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#128330 - 01/19/03 07:33 AM frequent capacitor failure  
amandaamisha  Offline
Joined: Dec 2002
Posts: 16
Dear Friends,
We have a circular wood cutting saw ,CHINA make whose capacitor is getting frequently damaged. The motor details are as follows.
MOTOR H.P. : 2.0 h.p.
VOLTAGE : 230 volts
SPEED : 1440 RPM
CAPACITOR 1 : 35 mfd, 440 volts ( frequently getting failed)
CAPACITOR 2 : 200 mfd, 275 volts ( running O.K.)
Somebody please help.

Tools for Electricians:

#128331 - 01/19/03 08:30 AM Re: frequent capacitor failure  
maintenanceguy  Offline
Joined: Dec 2001
Posts: 300
Southern NJ, USA
First, I'd check that the 35mfd cap is the right one for this saw. Could have been changed at one time with the wrong size.

If the 35mfd is right you can try a higher voltage capacitor, replace your existing one with that. Just keep the mfd rationgs the same and it will work fine.

But, 440 is the higher voltage caps I'm familiar with so you might not find a higher voltage cap available.

Sometimes two caps are used together on motors drawing large starting current. This divides the voltage between the two caps and puts less of a strain on each one. To do this, find two capacitor's whos mfd ratings add up to the 35mfd (Such as a 25 and a 10) and wire them in parallel.

to original cap terminals
| |
| |
|--||--| -->25mfd cap
| |
| |
'--||--' --> 10 mfd cap

#128332 - 01/19/03 08:32 AM Re: frequent capacitor failure  
maintenanceguy  Offline
Joined: Dec 2001
Posts: 300
Southern NJ, USA
Aparently the bulletin board software is set up to remove spaces from posts so my fancy drawing didn't work. But you get the idea.

good luck

#128333 - 01/19/03 10:51 AM Re: frequent capacitor failure  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
I have a couple of questions regarding this Motor:


[*]The RPMs listed is 1440, which sounds like a Shaded Pole Motor. Is this the nameplate RPMs at 60 Hz?,

[*]Is this Motor a Capacitor Start, Capacitor Run type (if not a Shaded Pole Motor)?,

[*]Is this Motor run at or near full load output for an extended period of time?,

[*]Is the Motor's frame hot to the touch?,

[*]Is the Motor large enough to handle the duty?

[*]Is the Motor being run on 208 VAC or 230 VAC?

If the RPMs listed is 1440 and the Motor is not a Shaded Pole Motor, maybe it's a 50 Hz Motor???

Please add as much information possible, so we can give you a more complete answer.

Scott35 s.e.t.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#128334 - 01/19/03 11:04 AM Re: frequent capacitor failure  
electric-ed  Offline
Joined: Jul 2002
Posts: 175
The motor appears to be a capacitor-start, capacitor-run type. The run capacitor is the one that is failing, so make sure that the replacement is rated for continuous duty, not an electrolytic type, which is normally rated for intermittent (starting) duty.


#128335 - 02/24/03 03:04 AM Re: frequent capacitor failure  
Andre M  Offline
Junior Member
Joined: Feb 2003
Posts: 8
Wheeling, IL
Definitely 1440 RPM motor is designed for 50Hz. Capacitor and motor inductance makes resonance circuit and if motor is runing on 60Hz, it is "closer" to resonance. Voltage on "run capacitor" is usually higher than line voltage (commonly known effect with resonance circuit) and closer to resonance can be twice or triple higher than supply voltage. There I see the main reason for damaging of capacitor.
Proper determination of capacitor needs engineering calculation. Ask manufacturer.

#128336 - 05/25/03 11:18 AM Re: frequent capacitor failure  
Ichabod  Offline
Joined: May 2003
Posts: 26
Statesboro, GA, USA
The original thread was posted from Kuwait, which is 50 Hz. So RPM sounds right. What is the failure mode of the capacitors? Are you sure the motor doesn't have a problem? If the original cap failed to ground the motor's auxiliary winding would have been damaged. First thing I would do is read voltage across this capacitor while the motor is running. The proper voltage should be about 400. And be sure it's the right capacitor. Running caps are most often oval and in a completely sealed aluminum can, which may be covered with paper. Starting capacitors are usually round and the terminal end has a pressure relief vent since they are electrolytic.


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