bob800
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Controlling fumehood fan speed
I recently built a small fumehood, and although it works just fine, there are many times when I don't want such a fast airflow. At first I thought
that I could just attach a dimmer in series with the blower, however on the Grainger page for my Dayton 1TDT2 it says "Note: Not for use with speed
controllers."
Why can't you use speed controllers? Does if have to do with the starting capacitor? Would I just have to start it on full power, and then bring it
down?
Thanks for any help
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Magpie
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Assuming you have the typical synchronous AC motor, the motor speed is controlled by the number of poles and the frequency of the power, ie, most
likely 60 Hz. Tim, correct me if I'm wrong here.
The single most important condition for a successful synthesis is good mixing - Nicodem
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bob800
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The specs on my blower can be found here.
Is there some device that can control that?
[EDIT] Ah, I think I found a fourm post on this here. Is that what your talking about?
[Edited on 17-10-2010 by bob800]
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bob800
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OK, I found an AC speed controller on Grainger: http://www.grainger.com/Grainger/DART-CONTROLS-Adjustable-Sp.... However, it says "Not Equipped to Handle Capacitor Start Motors". Which is what
mine is, I think.
[Edited on 17-10-2010 by bob800]
[Edited on 17-10-2010 by bob800]
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Magpie
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I'm 99% sure you have a synchonous AC motor. They're standard for the type of blower you and I have.
I believe there are devices that can control frequency, but you likely wouldn't like the price. I defer to others with more knowledge of this
subject.
The single most important condition for a successful synthesis is good mixing - Nicodem
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not_important
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Because different designs of motors need different things to control their speed. A synchronous motor (not your case) is locked to the frequency of
the AC power. DC motors are controlled by average voltage, this can be either a true DC voltage or by PWM.
The motor in your blower is described here : http://en.wikipedia.org/wiki/AC_motor#Permanent-split_capaci... Note that speed control is done via taps on the winding.
The controller you ref'd says "Not Equipped to Handle Capacitor Start Motors" and the motor in the blower is capacitor start.
[Edited on 17-10-2010 by not_important]
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bob800
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Thanks for all the help.
Perhaps I could turn the blower on direct AC, and then immediately switch it over the the controller. Would that work?
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entropy51
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That blower is rated at 549 CFM with no losses. With a reasonable face area your face velocity is only about 70 ft/min and would be less with losses
from bends and duct friction. That is not a very high velocity.
If you really do need to throttle the flow back, a damper in the duct between the hood and the blower, or a baffle inside the hood, would be a simpler
solution than trying to reduce the blower speed.
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Rosco Bodine
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It absolutely is possible to vary the speed either a shaded pole or a permanent split capacitor motor using a variac to vary the power input. It is
not true "speed control" as would be gotten using a speed sensor tachometer feedback into an active speed control circuit, which is a scenario where a
"speed" is selected and then the power is automatically delivered in the amount needed for a load which may vary or may not. If the nature of the
load is constant, or if the load is what is called a "fan load" or other type of "quadratic load" such as is presented by a centrifugal blower or
centrifugal pump, then a quasi-stable sort of variable speed "selection" can be achieved by simply varying the power input using a variac. A variac
provides a smooth waveform so there is no hum caused as would occur using a lamp dimmer with a PSC motor. A shaded pole motor can be used
better if the lamp dimmer type of control is used, but really a variac is the way to go. There is a significant drop in power efficiency of the motor
when it is operated away from its synchronous speed, as it is operating with a slip, a slippage of the magnetic field which caused by the load being
greater than the torque of the rotating field on the rotor....it is the same effect as a clutch slipping or a torque converter on an automobile
transmission, there is not full power available when
there is slippage between the driving force and the driven element. With a fan load, or any other quadratic load, there is a nature about that type
load which is geometrically proportional to the speed so greater speed requires much greater available power and a lower speed requires much lower
available power and this
allows for a sort of "dumb speed control" involving no active feedback speed control, but can be achieved simply by varying the power input using a
variac.
The load will then run at whatever speed corresponds to the power input available. Because the motor runs less efficiently at lower speed it needs to
be a good air over or air cooled motor to carry away any excess heat and the cooling requirement is then increased significantly. Many PSC motors
have sufficient headroom that they can be operated safely in a varied speed mode without any modification.....even if the motor is not rated nor
recommended by the manufacturer for such use ( and they won't ) .....they won't even talk about it 
But you own the damn motor so you can do your own tests and "rerate" the motor yourself for the particular use. If it is running too fast you can
slow it down using a variac. But then you need to monitor the motor temperature and see
if it is doing okay with the "off label" use at a lower speed and voltage. In a certain range of speed reduction the current rise due to inefficient
operation away from the synchronous speed may produce an excessive power dissipation which is
not allowable....so it is a technicians call on what is the allowable range by watching an amp reading and voltage reading and doing the math
....considering the cooling airflow as well and what the motor temperature is doing. This is why the manufacturers don't advertise or recommend such
speed control...because the average person doesn't understand matching load to motor and surveying the application to evaluate whether or not it can
be done. By the same token,
a motor manufacturer won't tell you that you can take a 240 volt 1/2 horsepower
PSC motor and run it load derated 75% on 120 volts ....but I have done that for years whenever the occasion has presented itself as a matter of
economy and convenience and never had any problem with such off label use. If you have an amp meter and a voltage meter and can do the arithmetic for
the application ....go for it. It isn't code but it works just fine.
There are available tapped multiple winding multi speed PSC blower motors,
and variable frequency AC motors and DC motors which are of course more elegant solutions. But there is a better than fair chance that the PSC motor
being used in a "fan load" scenario like a fume hood was a motor somewhat
larger than needed and is already running under rated even at synchronous speed for the application, and if that is the case as it is likely to be,
then that
PSC motor probably has sufficient "design capability latititude" to be speed
varied using a variac, at least across a fair range of speed variation, using only a variac. Some manufacturers motors are better than others for
this particular
"off label" kind of use. Some motors may do better with a different value capacitor and some will do best with the capacitor they are rated to use.
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bob800
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Quote: Originally posted by entropy51  | | With a reasonable face area your face velocity is only about 70 ft/min and would be less with losses from bends and duct friction. That is not a very
high velocity. |
My hood is only 2.5' sq, and I have less than 1' of "ducting" (a.k.a. a sealed wood box) with no bends. For most purposes, the air flow is fine. But it will blow my burner flame all over the place, and sometimes I only need a
very small face velocity.
Thanks for all suggestions and info. For now, I don't want to accidentally fry my brand new blower and have the entire hood rendered useless, but I
may try your ideas eventually.
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Rosco Bodine
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You could double sash the hood also, having a second intake at the top or have a movable adjustable louvered damper port in the upper part as a
supplemental intake.
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1281371269
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I have just bought one of these:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=370...
(the 6'' 305M^3 one)
Would I be able to cut the speed controllers from a standard 240v desk fan (the ones with set speeds e.g. 0,1,2,3,4) and use it for my fan?
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Mr. Wizard
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I enjoyed RBs description of using motors 'off spec' and he's right of course. Another cheaper method is just to limit the air flow with some kind of
baffle or choke on the input or outflow. This will also result in less current through the motor, as it is doing less work. Monitor to make sure air
flow is sufficient to cool the motor. Throttling the outflow may result in less turbulence in the hood than throttling the air getting in. You could
experiment with cardboard baffles first just to get an idea, then replace them with thin metal for safety.
I must add, in reviewing, the speed of the fan will remain the same, or slightly higher, but the amount of air it moves will be less, and at a slower
speed, except where the restriction takes place.
[Edited on 12-11-2010 by Mr. Wizard]
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watson.fawkes
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| Quote: Originally posted by Mossydie | | Would I be able to cut the speed controllers from a standard 240v desk fan (the ones with set speeds e.g. 0,1,2,3,4) and use it for my fan?
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Unlikely. Inexpensive cooling fans are wound with tapped windings so that by switching in more or less of
the coil you get more or less field strength, varying the speed. You could also open up the motor and add your own taps, if you're feeling
adventurous.
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