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

 

[itsmoked]

Tell'em better argumentative then "postal".

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[Skogsgurra]

But, jraef, you ARE "argumentative"! ;-)

Gunnar Englund

http://www.gke.org

 

[jraef]

No I AM NOT!

 

[Skogsgurra]

ARE, TOO!

Gunnar Englund

http://www.gke.org

 

[itsmoked]

YOU BOTH ARE!!!!


Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[waross]

I love the smileys, Keith
respectfully

 

[Skogsgurra]

I don't :-(

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Iamsorryto hearthatskogs!

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

 

[ozmosis]

ahhhh, holiday season again...
))

 

[Skogsgurra]

;-)

Gunnar Englund

http://www.gke.org

 

[cswilson]

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

 

[itsmoked]

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

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

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