Hello, I'm making a project which will heat a three phase induction motor (don't ask why), but before heating i must know the resistance of coil. For measuring the resistance I'm using resistor divider method and an Arduino.

Simplified diagram is shown.
L1,L2 and L3 are three coils of motor out of which only one needs to be measured, rest are identical. For this, connection is made from the same point where the 3 wires of motor will be connected on the pcb board for heating. The problem will arise when i try to power the coils for heating because of the previously made connections to measure the resistance as it will change the delta configuration of the 3 phase winding. So to avoid this i have included switches (S1 S2) which are controlled by Arduino. This will break the connection between 5V and ground. But I'm confused between using opto-coupler or relay in place of those switches. turned on opto-coupler will have some resistance which will interfere which the resistance measurement of coil but have smaller size but on the other hand relays have almost zero resistance but are bigger in size. If anyone has any better option please let me know. Thanks.

 

If you use a relay you will only need one for both S1 and S2 - a DPST relay.

 

Relay is better, opto-coupler is only if you want isolated connections..

 

Use low Rdson MOSFETs or SSR, you can get logic level MOSFETs in
some pretty small packages.

What is typical phase coil Resistance that you are trying to measure, its magnitude ?


Regards, Dana.

 

Use low Rdson MOSFETs or SSR, you can get logic level MOSFETs in
some pretty small packages.

What is typical phase coil Resistance that you are trying to measure, its magnitude ?


Regards, Dana.

The resistance will range between 5 to 20 ohms.
Thank you,
I'm going with dpst relay, as it will be cheaper and only take about 10 to 15 sqmm of real estate more on pcb board compared to a MOSFET.