Induction Motor Vector Control 1

[scottcash]

Hello
I am trying to model an induction motor in Simulink and have come across a few problems.

The model is a 3 phase induction motor based around a rotating reference frame on the rotor.

problem:
1- When starting from zero rpm, the id current is 0. This means the flux is 0, thus my starting torque is 0 and has to ramp up. Therefore i dont get full load torque from 0 rpm

2- The flux doesn't reach its desired value until base speed.

3- Since i am using a limited voltage source (300V), the torque dramatically drops off to 0 at base speed. (Flux weakening IS being considered)

The torque graph looks like the left hand side of a bell curve up to base speed, then drops to 0 soon after.

The vector control scheme i am using matches all the others in research papers.

Any advice would be appreciated
Thanks!

 

[waross]

It depends;
Is this DOL or VFD control?
Are you modelling applied frequency or rotor slip frequency?
You must have a difference between applied frequency and rotor frequency. This is called the slip frequency. Without a slip frequency there is no current induced in the rotor and no torque.
Take a look at the torque graphs on pdf page 7 of the Cowern papers:

http://www.baldor.com/Shared/manuals/PR2525.pdf

These graphs are for DOL starting. To use for VFD control, use 100% on the graph as the applied frequency. At full load the slip frequency will be about 2% or 3%, or 97% or 98% based on rated speed.
You can see that the curves pass the full load torque line at less than but very close to 100% speed.
The curves are accurate for evaluating slip frequency and torque at various different applied frequencies, but if the applied frequency is changed, the percentages become meaningless.
That is at 50% of rated frequency the slip frequency for a given torque will be the same as at rated frequency in Hz or in RPM, but the percentage base will have changed. That is 3% slip at 1500 RPM will be 6% slip at 750 RPM but in both cases the slip at full load will be 45 RPM slip and a rotor frequency of 1.5 Hz.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cswilson]

Why do you set id to zero at zero rpm? This makes the rotor completely unmagnetized.

Is what you are calling "base speed" really the no-load speed, where the back EMF matches the supply voltage so there is no "head room" to apply torque-producing current?

Curt Wilson
Omron Delta Tau

 

[Skogsgurra]

Thank you Curt, this was going a completely wrong way. And didn't know how to straighten it up. You did.

Gunnar Englund

http://www.gke.org

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