[AMPS] Fan motor capacitors

Peter Chadwick Peter_Chadwick@mitel.com
Wed, 7 Feb 2001 08:51:08 -0000

Steve says:

>Is the capacitance needed just to kick the motor 
>off in the right direction, or does it's value have any 
>effect on the power that the motor produces 
>when running? 

As usual, it all depends. Some motors use a centrifugal switch to switch the
capacitor out when it has got to speed, so the cap is just there for starting.
Usually, these caps are in the 20 - 100 mFd region, (at least for 50Hz/230v
supplies) and are often electrolytics, since they're not in circuit long enough
to get dangerously hot - unless the switch sticks.....

Other split phase motors use a cap permanently in circuit. The torque is
somewhat dependant on the value, because obviously, if it's too low, the current
falls. If it's too big, then the phase angle shifts round. Typical values are in
the 2 to 16 microfarad range - at least for 50Hz/230 volt circuits.  Such caps
used to be paper, often PCB impregnated, but these days, tend to be a synthetic
film type. You need a capacitor that can handle the current, of course. I've had
success in using quite small caps to reduce speed on fans, but you need to be
careful because  often the fan also cools the motor, and if you slow it too far,
overheating results. Generally, the value is fairly non critical. I found the
best way to slow a motor was to use an auto transformer.


Peter G3RZP

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