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Variable Frequency Drives

How does Vector Control compare to Servo Control? by jraef
Posted: 9 Dec 04

The following exerpts from Thread237-94601 can be helpfull in understanding how a Vector VFD can be compared to a Servo drive. Actual comments came from the members shown, chiefly sried, one of this Forum's frequent MVPs. To fully grasp this concept, please read the other FAQ titled "What is Vector Control in a VFD?"

sreid (Electrical)      May 14, 2004
The previous explanations are very good but slightly miss the fundamental problem that vector drives solve.  As stated in previous posts, to have a high quality servo drive one needs to know the magnitude and position of the flux vector on the rotor.  In, say, permanent magnet AC servos, the rotor flux magnitude is fixed and the flux vector is known.  In an induction motor, the rotor currents (and the rotor flux vector due to the rotor currents) are created by the stator currents and the rotor slip.

A vector drive uses a mathematical model of the motor to calculate and control the rotor flux position and magnitude based on stator currents and (often) motor shaft position.

jraef (Electrical)      May 14, 2004
So if I read you correctly, Vector drives actually out perform servos? Or are you just clarifying how they are capable of matching them? I am contemplating writing an FAQ for this forum on this (since I see it come up quite often) and I would like to include your comment as part of it. Maybe it deserves it's own FAQ though since servo applications are a horse of a different color compared to the majority of industrial Vector Drive applications.

sreid (Electrical)      May 14, 2004
I think at the practical limit, a vector drive would not have the performance of a permanent magnet AC sero because the flux vector for the induction motor is only an estimate.  One of the biggest problems is that the rotor resistance increases with temperature which changes the L/R time constant for the rotor.  This, of course, changes the motor model with temperature.

AC brushless servo motors become too expensive at higher horsepowers due to magnet costs;  Rare Earth magnets cost more than iron and copper per pound and the weight goes up by the cube of the dimensions.  The rotors of induction motors are, of course, iron and copper(or perhaps aluminum).  Induction motors are also rugged, brushless and mass produced.

There has always been a desire to get better, say, speed control of induction motors beyond what VF control can provide;  one way was slip compensation.  Also, it's handy to be able to control down to zero speed with torque available.  Vector Control allows these things to be done and be done now at affordable prices.

The cost today to build a vector drive is no more than any other drive.  The primary additive cost is the Engineering software development cost.  Vector Drives allow really good drive performance if you need it but things can be detuned in less demanding situations.  Most drives will allow one to run VF open loop if that is all that is required.

skogsgurra (Electrical)      May 15, 2004
Re: DC servo AC servo PM servo

I have tested an ordinary 0,75 kW AC induction motor together with an 1,5 kW NFO Sinus inverter and got a quite good speed loop performance. A 20 % step in speed reference gave me 20 % speed increase in 6 milliseconds. That is about as good as a DC servo used to be back in 1990. Siemens Masterdrive and an AC induction motor is also pretty good as a low end servo. I think that most vector drives perform well in this respect. But they cannot beat a PM servo motor. If you need sub-ms torque and ms speed response then you need PM servo (where the drive system is very similar to an ordinary PWM vector inverter).

sreid (Electrical)      May 16, 2004
The machine tool industry uses Vector Drives for the spindle in Milling Machines because you can do "Ridgid Tapping."  This is where the spindle is servoed in coordination with vertical motion of the part to be tapped.  Before vector drives, tapping was done as a secondary operation or special tap holders were use that had vertical compliance.

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