About 1.5 years ago I replace the blower motor on a 25 + yo american standard residential furnace. It is a basic GF unit with a 1/3 HP motor mounted on the blower cage driving the blower via belt. Checked balance on the blower, oiled bearings, checked startup and running current on the motor, installed some shock isolators on the cage frame - all appeared OK.
The motor replacement was a Dayton with the exact same motor p/n as the existing motor....somehow I don't believe the existing was the original motor...but anyway....
Recently got a call back and owner said motor 'sometimes hums' on startup. Of course, when I went there, it didn't 'hum.' Checked starting and running current - within spec. I then realized that there was no starting capacitor in the unit - motor is across the 110vac line with a relay contact. Seemed a little strange but not out of the ordinary. Attempted to verify if it should have a starting capacitor but the wiring diagram on the front panel had deteriorated, making it unreadable. Starting relay was solid with no chatter.
It seems the the motor is dying - true? Could I have just gotten a bad motor that is dying after 1.5 yrs of heating and cooling service?
And now I am questioning if there should be a starting capacitor to increase the starting torque. The current motor seems to start fine - a little groan under the load but comes up to speed quickly. ALso wondering if maybe the motor should be a 1/2 HP.
Yea, the entire unit should be replaced - the heat exchanger is thin but serviceable. They are an elderly couple and don't want to put the money into it.....
So, thoughts on if the motor is dying or other reasons for the hum that is intermittent?
Belt drive motors of that HP don't need start caps. You also don't have access to the start winding to put one in anyways!
I suspect the centrifugal start switch is sticking in the off position on shut down.
A new motor will remedy that.
I then realized that there was no starting capacitor in the unit - motor is across the 110vac line with a relay contact. Seemed a little strange but not out of the ordinary. Could I have just gotten a bad motor that is dying after 1.5 yrs of heating and cooling service?
And now I am questioning if there should be a starting capacitor to increase the starting torque. The current motor seems to start fine - a little groan under the load but comes up to speed quickly. Also wondering if maybe the motor should be a 1/2 HP.
Probably a shaded pole motor, never install a shaded pole on a belt drive, or any other configuration either.
"The critic is a prisoner to his own experiences and perspectives, erroneously believing his limited experiences are the sum of all truth". T.D. Jakes
Belt drive motors of that HP don't need start caps. You also don't have access to the start winding to put one in anyways!
I suspect the centrifugal start switch is sticking in the off position on shut down.
A new motor will remedy that.
After looking into it further, I agree with your assessment of a sticking start switch. Here is the motor specs:
Dayton Motor, 6K778, Belt Drive Motor, Split-Phase, Open Dripproof, 1/3 HP, 1725 RPM, Number of Speeds 1, 115 Volts, 6.6 Full Load Amps, 48Y NEMA Frame, Service Factor 1.35, 60 Hz, Cradle Mount, Ball Bearings, Auto Thermal Protection, Rotation CW/CCW, Ambient 40 C, Insulation Class B, Shaft Diameter 1/2 In, Shaft Length 1 1/2 In, Body Dia 5 5/8 In
Since it is split phase motor, the starter winding is disengaged when it reaches nominal speed.
Motor is out of warranty, so it is either buy replacement at $$$ (mfg in China), or get an AO Smith motor at $$$ (mfg in Mexico).
Thanks for your help....
-John
Last edited by jpsmith1cm; 01-04-2013 at 08:56 PM.
Reason: pricing
Another thought....uh...don't over-tighten the drive belt. Just tight enough to work, and nearly loose enough to slip is about right. Tighten as you would an automotive belt and you'll overload the new motor quickly. Old belt drive blowers do NOT like tight V-belts. Will overload the motor and ruin the bearings quickly!
Another thought....uh...don't over-tighten the drive belt. Just tight enough to work, and nearly loose enough to slip is about right. Tighten as you would an automotive belt and you'll overload the new motor quickly. Old belt drive blowers do NOT like tight V-belts. Will overload the motor and ruin the bearings quickly!
Thanks for the thought.
I typically tighten the belt with about 1/2" to 3/4" of 'play' (deflection). Any more than 3/4" and I see the belt bounce around so much that it looks like it will jump off the pully...
Should the belt be looser?
I don't want to add anything specific, but I have experienced, and continue to experienced problems with "ratings" on new electric motors. Some are under rated, some over rated. I've had motors burn when replaced to the exact nameplate specs. It probably stems from the fact that the "old motor" may have been under rated, and probably built better. The "new motor" is probably under rated and built like garbage. Anyway my experience tells me electric motors, with the same nameplate capacity, are not created equal.
I don't want to add anything specific, but I have experienced, and continue to experienced problems with "ratings" on new electric motors. Some are under rated, some over rated. I've had motors burn when replaced to the exact nameplate specs. It probably stems from the fact that the "old motor" may have been under rated, and probably built better. The "new motor" is probably under rated and built like garbage. Anyway my experience tells me electric motors, with the same nameplate capacity, are not created equal.
I've seen where people have been matching the motors by stack diameter and thickness instead of horsepower. Not sure how well it works in practice, but I understand where they are coming from.
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Originally Posted by three_jeeps
