R/C Tech Forums

Go Back   R/C Tech Forums > General Forums > Electric On-Road

Reply
 
Thread Tools Search this Thread
Old 03-13-2009, 12:51 PM   #61
Tech Fanatic
 
Join Date: Jun 2006
Posts: 872
Trader Rating: 35 (100%+)
Default

Novak 7.5 w/ sintered rotor, smooth can. I might have stopped a little early as I was getting a little nervous about the flywheel spin speed. Motor temp at end of run, 130deg.

Now I just have my own Novak 13.5 to compare to my friends Novak 13.5 and that is all I have, so I need to round up as many motors this Saturday that I possible can.

Attached Thumbnails
Novak Sentry as Brushless Dyno-novak750boost1.jpg  
__________________
High Desert Raceplace, Grand Jct CO, Sanwa Exzes Plus Stick Radio, Spektrum,Thunderpower 230g, Modified Tri-Nut Novak GTB2, Ballistic 4.5t 550, 5.83lb SCTE Ten, Novak Sentry Brushless Dyno, Crossweight Setup Station, Junsi 20A power supply Icharger 20A Charger,TP610C, 22B, Novak Edge, Novak 13.5, SC10, Havoc Pro SC XDrive, Ballistic 17.5 Matthew Joseph Cordova
mattnin is offline   Reply With Quote
Old 03-13-2009, 01:01 PM   #62
Tech Fanatic
 
Join Date: Jun 2006
Posts: 872
Trader Rating: 35 (100%+)
Default

I guess since I don't have any more motors to test, I will run a bunch of tests with the RS Pro boost timing at different settings.
__________________
High Desert Raceplace, Grand Jct CO, Sanwa Exzes Plus Stick Radio, Spektrum,Thunderpower 230g, Modified Tri-Nut Novak GTB2, Ballistic 4.5t 550, 5.83lb SCTE Ten, Novak Sentry Brushless Dyno, Crossweight Setup Station, Junsi 20A power supply Icharger 20A Charger,TP610C, 22B, Novak Edge, Novak 13.5, SC10, Havoc Pro SC XDrive, Ballistic 17.5 Matthew Joseph Cordova
mattnin is offline   Reply With Quote
Old 03-13-2009, 01:15 PM   #63
Tech Adept
 
Join Date: Mar 2009
Location: Metro New York
Posts: 139
Default

Predicting Peak Power from the Engineering Model.

I took Matt's first Sentry Dyno post for the Hacker 13.5, the one with voltage and current listed in the spreadsheet.

Vs - open circuit source voltage - 8.4 volts
Is - start/stall current - 106 Amps
Rs - internal source resistance - 0 Ohms

Ra - armature resistance - Ra = Vs/Is = 0.0792 Ohms

Knowing Vs and Ra, assuming Rs = 0, I apply a standard PMDC model (prefer to keep it to myself for now) formula to compute peak mechanical power output:

Pmax = 222 Watts

Now this compares with the Novak Dyno estimate of 215 Watts for this pull. That is less than 10% error and I did not account for the loss of power due to nonzero source resistance Rs in an actual battery. The calculation of Rs from the data set published may be possible, and I might even get the 215 Watts back exactly, which would be a good confirmation of the integrity of John's linear regression or maybe just an exercise in circular reasoning from the assumed data set, I can't decide which one yet.

Last edited by SystemTheory; 03-13-2009 at 01:39 PM. Reason: units error
SystemTheory is offline   Reply With Quote
Old 03-13-2009, 01:35 PM   #64
Tech Elite
 
Join Date: Aug 2002
Location: Houston, Texas
Posts: 3,777
Trader Rating: 27 (100%+)
Send a message via AIM to John Stranahan
Default

Matt-I think the max power box blew up. Check the power peak on your last post and the box result. Also if you have time. You have three runs on the same motor from your earliest work. We need max power numbers on those three runs. That will help us determine the precision of the measurement.

System Theory- Nice to see we are in the same ballpark.
John
John Stranahan is offline   Reply With Quote
Old 03-13-2009, 02:30 PM   #65
Tech Fanatic
 
Join Date: Jun 2006
Posts: 872
Trader Rating: 35 (100%+)
Default

John, I think the max box is working fine. Were you comparing max power to the torque axis?

Here is my own, newer novak 13.5 dyno'd at 0 deg and 30 deg timing, and compared. Notice my 13.5 at 0 deg to my friends 13.5 at 0 deg. The Novak at 30 boost revs almost to 30k rpms.

Attached Thumbnails
Novak Sentry as Brushless Dyno-novak13530boosttiming1.jpg   Novak Sentry as Brushless Dyno-novak1350boosttiming1.jpg  
__________________
High Desert Raceplace, Grand Jct CO, Sanwa Exzes Plus Stick Radio, Spektrum,Thunderpower 230g, Modified Tri-Nut Novak GTB2, Ballistic 4.5t 550, 5.83lb SCTE Ten, Novak Sentry Brushless Dyno, Crossweight Setup Station, Junsi 20A power supply Icharger 20A Charger,TP610C, 22B, Novak Edge, Novak 13.5, SC10, Havoc Pro SC XDrive, Ballistic 17.5 Matthew Joseph Cordova
mattnin is offline   Reply With Quote
Old 03-13-2009, 04:01 PM   #66
Tech Fanatic
 
Join Date: Jun 2006
Posts: 872
Trader Rating: 35 (100%+)
Default

John, I think you will be pleasantly surprised. I compared a previous hacker 13.5 dyno test to the one I have posted. The previous test showed that odd hump at the end of the run too, albeit more pronounced.

Here is the original:


Here is a previous test:
Attached Thumbnails
Novak Sentry as Brushless Dyno-hackerprectest1.jpg   Novak Sentry as Brushless Dyno-hacker1350-boost1.jpg  
__________________
High Desert Raceplace, Grand Jct CO, Sanwa Exzes Plus Stick Radio, Spektrum,Thunderpower 230g, Modified Tri-Nut Novak GTB2, Ballistic 4.5t 550, 5.83lb SCTE Ten, Novak Sentry Brushless Dyno, Crossweight Setup Station, Junsi 20A power supply Icharger 20A Charger,TP610C, 22B, Novak Edge, Novak 13.5, SC10, Havoc Pro SC XDrive, Ballistic 17.5 Matthew Joseph Cordova
mattnin is offline   Reply With Quote
Old 03-13-2009, 05:11 PM   #67
Tech Elite
 
Join Date: Aug 2002
Location: Houston, Texas
Posts: 3,777
Trader Rating: 27 (100%+)
Send a message via AIM to John Stranahan
Default

Damn! Thats really nice. Good Reproducibility. Need that third test.

I was looking at the wrong axis. Max box is OK. In excel all my axes are on the left.


So boost, moves the power peak slightly to higher RPM. It would improve speed on a long straight on a road course at the expense of some loss of efficiency and heat build up.
john

Last edited by John Stranahan; 03-13-2009 at 10:22 PM.
John Stranahan is offline   Reply With Quote
Old 03-14-2009, 09:30 AM   #68
Tech Elite
 
Join Date: Aug 2002
Location: Houston, Texas
Posts: 3,777
Trader Rating: 27 (100%+)
Send a message via AIM to John Stranahan
Default

Armature Rotational Inertia
.482 inch diameter 22.1 g

4.138 x 10^-7 kgm^2

I will add this to the spreadsheet in a bit. It raised the max power on my example from 215 to 216 W. The extra heavy flywheel makes this number less significant.

Please check my math.
john
John Stranahan is offline   Reply With Quote
Old 03-14-2009, 10:12 AM   #69
Tech Adept
 
Join Date: Mar 2009
Location: Metro New York
Posts: 139
Default

John, my equations and simulations predict no change in peak mechanical power due to a change in motor or load inertia, which I write Jeq = Jm + JL.

If you inspect the load line (torque-speed line) in my prior posts, if the input voltage is held constant, the mid-point of this line (peak power) does not change when the inertial load Jeq changes. What changes is the spin-up time, or the time to roll through this peak point on the way to equilibrium torque. When motor torque Tm = TL, load torque, equilibrium occurs.

An ineritial load exerts zero counter-torque TL at maximum angular velocity (it keeps spinning with constant angular momentum according to Newton's first law), so it should not change the final operating speed either, unless it puts some force on the motor bearings and this increases the angular damping loss. When you hang a flywheel on the output shaft it puts some stress on the bearings so top rpm might change a little due to that.

The voltage and current sensors are independent of the rpm sensor, so if the sample data are accurate, it is possible to use my peak power formula to help confirm the ballpark mechanical power in your spreadsheet model.

However it is difficult to estimate internal battery resistance from a regressive technique, so accuracy of the starting voltage and current would be absolutely critical to determining source resistance Rs. With the data from Matt's post of the Hacker this gives Rs = 0.006 Ohm, or about 1mOhm per cell for NiMH six cell stack (a bit low). Is it a high power Lipo? Also the Novak site says the current sensor sensitivity is +/- 0.5 Amp up to 100 Amp, so it is just above the rated sensitivity.

I get 205 Watts peak mechanical power trying to adjust for Rs, still within 10% of your 215 Watts and based on the independently sensed values in the electrical versus mechanical system.
SystemTheory is offline   Reply With Quote
Old 03-14-2009, 10:24 AM   #70
Tech Fanatic
 
Join Date: Jun 2006
Posts: 872
Trader Rating: 35 (100%+)
Default

John, I'll include the rotor inertia in the spreadsheet. All my previous tests were done with just one run based off a topped battery, plus include no current in the data. I have been taking the 3rd run and running that through the dyno because for some reason, I get higher RPMs at the 3rd run. I don't know if it is because the LIPO is 'warmed up' or something, but the 1st run is a good 2k less rpms than the 3rd run.

SystemTheory, the battery I am using is a 5200mah Tenergy 25C lipo rated at 125A continuous.
__________________
High Desert Raceplace, Grand Jct CO, Sanwa Exzes Plus Stick Radio, Spektrum,Thunderpower 230g, Modified Tri-Nut Novak GTB2, Ballistic 4.5t 550, 5.83lb SCTE Ten, Novak Sentry Brushless Dyno, Crossweight Setup Station, Junsi 20A power supply Icharger 20A Charger,TP610C, 22B, Novak Edge, Novak 13.5, SC10, Havoc Pro SC XDrive, Ballistic 17.5 Matthew Joseph Cordova
mattnin is offline   Reply With Quote
Old 03-14-2009, 10:55 AM   #71
Tech Elite
 
Join Date: Aug 2002
Location: Houston, Texas
Posts: 3,777
Trader Rating: 27 (100%+)
Send a message via AIM to John Stranahan
Default

System Theory-Thats not exactly the math check I wanted.

Because we are measuring power by spinning up an flywheel, the total inertia spinning up is important in the power number we get. Note it is only about a 1 % change.

I agree that if the motor has reached constant speed the rotor inertia will not limit power.

Matt- I do expect some change as the battery cannot be the same for each test. Thats why the three other dynos use a power supply to control motor input. Maybe two good runs is all that can be expected without recharging.
john
John Stranahan is offline   Reply With Quote
Old 03-14-2009, 12:45 PM   #72
Tech Adept
 
Join Date: Mar 2009
Location: Metro New York
Posts: 139
Default

John,

I realize ineritia J (or I, if you're a physicist) appears in your power formula, and I think so does the shaft speed difference delta-omega between sample times? Thus if J increases the acceleration rate, measured here as the delta-omega over a 0.1 second window, should decrease. In the continuous time system model, where sample time does not introduce calculation error, it is my understanding the peak mechanical power is limited by the battery circuit and design of the motor air gap, and does not change with inertia J, except that when the motor is designed, a good one makes more power with less Jm then a less optimized machine. That is to say, your peak power should have an expected error introduced by the long sample times, and I'm not sure data smoothing reduces this error, but I don't think actual power is effected at all by changes in Jeq.

Matt there are a number of factors related to heat that could add up to 2k extra rpm on the third pull. Probably a warmer, slightly weaker battery should reduce both the torque and speed, because voltage goes down slightly and resistance goes up at greater states of discharge. I am not an expert on lubrication, but warm bearings might have less damping, and this lets the motor rev to a greater rpm before torque equilibrium Tm = TL caused by the counter-torque in the bearings. That is where you're getting max rpms in the Dyno.

In addition the steepness of the torque-speed line changes with temperature. In a good reference I found online, the steepness of the line is a function of two engineering parameters Ra and k, where k is the air gap constant in volt-second/radian. According to this snippet from the motor expert below, a warm motor might start with less torque but gain more speed based on an increase in Ra and a decrease in k:

Quote:
A complication is that the resistance R rises with α=4 % per 10K and that the motor constant K falls with some percent per 10K by the temperature dependence of the magnet strength. Usually symbol for the relative decrease of the motor constant is kt [%/K]. Concerning this last dependency one has to refer to the motor data sheets, where one also will find a usual tolerance on the resistance R and motor-constant K of 5 to 10 %.
This reference also says that a better motor always has a steeper slope of starting torque over maximum speed, that is S = Ts/wmax is greater for a better motor. That means the cold motor with a fresh battery is probably coming up as the "better motor" on your first pull, and the hotter motor with a less fresh battery is coming up with a little degradation, but you have the data and I am just offering ideas based on studying one expert's advice on motor specification for control system engineers.

Last edited by SystemTheory; 03-16-2009 at 06:45 PM. Reason: typos; logical error corrected
SystemTheory is offline   Reply With Quote
Old 03-14-2009, 04:08 PM   #73
Tech Elite
 
Join Date: Aug 2002
Location: Houston, Texas
Posts: 3,777
Trader Rating: 27 (100%+)
Send a message via AIM to John Stranahan
Default

"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

Last edited by John Stranahan; 03-14-2009 at 04:19 PM.
John Stranahan is offline   Reply With Quote
Old 03-14-2009, 08:20 PM   #74
Tech Adept
 
Join Date: Mar 2009
Location: Metro New York
Posts: 139
Default

John, I just "lost" a very long post, carefully explaining the numbers and units behind my peak power estimates! Luckily I've been through Anger Management courses, and failed miserably

I see the logic behind your peak power calculation and respect the integrity of this approach. Instead of hit you with my calculations and comments, let me present the model in the attached SPICE circuit. This model solves the coupled differential equations:

Tm = k*Ia = Jm*(dw/dt) + br*w (1) feedforward equation
Ia = (Vs - k*w)/(Rs + Ra) (2) feedback equation reducing Ia

Tm - motor torque generated in the air gap
k - air gap constant expressed in SI units
Ia - armature current flowing out of the battery into ESC
Jm - moment of inertia of motor plus flywheel
dw - infinitesimal change in angular velocity
dt - infinitesimal change in time
br - bearing damping assumed to generate a linear feedback torque proportional to shaft speed
w - angular velocity at an instant of time
Vs - source voltage
Rs - source resistance
Ra - armature resistance
Vbe - back-emf voltage developed by generator coils in the air gap, Vbe = k*w

All units are Standard SI units, this sets k to the same value in the feedforward and feedback sources.

When I run this model, holding Vs, Rs, Ra, k, and br constant, then probe for the peak mechanical power in the "flywheel capacitor analog," this peak value does not change when I increase or decrease capacitance/moment of inertia Jm, it remains constant but the peak comes sooner or later in time. If I increase the damping loss br, this becomes a smaller resistor 1/br, it absorbs more torque and shaves power off the peak on the capacitor. Sorry if this is not clear enough, I'll try to explain better if you have questions.

PS - The power curve in the rotor/capacitor is always shaped like the sketch in BLUE.
PPS - By PMDC, I mean a brush motor model, but with a straight line torque-speed curve sampled on the Dyno, and smaller bearing losses (no brush friction), it appears the brushless BLDC motor behaves much like a brush motor in the dynamic system response to a step voltage (during a "Dyno pull").
Attached Thumbnails
Novak Sentry as Brushless Dyno-dynoequivckt.gif   Novak Sentry as Brushless Dyno-pmdc_sketch.gif  

Last edited by SystemTheory; 03-14-2009 at 08:59 PM. Reason: insert Vbe = k*w; add power curve sketch
SystemTheory is offline   Reply With Quote
Old 03-14-2009, 09:57 PM   #75
Tech Elite
 
Join Date: Aug 2002
Location: Houston, Texas
Posts: 3,777
Trader Rating: 27 (100%+)
Send a message via AIM to John Stranahan
Default

Ok. Thanks. Now I have something to study. I will take a look. At one time I used to do a ctrlC just before submitting a post to copy it into memory. I know the agony.



Here are some tips for all dyno users that I have learned through experience.

We would like repeated dyno runs to be within 2 Watts on repeated runs. Improvements are of this small size especially with stock motors.

soldering on leads heats a motor and causes low power.
Wait 5 minutes after soldering or make sure that two comparison motors are tested near the same wait time after soldering. Forget about using clips.

A motor test, heats the motor. Discard the first test. Do repeated tests every 5 minutes on the clock. This gives the motor the same time to cool and takes away that first high reading. Look out for "time trends" Power keeps going down with repeated test. This could be from heating (or with brushed motors from deterioration)

Use the same ambient temperature. Motors test better in the cold. I do my testing indoors at 70 F. Otherwise results don't compare.

And finally a safety point. Put a scattershield on that beast if you are using a flywheel at 7.4 V. I think we are reaching the limit of strength of the 1/8 inch shaft when spinning up a 3.5 motor with the tiniest imbalance. Matt noticed this already with the 7.5. I had a 3.5 really screaming on the Fantom with 7.4 volts.


john
John Stranahan is offline   Reply With Quote
Reply

Similar Threads
Thread Thread Starter Forum Replies Last Post
RC10B4/T4 Forum DerekL Electric Off-Road 14339 12-13-2017 11:20 PM
Dyno, Homemade, Using a Novak Sentry Data Logger, Continued, The Experimental Thread. John Stranahan Electric On-Road 344 12-30-2016 05:22 PM
CRC Battle Axe, GenXPro 10, 1/10th pan, Brushless, Lipo,4c, Road, Oval,TipsandTricks John Stranahan Electric On-Road 1776 03-01-2016 01:18 PM
Novak New Ballistic Motors 20 SMOKE Electric Off-Road 11 05-11-2009 03:58 PM
Idea to Use brushed motor dyno for brushless JevUK Electric On-Road 45 04-06-2009 10:52 AM



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off



All times are GMT -7. It is currently 04:49 AM.


We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
Powered By: vBulletin v3.9.2.1
Privacy Policy | Terms of Use | Advertise Content © 2001-2011 RCTech.net