Current Sensing of AC motor.

FvM said:

Reference voltage can be 5V or whatever the ADC needs. I suggested filtering the rectifier output, so the voltage won't be necessarily strongly pulsating. You'll adjust the secondary load to get suitable ADC input voltage at full motor current.

Click to expand...

What I am using this is to sense the current of 1 hp motor, so that I can actuate another motor depending on this current.

Current drawing of the motor is 5 A, At starting it would be approx 15 A, Let's take it 20 A. Working of the Motor would be at 5. I need to start another small motor between 3.8 A to 4.5 A... So I need to sense the current of the main motor, which is actually varying. Does it mean at secondary of CT voltage would also be varying.??

I have selected 1000/25 A CT. Turns 5:200. Wire gauge:: SWG 17: SWG 40.
I have calculated taking 15 A starting current of motor. So at secondary it would be 15*25/1000 = 0.5 A.
At working condition if 3.8 A current would be there. so at secondary it would be 3.8*25/1000 = 0.095 A i.e. 95 mA
At working condition if 4.5 A current would be there. so at secondary it would be 4.5*25/1000 = 0.1125 A i.e. 112.5 mA

As this current is less what would be output voltage at secondary? which would be converted to DC using rectifier and then would be applied to ADC which can be given to uC (MicroController AT89C51). So uC will actuate another motor depending on the voltage at secondary.

The circuit which I have understood is

CT primary is connected in parallel to main current carrying wire of motor.
CT secondary is connected to full wave bridge rectifier with RC filter parallel to it.
Rectifier's plus is given to ADC Vin+ and GND to ADC Vin-.
What should be at ADC Vref? This is also a doubt for me.

Another Question is if voltage at secondary of CT is varying then won't it be prob to apply fluctuating Vref at ADC?

I may be wrong at any place in my all explanation, If I am wrong rectify it and let me know.
The support you are showing is applicable.

Regards...

Can anyone suggest anything on this

ankitbajaj01 said:

Current drawing of the motor is 5 A, At starting it would be approx 15 A, Let's take it 20 A. Working of the Motor would be at 5. I need to start another small motor between 3.8 A to 4.5 A... So I need to sense the current of the main motor, which is actually varying. Does it mean at secondary of CT voltage would also be varying.??

I have selected 1000/25 A CT. Turns 5:200. Wire gauge:: SWG 17: SWG 40.
I have calculated taking 15 A starting current of motor. So at secondary it would be 15*25/1000 = 0.5 A.
At working condition if 3.8 A current would be there. so at secondary it would be 3.8*25/1000 = 0.095 A i.e. 95 mA
At working condition if 4.5 A current would be there. so at secondary it would be 4.5*25/1000 = 0.1125 A i.e. 112.5 mA

As this current is less what would be output voltage at secondary? which would be converted to DC using rectifier and then would be applied to ADC which can be given to uC (MicroController AT89C51). So uC will actuate another motor depending on the voltage at secondary.

The circuit which I have understood is

CT primary is connected in parallel to main current carrying wire of motor.
CT secondary is connected to full wave bridge rectifier with RC filter parallel to it.
Rectifier's plus is given to ADC Vin+ and GND to ADC Vin-.
What should be at ADC Vref? This is also a doubt for me.

Another Question is if voltage at secondary of CT is varying then won't it be prob to apply fluctuating Vref at ADC?

Click to expand...

Yeah your secondary voltage will also varying... first of all how did you connect the CT output to your uc?
You must rectify it and filter it before giving controller and after that you shd have a gain controlled amplifier which can be used to convert your .5A to 5V..
After that everything will come under controller programming...

So you must do the signal conditioning things well.. Your calculation of CT output may not be real so do it experimentally and find the real values...