42 Volt Limited Top Speed

[SWFLrunner]

So if you look at the tests for comparison;

Wheels off the ground
1. Speed limit off, max RPM limited to about 5600 RPM
2. Speed limit set to 7000, RPM limited to 6600 - makes sense 400 RPM safety
3. Speed limit set to 8000, didn't run it up here but I would figure it would go to 7600.

Aside from the unexpected result of no set speed limit, the off the ground results would be "normal".

Now following the numbers above for the ground speed tests, same state of charge same course, same controller settings, GPS measured, same device.
1. 22 MPH
2. 24 MPH
3. 25 MPH

The question is what factor would effect ground speed resulting from the speed limit increases when there is a load on the cart?

There has to be some variation in the voltage being thrown at the motor resulting from the speed limit changes. Maybe it's overcoming another factor. it's not brake resistance. Maybe it's overcoming an inefficiency in the motor to get this effect but the program changes are definitely effecting the ground speed.

[yurtle]

Generally, more load means more amps, for a given speed. Assuming you have a large enough controller (max amps), an increased load, withing reason, should still allow you to achieve the same top speed, though you may take a little longer to get there.

[SWFLrunner]

For the record my wife thinks I'm completely mental for wanting to know what's going on LOL, maybe she's right.

She though outboard prop calculations were nutty, I'll show her...

[yurtle]

Women. LOL. JUST JOKING. Or am I???

[JohnnieB]

The only thing that comes to mind is that the the field mapping is altered by the RPM limit setting. About the only easy way of determining if that is happening is to view data logs recorded at different RPM limit settings.

The amount of torque a series DC motor produces is proportional to the armature amps being drawn and the amount of armature amps that the motor can draw is inversely proportional to the RPM of the armature and the voltage appled. Basically the motor's RPM will increase until the torque it produces equals the mechanical load imposed upon it. Since a higher voltage will pump more amps through the same impedance, a 42V battery pack will cause a motor to reach roughly a 16.7% higher RPM than a 36V pack will if the mechanical loading is the same.

A sepex motor (IE: the type used in a PDS cart) works the same, but the field winding are excited separately from the armature windings, so the relative strength of the magnetic fields produced by the armature and field windings can be altered while it is in operation to effectively produce more high end torque, which in turn increases the max RPM reachable for the applied voltage. In a nutshell, a sepex motor can transformed from a high-torque motor into a high-speed motor by reducing the amount of field current. (At the expense of efficiency, so sepex motor tend to run hotter than series motors.)

The data logs will show what the minimum field current reached for the various RPM limit settings. The lower it is, the more the controller is shifting the motor towards higher speed.

[SWFLrunner]

Got the log files. The cart is hot off the charger after charging overnight, the Accusense reading fully charged.

Runs were made consecutively over the same road/direction, tires measure 18", all other variables in the controller unchanged aside from speed limit.

Speed readings in order they occurred, the log files match up to each pass here.
Pass 1 8000 speed limit, 25 MPH (actually read 26 but could be slight GPS error, call it 25) I have a video from this run.
Pass 2 7000 speed limit, 25 MPH, felt the same to me as pass 1 in acceleration and top end.
Pass 3 6000 speed limit, 23 MPH, noticeable drop off felt in acceleration and top speed.
Pass 4 no speed limit, 23 MPH, felt the same as the 6000 speed limit.

Pass 5 8000 speed limit retest, I did not log this one 24 MPH
Pass 6 8000 speed limit, 25 MPH, I did not log this one, could be some outside influence between these 2 runs or GPS error, don't know.

[JohnnieB]

I loaded the csv files into spreadsheets and did some sorting, editing (deleting extraneous lines) and number crunching.

The samples are short in duration, about a minute or less, and the lengths of time the cart is actual running is still less, but it appears the the RPM limit setting do effect low end torque (acceleration) and top speed even though the motor is not reaching the max RPM setting.

Also, the speeds I calculate from the max motor RPM at 100% throttle vary from the GPS speeds, as follows:
8k limit - 6068RPM Max = 26.12MPH
7k limit - 6159RPM Max = 26.51MPH
6k limit - 5678RPM Max = 24.44MPH
No limit - 3878RPM Max = 16.69MPH

The 7K setting appears to be faster than the 8k, but it is such a slight difference it might be due to wind speed or some other factor. With multiple samples of longer duration of each setting, I suspect they will average out to the same speed. What you are seeing is the max RPM that specific motor can reach working against that specific load at the voltage being applied. Since the motor can at that load and voltage can spin faster than 6000RPM, the 6k setting does effect the top speed.

As for low end torque: Max Armature amps and Field current for each RPM limit setting.
8k limit - Armature = 276.5A Field = 24.6A
7k limit - Armature = 242.2A Field = 19.0A
6k limit - Armature = 266.7A Field = 23.1A
No limit - Armature = 240.7A Field = 18.0A

[SWFLrunner]

I think the no speed limit is a bad log taken at the wrong time, I am going to rerun that one and run a 6500 log and see what comes up.

Did the pack voltage seem OK coming right off the charger and response to loads?

[JohnnieB]

Originally Posted by SWFLrunner

I think the no speed limit is a bad log taken at the wrong time, I am going to rerun that one and run a 6500 log and see what comes up.

Did the pack voltage seem OK coming right off the charger and response to loads?

Your pack voltage looks okay, pretty good in fact.
Never dropped below 37.0V in any of the logs and never dropped below 39.0V in any log while at 100% throttle. (Peak amp draw occurs at low RPM, often before throttle ramps up to 100%)

In and of themselves, the seven batteries in a 42V pack will drop in the neighborhood of 1.25V per 100A of current draw. The cables & connections, solenoid contacts and controller will typically drop about 1.3V/100A for a total of about 2.55V/100A. Looks like you drop about 2.6V/100A, so the resistance of your cables, connections, contacts and controller is about as low as it gets.

The thing to remember when setting the RPM limit is the cart will go faster downhill than it goes on level ground. If it is only you driving cart it isn't too hard to remember not to go downhill at full throttle. If there is more than one driver, the RPM limit ought to be set the a safe max RPM for going downhill.

I made the mistake of going downhill with my foot on floor to see how fast the cart would go. Based on the GPS speed and my 17" tire height, the motor was spinning at about 8000RPM when I lifted my foot and when the regen braking kicked in, the armature exploded. I slid to a stop from about 33MPH, leaving long black marks on the pavement and a short brown one on the seatcover.

[SWFLrunner]

2 logs for today

The 6500 RPM speed limit allowed the motor to spin up to about it's max RPM limit, just a little over 6000 RPM.

The no speed limit log confirms that the controller holds the RPM limit and top speed down on flat surface.

Thanks for the education on this stuff.