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What methods are there for continuously monitoring the current in a 3-phase servo motor for a CNC machine and giving an alarm when the current is less than some value?

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You need a current transformer (or three).

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If you connect a resistor to the transformer's secondary winding a current through the sensed wire will cause a voltage across the resistor. The voltage is proportional to the current so will be a sine wave.

If you'd rather like a DC output proportional to the current you can use an i-Snail-V current transformer. It's self-powered and the i-Snail-V10 version for instance gives you 5 V out for 10 A. Other versions with other sensitivities exist.

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  • \$\begingroup\$ The i-snail-V that @stevenvh suggested looks like a good option. If you use a plain current transformer (CT), make sure that its output is never open circuited since dangerous voltages can result on the open terminals (to the CT and/or user). If the CT is unloaded/open, then it should always be shorted. Special shorting terminal blocks are typically used to connect to a CT to allow a meter to be removed/changed. see: metersusa.com/Transformers/Data/ShortBK.htm \$\endgroup\$ – madrivereric Jun 18 '12 at 2:35
  • \$\begingroup\$ @madrivereric - I think the i-Snail-V is a great product, but it's not cheap. Since OP talks about CNC this is probably not a hobby project, and maybe he can afford it :-) \$\endgroup\$ – stevenvh Jun 18 '12 at 12:18
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A variety of current sense techniques exist.

Hall effect sensors are nice for measuring large currents through a wire at high voltages. Some have analog out, eating up one of the analog inputs on your microcontroller. Others have an integrated internal ADC, with digital pins that directly connect to your microcontroller. A few also have an integrated power FET driver, and are smart enough to unconditionally turn off the FET when it measures over-current. Perhaps you could somehow use that output to drive an alarm instead of a FET.

The Allegro Hall effect sensor chips look nice.

Related: How can I sense the motor's current?

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