Just curious...............

[Ken]

I do not need a blower motor for my furnace, but I have a question about such motors. One of my furnaces has a four speed motor. If I should search for a comparable motor (such as:

) it lists the motor as a 1075 RPM. I realize there are leads on the motor that can be wired so as to attain the different speeds, but what are those other speeds? I never see them listed in RPMs. Thanks.

[stanhvac1]

Speeds are Low, Med Low, Med high and High. Normally only two of the speeds are used, One for heating and one for cooling.

[stanhvac1]

RPM's would be approx. 850rpm, 975rpm, 1025rpm and 1075rpm

[Ken]

Thanks, that was exactly what I was wondering about.

[Wally W.]

How are these speeds are produced?

Synchronous speeds are 900 rpm for an 8-pole motor, 1200 rpm for a

6-pole motor.

Approximately 850 rpm and 1075 rpm are possible speeds for the above when accounting for slip.

That left me scratching my head about 975rpm, 1025rpm.

This suggests they may result from larger amounts of slip for an

8-pole motor:

Is the 850 rpm the result from even more slip on a motor that wants to run at 1200 rpm?

If so, the following quote from the link above may be of interest to the OP: "Consequently, if you bench test a multi-speed tapped motor using a tachometer or strobe, you will detect little variation between speed taps since there?s no load."

[stanhvac1]

I wasn't trying to get technical on this. Speeds given are Approx.for spec. CFM/SP loaded conditions. Normally you would use a two speed motor and adjustable pully to obtain the desired RPM for the blower CFM and SP.

[Tony Hwang]

Hi, Look at the schematics of your furnace. You use only two speeds(faster for cooling, slowe for heating. Speed is hi, mid-hi, med, lo to choose) Must be an old furnace using SC motor. New ones are DC motors for higher efficiency. Often variable speed.

[Wally W.]

For 1200 rpm synchronous speed:

RPM % slip

1075 10.4 1025 14.6 975 18.8 850 29.2

Page 23 from:

Modern multi-speed motors are really not multi-speed, but multi-horsepower. The speeds are simply taps at different points in the coil. The higher speeds are tapped at a point with fewer turns as compared to where the subsequent speeds are tapped in the same coil. ... This field weakening allows the load to slow the motor down each time a slower speed is selected. With no load mounted on the shaft, as with a bench test, no difference between speeds will be seen.

[Wally W.]

With speed so dependent on the motor load, adjusting the pulley would seem to involve more trial-and-error than for a motor that is more insistent about trying to run closer to synchronous speed.

[stanhvac1]

Wally, thanks for your input. Firt, the orig. post only noted a 1075rpm motor we do not know the HP, voltage, SF, slip rating, manufacturer or other needed information. Actually a 1075 motor is a 6 pole synchronous 1200 rpm rated motor with a full load rating of 1170 rpm. So a rating of 1075 rpm would indicate a eff. rating of 92%. Tap speed at 92% rpm's would be 1076, 990, 911, and 838 not taking into account for slip. You would only see this on a loaded motor. It would not show on a bench test on an unloaded motor where you would see the syn. speed rather than the loaded speed. These are only estimates and not actual since we don't have all the required information. I agree with you about the speeds being hp reduction verses rpm reduction.

[No Body]

And with variable frequency drive systems, you can maintain synch lock while running at different speeds.