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Current Sensing of AC motor.

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ankitbajaj01

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Hello,

I want to sense the motor load. what I want to do is I want to start and stop feeding depending on the load(Current) taken by motor.

For this I have searched various things and found the I to V converter an easy and chipper option to implement this.

Now issue is I am not able get where to start it.

What I know is I need to use Current Transformer from there Rectifier and from there OPAmp.

Can any one help me out in this.

You can even suggest the better option if any.

Motor is 1HP, 5 Amp, 230v, 50Hz.

Current which I need to sense is 3.4 A and 4.5 A. Also 7 A for overload stop.

Kindly help me out.
 

use your own current transformer with a U nail with number of turns (It will work sure)...

another option is putting a small resistor in series with motor...

next section is pre amplifier precision rectifier and so on....
 

kam1787 said:
What is a .................U nail ?! A heavy duty staple - like a fence staple?
Click to expand...

Exactly... with thin copper conductor about 1000 turns....

the main wire can go one rotation on the nail current transformer setup..

A mini current transformer ready...
 
Last edited:

First of all thanks for such a quick response.

I also need to control it with the AT89C52. so I need to give signal to it to inform uC(micro-controller) that the specified current has reached and so that uC can operate other things.

Is it possible to do so without Current Transformer.? Do I need to convert this current into DC?

Regards...
 

Micro controller can sense signals between 3 to 5V, and a current of 30ma ( i am not sure of the specification for AT89c52.check the datasheet for the exact values). so design a circuit to drop the current from the current transformer within these limits.....

if u are using a 100a/30ma CT, then at the secoundary it has a voltage of 30v and current of 30ma. if u give it directly to the controller , the controller will not work.

u can directly give AC and use the A/D conversion module in the controller if it has one. else you need to convert into DC.
 

Thanks Charu...

What I think is if I convert in to DC it cost much less. But ADC itself costs around Rs. 100. So I guess converting in DC would be good.

Another question I have is if I would be having the 230v, 4A current. Let Output of CT be 40 mA still AC... But what about voltage. Do I need to directly connect it to Full wave bridge rectifier?

And are we using voltage of this rectifier to give signal to uC(Microcontroller).?

Regards...
 

If you give it to a fullwave bridge rectifier you will get nothing... because the Turn ON voltage for diode is 0.7... first of all you have to amplify it and rectify with a opamp based precision rectifier... you want to give this into MU but for what???
 

hi

check the specification of the CT. the voltage will be specified in it

and ya rectify that voltage and by using some filter capaciters you ll get a Dc voltage

i would still recommend to use a A/D convertor module in the PIC because even by using a bridge rectifier there will be some ac riples.
 

venkadeshm said:
If you give it to a full wave bridge rectifier you will get nothing... because the Turn ON voltage for diode is 0.7... first of all you have to amplify it and rectify with a opamp based precision rectifier... you want to give this into MU but for what???
Click to expand...

I need to give signal to another motor to actuate it, depending on the ampere taken by main motor.

So how to utilize the output of the CT... Can you please elaborate it in detail... Because what I am not having much knowledge of how to apply it...

Regards...

- - - Updated - - -

charu2539211 said:
hi

check the specification of the CT. the voltage will be specified in it

and ya rectify that voltage and by using some filter capaciters you ll get a Dc voltage

i would still recommend to use a A/D convertor module in the PIC because even by using a bridge rectifier there will be some ac riples.
Click to expand...

Thanks once again Charu...

What I am doing for CT is I am giving turn ratio Np/Ns to supplier and he is supplying the CT of specified turn ratio... So kindly you elaborate about how exactly utilizing output of CT. I have taken 5/50 CT.

And how to utilizing output of CT...That also I wanna know...

Regards...
 

May anyone reply on this...
Click to expand...
To motivate others to answer your question, you should use a more intelligent way, not so obviously violating forum rules...
I think the forum rule gives a pretty strong hint how to do it better.

Thread bumping is not allowed
Don't bump your thread with useless followups like “please help” or “no one” etc. and don’t be in a hurry to get a reply, be patient. If you have any additional info to provide (like results gathered after the posted question or additional info) then you are welcome to post them in order to explain better your problem.
Click to expand...

I have seen a huge load of CT-to-µC interface circuits at edaboard. Searching a bit should help.
 

ankitbajaj01 said:
I need to give signal to another motor to actuate it, depending on the ampere taken by main motor.

So how to utilize the output of the CT... Can you please elaborate it in detail... Because what I am not having much knowledge of how to apply it...

Regards...
Click to expand...

If you are giving the rectified input to the micro controller without ADC means You will threshold (comparator) it with outside hardware, Is that OK..

for example you set a external comparator for 3V means if the voltage from the sensor is more than 3 you will get a high input and if not you will get low input.. If you want change the threshold value from 3 to 4 means you have to change it in hardware you will be unable to do it by software... so its better use an adc or choose a controller with internal ADC...

If you need not to change the threshold then go with a comparator......
 

In contrast to previous comments, I'm sure that a passive bridge rectifier, before the secondary shunt resistor, will work with a lot of industry standard current transformers. You get a certain non-linearity due to the diode voltage drop, but it can be easily compensated when chosing the analog threshold values. You have to review the CT datasheet for possible output voltage, or calcuate it if you made it yourself.

So the circuit would be comprised of
CT, secondary winding to bridge rectifier AC in, rectifier output to RC parallel circuit (shunt + filter).
CT secondary floating, rectifier minus grounded. Rectifier/RC plus to µC ADC input, through series resistor to protect against input current overload.
 

FvM said:
In contrast to previous comments, I'm sure that a passive bridge rectifier, before the secondary shunt resistor, will work with a lot of industry standard current transformers. You get a certain non-linearity due to the diode voltage drop, but it can be easily compensated when chosing the analog threshold values. You have to review the CT datasheet for possible output voltage, or calcuate it if you made it yourself.

So the circuit would be comprised of
CT, secondary winding to bridge rectifier AC in, rectifier output to RC parallel circuit (shunt + filter).
CT secondary floating, rectifier minus grounded. Rectifier/RC plus to µC ADC input, through series resistor to protect against input current overload.
Click to expand...

When we Interface Rectifier plus to ADC then, as it comes from the CT won't it be fluctuating DC voltage. So how to set ref voltage for ADC. Means from 0to?? V it would be 0to5 V. So what would be input Ref voltage which would be converted to the Voltage. As I am connecting Rectifier from CT whose current would be fluctuating as current at CT's primary side is also fluctuating... So my question is how to set input ref and of what value if the current is varying...
 

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.
 

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