R/C Tech Forums

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

Reply
 
Thread Tools Search this Thread
Old 10-13-2003, 10:19 PM   #1
Tech Initiate
 
Join Date: Sep 2003
Location: Nebraska
Posts: 43
Default T.C. motor, Volt and Amp draw

I just have two questions.

1) What is the maximum voltage and amperage a stock motor could draw under the conditions listed below.

2) What is the maximum voltage and amperage a modified (e.g. 9 turn or the like where run time is of no concern) motor could draw under the conditions listed below.

The car is any competitive touring car under racing conditions and the batteries would be six 3300 mah NiMH cells and a competition grade esc.
kcburnzz is offline   Reply With Quote
Old 10-14-2003, 08:53 PM   #2
Tech Regular
 
dpaton's Avatar
 
Join Date: Sep 2003
Location: Schaumburg, IL
Posts: 283
Send a message via AIM to dpaton
Default

Well, assuming that the ancient Midnight in my XXX-S is a reasonable specimine of a stock motor:

Rarm is ~0.8 ohms, the resistance of one of the armature coils
Varm is 7.2V, the nominal voltage of a 6 cell pack.

Theoretically, V=IR, so your current is Varm/Rarm, which in this case is 9A.

For a mod motor we only need to know Rarm to do the same calculation.

Unfortunately, because of the way that motors work, that figure won't happen much in the real world.

Every time the arm turns, the electric field in one of it's 3 coils is being broken down at the same time the adjacent coil's e-field is being set up. This takes a lot wf work on the part of the incoming current, because the coil is spinning in a magnetic field, which is trying to generate a voltage in the arm counter to the one the ESC is feeding it. The coil being town down also generates a flyback voltage spike as it's contact with the brush is broken and it's field starts to collapse, which messes up the nice little DC model we use even more.

ESCs use pulse width modulation to drive the motors. In PWM, the ESC outputs a stream of pulses of power to the motor that are all at full voltage, but vary in their width (duration). The width of those pulses determines the duty cycle of the ESC, and after a little bit of filtering (those big caps we all love and/or hate, and the inherant inductance of the motor's windings) it gets turned into a relatively DC looking voltage. When you throw it all together tho, things get complicated, what with Schottky diodes and motor caps and lions and tigers and bears, oh my!

In reality, the current and voltage at the motor terminals are wildly fluxuating, all the time, and computing any kind of figure is an exercise in controlled guessing. Brush arc, contact with other cars, etc, can all cause the current to vary quite a lot. I had a 2wd off road car I instrumented a while back and measured about 9A with a stock motor, but up to 141A in short bursts, due to brush bounce and me hitting a wall. This is one of the reasons current limiters are a good thing. :-)

Now my question: Why do you want to know?

-dave
dpaton is offline   Reply With Quote
Old 10-14-2003, 09:08 PM   #3
Tech Initiate
 
Join Date: Sep 2003
Location: Nebraska
Posts: 43
Default

well... what I really want to know is how many amp and volts can I generate with a modified motor. I have done a few non-scientific trials here and so far when I turn my chameleon at roughly 15-20k rpm I get back 7.5v and about 2 amps (which is more than I recieved with the stock motor. If I use a motor with fewer winds (eg 9T double ) at 20k rpm can I generate enough amps to run a hypothetical touring car under racing conditions.
kcburnzz is offline   Reply With Quote
Old 10-14-2003, 09:12 PM   #4
Tech Initiate
 
Join Date: Sep 2003
Location: Nebraska
Posts: 43
Default

also unfortunatly I do not have much electrical measuring equipment except a fluke 23 multimeter. My power source is simply my battery and esc from my race car, no turbo 35!!
kcburnzz is offline   Reply With Quote
Old 10-15-2003, 06:17 AM   #5
Tech Regular
 
dpaton's Avatar
 
Join Date: Sep 2003
Location: Schaumburg, IL
Posts: 283
Send a message via AIM to dpaton
Default

So you're looking to run a motor as a generator instead?

As a basic rule of thumb, if you're using a stock motor in the car, use something with ~10 less winds as the generator. With a 27t stocker, a 19t outlaw would probably be a good starting point.

That said, RC car motors make pretty poor generators. Generators are wound differently, with different timing and different magnet structures.

Also, when testing your setup, you'll need some kind of load to put on the generating motor. Usually a 1 ohm 10W resistor with a fan on it or an old 27t stock motor will be OK for short testing sessions. When you load the generating motor it will provide a more realistic measure of the real Vout and Iout, which will be somewhat lower than your initial measured numbers.

How are you spinning the generating motor, with a Dremel tool?

-dave
dpaton is offline   Reply With Quote
Old 10-15-2003, 10:54 AM   #6
Tech Initiate
 
Join Date: Sep 2003
Location: Nebraska
Posts: 43
Default

for now I am using a dremel but eventually I will power it with a small gas enigne. Any suggestions for a proper generator that is roughly the same physical size as an rc motor?
kcburnzz is offline   Reply With Quote
Old 10-15-2003, 11:36 AM   #7
Tech Regular
 
dpaton's Avatar
 
Join Date: Sep 2003
Location: Schaumburg, IL
Posts: 283
Send a message via AIM to dpaton
Default

A proper generator in 540 size is something I haven't seen before. Your best bet it to try and find one of the motors for an old style motor dyno.

If you can't find one of those, you'll need to get yourself a motor that you can set up to run in reverse, by spinning the endbell 180 degrees. You'll also want to move the timing back to near 0, since reverse timing will only decrease your power output. using an RC motor as a generator will be mainly trial and error with the timing adjustment, trying to get the best power.

Ideally you would use one a brushless motor, but that would also require conversion from the 3 phase connection on the BL motor to single phase for your load.

Finally, remember that the voltage you'll get out of a motor run as a generator will have a significant AC component. You cam measure this using the AC side of your Fluke. If you want clean DC you'll need a rectifier and some capacitors.

Powered by a gas engine....this sounds interesting. May I ask what the end result will be for? I'm having trouble imagining why you'd want to pursue something like this...

-dve
dpaton is offline   Reply With Quote
Reply

Similar Threads
Thread Thread Starter Forum Replies Last Post
motor amp draw Z-Fire Electric On-Road 0 05-04-2008 05:18 AM
motor amp draw Z-Fire Electric Off-Road 0 05-04-2008 05:12 AM
Motor Break DOES Higher AMP draw = better motor? Travis S Electric On-Road 13 02-12-2005 03:02 AM
Motor AMP Draw? atlrcracer Electric On-Road 13 07-13-2004 09:44 AM
Stock Motor Amp Draw warlord385 Electric On-Road 10 05-02-2003 01:11 PM



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 05:08 AM.


Powered By: vBulletin v3.9.2.1
Privacy Policy | Terms of Use | Advertise Content © 2001-2011 RCTech.net