Hi,what is the difference between vector drive and ordinary VFD? thanks

-- Raja (rs41203@yahoo.com.sg), January 06, 2005

Answers

Hi,

I think you might know about a Variable frequency drive. I will explain. In order to get a precise control over the speed of an elevator an induction/PM machine could be easily controlled by means of changing the frequency and voltage simultaneously applied to the armature. This is achieved by means of synthesizing a sine wave by chopping the fundamental waveform embedding pulse width modulation technique. This VFD is otherwise called as VVVF drive in the market.

Now VFD is classified into two form of control. a.) Scalar control b.) Vector control

In scalar control only the frequency and voltage is changed keeping all the other machine parameters unchanged. So the main disadvantage of a scalar controlled VFD is the speed is not controlled dynamically as equivalent as a DC machine.

In vector control the rotor flux are sensed by means of flux transducers or current transformers and calculated by implementing park and clark transformations (dq transformation) and make the AC machine dynamically equivalent to a DC machine so that the torque and speed are controlled brilliantly.

Finally all VFD's cannot become vector drive but a vector drive is a VFD.

Regards

-- javeed (javeed_skh@yahoo.com), February 21, 2005.

I would like to here a little more on this topic. What's the differance between open and closed loop in referance to a VVVF Drive? Thanks

-- Donald J. Pfuntner (dgelev3@frontiernet.net), February 21, 2005.

Hi,

Open and close loop control could be applied to scalar control. For example consider a water pump application and it runs in a particular speed and the speed needs to be changed not precisely but in steps. here open loop is sufficient and the change in speed could be varied manually by means of man-machine interface. Only K=v/f needs to be changed.

In vector controlled VVVF drive surely it should be closed loop since the parameters of rotor position and flux needs to be determined by the software algorithm. Also stator(armature) current needs to be determined. Hence the phase angle between the stator current and the rotor flux (d-q)axis would be extracted by means of an encoder and CT's and put into the processor to acheive the vector control.

Need not be induction motors it could be a permanat magnet synchronous motors in vector control. But a PM machine could not be controlled in an open loop way because the magnet positions should be determined by means of an encoder or hall effect transducer in order to control the initial torque(90deg)

regards

-- javeed (javeed_skh@yahoo.com), February 22, 2005.