Solving DC Motor Issue with Rotor Inertia (J) Change

In summary, the conversation discusses a simulation model for a permanent magnet DC motor and the results obtained. The main concern is the high acceleration of the motor even when loaded heavily. The group discusses the incomplete model and the need to consider the complete electromechanical problem, including the equations for current-voltage and speed-torque relation. They also discuss the need to model friction and its relationship with speed. The conversation ends with the user sharing simulation results and asking for suggestions on the high acceleration of the motor.
  • #1
zuq
6
0
Hi guys,

I have built a simulation model for a permanent magnet DC motor, but I am having trouble accepting the results.

dynamic equation states. machine accelerates as long as there is torque imbalance between elctrical and load torque, below.

dw/dt = (1/J)*(Telectrical - Tload - Tfriction)

I get acceleration of about 1140 rads/s^2 , which is crazy, even when the motor is loaded 100%.

So the only thing comes to my mind is: does the rotor inertia (J) change with the loading of motor? if so, how would I work it out?

Thanks in advance
 
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  • #2
Your model is incomplete. By your own equation, when Telectrical = Tload, the acceleration show go negative due to your friction torque term.

I think the bigger problem is that you have not looked at the complete electromechanical problem. You need to look at the equation for the current - voltage relation and also for the speed - torque relation. They should be coupled, and your don't show that at all. Get an electrical machines book and take a look there. You have some things to learn here.
 
  • #3
As an added question, How do you model the friction?
It obviously needs to be 0 when speed is 0, but I don't thinks its linear relation with speed, like T(friction) = K * w,
because, don't they say that dynamic friction is independent with speed?
or is it a step definition like,
T(friction) = T(applied) (for w=0 && T(applied) < T(friction_max) ) ------static friction
.....= Constant (for w>0 || T(applied) > T(friction_max) ) --------dynamic friction
 
  • #4
PM dc motor, you say... how do you calculate torque and counter-emf?

From machinery lab course i took ~1965

Counter-EMF = K X [itex]\Phi[/itex] X RPM , (K X [itex]\Phi[/itex]) determined by no load test

Torque = 7.04 X same K X [itex]\Phi[/itex] X Iarmature

So your torque falls off as speed increases because increasing counter-emf reduces armature current.
 
  • #5
How do you model the friction? Bearing friction i don't know
windage will follow fan laws
perhaps there's a MechE in the house?
 
  • #6
Hi guys..Thanks for your responses

oldengr36.. Yes you are right.I do have the complete mechanical plus electrical model.
And I thought about my wording of question a bit later, yes the speed would be negative.
I was trying to simplify the scenario to ask if J (inertia) changed with speed.

But our friend "I am Learning" has posed a new interesting question, that friction torque increases with speed! Which would make sense..I guess

Any ideas how to model that?

I took friction losses as a constant as I simply saw it in a paper and that is how they had done it. The way I found k for friction torque was:

Using the motor performance curves, I said the torque produced by motor at no load speed must be equal to friction torque. True?
 
  • #7
For an induction machine, the motor torque at no load speed is the sum of shaft friction and windage on the rotor, so your statement is true.

J for the motor does not change with speed. It could change with position and/or speed for some part of the driven machinery.
 
  • #8
Thanks oldenr63, my question still remains, why is the motor accelerating so quickly even loaded so heavily?
 
  • #9
At what speed is it accelerating so quickly?
 
  • #10
Thanks oldengr63. Please see the link below for simulation results. I have uploaded a picture of it on imageshack
http://img515.imageshack.us/img515/3706/dcmotorr.jpg

First graph shows armature current superimposed on my reference current.
Second is the speed of shaft in rads/s
and third is acceleration dv/dt in rads/s^2

At 0.5s current reference changes from 200A to 70A so motor decelerates.

To give you a bit of background, this is for a go kart.
Note that I am using current control, Motor is rated at 200A for 10minutes so I am giving it full torque/throttle situation. And you can see the results. Don't think I have anything wrong with my model.

Your suggestions are much appreciated.

Thanks again
 
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  • #11
I think the answer to your original question is simply that the maximum machine torque is well above the rated torque, and consequently the starting acceleration is much higher than you expected based on the full load torque.
 

FAQ: Solving DC Motor Issue with Rotor Inertia (J) Change

1. What is rotor inertia and why is it important in DC motors?

Rotor inertia (J) is a measure of the resistance of a motor's rotor to changes in speed. It is important in DC motors because it affects the motor's ability to accelerate, decelerate, and maintain a constant speed.

2. How can changing the rotor inertia help solve DC motor issues?

Changing the rotor inertia can help solve DC motor issues by altering the motor's response to changes in speed. A lower rotor inertia can result in quicker acceleration and deceleration, while a higher rotor inertia can provide more stable speed control.

3. What factors affect the rotor inertia of a DC motor?

The rotor inertia of a DC motor is affected by the mass and shape of the rotor, as well as the motor's design and construction materials. Any changes made to these factors can result in a change in the rotor inertia.

4. How can I calculate the rotor inertia of a DC motor?

The rotor inertia of a DC motor can be calculated by multiplying the mass of the rotor by the square of its radius. This value can then be adjusted based on the shape and design of the rotor.

5. Are there any potential drawbacks to changing the rotor inertia of a DC motor?

While changing the rotor inertia can help solve certain DC motor issues, it can also lead to other issues such as increased power consumption, decreased efficiency, and potential damage to the motor if not done properly. It is important to carefully consider the potential drawbacks before making any changes to the rotor inertia.

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