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
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 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
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
RE: Induction Motor Vector Drive - Torque control mode
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
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
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
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
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
Hey, by now you should know we are all about precision wording in here!
...except when we're not.
RE: Induction Motor Vector Drive - Torque control mode
RE: Induction Motor Vector Drive - Torque control mode
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
Barry1961
RE: Induction Motor Vector Drive - Torque control mode
Thanks to all for your replies.
respectfully.
RE: Induction Motor Vector Drive - Torque control mode
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Induction Motor Vector Drive - Torque control mode
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Induction Motor Vector Drive - Torque control mode
What's "SV" stand for?
RE: Induction Motor Vector Drive - Torque control mode
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
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
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
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>
RE: Induction Motor Vector Drive - Torque control mode
Gunnar Englund
www.gke.org
RE: Induction Motor Vector Drive - Torque control mode
RE: Induction Motor Vector Drive - Torque control mode
Gunnar Englund
www.gke.org
RE: Induction Motor Vector Drive - Torque control mode
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>
RE: Induction Motor Vector Drive - Torque control mode
respectfully
RE: Induction Motor Vector Drive - Torque control mode
Gunnar Englund
www.gke.org
RE: Induction Motor Vector Drive - Torque control mode
That's okay people, skogs is allowed his own opinion... A lot of old stodgy people dislike smileys.
lolo
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Induction Motor Vector Drive - Torque control mode
))
RE: Induction Motor Vector Drive - Torque control mode
Gunnar Englund
www.gke.org
RE: Induction Motor Vector Drive - Torque control mode
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
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Induction Motor Vector Drive - Torque control mode
Nice explanation BTW.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376