Motor speeds

marty D_2

I am curious about a lot of things and this is one of
them. We have/had 1725 rmp burner motors. Now we mostly
are installing 3450. I have asked a few fellow techs and
it seems nobody knows what determines the rpm. Any motor
guru's out there that can explain this. Is there a formula?

thanks

Marty

Robert O'Connor_6

It is dictated

bye the burner manufacturer. Usually the older burners get 1725 and the the newer ones get 3450. Have to watch your pumps on this too

Regards

Robert

ME

steve_60

why ac motors are 3450 and 1725 rpm

This website has a good explanation for you: http://www.aosmithmotors.com/html/motorDoctor/Determiningspeed.htm

The speed is related to the number of poles the motor has and the alternating current's frequency.

oil-2-4-6-gas

.

Boston Boiler_2

Motor & pump speed must match like Robert said and also rotation.

BB

Mitch_4

for every

pair of poles in a motor, you divide the # of pairs into 3600 (60hz = 60 cycles/sec X 60 =3600/min), so a 2 pole motor would be 3600 / 1 (pairs) = 3600 rpm, a 4 pole motor would be 3600 / 2 = 1800 rpm. These are true no-load values. In reality due to slippage, friction and loads, they are averaged so a true 1800 rpm results in approx 1725 because the motor will not spin as fast as the rotating magnet (Poles).

Clear as mud?

Mike

marty D_2

slippage?

Thanks,that basically answers my question, and the website
that I was directed to in previous response (thanks) was very helpful as well. I understand load and friction, but was is "slippage"

marty

Bluenose_24

higher static

I believe that they went to higher rpms and smaller diameter blast tubes for higher static pressure in the unit.

Firedragon_4

bluenose is the closest on this,

it has to do with velocity.

It takes a given amount of air to burn a gallon of oil, that's chemistry. To overcome tighter heat exchangers (pressure drop) you need more static pressure.

When burners went from 1725 to 3450 the speed (velocity) doubled. Then smaller housings, tighter fans, shrouds, baffles and gaskets everywhere tightened up the air leaks and continued to increase velocity. Add a PSC motor and you change air volume due to reduced air leakage again.

If you take new OEM spec guides from CCT and RWB and compare them to older ones (split-phase) you'll see the air gates and shutters are more closed now. BTW, it also prompted a change in the CCT EZ air shutter for low firing rates.

Air quantity hasn't really changed (chemistry), but burner design has changed the velocity up the wazoo (physics).

Mitch_4

In a situation where

THERE WOULD BE NO LOAD ON THE MOTOR, AND NO FRICTION, THE ROTOR COULD TURN AS FAST AS THE SHIFTING OF THE MAGNETIC FIELD IN THE STATOR. bUT THAT IS IMPOSSIBLE, SO THERE IS "SLIPPAGE" THE RATE AT WHICH THE ROTOR TURN IN REALITY.

bASICLLY THE MAGNETIC FIELD ROTOATES AT 1800RPM, THE MOTOR AT 1725, THE DIFFERENCE IS THE "SLIPPAGE"

mITCH.

Sorry, just saw the "caps lock" was on, but I'm to lazy to retype it.