Controlling fumehood fan speed

Quote: Originally posted by Magpie

the motor speed is controlled by the number of poles and the frequency of the power

Quote: Originally posted by bob800

Is there some device that can control that?

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.

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.

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?