John Stranahan

iTrader: (27)

"think so does the shaft speed difference delta-omega between sample times?"


I am going to outline our Power calculation method as this is precisely one of things that I tried to eliminate.

RPM is taken at discreet data points by the sentry. It turns out the RPM values are discreet as well and selected from a chart rather than calculated individually. The sampling process and selection from a chart causes some of these values to be high and some low of the actual value.

We change RPM to angular velocity (just changing units here)

We fit a polynomial model to the angular velocity data. The fit is good. Now we have an equation that describes the angular velocity vs time. No longer are we concerned with delta (value)/delta t. We can calculate pretty exactly the angular velocity at each time point. We get instantaneous angular velocities.

We take the derivative (using calculus) of this equation. Now we can calculate the angular acceleration. We get the instantaneous angular acceleration. No delta W/delta t is involved. We are working with instantaneous velocity and angular acceleration now.

We use two formulas more

T= I x angular Acceleration

The inertia from the armature is indistinguishable from the inertia of the flywheel in this spin up test. You must add it.
If you do a steady state test like the Competition Electronics dyno that measures torque with a sensor then you do not need to add or be concerned with angular inertia.

One last formula

P=Torque x angular velocity

In each case we have instantaneous values from our fitted equation and its derivative. This is responsible for a small improvement in accuracy and smoothness.

There was once a Tekin Dyno. It calculated power from simply an RPM drop as load increased from 10 to 20 amps. It agreed with no other dyno. It came with a three page paper explaining the theory. As far as I could tell the theory was just smoke and mirrors.
I can actually follow theory if care is taken in its presentation. All the pieces have to be there. Acronyms should be described on first use in a document or just not used. Units are important. A person should not assume the other person has the same reference in his possesion or have been through the same course. I have trouble with your stuff System-Theory. I have trouble with you getting a power number but using test values from Matt's Dyno run like resistances that are unavailable unless you do the dyno run. If you are doing a dyno run anyway then power can be calculated with the Physics printed above.
John