Can a variac be used to control a "rotor capacitor driven asynchronous motor"
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    Default Can a variac be used to control a "rotor capacitor driven asynchronous motor"

    Hello,

    I'm sorry if this has been covered previously. I'm having trouble finding answers using google and the forum search tool. Additionally, I am not very well versed in electric work or motors...

    I have a fan that is controlled by a "rotor capacitor driven asynchronous motor". That was the answer I got when asking the manufacturer if the motor is a permanent capacitor motor.

    Can I safely control the speed of this type of motor/fan using a variac?

    Thank you!

    P.S. I have tested it, and I am able to control the fan with the variac. But I do not know if it is safe. I also wonder what the "Working hours" means on the variac controller. I plan on using this 24/7

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    That sounds like a fancy way of saying it's an induction motor with a capacitor that provides the phase shift that establishes rotation. Which means it's like a billion other electric motors.

    Since you provided no other information as to the motor size or type there's no further helping you. But I suspect you have a single phase motor due to the cap and that's not gonna work with a variac.

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    why do you want to slow down the fan ?

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    I can try to provide more information...here is a link to the fan(i hope im not breaking any rules here. If i am, I am sorry and will remove the link asap) Amazon.com : Active Air 4 CFM Inline Fan, 6-Inch : Garden & Outdoor
    If that doesn't help I can provide more info from the instructions/packaging.

    I want to slow the fan down because it is moving too much air, but would like the ability to use it at full capacity at times (if possible I don't want to go to a smaller fan). Now apparently this is a common problem for marijuana growers. Googling this particular issue links me to a bunch of marijuana forums. That was where I read the suggestion to use a variac...but I'd rather get the opinion of this forum before i start a fire.

    What is throwing me off here is that my research says a typical induction motor will maintain the same RPMS even if voltage is reduced. However, the variac definitely controls the speed of the fan.
    I'm sure there's a missing link in my knowledge involving resistance of the fan blades, torque, and relationship to the motor. I guess that's why I'm on here though

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    once the motor is over a certain size then using a variac to slow it down will increase the operating temperature of the motor and you risk burning it out. (actually, this is the case with any motor but small fan motors operate as high as 50% slip anyways and they have sufficient surface area for cooling)

    for example, i have a 12 inch centrifugal forward curved fan blower from a household sized furnace, 1/4th hp 230v motor. operating the motor at 120vac means the motor never got up to synchronous speed and it pulled 740 watts from the 120v outlet.

    with a larger pulley on the blower, and a smaller pulley on the motor (maybe half the rpm on the blower) the motor only pulled 70 watts from the outlet on half the design voltage and it blows enough air for what i need it to do.

    you are much better off changing the motor if its larger than about 1/6th hp.

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    First off, I followed your link, and it shows it available with a speed control.
    Use that one, not a variac.


    Are you using this in a grow operation ?

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    Quote Originally Posted by digger doug View Post
    First off, I followed your link, and it shows it available with a speed control.
    Use that one, not a variac.


    Are you using this in a grow operation ?
    its almost certainly a triac.. aka dimmer switch.

    and i was thinking the same thing...

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    The speed controller it shows with the fan causes a loud humming noise, which didn't sound "healthy" to me...are you sure it is okay?

    I am using this for a charcoal filter, much like they use in grows. But it's just for a vermicompost bin I use for my garden. I think it smells nice and earthy, but my girlfriend doesn't agree :/.

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    An easy way to accomplish what you are trying to do, is to merely restrict the air flow on the inlet side of the blower, as long as there is enough air flow to keep the motor cool. Some blowers are even sold this way, as this one, shown with the valve completely closed.
    Buy OEM Specialty Blowers - Free Shipping over $5 | Zoro.com

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    Quote Originally Posted by cberg View Post
    The speed controller it shows with the fan causes a loud humming noise, which didn't sound "healthy" to me...are you sure it is okay?

    I am using this for a charcoal filter, much like they use in grows. But it's just for a vermicompost bin I use for my garden. I think it smells nice and earthy, but my girlfriend doesn't agree :/.
    You could simply plumb it into your stack vent....and dispense with the charcol
    filter.

    Unless the smell would attract a DEA agent.

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    As long as the motor is small enough to not need a start switch, you can use a variac. In fact a variac is better as it does not have sharp pulses that cause noise and heat. 4 cfm to me sounds like a muffin fan or thereabouts.

    Tom

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    Do you really have a variac? They are pretty expensive, and not the kind of thing I would expect someone who doesn't know a lot about motors to have.

    This is a variac:
    171.jpg

    I suspect you have a triac-controlled lighting dimmer, the kind you would use in place of a light switch.

    Like this:
    201310281923042033_1.jpg

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    Quote Originally Posted by johansen View Post
    once the motor is over a certain size then using a variac to slow it down will increase the operating temperature of the motor and you risk burning it out. (actually, this is the case with any motor but small fan motors operate as high as 50% slip anyways and they have sufficient surface area for cooling)

    for example, i have a 12 inch centrifugal forward curved fan blower from a household sized furnace, 1/4th hp 230v motor. operating the motor at 120vac means the motor never got up to synchronous speed and it pulled 740 watts from the 120v outlet.

    with a larger pulley on the blower, and a smaller pulley on the motor (maybe half the rpm on the blower) the motor only pulled 70 watts from the outlet on half the design voltage and it blows enough air for what i need it to do.

    you are much better off changing the motor if its larger than about 1/6th hp.
    I believe that centrifugal blowers are not designed to be run with no restrictions of air flow but require some restriction to keep the load on the motor within specs. I have recycled a few and some motors would trip the overload until I blocked part of the output opening.

    Bob
    WB8NQW

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    It sounds like a "PSC" motor, with one start/run capacitor.

    YES, those will change speed with voltage, whether via a triac control or a variac. BUT.... that is if you have a load on it. With no load, they will end to run at nearly synchronous speed, over a range of voltages.

    Quote Originally Posted by blcksmth View Post
    I believe that centrifugal blowers are not designed to be run with no restrictions of air flow but require some restriction to keep the load on the motor within specs. I have recycled a few and some motors would trip the overload until I blocked part of the output opening.

    Bob
    WB8NQW
    That can be the case. The max load on the motor with some sorts of fan, is when the fan is running with no restriction. Many HVAC squirrel cage fans are like that, they need some restriction or they may be overloaded.

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    I think you are going to have to try it and see. You can monitor the temperature of the motor and of the variac, if it is really a variac, for a while. If either gets too hot to keep your hand on, then it will probably burn up. But if you can leave your hand on them for a minute or three after they have been running for a while, then they are probably OK.

    As for starting a fire, keep any other combustibles away from them, at least while testing. I would keep them clear even after that. There is not a lot in a motor or in a speed control to feed a fire so if there is a foot or two around them and several feet above them that is clear, they should be able to get hot without starting a fire beyond their outer shell. Inside them is another question. Oh and if they are on a combustible floor, put a sheet of metal under them that extends about a foot in every direction.

    At first I would check every five minutes. If that goes OK for a couple of hours try every 15. Then every 30, every hour, and then several times a day. Keep checking at least once a day for a month or so. By then you should have a pretty good idea how things are going. If you change the speed setting, check more frequently again. In other words, start over.

    If it is actually a variac, be careful where you touch it. A variac will have coils and connections that will be electrically hot and cause a shock. Only touch a grounded cover.

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    To answer the original question instead of all the gratuitous advice, you can control a capacitor run induction motor powered fan. What people are not considering is that the load of such a blower is proportional to the square of the speed. Running at full voltage, an induction motor runs close to synchronous speed with a little slip to induce current in the squirrel cage bars. When you reduce the voltage, it drops completely out of lock and slows down until the air load has fallen to match the power output of the motor.

    For years I had a blower on an air conditioner set up that way to reduce the noise when less cooling was needed. It gave smooth control over the whole speed range. Since the seller shows a speed control, we can assume that sufficient air will flow over the motor at reduced speed.

    Initially, check the current drawn and monitor motor temperature for a while. It is almost certain to work well. It may be necessary to restrict the inlet or reduce the maximum voltage setting to keep the full power current below rating.

    Bill

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    Thank you all for your help so far.

    If I take apart the motor and attach pictures would it make much of a difference?

    Also, I am 99% certain that I am using a Variac...it's amazing how anything can be found on amazon these days...Amazon.com: Metered Variac Variable 5W 11V AC Transformer Auto Regulator -13V 5VA US PLUG: Home Audio & Theater

    I will test it out as EPAIII has suggested.

    Thanks!

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    That is definitely a variac. Note that it goes up to 150 volts, an overload for your motor.

    Bill

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    Quote Originally Posted by TDegenhart View Post
    As long as the motor is small enough to not need a start switch, you can use a variac. In fact a variac is better as it does not have sharp pulses that cause noise and heat. 4 cfm to me sounds like a muffin fan or thereabouts.

    Tom
    Exactly. I installed variacs in a photo-lab to make the blowers variable speed with no noise or vibration. still working perfectly after 15 years. yes, you need to be aware of static pressure and minimum speed for motor cooling etc, but it's generally not the problem on small systems.

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    " Running at full voltage, an induction motor runs close to synchronous speed with a little slip to induce current in the squirrel cage bars. When you reduce the voltage, it drops completely out of lock and slows down until the air load has fallen to match the power output of the motor."

    Huh? An induction motor rotor never has any 'lock'. It's always slipping. Any when you reduce voltage, it seeks to maintain a running condition by pulling more amps, not losing RPM. With a tiny motor like we are talking here, it probably will lose some RPM because there is not gonna be a ton of margin either way.

    As others pointed out, a motor this small and crummy will not even be running close to synch speed anyway...it'll have a significant amount of slip.


    As for the OP's girlfriend not liking the smell, I wouldn't worry about that; all he should care about is what his wife thinks of it.


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