Variable Voltage VFDs and RPM Control

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Anonymous Poster	Variable Voltage VFDs and RPM Control 06/11/2009 1:24 PM
	IN VVVFD FOR RPM CONTROLLING WE CAN DO BY VARYING FREQUENCY... i.e BY THE FORMULA RPM=120XF/P..OK. BUT IF WE WANT TO CONTROL THE RPM BY VARYING VOLTAGE THAN WHAT IS THE FORMULA FOR RPM CONTROOLING BY VARYING VOLTAGE????? PLEASE GIVE ME THE ANS.
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JRaef Guru Join Date: Sep 2006 Location: California, USA, where the Godless live next door to God. Posts: 4665 Good Answers: 804	#1 Re: Variable Voltage VFDs and RPM Control 06/11/2009 6:44 PM
	Not enough information. But if you are referring to standard AC Squirrel Cage Induction Motors, you cannot vary the speed only by varying the voltage. Voltage will only cause a loss of torque, and torque varies by the square of the voltage reduction. So if the voltage is reduced to 80%, torque is reduced to 64%; 50% voltage nets 25% torque etc. etc. Now, an AC motor will slow down if the torque is insufficient to maintain the load rotation, but when that happens, the slip increases, causing the motor to pull more current and it over loads. By the way, if you ONLY vary the frequency, you also cannot do that without causing problems. That is why it is called VV VF; you must vary them TOGETHER. And to maintain the proper amount of torque, you must maintain the same V/F ratio as what the motor was originally designed for. __________________ All I every really wanted to be, was... A LUMBERJACK!.
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Anonymous Poster	#2 In reply to #1 Re: Variable Voltage VFDs and RPM Control 06/11/2009 9:55 PM
	Thankyou for providing me the information? but please tell me what more information need you ? is there any formula for both vv & vf for contrroling the rpm of motor???
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JRaef Guru Join Date: Sep 2006 Location: California, USA, where the Godless live next door to God. Posts: 4665 Good Answers: 804	#5 In reply to #2 Re: Variable Voltage VFDs and RPM Control 06/12/2009 11:44 AM
	The reason I said you did not provide enough information is that you forced me to guess that you were referring to an AC Squirrel Cage Induction Motor, as opposed to an AC Wound Rotor Induction Motor, or an AC Synchronous Motor, or an a Brushless DC Servo Motor etc. etc. etc. Others have answered the rest. If you don't yet grasp the concept, you need more help than you can attain here. __________________ All I every really wanted to be, was... A LUMBERJACK!.
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dbt Associate Join Date: Sep 2008 Posts: 31	#3 Re: Variable Voltage VFDs and RPM Control 06/12/2009 8:10 AM
	The motor turn by given formula. it means the frequency of AC current is the key thing. you can not control RPM by VV. Voltage is just a factor to be sure the motor torque is stable.
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sb Guru Join Date: Mar 2007 Posts: 2550 Good Answers: 103	#4 Re: Variable Voltage VFDs and RPM Control 06/12/2009 8:35 AM
	As the frequency is varied, the voltage is too varied to keep the V/f constant. This value V/f is proportional to the magnetic flux, and must be kept constant to avoid the oversaturation, as well as the under (which will obviously reduce the motor power) __________________ Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches
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Anonymous Poster (1); dbt (1); JRaef (2); sb (1)

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