Induction Motor Vector Drive - Torque control mode 4

[MedievalMan]

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

 

[Skogsgurra]

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

http://www.gke.org

 

[waross]

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

 

[MedievalMan]

Thanks for the info!

 

[alexit]

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

 

[Skogsgurra]

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

http://www.gke.org

 

[jraef]

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

 

[waross]

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

 

[Marke]

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

 

[jraef]

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.

 

[DickDV]

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.

 

[Skogsgurra]

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

http://www.gke.org

 

[Barry1961]

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

 

[waross]

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

 

[itsmoked]

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"


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

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

 

[dpc]

jraef,

What's "SV" stand for?

 

[jraef]

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

 

[dpc]

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.

 

[jraef]

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".