VSD

It is the otherway round. The V/f strategy is the basic one. (also called the scaler control)Then we have two types of vector controls

With sensors (encoders) The encoder based system optimizes by capturing the shaft speed and position feedback from an encoder. and

sensorless - try to optimise performance by mathematically determining the motor characteristics and operating parameters (electrical) and controlling VFD output accordingly

The vector controls are able to tune the output based on the exact motor characteristics which varies a bit (which may infact be significant) due to load and temperature variations. And this is not only to tune up the Voltage but the frequency too.

Not bad. But here's the way I like to teach it when I give seminars to non-engineers.

With a Scalar drive (V/Hz only), the VFD TELLS the motor what speed to run, but that's it. It has no idea if it is actually running that speed or not, it is making an ASSUMPTION. If something interferes with that, such as the issues raised above, it goes blissfully along with no idea that the motor is not performing as ASSumed. It's like telling your child what to do over the telephone without any clue if they are actually doing it.

A Vector drive on the other hand has some sort of feedback loop that looks at the motor's performance and allows the VFD to tweak it's output in order to maintain the desired effect, be it torque or speed (or both). So the VFD tells the motor what to do, then looks at the motor and if there is an error, makes adjustments to correct for it. So this is like telling your child what to do while you are standing there watching them and when they screw up, you correct them.

The FORM of that feedback loop is the difference between Sensorless and True Vector drives. True Vector drives use an absolute encoder feedback to judge motor performance, which is more accurate. Sensorless Vector drives have no external feedback device, but rely instead on a mathematical model of the motor by observing the current signature as the power is fed to it. Essentially if you can monitor, slice and dice the current waveforms with enough accuracy, you can essentially watch the rotor bars pass through the stator fields because of the effects they have on the way the stator coils react to them. So they use that to watch and keep track of the rotor position in lieu of an encoder (this is a HIGHLY non-technical explanation of a very complex process).

The only problem with Sensorless Vector is that unlike an encoder it is not absolute, which means that when the motor starts off after power is first applied, it does not know where the rotor is for a brief instant. Generally this isn't a problem, but on HOIST applications, it can be. If your hoist is holding a load with brakes and you release the brakes to lift or lower it, the motor torque MUST be there at that instant. In the fraction of a second a Sensorless Vector drive takes to determine and calculate the vector agnle, the load may run away and be impossible to stop again.

By the way, the Altivar 71 IS a Vector drive, it can be Sensorless Vector or True Vector with the addition of an Encoder Input card accessory. maybe someone programmed it to turn off the Sensorless Vector, I don't know, but that would be foolish given that it could be used and should be in that application.

Ahhh, you left out the detail that you had one drive and multiple motors. Yes, in that case you cannot use vector control of any kind, your only choice is Scalar control (V/Hz).

If your other system is one drive for multiple motors and they have it in Vector mode, you are destined for problems. The modeling algorithm cannot adjust for all the variables that exist in the power circuit for anything other than one motor and may go completely bonkers on you one day. I've seen it happen may times. it can be commissioned this way and run for a while, but as soon as ANYTHING changes in the circuit, the VFD loses it.

I think we're all agreed re the v/f and open loop vector ideas and explanations.

However, in my applications, closed loop vector drives use incremental encoder feedback, not absolute, when controlling asynchronous Induction Motors. When absolute is used, it's to gain positioning information rather than the motor control aspect.

Absolute feedback is needed when controlling synchronous motors in order to maintain field to pole position. There are, however, various means to still utilise incremental with updates (open or closed loop) to maintain good control.

In a hoist application, it's essential to develop torque before lifting the brake, this can be done sensorless (and commonly is) with the appropriate protection devices in place. It is easier and preferable to start at zero speed using feedback but many customers choose a lower cost solution. (but the question referred to travel drives on multi-motors)

The simple explanations and analogies re v/f and open (or closed) loop vector are already posted.

In your application, the feedback for sensorless vector control is the summed current of all four motors, so if one was stalled or even missing, the drive makes its computations based on the total information regardless. So, if the information is rather meaningless, so is the drives computations and adjustments.

In short, when connecting more than one motor, it makes no sense to use vector mode as the mathematical model is being fed erroneous information. So, for example, if one motor load increases, the drive may up ALL voltages causing problems.

So, I recommend v/f control in this case and this offers the advantage of you setting the relationship (fixed) so you can make more sense of control information if needed.

(Normally, vector mode should be superior in most single motor applications particularly in response to low speed operation and / or significant and faster load changes.)