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Topic: Power,Voltage,Current & Resistance of a motor (Read 1 time) previous topic - next topic


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 :P


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.



Apr 05, 2012, 01:41 am Last Edit: Apr 05, 2012, 01:44 am by DVDdoug Reason: 1
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.


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

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