Properly Measuring Stall Current on DC motor

[172heavy]

I have two motors that normally run on 24volts, these are Jazzy Jet 3 motors which have no data! I am trying to determine what the stall current is so that I can select a proper driver etc. The wires that supply power to the motors are 14ga. I measured the resitance across the A and B terminals of the motor and I can't get a consistant reading, 2 to 4.5 ohms. I was told that I could hook the motor up to a lower voltage and extrapolate the stall current. I hooked it up to a 9 volt battery the motor was locked and the current that I measured is 3.9 amps at 8.7 volts. 8.7v/3.9amps yields 2.2 ohms. 24 volts/2.2 ohms = approx 10.9 Amps stall current.

Is this even close to being an accurate way to derive this value?

 

[MikeHalloran]

I'd call the resistance 2 Ohms and work from there.
I.e., I'd want a driver that could supply 12 Amps and then some.



Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

The voltage drop across the brushes is non-linear. Measure the resistance across the armature. The calculated current will be a few percent high due to ignoring the brushes. Should be safe for selecting a driver.
(If this is a four brush motor, let us know.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[172heavy]

When you say calculated current do you mean what I calculated/measured. Or what I should be able to calculate after measuring across the armature?

Thanks.

 

[Skogsgurra]

A question

Are you planning to run with stall current at all? That is usually never done except if you are running without a controller (i.e. connecting directly to a stiff DC source).

But, you have those 24 V controllers now, haven't you? Just let them handle stall situation by setting the current limit to a safe value - twice or thrice the rated current is usually OK with these low volt motors.

A tip: Get a DC clamp and some kind of scope/recorder/USB data taker and measure. You will need that later in the project so why not get it now and benefit from the good learning such equipment will give you?



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Applied voltage divided by measured armature resistance will give a safe value for stall current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

PS: What Gunnar says.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[172heavy]

Yes, I did purchase the correct controllers.

I have a DC clamp and a USB Scope. It is not a real fancy one, it is the PropScope from Parallax. It seems to do the job. I use it quite a bit with the different timing circuits serial communication etc. that I have messed with in the past.

I will measure the armature resistance and see where I end up.

Thanks for the input.

 

[172heavy]

I have a basic question here, what exactly does an H-bridge controller IC do for me? If I were to build a simple h-bridge from an N type enhansement type MOSFETS where would the controller come in?

I know this could turn into a much deeper discussion, that is not really what I am looking for, I have ordered a few books on the subject and I plan on doing my homework here, I don't think the controllers I purchased are up to the task.

I have not completly written then off but they are generating a lot of heat and I havent even applied a load yet.

I am going to shorten my power leads and try to go up to 12g wire in a effort to control the heat.

Thanks,

Josh