What factors affect the rotating speed of a motor?

[mainer0311]

This is my first post, so I'll give a bit of an intro. I am an marine engineering cadet at mass maritime academy in my junior yet. I took physics II last semester and this semester I am taking electrical machines.

In class today we were talking about how fast motors can rotate and came up with this equation:

Ns = 120f/p Ns being the speed of the motor, f being the frequency and p being the number of poles.

with this equation using 60hz and a 2 pole motor it says that a motor will only rotate at 3600 rpm's.

I know this isn't true because wood working routers rotate well over 15,000 rpm.

I asked my Proff how this worked and was told to try and find an answer for next class.

I haven't been able to find anything searching google about it. Does anybody know how this works?


Thanks Cooper

 

[tvavanasd]

Different types of motors have different speed and torque characteristics.
The equation applies to AC induction motors, which run asynchronously at just under that suggested. It also applies to some synchronous motors.
Universal motors, DC brush commutated motors and many others do not obey the equation.

 

[Perfection]

They use Universal motors which are different from whatever the heck kind of motor your prof is talking about:

http://en.wikipedia.org/wiki/Universal_Motor#Universal_motors

Edit: or what the TV said. :p

 

[mainer0311]

So the speed of an induction motor is dependent on the frequency of the power going to it?

And these tools that require speeds greater then 3600 rpm use a universal motor?

this is making more sense, thanks.

 

[SirAskalot]

Or a frequency converter. Or gears and pulleys. It all depends on the system being powered.