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difference between V/f and vector control for induction motor

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#1 himadri.bdas


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Posted 15 October 2003 - 10:40 AM

Can any body tell me the basic difference between v/f control strategy and vector control for induction motor .which is better for fluctuating load like traction application.

#2 grobert


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Posted 17 October 2003 - 09:06 PM

Its been awhile but I'll try. V/F is variable frequency . All ac drives vary the frequency to control the speed of an ac induction type motor. They internally use math to decide when the electronics should fire the signal to the IGBT telling it to produce a PWM output. A VFD may use vectorial summation to internally determine what and when a motor needs to accomplish the task at hand.

As far as loads, most drives are offered as variable or constant torque or both. You have the former and should select a drive as such. Hope this helps.

#3 jraef


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Posted 19 November 2003 - 07:06 AM

Nothing wrong with the above reply, but there is a little more to the difference.

A "standard" PWM drive is, as you said, a V/F drive, but I like to use the term "Scalar" drive since the output V/F pattern is always scaled the same. Basically a vector drive is also a V/F drive. The difference, as mentioned above, is in the added vector calculations going on. It is best demonstrated by example.

A 460V scalar drive is told to run a 460V 60Hz motor at 1/2 speed. The output V/F pattern is always at the same scale, 7.67 Volts per Hertz (derived from 460 / 60) so the output voltage is 230V. This is fine, until we add the load to the motor. Now the motor is dragged down to 40% speed, even though the VFD output says to turn at 30Hz. The increased slip at that speed attempts to move it back to the desired speed by increasing current draw, but at the fixed V/F pattern there may not be enough response.

Enter the Vector Drive. It "knows" that the motor response to the applied load is not the same as the commanded speed. A "Closed Loop" vector drive does so by looking at a rotor shaft position model from an encoder input on the motor. An "Open Loop" vector drive does so by looking at the motor current reponse and compring that to a previoulsy created mathematical model of the motor response curves. In either case, the VFD microprocessor then calculates the proper vector of Volts and Hertz to alter the torque outout response of the motor to achieve the desired 50% speed. Both can do this at accuracies of + or - 0.01HZ or better! The difference is in what they can do at Zero Speed. An Open Loop vector has difficulty modeling the current response to a locked rotor since it has nothing to compare to yet. A Closed Loop drive knows right away what the motor needs because ithe shaft encoder tells it what is happening immediately. This last point is debated frequently amonst VFD manufacturers because some Open Loop manufacturers want to make us believe that they can do it without the encoder. I leave that up to the salesmen.
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