios so that the total voltage across the side of the transformer feeding the residence is 240V. There is a tap in the middle of that side which is bonded to earth. You now have three conductors to drop to the residence one at 0V, one at +120V, and the other at -120V. At the residence, the conductor that was grounded at the pole is grounded again at the residence. You now have two ungrounded conductors and one grounded conductor in the panel. You can either grab the two ungrounded and have 240V or you can grab the grounded and one of the ungrounded and have either +120V or -120V relative to the grounded conductor. If you didn’t ground at the service entrance, you’d have a bias because of impedance between the pole and the residence and, if you look inside the residence some distance away from the panel, there will be a voltage drop across the neutral (outlet vs. panel) just because it has some resistance. So, again you’ll see a “bias”….a couple of volts relative to ground, but only when there is current on the neutral / grounded conductor. This isn’t DC, though and I don’t think I’ve ever heard anyone call it a bias. (Okay, the electricians can jump on me now and tell me where I butchered it 🙂

  • This reply was modified 11 months, 1 week ago by Ed.