×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Motor Inertia Simulation

Motor Inertia Simulation

Motor Inertia Simulation

(OP)
I am trying to find an equation that simulates the inertia of a motor using on a brake dynomometer.  Currently I have:

I=(Pm*RR*9549.3)/(RPM*a*g)

where:
I - Available inertia, kg-m-s^2
Pm - maximum motor power, typically 150% of rated power, kW
RR - I am unsure of what this truely is but my guess is Rolling Raduis of the wheel.
RPM - rotational speed of interest, rev/min
g - acceleration of gravity, 9.81 m/s^2
a - Vehicle acceleration, m/s^s

My calculations show that the units do not work out assuming that there are no units associated with "9549.3".  I found this formula on a brake dynomometer testers and builders website called Link Testing.

If anybody has any ideas on the validity of this formula, or any other formulas that may be useful please let me know.  

Thank you

RE: Motor Inertia Simulation

The units work out. Power has the dimensions Watts or Newton-meters or kg-m^2/sec^2. The constant 9543.3 is 1000 x 60 / 2 x Pi to convert RPM to radians per second and kW to W.

RE: Motor Inertia Simulation

Should have said Watts = Newton-meters/sec = kg-m^2/sec^3.

RE: Motor Inertia Simulation

(OP)
I do not see how the units work out.

((kg*m^2/s^2)*m*(rad/s))/((m/s^2)*(m/s^2)=kg*m*s not the units of inertia.

or bring the rad/s to the denominator instead of the numerator and you get - kg*m*s^3.

Maybe I am being ignorant but I do not see how the units work out.

RE: Motor Inertia Simulation

(OP)
Inertia is kg*m/s^2

RE: Motor Inertia Simulation

(OP)
okay
now that is correct

RE: Motor Inertia Simulation

I had the inertia worked out to kg-m-s^2. Now that I have looked at it again, I think that it should work out to kg-m^2.

Does the equation we have been working on suit your problem if we get it right? Can you explain what data you are starting with and what you actually want to calculate?

RE: Motor Inertia Simulation

In any calculation like this, from a canned formula, you must be very careful about how the moment of inertia (MOI) is expressed. (By the way, because there is a lot of confusion when people are not explicitly clear about terminology, I prefer to use MOI for rotary systems rather than just "inertia".)

MOI can be expressed in units of mass*length^2. In SI units, this is kg-m^2.

MOI can also be expressed in units of force*length*time^2. In English units, you often see lb-ft-s^2. Since force is mass*accel = mass*length*time^-2, you see that the two methods of expressing MOI are equivalent.

It is critical to realize that while we use pounds and kilograms in common usage as interchangeable, in technical usage they are not. Kilograms are mass units, and pounds are force (weight) units. Where the confusion really comes in is when MOI is expressed in "hybrid" units, such as kg-m-s^2, as in this example. The intent is to use metric units in a form that is familiar to people who are more familiar with English units (or vice-versa).

IMHO, this typically creates far more problems than it solves. The "kg" in kg-m-s^2 is not a mass unit, but a force unit. It is the downward force exerted by a mass of one kilogram in normal earth gravity of 9.81 m/s^2. If you have to use these units, I believe you should label this "kgf" to make sure you distinguish it from the real mass units of "kgm". Remember that 1 kgf = 1 kgm * 9.81 m/s^2 = 9.8 N.

In your example, the factor of 1/g is in the equation to convert the force units to mass units.

Curt Wilson
Delta Tau Data Systems

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources