# vector flux, flex vector, variable frequency question



## nolabama (Oct 3, 2007)

The three terms above, some one shed some light on the first two. I understand the variable frequency, but my engineers says we are gonna go from a sensorless vector variable frequency to a vector flex ? ? ? I dont know enough to pose a proper question....


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## JRaef (Mar 23, 2009)

I think you are getting some of the terms you heard a little mixed up, they sound very similar and people often get them confused (meaning the people who mentioned them to YOU). There are also issues with different manufacturers trying to differentiate themselves by using versions of the same terms to make it appear they have something better than the other guy, they don't. I'll see if I can help.

All are Variable Frequency Drives, that's just a generic term. The difference is in the amount of control you want.

A "basic" control method is called "Volts-per-Hertz (V/Hz)" or sometimes called "Scalar" control. This is the base issue of just increasing the voltage (V) and frequency (Hz) together at the same rate to make the motor produce it's designed torque. As long as you maintain the V/Hz ratio that the motor was designed for, it is _*capable*_ of producing it's rated torque at any speed. This was how all drives were at first. But notice that I highlighted "capable". Just because the motor is made capable of doing it, doesn't mean it really did. The problem is, although the VFD can TELL the motor what speed to run at, it has no idea if it is actually running at that speed. So as the motor is loaded, it slows down and although the drive is looking at the current in terms of protecting the motor, it isn't doing anything else to try to prevent it from slowing down under load. For a lot of simple applications, like pumps and fans, this is fine. But when you get into machines, sometimes it isn't and you want ACCURACY, either in speed, torque or both.

A "Vector" drive adds a level of intelligence to the VFD so that it DOES watch what is happening in the motor, and it then alters the output waveform of itself to tweak the motor back into performing as you want it to. It is tweaking the VECTOR of the current and/or voltage on the fly. To do that, it needs some sort of feedback from the motor. That's when the TYPE of Vector drive splits off.

Sensorless Vector Control (SVC, also sometimes called Open Loop Vector) is not really sensor"less", it just uses sensors that are INSIDE of the VFD. By using a high speed processor (computer) in the drive, it monitors very tiny little effects of current in the output to the motor and compares them to a mathematic model of the motor that it has created (in setup). If what it sees from the motor is not what it has determined it SHOULD see, it tweaks the vector(s) to bring it back into line. All of this happens VERY very quickly inside of the drive. The one drawback is that in order for this to work, the VFD has to "see" the motor and changes in the motor. So the slower the motor gets, the less control a SVC drive has over it. The rule of thumb is "down to 1Hz".

A Flux Vector drive (sometimes called Closed Loop Vector, and FOC or Field Oriented Control is similar) is essentially the same thing, in fact it actually came along first. It doesn't rely just on monitoring the current from the motor, you actually put a speed sensor ON the motor shaft, usually an absolute encoder. The processor in the drive then looks at the absolute _*position*_ of the motor shaft at any given moment and tweaks the vector based on that, regardless of current etc. It is much more accurate, as in nearly Servo Motor accurate (that's essentially the same thing a Servo Drive/Motor is doing as well). The main benefit is that it can accomplish the best performance at ANY speed, even at Zero Speed, the "holy grail" of VFD control. Seems odd, but think of this application. You have a hoist at Boeing that is holding up a 747 fuselage in the assembly line. When you want to lift it higher, you release the brake and at that moment, if the hoist motor is not putting out MAXIMUM torque at zero speed, the load starts to drop and once started, will need a LOT more HP to overcome the inertia of it falling to the ground. Very bad. So a Flux Vector drive with a "torque proving" feature will make sure that BEFORE you release that brake, the motor ALREADY is producing enough torque on the hoist shaft to keep it from moving.

So the big difference is in the type of speed sensor involved. Sensorless = internal to the drive, has some limits as to how low you can go. Flux Vector means an external encoder for position sensing, no limits to what can be done with it.


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## nolabama (Oct 3, 2007)

I can see my need for maximum torque at zero speed. Thanks man for that explanation.


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