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Current Limit


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#1 jigtic

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Posted 14 March 2011 - 06:42 PM

Hello Friends,
I am new to this industry. correct me if i am wrong.
I have a project where i ahve to Limit the output current by changing Output voltage in constant torque mode.
Unfortunately i am having problem to change the output voltage(Ristricted Code). I made code by changing freq. but is not acceptable.
Can some one tell me other ways, how i can control the Current? Can some one recommend me some books or good study links for Motors and Drive controls?

Any help is appriciated.

Thanks

#2 jOmega

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Posted 15 March 2011 - 05:50 PM

QUOTE (jigtic @ Mar 14 2011, 01:42 PM) <{POST_SNAPBACK}>
Hello Friends,
I am new to this industry. correct me if i am wrong.
I have a project where i ahve to Limit the output current by changing Output voltage in constant torque mode.
Unfortunately i am having problem to change the output voltage(Ristricted Code). I made code by changing freq. but is not acceptable.
Can some one tell me other ways, how i can control the Current? Can some one recommend me some books or good study links for Motors and Drive controls?

Any help is appriciated.

Thanks



Is this a "work" ...i.e., design project as in designing a controller ? or is this a "school/class" project ?

#3 jigtic

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Posted 15 March 2011 - 08:59 PM

This is my self study and i am trying to understand motor and drive stratagy very deeply as i want to go to this industry?
And just for my knowledge, Does it matter that it is work related or self study? I think this is learning forum for everyone. isn't it?
Please suggest other way to control output current than Voltage and freq.?



#4 jOmega

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Posted 18 March 2011 - 05:31 AM

QUOTE (jigtic @ Mar 15 2011, 02:59 PM) <{POST_SNAPBACK}>
This is my self study and i am trying to understand motor and drive stratagy very deeply as i want to go to this industry?
And just for my knowledge, Does it matter that it is work related or self study? I think this is learning forum for everyone. isn't it?
Please suggest other way to control output current than Voltage and freq.?


In answer to your question, it does make a difference in how people will respond to your questions.

There is a section of this forum that is set-aside for Students
Click on this link http://www.lmpforum....dents-f102.html or scroll down to the Students section.
Within the Student section there are two sub-sections:
    Electronic
and
    Electrical
...where students are encouraged to post their questions.
The Electronic sub-section is a Discussion area for students questions about electronics.
The Electrical sub-section is a Students discussion area for electrical topics.

Ok, 'nuff said about where to post ....

Help us to understand better what you are asking specifically ......
Are you asking how Current Limit is achieved on a Variable Speed Drive that is controlling a three-phase— asynchronous (induction)—motor ?
If that is so, then the answer is .... that both voltage and frequency are altered together so as to limit the current being supplied to the motor.

Your original post stated:
"I have a project where i ahve to Limit the output current by changing Output voltage in constant torque mode.
Unfortunately i am having problem to change the output voltage(Ristricted Code). I made code by changing freq. but is not acceptable."
Are you working with a specific piece of equipment ... ?
If so, could you please provide us with specific detail as to
  • what that equipment (drive / controller) is ?
  • who made it (manufacturer) ?
  • what the model number is ?
  • etc... ?

Variable Frequency Drives (inverter drives) will have a Current Limit Adjustment. Some drive controllers have a Torque Limit adjustment. DC Drives also have a Current Limit Adjustment.

Soft Starters operating at fixed frequency, limit the motor current by reducing the voltage supplied to the motor.

DC Drives limit the current by reducing the voltage applied to the motor.

Variable Frequency (inverter) Drives limit the current by reducing the voltage and frequency together that is supplied to the motor.

Consider the case of the Variable Frequency Drive. If you just reduce the voltage to the motor when the current becomes excessive, the rotor slip will increase... causing the motor to draw more current. If you simultaneously reduce the voltage and the frequency in the same proportion, you will reduce the slip... thereby reducing or limiting the current .....
Slip is defined as the difference between the applied synchronous speed as a function of applied frequency and the rotor speed.
see: http://www.engineeri...slip-d_652.html








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