I had a look at a huge windmill and was wondering how fast the blades are spinning. Obviously, the distance the blades make is different on the outside than on the inside. Is the speed of a rotating object always measured in rpm and not in mph?
In: 3
If you’re doing any serious math related to rotation, radians per second is the best. Absolutely anything else will require some conversion factor. That’s not to say they aren’t common. For instance, the number of poles in an induction motor is typically 120*f/s where frequency is in Hz and speed is in rpm. But that 120 factor is to make the formula work with rpm.
RPM is often used because it’s a more familiar number to work with. If I say something is spinning at 50.26 rad/s, that is going to be tough to work with mentally. But convert that to 480 RPM and you can already have an idea how fast that is. You know it’s slow compared to an engine, but maybe pretty quick if it’s a carnival ride. You can also use your multiplication tables to get to 8 rotations per second which is even more understandable to a human.
The one and only reason anyone should ever report a rotational speed in distance per time (like miles per hour) is a shock value. A news story might use it because it sounds crazy or deadly or impressive and they know it’ll get more clicks. But no engineer, mathematician, mechanic, or really any industry professional will ever use mph to represent rotational speed.
If you’re doing any serious math related to rotation, radians per second is the best. Absolutely anything else will require some conversion factor. That’s not to say they aren’t common. For instance, the number of poles in an induction motor is typically 120*f/s where frequency is in Hz and speed is in rpm. But that 120 factor is to make the formula work with rpm.
RPM is often used because it’s a more familiar number to work with. If I say something is spinning at 50.26 rad/s, that is going to be tough to work with mentally. But convert that to 480 RPM and you can already have an idea how fast that is. You know it’s slow compared to an engine, but maybe pretty quick if it’s a carnival ride. You can also use your multiplication tables to get to 8 rotations per second which is even more understandable to a human.
The one and only reason anyone should ever report a rotational speed in distance per time (like miles per hour) is a shock value. A news story might use it because it sounds crazy or deadly or impressive and they know it’ll get more clicks. But no engineer, mathematician, mechanic, or really any industry professional will ever use mph to represent rotational speed.
The tips of those blades are moving WAY faster than you suspect.
If you have only MPH then you need to know at what distance from the center that measurement was taken.
I was driving home the other day and notice that one of the turbines I passed took about 4 seconds to complete one revolution. From this it is easy to determine that in one minute the blade would make a complete revolution in 60/4 = 15 revolutions in one minute.
In one hour that would be 60*15 = 900 revolutions.
The average span (diameter) of a wind turbine in Europe is 113m or 370′. The circumference of a circle is pi*d so the tip of that blade travels pi*370′ = 1162′. 1162′ is .2201 miles (the blade tip is traveling almost a quarter of a mile with each revolution!)
In one hour the blade tip will travel .2201mi * 900 = 198.1 miles. So the tip of the turbine blade was traveling almost 200 mph! This is one reason they frequently kill birds.
The tips of those blades are moving WAY faster than you suspect.
If you have only MPH then you need to know at what distance from the center that measurement was taken.
I was driving home the other day and notice that one of the turbines I passed took about 4 seconds to complete one revolution. From this it is easy to determine that in one minute the blade would make a complete revolution in 60/4 = 15 revolutions in one minute.
In one hour that would be 60*15 = 900 revolutions.
The average span (diameter) of a wind turbine in Europe is 113m or 370′. The circumference of a circle is pi*d so the tip of that blade travels pi*370′ = 1162′. 1162′ is .2201 miles (the blade tip is traveling almost a quarter of a mile with each revolution!)
In one hour the blade tip will travel .2201mi * 900 = 198.1 miles. So the tip of the turbine blade was traveling almost 200 mph! This is one reason they frequently kill birds.
The tips of those blades are moving WAY faster than you suspect.
If you have only MPH then you need to know at what distance from the center that measurement was taken.
I was driving home the other day and notice that one of the turbines I passed took about 4 seconds to complete one revolution. From this it is easy to determine that in one minute the blade would make a complete revolution in 60/4 = 15 revolutions in one minute.
In one hour that would be 60*15 = 900 revolutions.
The average span (diameter) of a wind turbine in Europe is 113m or 370′. The circumference of a circle is pi*d so the tip of that blade travels pi*370′ = 1162′. 1162′ is .2201 miles (the blade tip is traveling almost a quarter of a mile with each revolution!)
In one hour the blade tip will travel .2201mi * 900 = 198.1 miles. So the tip of the turbine blade was traveling almost 200 mph! This is one reason they frequently kill birds.
Thank you so much, I am impressed and love this sub!
Thank you so much, I am impressed and love this sub!
Thank you so much, I am impressed and love this sub!
If something is rotating, revolutions per minute is the only measurement that makes sense because the entire length is moving at the same speed (in rpm).
Every point on the blade (in your example) moves around in a circle so the actual distance in metres, miles or whatever depends on how far that point of the blade is from the centre of the circle (the tip of the blade is moving faster in mph than the middle of it is).
If something is rotating, revolutions per minute is the only measurement that makes sense because the entire length is moving at the same speed (in rpm).
Every point on the blade (in your example) moves around in a circle so the actual distance in metres, miles or whatever depends on how far that point of the blade is from the centre of the circle (the tip of the blade is moving faster in mph than the middle of it is).
Latest Answers