PM DC Motor Model Trouble

[Gerry M]

I'm currently using Multisim 12 while attempting to utilize the DC_MACHINE_PERMANENT_MAGNET model found in Electro_Mechanical > MACHINES.  I wish to simulate several different conditions relating to both power input (including precharged caps) and motor characteristics and effectively measure torque and speed responses.  I have some experience with circuit simulation in the past but I am a little rusty.

My first question deals with the dc motor model I am using.  It comes with 4 outputs, those being Te (torque), theta (rotor position?), Ea (back emf?), and omega (angular speed?).  How do I go about measuring these?  

Most of my attempts have been centered around a DC sweep analysis with a 48V battery source connected to the motor.  I haven't been able to get the arbitrary load model to function correctly but I do believe I have gotten the incremental sensor to work as it is recording a sensible RPM output.

I have been primarily referencing figure 2 of this link http://zone.ni.com/devzone/cda/tut/p/id/13721

For the purpose of my simulation, I do not require the h-bridge control inputs let alone the digital control via LabVIEW.  I do believe I have exhausted my search for more relevant information, please prove me wrong.

If there is a better program for my purposes please let me know.  Any assistance will be appreciated.

Thanks

Gerry

[OStep]

Gerry,

You may find that the help for this component (click Help in its properties dialog) as well as the help for using machines in general (titled "Machine Modeling") might be of use. I'll try to answer some of your questions directly in here however.

The following is a general representation of the internals of all machines models:

Regarding the four pins that you speak of...

The voltage on the 'omega' pin is the speed in rad/s. This pin corresponds to the 'omega' node in the figure above. Note that this pin, unlike the Te,Ea, and Theta pins, is part of the dynamic electromechanical model. If you have some external mechanical load, you would attach it to this pin. This in turn would affect the voltages/currents/speeds/torques of the system. However the 'Te', 'Ea' and 'theta' are measurement pins only and are included primary for convinience. The respective quantity is read as a voltage and in general you would simply probe these pins using the grapher or an instrument. The measured quantities are all documented -   'Te' is the generated electromechanical torque (i.e the value of the internal Te source in the figure above), Ea is the back-EMF, and theta is the shaft position (in radians).

I've also attached a circuit that traces the speed/torque characteristic of the motor. 

Let me know if that helps.

[Gerry M]

Max

Thank you for the quick response.  I now understand how to properly apply the motor models.

I have another question in regards to graphing certain parameters.  I have attached a performance sheet for a specific motor and would like to replicate the parameters displayed.

My questions then are this:

• Would a current source in series with the motor be needed to perform a 0 to 300 A sweep on the motor?
• And how would I go about maintaining this sweep at a constant 48 V input?
• As shown in the attached sheet, input and output power are compared and represented by an efficiency curve.  To measure power output I would assume it would be the Torque*rpms, would this be with no load?

Regards

Gerry

[OStep]

• Would a current source in series with the motor be needed to perform a 0 to 300 A sweep on the motor?

Yes if you are attaching a current source to the shaft (omega) pin, you are simulating a constant-torque load. You can sweep this current source to simulate the conditions at various constant torque loads. 

• And how would I go about maintaining this sweep at a constant 48 V input?

Attaching a 48V voltage source to the armature will maintain this voltage at this value throughout a sweep (unless of course you are sweeping this very source)

• As shown in the attached sheet, input and output power are compared and represented by an efficiency curve.  To measure power output I would assume it would be the Torque*rpms, would this be with no load?

Power(W)=Torque (Nm) * Speed (rad/s)

The current source value is assumed to be torque in Nm and speed read off the shaft pin is in rad/s. So you only need to multiply the two to get power. This will be the power at that load condition.

See attached circuit. I am sweeping the torque (current) source and outputting power at two stages in the system: internal power generated by the motor- V(Te)*V(omega) and the power at the load - I(i1)*v(omega). The small difference between the two is the result of the power lost to the shaft friction. Remember the the Te pin is a measurement pin that represents the torque generated by the motor. 

[OStep]

Any luck?

[Neil_1234]

Hello,

I am still puzzled in measuring Torque in N.m. and Speed in rpm. The torque and speed in the example from the previous post is available only as voltage readings. How would we convert these into N.m. and rpm.

Thanks

[Tayyab_R]

Hi,

Please take a look at the attached file. In general, you can try to look at the help for a component to make sure what each output is. You can double-click on the component and then click on Help. 

In this case, the output is in Nm. What that means is if you see 10V on the oscilloscope, that refers to 10Nm.

Hope this helps.