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Induction Motor Vector Drive - Torque control mode
4

Induction Motor Vector Drive - Torque control mode

Induction Motor Vector Drive - Torque control mode

(OP)
I want to run the Baldor vector drive ZD18H205–E in torque operating mode.  

The induction motor I'm using it with is a M3538 (0.5 HP).

The only information I enter into the vector drive is the nameplate data on the motor.  The unit performs calibration tests, and measures the input voltage and stator current during operation.

Now, my concern here is how the torque operating mode  works in practice (I've read the manual to no avail.)   I'm hoping some motor experts here might be able to shed some light on this situation.


What I want to do is: use the +-5V input signal to the drive, and this signal should be proportional to the amount of torque the motor develops(the manual includes a setup to do this), regardless of what speed it's driving the load at.     

How does the drive do this given the non-linear shape of the speed vs torque curve of an induction motor?  Or is it just controlling the stator current (not the torque)?   

Thanks in advance,

Matt Lawson

 

RE: Induction Motor Vector Drive - Torque control mode

It actually controls torque. And usually in a very good way, too.

It does this by keeping the current vector that is "in line with" the voltage. In other words, it controls the part of the current that has power factor 1.00. This current is usually called Iq (which should be mentioned in your manual). The other part of the current, the Id, is the magnetizing current. It is 90 degrees after the Iq. The total motor current (the one you measure with a clamp-on ammeter) is the vector sum of the two, I = sqrt(Iq^2 + Id^2). The calculation of the necessary magnitudes of these two vectors (hence vector control) is usually done with very minor errors. You can expect the torque control to be accurate to better than five percent. It is fast, too. Typical torque response is in the "few milliseconds" range. Some even have sub-ms.

Gunnar Englund
www.gke.org

RE: Induction Motor Vector Drive - Torque control mode

Hello skogsgurra;
Can torque control be used to prevent damage to driven machinery in the event of a jam-up, and stalled motor condition?
respectfully

RE: Induction Motor Vector Drive - Torque control mode

(OP)
Thanks for the info!

RE: Induction Motor Vector Drive - Torque control mode

Waross,

Torque control is usually used for driving conveyer belts and such. Speed is allowed to vary from motor min (or zero) to motor max but torque is held constant at input value. In the event of a jam the motor will reach min speed or zero then either it will maintain the set torque or will trip (maybe immediate, or maybe when motor temperature hits upper limit.)

Alex

RE: Induction Motor Vector Drive - Torque control mode

Matt and waross,

As Alex says. And, yes. You can use the 5 V analogue input to set the torque and prevent the mechanical parts. Just go on and test it. You will be positively surprised how well it works.

Gunnar Englund
www.gke.org

RE: Induction Motor Vector Drive - Torque control mode

Sematics issue:
It should be noted that with vector drives you can have speed control or torque control, not both simultaneously.

When in speed control however, the torque precision is maintained at the highest degree. You can also establish torque and/or speed limits along with a variety of ways to have the drive respond to those limits, such as reduce speed, maintain speed and lower torque, coast, alarm, trip etc. etc.

Vice versa for being in torque control as well, i.e. speed limits, trips etc.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Induction Motor Vector Drive - Torque control mode

Thanks guys. I see from your answers that I was imprecise
in the wording of my question.
I should have asked,
"The speed is controlled by a PID loop. Can I set a torque limit to prevent machinery damage when a block of wood in the sawdust jams the screw conveyor?"
respectfully

RE: Induction Motor Vector Drive - Torque control mode

Hello waross

Yes, many drives have a torque limit when in speed control mode. - Check with the particular model in question.

Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Induction Motor Vector Drive - Torque control mode

Quote (waross):

Thanks guys. I see from your answers that I was imprecise in the wording of my question.

Hey, by now you should know we are all about precision wording in here!

...except when we're not. wink

RE: Induction Motor Vector Drive - Torque control mode

waross, you may want to check the capabilities of your drive.  Some now have the ability to bring the torque limit level out to one of the analog inputs so you can control the torque limit level "on the fly" with a 1-10V or 4-20ma signal.

RE: Induction Motor Vector Drive - Torque control mode

Yes, good thing to do. But what you see is the drive's own thinking about what torque it is outputting. How do you realle, REALLY, know what the real torque is?

Answer: Attach a beam to the shaft. Make sure it cannot move more than a few mm. Find out what the torque is by loading the beam with known weights. Do not forget to include beam's weight at the correct distance from shaft centre. And, do not forget to start with a low torque limit setting.

Gunnar Englund
www.gke.org

RE: Induction Motor Vector Drive - Torque control mode

If a sudden jam occurs you may need some other torque limiting device to dissipate the kinetic energy of the rotor and drive train.

Barry1961

RE: Induction Motor Vector Drive - Torque control mode

Good point Barry1961.
Thanks to all for your replies.
respectfully.

RE: Induction Motor Vector Drive - Torque control mode

Wow skogs, sounds like a lot of fun...  "Whoops!! Torque limit set too high!  "Someone look for the beam, I'll search for the motor"lol


Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Induction Motor Vector Drive - Torque control mode

I have a digital torque wrench attached to a round block with a 1/2" square hole in it on the end of the motor shaft. Of course, I am only looking at small amounts of torque. I'm using it to test repeat accuracy on a new SV VFD design. It's amazing to me just how repeatable it is, so far the deviation has averaged less that 5% at zero speed. I know that's not what SV drives are intended to do, but I am impressed nonetheless. I have always loved VFDs, but the latest generation of vector drives is really impressive, almost regardless of make.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Induction Motor Vector Drive - Torque control mode

jraef,

What's "SV" stand for?  

RE: Induction Motor Vector Drive - Torque control mode

dpc,
SV = Sensorless Vector. See my FAQ. FAQ237-1062
I'm trying to push an agenda to pare down the countless "marketing terms" used in the industry that try to obfuscate what mfrs. have to offer. My opinion is that marketing gurus are trying to differentiate their product in an industry  that is homogenizing by coming up with multiple new names for the same thing. It only serves to confuse people.

SV has also been called:
Open Loop Vector - OLV (technically incorrect because there is a loop, it's just inside the drive),
Sensorless Flux Vector - SFV (confusing with Flux Vector, which is typically reserved for Closed Loop with an encoder),
Space Vector Control - SVC (a term applicable to all PWM vector control and also ambiguous with some DC control methods),  
Vector Torque Control - VTC (too ambiguous, could be either)
Direct Torque Control - DTC (proprietary to ABB and controversial because ABB is big and throws its weight around, insisting that it's different / better).

Flux Vector (FV) is what I am now calling the Closed Loop Vector method that uses an encoder feedback loop. Closed Loop Vector is OK, but as I said, technically any vector drive is closed loop. I have vacillated on using Field Oriented Control - FOC because that does accurately describe the difference, but I see more people using FV and I like 2 letter acronyms better, so I'm going that way. Besides, if you sound out FOC it can distract attention away from your discussion topic.

I also like using "Scalar" to describe the old V/Hz drives because ALL drives are technically V/Hz drives but again, V/Hz wins out by popularity. I recently saw one mfr. try to use IFD (Inverter Frequency Drive) to describe V/Hz, which again is too generic and technically could mean any drive.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Induction Motor Vector Drive - Torque control mode

Thanks jraef - good info.  

Speaking of terminology, I worked with an engineer who insisted that "Adjustable Frequency Drive" (AFD), was more correct than "Variable Frequency Drive" (VFD).  The logic is that "adjustable" implies a parameter that you have control over, where "variable" could be something was not controllable, but just "varied".  But VFD seems to have won the TLA war.  

RE: Induction Motor Vector Drive - Torque control mode

I've made that argument myself, and pointed out that VFD also stands for Volunteer Fire Department, the guys who respond when your VFD dies a violent death! I acquiesced years ago.

The other one I like to engage in with people is using the term "inverter" do describe a VFD. Technically, that is less than 1/2 of the total package, and inverter is also used to more accurately describe a DC-AC conversion system as used in boats and cars. Unfortunately my boss and bosses boss both use inverter instead of VFD, so I get myself reprimanded occasionally for being "argumentative".

RE: Induction Motor Vector Drive - Torque control mode

Tell'em better argumentative then "postal".

Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>

RE: Induction Motor Vector Drive - Torque control mode

But, jraef, you ARE "argumentative"!  winky smile

Gunnar Englund
www.gke.org

RE: Induction Motor Vector Drive - Torque control mode

No I AM NOT!

RE: Induction Motor Vector Drive - Torque control mode

ARE, TOO!

Gunnar Englund
www.gke.org

RE: Induction Motor Vector Drive - Torque control mode

YOU BOTH ARE!!!!machineguncurse      hammer flush2


Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>

RE: Induction Motor Vector Drive - Torque control mode

I love the smileys, Keith
respectfully

RE: Induction Motor Vector Drive - Torque control mode

I don't  sad

Gunnar Englund
www.gke.org

RE: Induction Motor Vector Drive - Torque control mode

shockedIrainbowampokesorryflush2toinfinity hearlickfacethathairpull3skogs!hammer

That's okay people, skogs is allowed his own opinion...  A lot of old stodgy people dislike smileys.

lightsaberyodabravoponderbatsadeyesshadessadupsidedown yinyangnosmileypeacecheersturkeycowheartreindeer     snailcyclopshippypacmanpc2vikingviking2alienworm

lolo
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com      

RE: Induction Motor Vector Drive - Torque control mode

ahhhh, holiday season again...
))

RE: Induction Motor Vector Drive - Torque control mode

winky smile

Gunnar Englund
www.gke.org

RE: Induction Motor Vector Drive - Torque control mode

Kids, kids!!!

To return to a couple of serious points, MedievalMan asks in the original post, "How does the drive do this [maintain commanded torque over speed] given the non-linear shape of the speed vs torque curve of an induction motor?  Or is it just controlling the stator current (not the torque)?"

MM: The classic non-linear torque-speed curves for induction motors are for constant electrical frequency, typically 50 or 60 Hz. You are better off viewing the horizontal axis as (inverted) slip frequency, not as motor speed, because that is what is important for torque generation.

An important part of what vector control does is to limit the slip to the quite-linear right-hand end of this curve. Now, in open-loop operation at a fixed electrical frequency, additional load torque causes a deceleration that increases the slip frequency, causing operation to move up this part of the curve to generate a countervailing electromagnetic torque at a slightly lower speed.

In vector control, whether using a shaft sensor or deducing speed from electrical sensors in the drive ("sensorless vector"), we turn this process around. To get the torque that we desire, we compute the slip frequency we need to get that torque. (The torque/slip-frequency ratio is the slope of that part of the curve.) We then create an electrical frequency equal to the mechanical frequency (i.e. rotor speed) plus the slip frequency.

Many commercial vector drives can be operated in either torque mode or velocity mode. If operated in velocity mode, there is an outer velocity loop, usually proportional plus integral, around the vector calculations, that determines how much torque is required to maintain the commanded velocity. If put in torque mode, this outer loop is bypassed, and the external signal is the actual torque command. In either mode, current feedback and the motor model are used instead of actual torque sensing.

Curt Wilson
Delta Tau Data Systems


RE: Induction Motor Vector Drive - Torque control mode

Thanks Curt. Nice explanation.  I hadn't heard it put that way before.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Induction Motor Vector Drive - Torque control mode

Ah, a word from the venerable Delta Tau, one of the originators of industrial grade processors with enough kahunas to do closed loop vector control on VFDs long before any of the major VFD players were capable of doing it themselves. I used to see a number of your front ends on various brands of VFDs in order to turn their hardware into something that worked for the end user. A Cutler Hammer salesman once bragged to me about how many VFDs he sold to an OEM machinery mfr though a Systems Integrator. I pointed out that for the most part, the SI was gutting the C-H drives to get the power train, then putting a Delta Tau processor on it! It was more your drive than theirs as far as functionality went.

Nice explanation BTW.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

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