To external capacitor or not?

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ritelec

ritelec

Senior Member
Location
Jersey
Hello.

In researching a replacement motor for a gabble whole house "exhaust" fan :- ) , it appears in a grainger book that there are 1/3 hp motors and 1/3 hp motors with a capacitor mounted on the side of it

Why or why not the capacitor ?


I'm pretty sure it's for starting but the one without a capacitor screwed to it starts without one.

So why screw this extra appendage to the motor?

Thank you
 
kwired

kwired

Electron manager
Location
NE Nebraska
ritelec said:
Hello.

In researching a replacement motor for a gabble whole house "exhaust" fan :- ) , it appears in a grainger book that there are 1/3 hp motors and 1/3 hp motors with a capacitor mounted on the side of it

Why or why not the capacitor ?


I'm pretty sure it's for starting but the one without a capacitor screwed to it starts without one.

So why screw this extra appendage to the motor?

Thank you
Click to expand...
One without capacitor is probably a shaded pole motor, one with capacitor is probably a PSC motor. Shaded pole likely costs less, PSC likely operates more efficiently. Bearing failure is usually happening more often then capacitor failure regardless which one you select.

Use your own discretion in selection.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
140513-1201 EDT

ritelec:

Following is a relatively good discussion on single phase motors from a general perespective. But it does not answer your question.

Single phase motors are always a two phase motor for starting, and capacitor run motors are two phase for start and run. Split phase are also two phase for starting. There is no other way except more phases and/or an external rotator to start rotation (a rope).

http://www.industrial-electronics.com/elecy4_22.html

Space inside the motor is going to limit capacitor rating and thus starting torque. There may be cost factors relative to performance that determine the design.

You did not supply a specific reference in Granger. To compare the motors get the manfacturer's specifications and look for the differences.

.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
A quick look at a Grainger catalog confirms that if you searched for "fan motors" at 1/3HP 115V, you get a choice of Split Phase (SP) or Capacitor Start (CS). So the one with the lump on the side is the CS, the cheaper one without it is the SP. Split Phase motors provide less starting torque, so in a belt driven fan it will take longer for it to get up to speed. Probably not a big deal for a gable fan application, but SP motors will also present a significantly higher starting current surge compared to a CS motor, which can be an issue if the motor takes a long time to accelerate; i.e. a large blade diameter.

These are issues taken up by the initial design engineer for the entire fan, and it's unlikely you are an ME. If I were doing this, I would replace it with whatever came out of there.
 
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