Actions
Forum Thread Details
  • Replies 16 replies
  • Subscribers 73 subscribers
  • Views 2733 views
  • Users 0 members are here
Related

Can a motor controller be seen as a Current controlled source?

mouch

Hi,

 

For a Simulink model I would like to use a very simplified model of a motor controller based on a capacitor and a current controlled source. I am assuming the use of an 3 phase induction motor with motor controller based on a DC to AC inverter.

From what I understand, under regenerative braking the motor generates current which via the inverter  is turned into a DC voltage and current source, which I think can be approached using an Current controlled source and an capacitor.

 

To speed up my simulation, I do not want to simulate the full motor control and motor. I want to assume that the motor and controller can deal with what is expected and that I only see the current and voltages at the DC bus - to which I connect my batteries.

 

To this end, I have calculated the power profile at the bus (as seen by the batteries) based on a drive cycle, vehicle parameters, etc. This powe demand I divide by the voltage I expect at the bus to get the current which I then supply to the current controlled source and capacitor which then get connected to the rest of my model. For example, the battery.

 

My question is:

1. Can a motor controller be seen as a Current controlled source?

2. Are there any oversights I need to take in account if I want to take this approach?

 

Cheers,

  • in reply to mouch

    Hello Dirk,

     

    I'm unconvinced that you can model the controller this way but it does depend on how you control the current.

    Your capacitor is being charged by a constant current but that means that the power going into the capacitor increases as the voltage across it rises.

    So I think that with the model you have shown me the energy in the cap would increase according to t^2 for constant power from the motor - so the current controlled source is making energy.

     

    I'm not sure - it's all in the blockset - in real life there are no current controlled sources image

     

    Try running the model - do you get sensible results ?

     

    Michael Kellett

  • in reply to michaelkellett

    The way I see it under normal conditions:

    The throttle is pressed and the motor controller starts to move the motor (through FOC or whatever). This makes that the motor controller demands a current from the battery (the input to the contrller is a capacitor for some smoothing and voltage measuring).

    So, so far a current controlled source should work. Simulations show a reasonable match to other simulated outcomes in literature.

     

    Under regen conditions (and this is where I am having some doubts, but nothing that realy would stop use of the current controlled source).

    The brake is pressed and assuming that we are not going to recover all the energy but only a part of it. The system is going to react in reverse with current flowing out of the controller. The controller I am using on my dynamometer has an "over voltage under braking" protection which monitors the voltage across the input capacitor and when that goes to high it starts burning of current. In other words, 

    So I think that with the model you have shown me the energy in the cap would increase according to t^2 for constant power from the motor - so the current controlled source is making energy.

    The energy that flows into the capacitor is then further transported to -for example- a battery pack.

     

    I got the idea from a paper I read, but under buck (regen) conditions I was encountering some behaviour that I could not explain (was not explained in the paper and I was surprised the author had not made a note on that behaviour). The author connected a buck converter to a bus (with the bus simulated by a curretn controlled source) and only used current control to control the current through the converter. But I found that if the input capacitor isnt monitored the voltage will go sky high or drop to input voltage and the converter will stop functioning. The explanation to this effect is simple: because of the switching behaviour of the converter you do not know exactly how much current you draw out of the capacitor and how much will flow into it. And the tipping point is very steep. i.e. too much current and the capacitor voltage will drop; too little current and the capacitor voltage will keep rising (as per your quote again).

     

    So, all in all, I think it can be done but I wondered why this wasn't reported in the paper. Something I considerd fairly important to the overall functioning.

    My question, here was to find out if more people had looked at this before and if there are serious objections to the use of a current controlled source as a motor controller (in simulation).

     

    cheers,

  • in reply to mouch

    I'm sorry but I don't think I can help any more   - without seeing the complete model and some actual data I'm just guessing what you are doing.

     

    Michael Kellett

  • in reply to michaelkellett

    Lol think I just found my answer:

     

    http://www.dspace.com/en/inc/home/products/hw/simulator_hardware/simulator_specific_hardware/electronic_load_module_30v.cfm

     

    quote:

    It can work as both a current sink and a current source to provide bidirectional current flow, i.e., it generates or consumes real current on ECU motor outputs.

     

    I think this answers my questions.

  • in reply to mouch

    I'm happy if you're happy but the dSpace device is an electronic load intended to simulate a motor not a controller.

     

    Michael Kellett

  • in reply to michaelkellett

    True, but what is an inverter but a set of switches that create a path for the current to flow (there is not voltage involved - assuming ideal switches equals no voltage drop). The input capacitor would then see only a sink or a source of current.