Vector OR DTC drives

[HanyHamed]

hello
what is the main difference between Vector and DTC drives from application point of view. is DTC added some thing to the performence in some applications and if yes what type of application the Vector Drive could not be used and DTC is the solution?
thank you

 

[jraef]

Good question. The problem is that DTC is a marketing term exclusive to one brand of drive, so any answer is potentially an advertisement for that brand. I for one will not red flag that however, because I am interested in an answer since they tend to make a big deal of it in the marketplace.

My understanding of DTC is that it is essentially a "tweak" of the SVC (Sensorless Vector Control) algorithm that provides additional precision. Kind of like saying, "I can split this proton into quarks!" They claim to be able to provide equal performance to FOC (Field Oriented Control) without needing an encoder feedback. I have seen demos and it is impressive, although I have seen demos of many SVC algorithms and can attest that indeed, some work better than others do. I have, in fact, seen one SVC demonstrated that out performed DTC in terms of regulation, load step change response and overshoot, and they were not making a big deal of it!

Where the DTC proponents take it one step too far IMHO however, is when they claim that DTC can provide the same torque proving capability as FOC in hoist applications, where the VFD must prove and provide full torque at zero speed so as not to drop the hoist load when the brakes are released. DTC requires that the VFD create and maintain a very complex mathematical model of the motor operating characteristics. I have heard (and in one case personally witnessed) that under the right circumstances the VFD can lose that motor model and have to recreate it. The one I saw was on a PD down-hole oil pump, so it was really no big deal. That delay however could be potentially disasteros for anyone relying on that drive to hold a hoist load, because once a load starts to drop freefall, it is infinitely more difficult to stop it. If you do a keyword search on this forum for "hoist VFD" and "vector drive" you will find other threads where we have discussed this. Unfortunately, some of the responses I recall that mentioned DTC have apparently been removed, probably because they are commercially biased.

I still maintain that I will not stand under a load being held by a DTC drive when the brakes are released. I have done so many times with good FOC vector drives with encoder feedback.


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[Marke]

Hello DriveMaster

A sensorless vector drive uses a mathematical modle of the motor and current/voltage measurements to calculate the gap flux, peformance etc. The model is fed with historical information and so is responsive to what has happened in the previous cycle etc.

The DTC principle is a predictive method. It also uses a mathematical model, but measurements are made at a very high rate and the model is updated with instantaneous information that is compared with predicted information.
The net result is similar except that the DTC can theoretically be faster responding to transients and because it is constantly updated against a reference, it can theoretically operate down to a lower frequency without an encoder.

In most situations, I would not expect that you would find much difference.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

I think that jraef covered it all. I could add that the TLA DTC stands for "Direct Torque Control" and that it was introduced some ten years ago. That was when sensorless control seldom worked well below around 5 Hz motor frequency while the DTC actually seemed to work well down to 0 RPM shaft speed and full torque.

The salespeople got carried away at that time and told visitors at the trade show in Gothenburg where DTC was introduced that the speed error was 0.000 percent at full torque. We all (I hope) know that that is impossible without an encoder and I asked the ABB guy to do a test. He agreed and I told him to set speed reference to 0.000 percent. He also agreed that any deviation from the set speed would be a speed error.

I then took a good grip around the exposed shaft (yes, i was ready to let go if things were going nasty) and turned the shaft slowly - but visibly - to the unaided eye. They changed the marketing slightly after that.

The DTC works without the then classic modulator. It uses a bang-bang controller that keeps the flux vector length within two limits that can best be described as two concentric rings. There is no fixed switching frequency - it depends on load and set distance between the two circles.

I have not looked any deeper into the DTC. I think that there is something to be found on the Net.

I also agree with jraef. DTC is good but not a miracle. ABB tends to use TLAs a lot. Remember the RFE? It was supposed to eliminate all problems with resonating loads in drives. I haven't heard about it at all after its introduction in the late 20th century. Anyone with any experience from RFE out there?

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

...and MarkE...

Gunnar Englund

http://www.gke.org

 

[HanyHamed]

thnak you so much for your valuable answer.

 

[cswilson]

I'll throw in my concurrence with jraef and skogsgurra. Several years ago I had the opportunity to assist a colleague who was using a DTC drive as a large servo drive, with shaft encoder information used by the servo controller, which was outputting analog torque commands to the DTC drive. The data logging capability of the controller showed us very clearly that the vaunted motor model lost it every time we went through zero velocity (even if quickly through), resulting in a noticeable hiccup in performance. Other than that, it was well behaved.

They also claim their control scheme for the power transistors results in better performance. Their marketing guys claim there is no modulation, but that's bull. Essentially, they modulate their power transistors like a single-bit DAC, deciding at short intervals (every 20usec, IIRC) whether each transistor should be on or off for the next interval. I have not been able to confirm or rebut this claim to my satisfaction, but the power guys in my group are dubious.

 

[niallnz]

From the ABB ACS 600/800 standard catalog, the open loop speed accuracy is 10% of motor slip, and closed loop it is 0.01% of nominal motor speed.

Typically a DTC drive is best in high torque applications such as extruders and hoists. I've used a DTC drives on a couple of 150Ton cranes. Part of the certification requirements for the cranes were to lift a 125% of SWL load and hold it on the motor/drive for 10 minutes. The gap under the load was repeatedly measured to see that there was no creepage; of which there was none.

Personally I wouldn't walk under a hoist at all, DTC, FOC, DC or slipring AC. It's a risk that is entirely avoidable.

DTC was mathematically developed by a couple of Germans, F Blaschke 1974, and M Depenbrock 1985, ABB commercialised it with ACS600 range in 1995.

 

[DickDV]

I use DTC drives in jet aircraft hydraulic pump test cells where the operating speeds are in the range of 12000-18000rpm.

We routinely meet a performance spec of not more than 25rpm speed error under no load/full load step changes with steady state speed error of not more than 2 rpm.

This is without an encoder on the two pole induction motor.

We have not been able to reach performance at this level from any other sensorless AC drive technology.