# Thread: universal motors

1. Professional Member
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on a universal motor (condenser motor)
it can be used for 1/6 up to 1/3 hp by changing speeds..
and caps the motors are rated at 1075 rpm with 2 speeds
if lower speed is used for 1/6 hp does it drop the rpms
or just amps

i should know this but i don't so go easy on me!!

2. Horsepower has nothing to do with speed. 1/6th will pull less current and drive smaller devices than a 1/3rd, but the speed will be essentially the same.
Speed on ac motor is dependent upon no.of poles, and freq.

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bornriding is right on a condenser,horsepower tells you how big of a blade it can pull, and rpm's tell you how fast it is going to go.

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I don't necessarily agree with you guys.

Yes the speed on a motor is mainly controlled by the formula: RPM =(120*60)/#poles. Where the 60 is the line freq of 60 hz.

So, a 2 pole motor basically runs at 3600 RPM. But then there's slip. Slip is the diference between armature speed and the electrical field rotation speed (or synchronous speed). For a motor to have any torque there will always be armature speed<synchronous. Synchronous motors are the exception, but are designed to operate at synchronous. The more load, the more slip a motor will have.

Now, those multi-hp motors are not changing the number of poles. The speed taps are taps on different places of the armature coil; a form of field weakening. This reduces the effective hp the motor can provide. So what you basically have a controlled slip motor to get the RPM you want. The motor must have the correct load to get the desired RPM. Unload this motor and the RPM will be the same for all the taps; which is not the same for a true multi-pole motor.

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ok i think i have it...
so changing the speed taps on a multi hp motor only changes hp and not rpm....

i understand all about the rpm and hp was just unsure about if the rpms were changed by the speed taps when the hp was changed....

so that being the case then why on a blower motor when speed taps are changed you get either a low speed med and high speed difference and doesn't it change rpms when speeds are changed...

6. The rpm will change on those motors.

We switched tho them about 6 monthes ago.

Follow the instructions with them for the hp, and rpm you need.

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i've read the little book that comes with them but all it tells me is about hp not rpms
we use the first call motors from lennox

thanks i will try to fine more info from lennox

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Originally posted by stkepo
ok i think i have it...
so changing the speed taps on a multi hp motor only changes hp and not rpm....

i understand all about the rpm and hp was just unsure about if the rpms were changed by the speed taps when the hp was changed....

so that being the case then why on a blower motor when speed taps are changed you get either a low speed med and high speed difference and doesn't it change rpms when speeds are changed...
HP=Torqe RPM=speed

So if you increase or decrease HP it won't affect the motor RPM's.

9. The ones we use are ao smith. Sourse one.
You can change the hp, or rpm, or both.

10. Originally posted by beenthere
You can change the hp, or rpm, or both.
Actually, all you can only lower is the HP, the free RPM stays the same. "Free RPM" is the rpm with no load on the shaft.

The only way to lower the free RPM is to change the number of poles, change the voltage, or change the frequency.

Some of the motors I have seen allow you to fine tune the operation of the motor a little by changing run capacitors.

Lowering the HP, but keeping the same load, will lower the speed due to increased slip.

The HP taps on these universal condensor fan motors are essentially the same thing as the "speed" taps on a multispeed blower motor, wich actually only change the HP of the blower motor.
Completely unloaded with no load on the shaft, the motor will spin at the same speed, regardless of wich tap is used, same with a blower motor.
With the same load on the motor, the RPM will be higher with the hither power taps and lower with the lower power taps.

I personally don't use these motors, exept when given one by the OEM as a replacement.

I keep roughly 20 different direct drive blower and condensor fan motors on my truck, about 24 total, and I go get an OEM motor if one on my truck isn't a nearly identacle match. Fortunatly just about every OEM has parts distribution within about a 15-20 minute drive of nearly my entire service area.

If you insist on using the muliti HP universal motors, note that they come in 1075 and 825 RPM varieties, and in different ranges of HP. They also have more than one height of motor and choices of sleeve or ball bearings.
Generally it is not good to use the motor that goes up to 1/2 hp to replace a 1/5 hp motor, even though the motor is rated to do so. Using the lower HP taps on them will result in a motor that runs a bit hot, and shortens the potential lifespam of the motor.
In order for these multi HP "universal" motors to be worth stocking, you need to stock 6 different part numbers on your truck.

It will always, 100% of the time, be better to use a motor with the same speed, horspower and electrical ratings as the motor you are replacing.

[Edited by mark beiser on 07-20-2005 at 01:45 AM]

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Here's a link to Fasco. All your questions will be answered once you download the PDF file.

http://www.fasco.com/fasfacts.asp

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tell you guys the truth my service manager ordered a much of those motors and the get u thru a cooling season but they dont seem to last ong i think is beter to keep single horsepower motors on van i know someone will say i can use it for more units but its just what i think

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thanks for the link it's a big help......

thank all for the knowledge
14 years and i'm still learning
guess i always will

thanks again!!!

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