slowing down split phase motor.
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  1. #1
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    Default slowing down split phase motor.

    I have a split phase fan motor with a capacitor. No centrifugal switch. Can I run it with a SCR to lower the speed and noise a little bit?
    All the info I found talks about the centrifical switch limiting lowest rpm and do not do it. It is cooled by being inside a squerell cage fan. RPM will not be less the around 75% of nameplate.
    Bill D

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    Quote Originally Posted by Bill D View Post
    I have a split phase fan motor with a capacitor. No centrifugal switch. Can I run it with a SCR to lower the speed and noise a little bit?
    All the info I found talks about the centrifical switch limiting lowest rpm and do not do it. It is cooled by being inside a squerell cage fan. RPM will not be less the around 75% of nameplate.
    Bill D
    Just get a multi-speed replacement motor or squirrel-cage entire from Zoro/Grainger that is ready-made for that service.

    Present air-handler, my one is 3-Speed. Downdraft vent fan of the Whirlpool kitchen range is a 2-speed, so is the Broan overhead vent. York outside AC unit fan is 3-speed. ISTR even 5 speeds exist?

    Same again ceiling fans, floor fans, etc. 3-speed, nearly all of them.

    Dirt common for air-movement, and the work has already been done. Why mess?

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    At 1/3rd hp and below you can use a triac dimmmer to slow a fan motor, even a somewhat enclosed motor and it wont overheat. The reason why is because a 1/3rd hp motor is about 50% efficient and a fan has a cubic power curve.

    Above 1/2 hp the motor tends to over heat, but if it is cooled by the moving air it probably wont.

    Celing fans are not 3 speeed. They are 3 voltage, run them at 180vac on low speed and they will spin as fast as high speed.. And will actually use less watts to do so because copper loses are lower (the coil is tapped like an auto transformer) they are pancake motors with lots of surface area and dont warm up much.

    Modern ceiling fans are inverter driven permanent magnet motors and consume like a tenth of the power.

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    Johansen thanks for the info. It is rated at 1/250hp which seems very low to me. I would have guessed 1/8 hp or less. But it is only 950 rpm so maybe that is reasonable for the size. But it sounds like the speed control will work fine at reasonable rpm's.
    Bill D

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    A capacitor FAN motor with no centrifugal start switch absolutely CAN be slowed by adjusting the effective voltage, which is what a triac control does.

    That is the only case, though, because the cubic fan curve means that it will always find a speed that balances the power. Other loads won't do that, it is still an induction motor and "wants to" run near synchronous speed. The fan load just lets the motor have increased slip, and provides a generally constant load at any speed.

    Yes, the increased slip can cause more heating. The motor ought to be designed for that, and a "fan duty" motor should be OK with it. The actual voltage control does the same thing, and also has increased slip.

    Most are "air over" motors, meaning that they need to be in the airflow to be cooled. Some belt drive types are not air-over, and have to have their own cooling.

    While some older motors had an external inductor etc to adjust voltage, most these days adjust the number of turns in the winding to set speed. That has the same effect.... more turns makes the fan "designed for" maybe 140 or 160V, so it has less power at 120V.

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    Quote Originally Posted by thermite View Post
    Just get a multi-speed replacement motor or squirrel-cage entire from Zoro/Grainger that is ready-made for that service.

    Present air-handler, my one is 3-Speed. Downdraft vent fan of the Whirlpool kitchen range is a 2-speed, so is the Broan overhead vent. York outside AC unit fan is 3-speed. ISTR even 5 speeds exist?

    Same again ceiling fans, floor fans, etc. 3-speed, nearly all of them.

    Dirt common for air-movement, and the work has already been done. Why mess?
    My air handler motor is Variable DC, my outside condenser fan is also variable DC and the compressor is.... you guessed it Variable Dee Sea!

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    Slight misnomer there, as they're brushless DC - AKA electronically commutated, or VFD-driven AC motors, usually permanent magnet synchronous AC.

    You wouldn't use brushed DC motors for 24/7 low-maintenance tasks like that and certainly not in a sealed system like a compressor.

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    Quote Originally Posted by SomeoneSomewhere View Post
    Slight misnomer there, as they're brushless DC - AKA electronically commutated, or VFD-driven AC motors, usually permanent magnet synchronous AC.

    You wouldn't use brushed DC motors for 24/7 low-maintenance tasks like that and certainly not in a sealed system like a compressor.
    Good point. I was just going by what it says on the box!
    The fans start up so quiet and slow, then slowly ramp up to demand


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