[Amps] Shaded pole motor speed

[Will Matney]

Martin,

Most of the small cheap variable speed controls use a SCR and an oscillator to 
do a pulse-width control of the motor. What I've seen on some amps that use a 
speed thermostat, they just use a wire wound resistor in series with the motor 
winding. They short this resistor out with a thermostat allowing the fan to run full 
speed when the temperature gets hot enough to close the thermostat. To get the 
speed you need, you'll need to replace the 60 Hz motor with a 50 Hz one. Air flow 
can also be increased by the angle of the fan blades, and the diameter of the fan 
blade. Here, heating and air conditioning suppliers stock both motors and fan blades. 
The Dayton types use both shaded pole and squirell cage motors. The smaller of 
them use the shaded pole type.

Best,

Will

*********** REPLY SEPARATOR  ***********

On 8/23/06 at 9:56 AM Martin Sole wrote:

>For some time now I have been thinking about ways to improve the airflow 
>through the tubes in my amplifiers. I have a total of four amps, a 30L1, 
>two Alpha 78's and a Henry 2K classic. In all cases I am most 
>unimpressed by the amount of air being moved and I consider all of them 
>to be essentially noise free so there is room to move on that front.
>
>Two factors that seem to contribute to my lack of happiness in this 
>matter might be, a) our AC supply is 50Hz so the motors are running at 
>20% less speed than they would on 60Hz (all the motors are marked 
>50/60Hz), and b) the ambient room temperature is about 32-34 degrees 
>celsius.
>
>Use of the amplifiers for anything more than a few minutes with 
>processed SSB at typical short contact duty cycles, but non contest, 
>results in exhaust air which is too hot to hold ones hand over for more 
>than a second or two. In standby with the amp on there is enough heat 
>from the tubes to warm the airflow. What I desire is to have not more 
>than ambient air temperature during standby.
>
>The Collins and the Alphas use simple shaded pole motors, I'm not sure 
>about the Henry, it is a Dayton blower. My understanding of these is 
>that frequency is the variable that controls speed. How can I vary the 
>frequency and drive these motors at say 60 or more Hz? I have seen 
>shaded pole motor speed controls for sale and they seem simple enough 
>given their price but what is inside them? Can I homebrew something to try?
>
>I don't really want to go the route of extra external fans if it can be 
>avoided.
>
>Martin, HS0ZED
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