Motor current simulation

[fubuaks]

Hi,
In my project i require to take the current reading of a 20hp motor. The maximum current of the motor is 30a, so i've bought a 30A/30mA current transformer.
But right now i m at testing stage so i need a way by which i can simulate the motor current, ie, a source that could provide a maximum of 30A of current.
Can anybody give me some idea about this source?

 

[kchriste]

You could wind 10 turns through the current transformer and supply 3A. That'll look like one turn of 30A to the CT.
Or simply leave the CT out of the circuit for now and supply 30ma of current directly from an isolated source.

 

[fubuaks]

yes dats true but how do i supply 3A ac current....?

 

[kchriste]

It depends on what equipment you have available to you. In my shop, we have a transconductance amplifier that can convert 0-20V to 0-20A AC at various frequencies. What frequency range do you need? What test equipment do you have available? Signal generators, even AF amplifiers, etc.
Surely you have something that can supply 30ma AC?

 

[fubuaks]

We have signal generators, variac etc in our college. Can 30ma be given through one of these?

 

[Ubergeek63]

Hi,
In my project i require to take the current reading of a 20hp motor. The maximum current of the motor is 30a, so i've bought a 30A/30mA current transformer.
But right now i m at testing stage so i need a way by which i can simulate the motor current, ie, a source that could provide a maximum of 30A of current.
Can anybody give me some idea about this source?

why are you using a single phase at 20HP?

 

[kchriste]

We have signal generators, variac etc in our college. Can 30ma be given through one of these?

The signal generators should be able to supply 30ma of AC current. Find one capable of driving a 50Ω load at 1.5 Vrms or better.
For more current, say 3A at your line frequency, you could use the variac and a transformer with a 3-5A low voltage secondary.

 

[fubuaks]

Thanx for ur replies....m connecting 3 bulbs of 200w,200w nd 40w in parallel and then i'l put 15 turns on the CT....so current will step down from 2A to 30mA. Is it correct way of doin.....?

 

[kchriste]

That'll work if your mains voltage is 220Vac. Just be careful when handling mains voltage.

 

[aljamri]

Or use current source if available, where you can vary your input current.

 

[tcmtech]

unless you are running that 20Hp motor off of a current limiting VFD unit expect that you may see 4 -5 times that 30 amp rating on the start up.
Three phase motors have high inrush currents while getting up to speed. It only last for few seconds but its more than enough time to possibly smoke components on a 30 amp current transformer or transducer.

 

[fubuaks]

thnx......i'l take into account that current inrush thing...where can i get a current source.....?

 

[fubuaks]

Now i want to measure this current, ie, hve to convert it to dc vtg and than give it to ADC. How do i do that. Will a simple bridge rectifier suffice.....?And wat about the diode losses....? Can i get a linear relation between input AC current and DC vtg...?

 

[kchriste]

Do you need a true RMS reading or will an RMS value estimated from the peak value work for you?

 

[aljamri]

...where can i get a current source.....?

it is a lab instrument like signal generators, else you can do a simple one. I've googled for current source circuit and here is the results:

http://www.google.com/search?hl=ar&q=%22current+source+circuit%22&btnG=%D8%A8%D8%AD%D8%AB%21&meta=&aq=f&oq=

 

[fubuaks]

@kchriste....the rms value extracted frm the peak value will do......how shud i do it.......m thinkin of using a full wave precision rectifier as i hve to read vtgs less than 0.7V....but m not able to build it using lm324.....Some Vref is the issue, but i cant make out the problem exactly.....Can nybdy pls provide me some help.......!

 

[kchriste]

Post your schematic and we should be able to figure it out.

 

[fubuaks]

This is the ckt that i am using.....As LM324 is a single supply OP amp, I've connected only positive supply, so i need to connect ny virtual grnd or something......?

 

[kchriste]

To use the LM324, you would need a negative supply connected to pin 11. If you want to use a single supply, some OpAmps can do what you want and the bonus is that you need no diodes:
**broken link removed**
The caveat is that you must use specific OpAmps for this to work.

 

[fubuaks]

Thanks this ckt is workin....but wat abt lm324, on its datasheet its written that it works on single power supply.....dats the USP of lm324....If -ve supply is mandatory than there's no diff between 741 and 324.....!