Speed control for Wind Tunnels?
Posted 07 June 2005 - 11:07 AM
The application is amateur wind tunnels for modellers. Here we have a fairly typical fan control requirement where the torque required drops as the speed goes down. The trouble is that we are looking for precise control of the revs, and a very wide speed range - fron 1440 down to perhaps 60rpm or lower. The precise rev control is needed to reproduce similar airflow as the test item is re-positioned in the test section, while the tunnel becomes useful for a wider variety of purposes the more the air speed can be varied. One of the most effective flow visualisation demonstrations I have seen was performed with a fan running at about 12 rmp, rotated by hand with a stick! It would have been much better if an easy variation between 10 and 20 rpm were possible.
The other problem is price. Amateur hobyists are not going to spend around £1k on a variable frequency controller, which seemed to be the only technology capable of providing this sort of control when I first wondered about it as a young boy in the '60s.
Most practical fan/motor combinations I have considered for this application are induction systems. Cheap brush motors are usually a bit small, and it's ideal to buy the motor/fan as a combination, rather than struggle to connect the two and balance them. Maximum sizes are probably a 3/4hp motor spinning a 30" fan. In practice, to keep the costs down, a second-hand ventilation axial fan unit will probably be the obvious unit. I have considered getting a fan controller from Maplins, but this will not give the really low speeds.
Professional tunnels often use a mixture of electronic and mechanical systems, especially variable-pitch propellors, which can offer immediate response times (especially if you are using a jet turbine as the prime mover!). For the amateur, mechanical answers suffer cost, complexity and long set-up times, particularly if something like a pulley train is used. I have seen smaller professional systems using centrifugal fans and damping the output with a flap to control the airflow.
Most existing wind tunnels use old control technology - they are often old themselves and there isn't a continuous demand for new ones! The lack of demand means that there is no commercial pressure to address the issue. Perhaps modern control technology using microprocessors could come to the rescue?
Posted 07 June 2005 - 07:19 PM
Welcome to the forum.
If you wish to have accurate speed control, you will ned to either use some form of synchronous system with a variable synchronising source, or you will need to use a closed loop system.
The AC Induction motor is a pseudosynchronous system and by varying the frequency of the supply, you can vary the speed of the motor over the range that you suggest. This is a simple system (unless you wish to build the speed controller your self, then it is very complex!!).
D C motors and universal motors (brushed AC motors as used in hand tools etc) are not directly speed controlled. Varying the voltage applied to them varies the torque that they produce. This will cause a change in load speed untill the load torque equals the torque generated by the motor. For accurate control, you need some form of speed feedback into the control.
I would expect that the cheapest means of achieveing your requirement is to use a small three phase AC induction motor and inverter. Depending on the size, you should be able to achieve less than the 1K you refer to.
Can you give us some indication on sizing and perhaps we can help further.
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