motor experts? Change Rpms

[nesc39]

i recently got into the electrical field 1 year ago and am currently in school but beed help with a motor i want to use in a hobby

The motor i need to use should on turn anywhere from 10-20 rpms. My question is..............how are different motor speeds built? how do they work?. Some motors turn 10,000rpm and others only 2rpm. How do they change the rpms. Is there any way i can increase or decrease the rpms in a given motor without using gears?

 

[mdshunk]

For an AC motor, a VFD would be the choice. For a DC motor, there are more methods, but a DC motor "drive" is the typical choice. If you wanted to custom wind a motor, more or less "poles" would get you different RPM's at full voltage without using a VFD.

 

[Rampage_Rick]

For an AC Sychronous motor the RPM is determined by the number of poles vs the AC frequency. A 2-pole motor on 60Hz will rotate 60 times per second, or 3600 RPM. A 4-pole motor will rotate 30 times, or 1800 RPM, and an 8-pole motor would run at 900RPM. Generally speaking any other speeds are done via gearing or by changing the frequency (which is where VFDs come into play)

A 3-phase 4-pole (per phase) synchronous motor will rotate at 1800RPM at 60Hz, but at 20Hz will rotate at 600RPM. In theory you can adjust the RPM from 0-1800, though many motors have a minimum frequency in order to maintain rotation.

DC Brushless motors actually operate in a very similar principle to this, as they are in fact 3-phase motors.

http://www.allaboutcircuits.com/vol_2/chpt_13/2.html

 

[zbang]

Induction motors will rotate somewhat less that the synchronous speed would indicate (i.e. the 1725 rpm motor) due to "slip". And any fractional horsepower motor less that about 1200rpm is probably geared down (since more poles means more space, small motors usually have fewer poles).

Spend some time poking around on wikipedia.org, start with "electric motor".

If you need 10-20 rpm, look for a small geared motor. C & H Sales in Pasadena CA seems to always have lots of varous surplus motors in stock.

 

[LarryFine]

If you need 10-20 rpm, look for a small geared motor.

Definitely.

Nesc, can you describe the load, how heavy, what kind of motion, what is the appliction, how will it be mounted, what kind of output shaft or lever, etc?

 

[zog]

N=(120*f)/P

N=Speed
P=Poles
f=Freq

You either change the number of poles or change the freq