# Voltage sampling from 220 V AC to 5 V DC

I made a 1 phase to 3 phase inverter using an Arduino and an IR2104 to control the speed of an induction motor and it is working perfectly.

To complete the protection scheme, I need to monitor the voltage of each phase to protect the motor if one phase is lost due to a malfunction of an IGBT or something else.

In addition, I need to monitor current for overcurrent protection.

In case of voltage I know that it is possible to use an optocoupler to convert high-level voltage to logic level 5 V DC. Is there any better and simpler practical method to monitor voltage and current of each phase for such an application?

• It depends. Post a schematic for what you've got. Commented Jan 6, 2019 at 17:36
• Monitor ground current imbalance from current sensor to neutral, rectified and averaged for each cycle and compensate balance error next PWM cycle for V on A,B,C if necessary and if sustained more than x Cycles , check voltage , disable all , and check again. Then report either low Vdc, low AC voltage or imbalance error or excess current or excess current imbalance error or driver fault whichever is root cause. Is this delta or WYE? Commented Jan 6, 2019 at 17:42
• @Transistor, schematic was added for one phase. Commented Jan 6, 2019 at 20:00
• @Tony, the connection is delta, can you please explain more about your described method, or provide me web resources to read more? Commented Jan 6, 2019 at 20:03
• I only speak from experience, not web sources but I am sure you can find some. I'll try to sketch something Commented Jan 6, 2019 at 20:04

Since you are already using an Arduino, your best bet would be to use a Current Transformer that will sense the current being delivered to your motors. This is a safe, and non-invasive way to monitor the Vac that you are consuming. It usually looks kind of like this.

It's not the cheapest way, but those things don't really cost that much in the first place.

• A CT is a valid solution, but measures current rather than voltage. If you know that the power factor is good and that a load will always be present then it’s fine, otherwise it may be better to use a single phase transformer between each phase and neutral, then measure the voltage on the secondary.
– Frog
Commented Jun 25, 2021 at 21:30

simulate this circuit – Schematic created using CircuitLab