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#102736 - 10/09/06 06:42 PM Motor RPM
bigrockk Offline
Member

Registered: 01/25/03
Posts: 175
Loc: Middle of Canada
I gave a Millwright friend of mine an old commercial furnace motor (120v) that he wanted to use for a project.
It is a four-speed motor and if I recall correctly it has seven leads, two for the cap, one for neutral and the four remaining are labeled low, med low, med and high speed.
We bench tested the motor with the intent to use a tachometer to test the rpm at each of the four speeds.
The surprising part of all this is that we got 1200 RPM no matter which lead (low, med low, med or high) was hooked up to the supply.
When we hooked it up for the various speeds we tied one lead to the supply and capped the other three.
Should the three "speed" leads not being used be tied together rather than tied back separately to get the various speeds?
I am pretty sure that when the motor was in service the high-speed lead was used and the other three were tied back separately but my memory isn't what it used to be...
Thanks for any input.

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#102737 - 10/09/06 08:18 PM Re: Motor RPM
Rewired Offline
Member

Registered: 01/01/06
Posts: 567
Loc: Hamilton, Ontario, Canada
As far as I know, the leads you are not using get capped and tied back individually, at least thats how it is with my furnace motor. Your speed measurement was with no load correct? Place a load on the motor and try it again, there should be a noticeable difference in speeds with a load Vs no load, at least for THAT type of motor.

A.D

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#102738 - 10/09/06 10:21 PM Re: Motor RPM
bigrockk Offline
Member

Registered: 01/25/03
Posts: 175
Loc: Middle of Canada
Yep you are correct speed measurement was with no load.
Funny you should mention trying it with a load as i was thinking the same thing at the time but didn't have anything to "load" it with. Then the more I thought about it I thought there should be some speed change even without a load and wouldn't you think it would rotate at more than 1200 rpm in high speed?
I will see if I can get him to retry with a load on the motor and see what happens.
Thanks for the input.


[This message has been edited by bigrockk (edited 10-10-2006).]

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#102739 - 10/09/06 10:47 PM Re: Motor RPM
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Sounds like a PSC Motor (Permanent Split Capacitor Motor).

Without the Blower on the shaft, the Motor will spin at high speed - regardless of the Speed Selection Lead used.

This is because the Motor draws input True Power (Wattage), according to the amount of Work required to do.

Simply, this means that if the Motor is connected through the "High" Speed lead, it will attempt to develope the highest amount of Work, by spinning the Blower at whatever RPMs that would result in the maximum rated Horsepower + speed of the Motor.

In other words, it would move a volume of air, which would require "X" amount of Horsepower, and this HP would be drawn from the Power Supply, in the form of Wattage, within a "Package" of Line Volt-Amps.

For any "Low Speed Setting", the input Voltage is reduced via an Autotransformer, wound into the Motor's Stator Assembly.

Now the level of input Watts / VA's are restricted by the Lower Voltage applied to the Motor's Stator, and as a result, the output Horsepower is reduced - which ends up slowing down the rotation of the _Fixed Load_; in this case is the Blower.

So, in more simpler terms:
The Blower's Speed is reduced because the amount of True Power which is drawn from the Power Supply, is limited.
The Blower will spin up to the speed which moves a given volume of air per the drawn input True Power (Wattage), which is carried to the Motor in the "Complete" Apparent Power "Package" (Apparent Power = Volt-Amps).
The output "Horsepower" developed by the Motor is transposed to the air, and is equal to the input True Power (Wattage) drawn from the supply (plus losses and other stuff!)

Simple, isn't it

You should cap off the unused Speed Leads, so they do not end up becoming an active circuit of their own.
No need to include them in any connections - just cap them so they do not end up drawing current by means of a Ground Fault, or even between themselves.
These leads are taps off the Autotransformer, so each lead is an active circuit by its self.

If you only want to use - say the Highest Speed Lead on this Motor, then cap off the remaining leads, and connect the Ungrounded Branch Circuit Conductor to the "High Speed" lead.
Capacitor leads are not connected to the Motor's "Input Leads" - they only connect to the Capacitor.
The remaining Motor Lead should be Terminated to the Circuit's Grounded Conductor (if used on L-N 2 Wire Circuit), or the Branch Circuit's other Ungrounded Conductor (if using an L-L 2 Wire Circuit).

Lastly, the remaining speed leads should be capped off separately - not all connected together.
If you have ever worked on a "Multi-Tap" Ballast, the same thing is done: cap off unused leads individually.

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#102740 - 10/10/06 07:56 AM Re: Motor RPM
bigrockk Offline
Member

Registered: 01/25/03
Posts: 175
Loc: Middle of Canada
Thank you "Scott35" for such a detailed answer I do appreciate it and understand it now.

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#102741 - 10/15/06 08:33 PM Re: Motor RPM
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
bigrockk:

 Quote:

Thank you "Scott35" for such a detailed answer I do appreciate it and understand it now.


Glad to be of help!

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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