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How to easily measure stall current of DC motors?

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Nicksan

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Ive googled and yahoo`ed and searched here. I found one link, but its in some strange, proprietary format I cannot read. PPT whatever that is.
 
JUst clamp the shaft and manually pulse the power to it once quickly while measuring the current.
 
With a ammeter in series with the motor and battery? I think thats how I did it a long time ago.

:?:
 
If you are applying DC then the stall current is simply V/R. Where V is applied voltage and R is the static resistance of the motor.

Mike.
 
Pommie said:
If you are applying DC then the stall current is simply V/R. Where V is applied voltage and R is the static resistance of the motor.

Mike.

DOes that resistance correspond to stall impedance?
 
dknguyen said:
DOes that resistance correspond to stall impedance?
Aren't DC impedance and resistance the same? When you measure the resistance you are applying a voltage and measuring the current with the motor stationary. The magnitude of the applied voltage is irrelevant.

Mike.
 
Pommie said:
Aren't DC impedance and resistance the same? When you measure the resistance you are applying a voltage and measuring the current with the motor stationary. The magnitude of the applied voltage is irrelevant.

Mike.
DC impedance and resistance are the same. WHat I meant was does the resistance measured represent the impedance of the motor at stall? As in can the stall current be calculated from it. WHich is a bit strange in itself since that would mean the voltage you use determines the stall current (it may, I'm not quite sure.) I was always under the impression that a DC motor's current is representative of torque independent of the voltage being applied.
 
Torque doesn't matter when the motor is stalled because the shaft isn't rotating which means no work is being done. The motor's current is dependant on the voltage applied, the speed of the shaft and the load resistance.
http://www.ezonemag.com/pages/faq/a405.shtml
Has some quick ways to calculate the three motor constants from which you can use to determine pretty much anything you need to know about the motor excluding things like hystersis inertia and heating losses.
 
As long as you are using DC then the resistance is the same as the impedance at stall and can be used to calculate stall current.

Torque is proportional to current.
Current is equal to (applied Voltage - back EMF)/resistance.

As back EMF is proportional to RPM then when the motor is stationary I=V/R.

At the other extreme, when free running, the back EMF almost equals the applied voltage and therefore the current is minimal. If you apply a load to the free running motor, the RPM and therefore back EMF reduce and the current consequently increases.

There was a really good article on 4QD which explained the relationship between all motor parameters in terms of simple motor constants. They now appear to only have a simplified version here.

Mike.
 
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