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

If i'm not mistaking regerative breaking requires a separate system from the motor's controller to capture and store the energy. This system need to know when the power to the motor has stopped so it can engage and take the power from the motor, rectify it back to DC and charge the batteries.

A motor controller for a 3 Phase induction motor is not a current limiting source. That kind of controller pulses the coils in the motor to make it spin. Those pulses are at a constant current and voltage. What controls the speed is the amount of time between the pulses.

If you use a basic DC motor then your motor controller could be a current controlled source but it wouldn't do well as a generator.

On the DC bus side, you will need to take into account the internal resistance of the batteries because the supply voltage dips as current increases because of this internal resistance.  When the system is in regen the voltage climbs relative to this internal resistance as well. 

The regen in most controllers is a side effect of the flyback diode on the switching device the controller will regen unless the IGBT mosfet or whatever actively keeps the potential of the buss close to the potential of the reactively generated voltage.  

As to treating a motor controller like a current controlled source it would be a very simple simulation.  It would be treating the system like a constant load like a pump, and it could work where if the current commanded falls below a certain point current will flow out of the controller onto the bus.  

Another thing to take into consideration is the ripple on the DC bus controllers cause by switching lot of amps.  Some component blow fuses for no apparent reason because of this and need inductive reactance added to the input path to bring the ripples to an acceptable level. 

Good luck,

Michael Pilcher wrote:

 

If i'm not mistaking regerative breaking requires a separate system from the motor's controller to capture and store the energy. This system need to know when the power to the motor has stopped so it can engage and take the power from the motor, rectify it back to DC and charge the batteries.

 

A motor controller for a 3 Phase induction motor is not a current limiting source. That kind of controller pulses the coils in the motor to make it spin. Those pulses are at a constant current and voltage. What controls the speed is the amount of time between the pulses.

 

If you use a basic DC motor then your motor controller could be a current controlled source but it wouldn't do well as a generator.

thanks for the reply. So, if I assume that a motor controller with a motor act like they can deal with the speed and torque I am demanding than this would translate to a power equivalent profile over time. This power profile can than be simulated using a fixed voltage and a changing current, right?

If the same assumtions are true for the motor controller with motor under regen braking (not interested in how it works, but just that it works) I can then say that at a (assumed) fixed voltage the current follows a similar power profile (albeit with a different sign to indicate that it leaves the controller instead of entering).

cheers

forrest2192002 wrote:

 

On the DC bus side, you will need to take into account the internal resistance of the batteries because the supply voltage dips as current increases because of this internal resistance.  When the system is in regen the voltage climbs relative to this internal resistance as well.

So, if there is no battery connected but a super capacitor then the voltage keeps rising (provide the initial voltage of the super cap is at bus level and the voltage level at the controller terminals is higher)?

At the moment I am ignoring ripple from the motor controller, I am just looking at the curretn profile in and out. This is what I need for my simulation.

cheers

I'm finding it very dificult to understand exactly what you are doing  - can you post the model ?

Michael Kellett

Michael Kellett wrote:

 

I'm finding it very dificult to understand exactly what you are doing  - can you post the model ?

 

Michael Kellett

See attached.

The idea is that for a given drive cycle and vehicle I get a power profile that the bus would see. I can then focus on simulating what a battery and converter need to be able to deal with. And research any problems that might arise.

The reason for simplifying the motor controller + motor + vehicle parameters is to speed up the simulation time. Simulating converters is already hell in terms of time especially since I want to look at whole drive cycles. As such, any saving in terms of simulation time is good.

Hope this explains it further

cheers

Attachments:

simulation.docx

Hello,

I don't think that the capacitor and the CCS are standard Simulink blocks - can you tell me where they came from ?

Michael Kellett

Thats right they are blocks from the SimPowerSystems toolbox. Which is an expansion to the simulink block set. It allows for simulation using a more electronic approach. I was initially looking to build the whole lot in simulink but then the bi-directional nature of a converter would become problematic. In this case, I can use Simulink for the control strategies while taking a electronic approach to the circtuiry elements.

Hello Dirk,

I'm sorry but I can't really help you with the PowerSystems blockset.

(I don't have it and I've never used it so I would be shooting in the dark !)

Michael Kellett

Hi Michael,

I understand. But the question was not about the toolbox, but about whether a motor controller can be seen as a variable current source with a capacitor. I think it can based on a setup of an inverter.

An inverter has basically very little resistor  but with a capacitor in front it can supply high current to the motor (in an induction motor current is the defining factor not so much voltage). Under regen conditions the motor basically becomes a voltage source (l believe the voltage raises as a consequence of the back emf increase). Any current into the capacitor raises the voltage of the capacitor (in my assumptions I am assuming that there are no problems with saturation in the motor).

According to these assumption I should be able to see the motor controller as a variable current source.

Is this more or less correct or are there very large discrepancies in my thinking?

Cheers