Calculating Motor Efficiency: A Step-by-Step Guide

good morning today our goal is to calculate the efficiency for a motor for several different masses

let's watch the video so there's our motor and the goal is to determine the

efficiency of that motor while lifting an object the motor is powered by three d-cell batteries

now a motor converts electrical energy into potential energy and so the question is how efficient is

it at converting electrical energy into potential

energy that's our goal today to measure electrical energy we'll need to measure voltage

that multimeter is set to measure voltage in addition we'll also have to measure

current that multimeter is set to measure current

now we need voltage and current to determine power however that's not electrical energy

in addition to power we'll have to determine the time it takes for the motor

to lift the object for potential energy we'll have to know the distance that the motor lifts the

object through for every trial today it'll always be 50 centimeters or 0.50 meters

in addition we'll have to measure the mass and so let's start off with that please

record these five different masses these are the masses today that we're going to use to

determine the efficiency of the motor and here's the table i'd like you to complete today

please copy the title please copy the units for the headings and notice the headings mass

slow motion time voltage and current notice for slow motion time i'd like you to complete four trial runs here's the

data for the 11.86 gram mass notice i've rotated the camera by an angle of 90 degrees

it's easier to see the meter stick this way and it's also easier to see when the mass

hits the 30 centimeter mark and the 80 centimeter mark so you're going to start your stopwatch

at the 30 centimeter mark you're going to stop the stopwatch at the right arrow the 80 centimeter mark

and in total that's a distance of 50 centimeters in addition you'll be doing this four

different times so i'll play the video for you now start and

stop so please complete four trials with your stopwatch

and notice the video was recorded in slow motion we'll deal with that later on

to record voltage and current i use the second camera let's watch the video this video is not

recorded in slow motion it's recorded in real time now maybe a little challenging to see

that in real time and so what you should do is play the video frame by frame

what i mean by that is to use the period key that moves the video forward by one

frame or to use the comma key that moves the video backward by one frame

i'll show you what i mean now here i record this frame by frame and this is something you could do with

any video on youtube now when you play the video frame by frame you'll notice that the most

consistent voltage is 4.28 volts and the most consistent current is 0.16 amps

so for your first data point after playing that video frame by frame that's what you should come up with for

voltage and current so there's the first row of data filled out for you

of course you should also complete the four slow motion trial times

now before we continue the experiment you may have noticed that the voltage drops significantly

prior to turning on the motor it was 4.58 volts but then while the experiment was

running it was only 4.28 volts so the question is why is there this voltage drop

well the problem is the current itself and the internal resistance of the batteries

every battery has some internal resistance what does that mean well as the current increases the effect

of the internal resistance on the overall voltage increases so the more current there is

the greater drop in voltage there'll be due to the internal resistance and here we have a drop of around 0.3

volts and this happens because of the internal resistance of the battery

all right here's our next data point the 22.94 gram mass here comes the video get ready with your

stopwatch here we go right now remember to start now at the green arrow

and stop at the now at the red arrow please complete four trials here comes the voltage and the current

remember this is in real time remember to play it back frame by frame the period key moves the video forward

by one frame and the comma key moves the video backward by one frame

here's the next data point 33.8 grams get ready with your stop watch remember four trials

and here's the voltage and current again use frame by frame to determine the most consistent voltage

and the most consistent current here's the next data point 44.59 grams get ready with your stopwatch

start your stopwatch right now the 30 centimeter mark and stop the watch

at the 80 centimeter mark here's the voltage and current for that data point

again if you play frame by frame it's easier to determine the voltage and current

here's a 61.10 gram data point get ready with your stopwatch

remember always for trial times here's the voltage and current for this data point

once again please use the comma key and the period key to advance the video one frame at a time

either forward or backward to get the most consistent voltage and current

so at this point you should have completed this entire table now we're going to take that table and

process the data to ultimately determine the final column which is the percentage efficiency

here are the formulas to assist us to get the average time we need to take the average slow motion

time and divide by eight this is because the video increases the length of time by a factor of 8

when we record things in slow motion power is the product of current and voltage

input energy is the product of power and average time and output energy is the mass multiplied

by 9.8 the acceleration due to gravity multiplied by the height that it was lifted by always 50 centimeters or 0.50

meters and here's our final formula for percentage efficiency

it's output energy or more commonly called useful output energy times 100 divided by the input energy

so hopefully you enjoyed today's lab activity hope you have a great day bye