Sizing an electric motor

[philjc]

I am a structures guy and need help sizing an electric motor for a project I am working on. Things I know: torque required, 110 volts (standard) and 220 (optional), direction is ccw or cw, it will drive a chain or belt, and it will run for 1-6 hrs a day. Do not know: HZ, Amps or anything about electrical. Id appreciate the info.

 

[desertfox]

Hi philjc

The power of a motor in watts is given by the formula:-


P=(2*3.142*r.p.m*torque)/(60)

where r.p.m. = revs per min of motor


If you know how long you need the motor to do one operation
of whatever its driving you can get the power by converting this to r.p.m . For example if the shaft of the motor needs
to go through 180 degrees to perform the task and I want say
sixty operations a minute, then that would convert to 30r.p.m and say the torque required was 10Nm ten the power would be:-


p= 2*3.142*30*10/(60)= 31.42 watts

This calculation is not allowing for any ineffiency so you will need pick a larger power motor than the calculation actually gives you but it will put you in the right ball park.

regards desertfox

 

[philjc]

Thank you Desertfox, I think that will get me going. Any good reference website/reading material on designing mechanisms (drive design, gears, cranks, chains, sprockets)that you can tell me? thanks

 

[desertfox]

Hi philjc

Glad I was of help, I don't know of any sites other than
trying some engineering calculators online specifically do a search for "martindales calculators"


regards desertfox

 

[gcaudill]

How about some more.

Motor HP = Torque*RPM/5250 where:

Torque = torque required to move load
RPM = motor operating speed (3600/1800/1200/900/600... RPM)
5250 = conversion factor

Knowing the selected motor RPM and required motor torque you can size the motor HP. Then there are several issues to investigate ...
1.) Motor voltage (assume single phase) 110V or 220V.
2.) Motor running torque
3.) Motor starting torque
4.) Motor pull-up torque (region between locked rotor and
rated RPM)
5.) Motor starting current
6.) Motor running current
7.) Frame type
8.) conductor sizes
9.) Overcurrent and overload protection
10.) A reversing contactor to change from CW to CCW

... to name a few.

 

[ccw]

Hi philjc

Peruse a copy of NEMA MG-1 and MG-2. The sizing logic is all laid out there. One of the frequently missed mechanical requirements is the driven system start-up inertia. Many times transient characteristics are more important than steady state (at-speed) operation.