[Amps] Shaded pole motor speed

Martin Sole msole at loxinfo.co.th
Thu Aug 24 09:12:28 EDT 2006


Thanks for all the interesting replies on this. I have had a quick mess 
with a variac. Motor starts at about 70 volts and speed seems to 
stabilise about 90 to 95 volts. It's a 110 volt motor so I only went up 
to 130 but saw no obvious increase. Definite variation in the 70 to 90 
volt range though. This is great for slowing the motor down but I need 
more speed not less so frequency change seems the only solution. a 60+ 
Hz oscillator driving a big transformer push pull should do it but 
apparently the motor causes a big sag that stops it from starting, hmm? 
Seems like a replacement motor is the only sensible answer after all. 
Anybody know of a direct replacement that would fit the Alpha 78 and 
produce more than the very weak draft I get at the moment?

Actually I'm starting to think I might be better off with moving on both 
78's and the Henry, together with various other parts I have there is 
probably enough for a serious amp fund. Makes me want to ask another, 
unrelated, kinda, question. What would you buy? I've always thought the 
Acom 2000 was about ideal for my 'perceived' needs. I did just go in 
search of them but not even sure if they still exist or if the 2000 is 
still made. Their web site is a disaster.

Cheers
Martin



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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