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[on:]

So i connected a 12v motor on a pretty big car battery the other day (12 volt,70Ah). When the motor was rotating with its shaft free (no load) the voltage across its terminals would be 12V and the current drawn 200mA. When i squeezed the shaft with my fingers (a significant amount of load applied) to the point where the motor was ready to stop, the voltage would be 8V and the current drawn would be 1A.

My Questions:
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>Could you please explain this to me in terms of both the power law (Power = Voltage x Current) and Ohm's law (Voltage = Current x Resistance)?

>Is there such a thing as lower resistance for motors? (if the equation V=IxR is applied, in the first case [no load] the resistance is 60 ohms which then drops to 8 ohms in the second case [full load])

>Why did the voltage drop from 12volts to 8 volts? (i was using a pretty huge source of stable DC energy)

>Am i right on this? : "The only two factors determining the current drawn by something is the voltage supplied to that something and the resistance of that something. If the voltage is steady (or drops), the resistance has to also drop in order for the current drawn to increase "

Any "light" to my question is well appreciated. Thank you in advance

[Reply #1 on:]

Ohms law does not apply since the motor is a non-linear device.

Firstly if your battery voltage fell to 8 volts, your battery is dead.

When a motor spins it generates a back emf which opposes the supply voltage, current drawn is determined by supply minus back emf.

When you apply load to the motor, its speed slows so back emf reduces, therefore supply minus back emf increases and hence current increases.

The resistance of a motor winding plus brushes plus commutator simply governs the maximum current that flows (as determined by ohms law) when the motor is at a complete stop.

After the motor starts to turn ohms law goes out the window

[Reply #2 on:]

Jeezum Crow (That's VermontSpeak), JackRae, I jumped in here to answer this cool question and you already covered everything so well.

Time for the War of the Gods... 

[Reply #3 on:]

Thank you very very very much
Regarding the battery thing...
I measured the voltage at the terminals of the motor not the battery (dunno if this makes any difference). I think the battery is good since its able to start a 2liter diesel engine easily. Could the reading have changed (dropped from 12V to 8V) as a result of the back EMF induced?

Again thanks for the help...(Is there a way to vote for the best answers btw?)

[Reply #4 on:]

Back emf does not affect the motor terminal voltage (you might even consider it as a virtual thing)
At only 1 amp of current, to drop to 8 volts, your line resistance was 4 ohms (12-8)/1    Note that ohms law applies to wire and dodgy connections  Therefore you must have been using either microscopic wire or your connection was extremely poor.  If the battery can start an engine then it can deliver several hundred amps at almost full terminal voltage.  I suggest you check your motor/wire connections and repeat your tests.

[Reply #5 on:]

True... the wire was indeed tiny(1mm diameter) and the connections just touching  .One of the motor terminals was connected on the chassis (ground) through the tiny alligator clip wire and the other pole of the motor touching the + pole of the battery.  Could you please explain the wire/line thing to me?

If i got this straight you mean that because of the tiny wire and the bad connections, the wire's resistance increased when the 1A was drawn and therefore more voltage was used by the wire. So basically with 1A of current, the resistance of the wire became comparable with the resistance of the motor and therefore i had created a voltage divider circuit. Am i correct on this? Please shine some light on this as well

[Reply #6 on:]

Please stop considering the motor as a resistor - it is anything but.  Yes it does have resistance but it is not constant and "varies " with applied voltage - and this is what causes confusion to beginners.  Resistors are passive devices with linear voltage versus current characteristics whereas a motor is an active device since it generates voltage (back emf) when it rotates. Your poor connections, probably the clips, formed a resistor is series with the motor.  As you loaded the motor it demanded more current and the poor connections (resistance) dropped some of the supply voltage.

If you get good connections between battery and motor you will find the motor voltage remains almost constant at 12 volts , even when you load the motor.  Since the motor supply voltage is constant, its speed will (should) remain fairly constant. You will also find the motor now has much more torque and draws much more than the 1 amp you previously measured.

Your 1mm dia wire is good enough for a rough test, providing the length is reasonably short, say less then a couple of metres.

[Reply #7 on:]

Thank you very much for the info sir

[Reply #8 on:]

 A 4-volt drop with a small motor on a car battery is rather "shocking".     

wire became comparable with the resistance of the motor and therefore i had created a voltage divider circuit. Am i correct on this?

EXACTLY!   If you have a small battery that can't put-out the required current, so that the voltage drops when you apply a load, we attrbute this to the internal resistance of the battery (or power supply). 

Just to clarify something - Ohm's Law ALWAYS HOLDS!   But, it can get tricky with non-linear devices and loads (motors, semiconductors, LEDs, etc.) because the resistance/impedance changes under various conditions.    When you put a heavy load on a motor, the impedance drops and you get more current.   The Ohm's Law relationship between current, voltage and impedance is still true...  If voltage is held constant and current increases when you put a load on the motor, impedance must have decreased.   

Another time Ohm's Law can get confusing is with capacitors and inductors in AC circuits, where current and voltage are out-of-phase.    If you use a multi-meter to measure voltage and current, you might think Ohm's Law is wrong (because you are measuring the average or RMS voltage and current).   But, at any instant in time, Ohm's Law holds.

[Reply #9 on:]

Definition of Ohm's law :

Ohm's law states that the current passing through a device is directly proportional to the potential difference applied.

So where in a motor is the direct relationship between voltage applied and current flowing  (there isn't one)

Many people confuse the law on the basis that there is a relationship between voltage and current, as there obviously is.  But in many devices this relationship is NOT directly proportional - which is the basic requirement for the defined application of Ohm's law

[Reply #10 on:]

Basically ohms law applies to conductors like metals and sometimes to semiconductors (a single piece of uniformly doped semiconductor bonded to the right kind of metal wires, for instance).  It doesn't apply to more complex semiconductors (diodes, transistors), or inductors / capacitors / transformers / motors.

It also only holds for fixed temperature and normal current-densities (the kind of high current densities that begin to show non-linearity are actually pretty extreme and lead to vaporization in very short time scales!)

For linear behaviour the average drift velocity of charges carrying the current has to be significantly less than the thermal velocity of the charges - for standard copper wires in normal use the drift velocities are measured in mm/s whereas the thermal (Fermi) velocity of electrons in copper is 1570 km/s, 9 orders of magnitude higher!