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reducing RPM's

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#1 Herman


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Posted 09 September 2002 - 11:14 AM

Can I safely reduce the motor (120V, 60Hz) rpm on a 10" attic fan (1140cuft/min) by simply reducing the current? I want to reduce the speed by about 50%. Would a dimmer like switch suffice?

Thanks in advance.

[Edited on 9/9/02 by Herman]

#2 marke


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Posted 09 September 2002 - 07:38 PM

Hello Herman

As this is an integral fan / motor unit, there is every godd chance that you can vary the speed of the fan by varying the voltage applied to it. There are small triac based fan speed controllers for fans that are very similar to light dimmers, but they are not quite the same. They have additional phase shift built in to the tribber circuit. If you try a light dimer, you may find that it doesn't work, however I am sure that the light dimer manufacturer would also make a fan controller.
Best regards,

#3 Crowbar


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Posted 15 September 2002 - 12:02 AM

You could also insert a correctly sized capacitor in series to limit current and therefore reduce the speed of the fan. That's how many 2 and 3 speed fans operate. Sometimes a triac based solution of control can result in unwanted side effects, a big one is noise - sometimes you'll get a hum like you wouldn't believe.

#4 kgm2844


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Posted 21 September 2002 - 10:02 PM

You can use triac type control to slow down your attic fan. However there are a few guidelines you must follow.

1-You must apply 90 to 100% voltage at start to ensure proper start acceleration (the fan starts with a load applied).
2-You cannot allow applied voltage to drop below 80%. This should satisfy your half speed requirement.
3-The rating on the dimmer must be at least 6 times of rated full load current. Motors have a higher starting current than when running.

#5 grobert


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Posted 08 October 2003 - 01:48 AM

Most light dimmers have their current ratings in resistive loads. You have a inductive load. THe inductance of the motor can / will fry standard dimmers at some point(if not right away). The inductance is a voltage that the free spinning motor generates once power is removed and can destroy solid state resistive dimmers. Get a inductive rated dimmer (like a ceiling fan) rated for the proper HP or current and you will be fine.

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