Hello to all forum participants. I need to make a three-phase motor start and stop detector with the help of arduino. The motor is 7.5 kv, and with the help of contactors (which do switching) the star and then the delta mode are started. I connected one phase (L1) and N from the same installation to an AC/DC adapter that lowers the voltage to 5V, then I lowered it to 1.2V and connected it to an optocoupler. I connected the 3.3v from the Arduino to an optocoupler and back to the analog pin to detect the start and stop state of the motor. The problem is that the contactors (which do the switching) in the distribution cabinet are buzzing. As if there is an imbalance even though it is a small step down rectifier AC/DC adapter. Does anyone have any better ideas to avoid the problem I'm having. Is there maybe another better solution?
A descriptive connection is given in the attachment.
It depends what you really want to detect? If you want to detect if the motor is running then you could add a tachometer using a simple optical sensor or a Hall effect sensor. You also get to know how fast it is going. You seem to be trying to detect when the motor is powered. You could use a current sensor to do this, or a physical switch in parallel with the motor switch. You could probably even detect the magnetic effect of the motor itself.
The goal is to detect when the engine is running.
Current sensor is good options!
Will this sensor do the job and how do I get power to it from the 3 phase motor?
Is the input adjusted for a 7.5 kw motor
I’m not sure. You would need to look at the data sheet. You can also get a sensor that simply clamps around the wire
You can also get a sensor that simply clamps around the wire!
This is nice solution!
This sounds like a synchronous motor probably connect to an air compressor doing double duty as power factor correction. We had one like that in a foundry, worked very well, changed the exciting to control the power factor. If so check the control portion it may already be doing what you want and have the signal available. If not the current transformer (clamp around wire), split core would be the easiest to install. Be sure the ballast (load)resistor is connected, if not you can get a very nasty shock. The output of the current transformer will be AC not DC so you need to convert that so you can detect it with your arduino. Because of the 7.5 kv be extremely careful and be sure you are with somebody when working on this.
Thanks for your reply. It is an engine that drives a large air compressor. For some reason I can't directly take L1, and PE or N from the motor and connect it to the transformer and get 5VAC. Between each L and N or PE there should be 220V, what is the imbalance in the contactors?
The noise from the contactors will be from slightly loose winding in the operating coil
and nothing to do with the load or any imbalance .
Your best bet, as said by previous replies , is to use a current transformer on one phase , and process it’s output to give a DC signal .
If you just want to know the contactors is “in “ drive a separate relay from the contactors coil connection .
Only do this stuff if you are suitably qualified
Hi, @marestudio
If you remove the 240V step down completely from the circuit, so there is no monitoring, does the contactor still buzz?
Can you please tell us your electronics, programming, arduino, hardware experience?
How?
Why are you connecting to PE, earth, on the 240V/5V supply from neutral?
Can we please have a proper circuit diagram?
An image of a hand drawn schematic will be fine, include ALL power supplies, component names and pin/terminal labels.
Thanks... Tom... 
Do the contactors have auxiliary contacts available?
Problem is fixed!
As I said before, I took the phase from the connector on the motor (which was unmarked) then connected it to the step down AC/DC 230v to 5v rectifier as well as the N from the terminal box. It was guessed that it is the phase that is connected to the K2 or K3 relays that the motor uses to start in star mode (K3) and later in delta mode (K2). To make the story even more interesting, the phase I chose is also connected to a transformer (15 - 0 - -15) whose voltage is used as a control signal for the contactors (25v DC). That was the reason for the disturbance in the control voltage and the buzzing of the contactor! Then, instead of the phase from the motor, I took the phase after the main switch on relay K1 (from the output part), took a phase that is not the same as the transformer, connected to it the step down as well as the power supply of the microcontroller itself. That way I fixed the problem that occurred!
No!
I don't have much experience with Arduino, I've worked more in Microputhon and with ESP microcontrollers. The principle is almost the same, digital and analog inputs and outputs, some processing of recorded data... I have a feeling that programming in the Arduino IDE (C programming) results in much faster operation of the microcontroller...
The voltage is reduced from 5v to 1.2 by a resistor, for the reason that it later goes to the optocoupler, and the bulb in the optocoupler works at about 1.2v!
There is no connection between PE and N lines!
I'm in a huge rush right now, as soon as I get some free time I'll draw a complete schematic!
It is a simple task, when there is voltage in the phase, turn on the relay (heater via the relay), when there is no, turn off the heater and send an SMS to the user that the process is complete!
No!
Hi,
I would say they do, but are probably all used in the lockout system for the star/delta switching circuitry.
Tom....
Tom....
It's a drawing error, I say I'm in a rush for time! The line and color clearly show the N...
That's right, as can be seen on the K1 contactor!
Rush + 3ph = danger.
Don’t rush
Hi,
What are your electrical qualifications?
OH&S and industrial safety would have something to say about non qualified personnel connecting anything to a major 3phase industrial/commercial installation, let alone your insurance company.
Tom...
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