[Milsurplus] 400Hz fan run(start?) cap help needed

[Bruce Gentry]

Tom Norris wrote:
> I have some gear with small 24 volt 400 Hz fans installed.  I had one of the fans seize - taking with it the fan inverter board - and have yet to find an exact replacement.  One replacement of the same series (Rotron AXIMAX2HT/990YH) I found was 48volt 400hz.  However, the capacitor specified for this fan is 0.68 mf, where the 24 volt cap was 12 mf.  
>
> One option is replace the things with DC fans, and have found some with similar pressure and flow specs, but I'd like to replace the fan in one of these amps with something close to original so I can compare various circuit temps between the two types of fan. (I can place thermocouples all over the place and compare temps, fairly easy)
>
> Any comments on running this fan at a reduced voltage?  What would you recommend for the cap value if ran at 24 volts rather than 48?
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Use whatever size condensor  the motor is rated for . It's purpose is 
phase shifting the current in the start winding, and it works in 
conjunction with the inductance of the winding. 12 MFD does seem far too 
large for a motor of that size on 400 cycles. As for reduced voltage, 
that is not a good idea with AC motors. If you can only give 1/2 the 
rated voltage, then only 1/2 the rated frequency should be applied, 
making the motor  run at 1/2 speed.  Giving a fan motor reduced voltage 
at rated frequency can easily cause it to draw very excessive current 
because it will never come up to speed and will be constantly struggling 
to start. The motor windings should be reconnected- a very demanding, 
precision job- or a transformer added to double the voltage if you want 
to run this motor from the existing power source.   Are the windings in 
the original motor fried? It may be easier to find and replace the 
bearings if the windings have survived.   Bruce Gentry