Electric Automation Forum
Forum » Variable frequency drive » DTC vs. classic PWM vector VFD
Topics: DTC vs. classic PWM vector VFD on Variable frequency drive
Start by
08-26-2013 09:10 PM

DTC vs. classic PWM vector VFD

Hi all.
In ABB documentation on DTC, they claim tremendous improvements with respect to PWM driven inverters, due to their motor model and switching logic.
Has anyone ever compared (fairly) a state-of-art "traditional" drive (PWM vector controlled, let's say, for example, Yaskawa), to the ABB competitor?
It would be really interesting!

Thanks in advance,
08-26-2013 09:10 PM
Top #2
08-26-2013 09:10 PM
ABB, Hitachi and Yaskawa stay on the cutting edge of technology. After looking over the ABB technical guide book they admit that the limiting factor is the motor. It is great to have a 25 microsecond update time but the inertia of the real world limits rate of change.

Sensorless Vector or sensorless feedback has been around awhile. The technology leaders keep coming up with better software and control algorithms to improve performance, which is great. In most applications the VFD far exceeds the motor capabilities. Once you can go from full torque at zero speed to fast enough to explode the rotor the advantages of cutting edge improvements are limited.

I have done torque control with Hitachi drives a couple of generations back on gauze winding applications that held constant tension in grams that worked great. All of the leaders can be used on hoists, catch a spinning fan, reverse a fan or provide current faster than the motor can respond to.

I think the advantage to most users is the ease of setup. What skill level technician or engineer is required to get maximum performance? What range of applications can it do well right out of the box? This is a big concern for users and OEM's. When Hitachi came out with continuous adaptive tuning to eliminate the need for a separate motor tuning procedure it made setup easier. ABB coming out with new and better controls will make setup even easier, which is great. But if you want to see a measurable physical increase in performance between any of the leaders once the drives are properly setup you will need to look very, very closely.

I can see onboard software in the future which could walk a very low skilled maintenance person through setup once they have power hooked up in 90% of applications.
08-26-2013 09:11 PM
Top #3
08-26-2013 09:11 PM
ABB is moving much on their products, are mainly aimed at energy saving up to 60% in frequency converters and a 20% energy savings in electric motors. In turn reducing maintenance costs and increasing productivity.
08-26-2013 09:12 PM
Top #4
08-26-2013 09:12 PM
Hello Sandro
The torque / speed control accuracy or response time depends on motor model accuracy, voltage and current measurement accuracy, the frequency of the calculation update and the delays in the measurement, calculation and control. This is if we only talk about the Variable Frequency Drives role. How to define modulation pattern to execute the needed output voltage based on measured values, calculated values and reference in the process of motor control has been solved in multiple different manners by different manufacturers. Technically the method of modulation is not that relevant for control accuracy. Sales promotion / marketing communication wise it has proven to be very effective way to differentiate.
08-31-2013 10:13 PM
Top #5
08-31-2013 10:13 PM
DTC is a wonderful sales pitch by ABB.

DTC is good - very good, but in the 'real world' does it deliver significantly more than a traditional vector control?

In my experience it depends on the motor and the application.
I am yet to see an application where DTC could achieve something that good vector control could not. But then again I haven't seen all applications.

I have had a drive running a 0.3Hz in open loop vector mode, with enough torque to perform the required job adequately. This was a high quality drive with excellent auto tune performance giving it a very accurate motor model. No DTC.

I once had a potential customer tell me they only use ABB for their application because it's DTC allowed it to do what other drives could not on their application. Then I found competitors drives being used successfully in precisely that application. Sounds like excellent sales work by ABB (not that there's anything wrong with that).
08-31-2013 10:13 PM
Top #6
08-31-2013 10:13 PM
To answer Sandro's question directly, I once worked for a drives company that did precisely that comparison for 'in house' use only. They used 4kW 4 pole motors - the same brand - for the test.

The results showed only slight performance benefit from the DTC control over the company's drive, and this was at very low frequency in a very small band. Certainly not enough for any customer to see any tangible benefit, unless you wanted to operate extensively in this small band of frequency.
08-31-2013 10:14 PM
Top #7
08-31-2013 10:14 PM
I think that the question is too general. I think that the question should be divided to operating points (as you can see from the answers above, the frequency appears a lot...). So I would suggest, if you don't mind, to refine the question - Low to medium frequency (up to let's say, 100Hz). then up to, let's say 1kHz of electrical frequency. Now torque should be added too... so you actually get a comparison matrix, which takes into account operating frequency and torque.
For example, if the electrical frequency is low, why waste energy on PWM switching losses ? the DTC will be much more efficient. However, in high frequency rotation, could be that PWM will be more efficient..
and we haven't started talking about field weakening range...
Reply to Thread