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Part of a project involves knowing if an AC motor is running or not, with that bit of information read by an Arduino. I understand there are several different paths I could take to detect this.

The motor is controlled by relays with a 24V coil, so I could use a voltage divider of resistors across the coil of the relay. Or I could put a small-value current-sense resistor in series with the relay coil.

However, this question is about another idea:

The motor is a 120VAC (US mains) device. Could I wrap a couple of turns of small-gauge wire around one of the wires delivering mains power to the motor, rectify and smooth it and detect the resulting voltage using an opto-isolator?

The schematic below shows an inductor, but that is really the couple of turns around the wire.

schematic

simulate this circuit – Schematic created using CircuitLab

Is this idea even viable for detecting (not necessarily measuring) AC power/current?

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Could I wrap a couple of turns of small-gauge wire around one of the wires delivering mains power to the motor, rectify and smooth it and detect the resulting voltage using an opto-isolator?

You are mistaken on how you think a magnetic field is produced by a wire and how to detect it with a coil. Basically, a wire produces a magnetic field like so: -

enter image description here

Now if you wrap wires around the current carrying conductor your wire loops "cut" no net magnetic field lines and no voltage will be induced. Field lines need to "cut" through the plane of each wound turn as per below: -

enter image description here

If you rotated the above coil through 90 degrees (moving the coil from the X=0 plane to the Z=0 plane), field lines no longer cut the coil and no voltage is induced. Ditto if you rotated the coil from the X=0 plane to the Y=0 plane.

So, with wires wrapped round the conductor you get no cutting-action. Unfortunately, the symbol for a current transformer does show it (incorrectly I might add) as wires wrapped around a central conductor but, in reality, the wires are wound in such a way as to run parallel with the current carrying conductor like so: -

enter image description here

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You would need a current transformer, rather than simply making a few turns around the wire.

A current transformer consists of a ferrite ring with the secondary wound on it. The primary wire, carrying the current to be sensed, is passed through the ring. The seconodary winding must be terminated with a "burden resistor", and you measure the voltage across that resistor. If you do a web search for "current transformer" you should find lots of information and illustrations.

Sensing the coil voltage on the controlling relay just tells you that the motor should be running. The motor may not be running if the AC has failed, or the relay has failed, although power is available to the coil.

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  • \$\begingroup\$ Can you describe (help me learn) what a current transformer does which makes it work, and what would a few turns around the wire give me? \$\endgroup\$ – jose can u c Jan 16 '17 at 21:13
  • \$\begingroup\$ I wouldn't do a current transformer. The open-circuit voltage on those is astounding. A relay-state answer is wrap the supply wire around a reed switch. \$\endgroup\$ – Harper Jan 16 '17 at 21:58
  • \$\begingroup\$ @Harper: As I said (but didn't emphasize) current tranformers MUST be terminated with a burden resistor, which will keep the voltage down to a reasonable level. When properly connected, they are not hazardous. \$\endgroup\$ – Peter Bennett Jan 16 '17 at 22:42
  • \$\begingroup\$ There are plenty of CT's that have the burden included or have protection on the outputs. Search for devices like the "SCT-013-000" they are made by YHDC. You might read this to help you understand: openenergymonitor.org/emon/buildingblocks/… \$\endgroup\$ – Jack Creasey Jan 17 '17 at 1:00
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If you don't need an exact measure of the magnetic field from the current use a hall effect sensor. DC or AC fields can be measure and the sensor is smaller. The current in the wire can even be measured. Here is an example

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