Baldor Capacitor size

[4020guy]

Just got a 5hp Baldor motor from an old barn cleaner from a friend. It is a late 60's, early 70's model spec 37NO3W115, frame 215. I changed the bearings but the 2 start capacitors are done, not burnt but corroded. Of course the lable showing the mfd is toast as well. The motor shop sayes it needs a catalog number but there isn't one on the name plate. Went to the Baldor site but couldn't find anything. Anyone have any ideas? It is single phase. Thanks in advance.....Ron
 

[buickanddeere]

Actually, the largest capacitors , of the proper voltage that will fit under the cover.
Caps must be rated for 370 or higher voltage.
The start caps in our electric feed mill used to pop about every 8 months until we found units rated at a higher voltage than the factory caps.
Having more mfd capacity than factory will improve starting torque.
 

[guido]

Hello 4020guy,
Buickandeeere is correct on both counts.
However MFD's increase should be limited to about 20%
Guido.
 

[4020guy]

Thank-you B&D and GUIDO for your expertise. Going through my spare capacitors and found a 216-249mfd one about one half inch smaller in diametre than the ones in the motor and the same height. So I'm thinking one about 300 to 325mfd should do the trick. Buildin a new compressor with all new US and Canadian componets[except the motor]. Should only cost about twice what a person can buy one for....Ron
 

[guido]

Hello 4020guy,

The voltage across the capacitor is affected by the start windings, and the line voltage. Voltage across the capacitor is a good indicator of proper capacitor size.

Guido.
 

[4020guy]

Guido, so at 240volt input, should the voltage measured across the capacitors be what and when should it be taken? The start capacitors are in parallel. I've never have had capacitors with paper lables with the ratings faded away. Thanks again....Ron
 

[Bus Driver]

Could you contact Baldor? My bet is that they will tell you the original specs for the capacitors.
 

[guido]

Hello 4020guy,

Voltage at the capacitor should be close to the input voltage. Too-much of a voltage drop will make the motor labor when starting.
When the voltage goes way down, then the motor won't start. Capacitors in parallel are used for lack of space at times. When they are parallel you add the MFD's to get the total MFD's capacity, just like a car battery amp capacity.
When the capacity in MFD's is right, you should see a little voltage drop. There is a formula to determine the size of a capacitor in a circuit.
If you get stuck let me know.
Guido.
 

[guido]

Hello 4020guy,
I just found more info for you.
A 5HP motor requires a 10 MFD's capacitor for each 120v leg for a 3600 R.P.M. motor.
@ 900 R.P.M's it requires a 33MFD's capacitor
for each leg. SO if you start with 66 MFD's capacitors rated at 370V would give you a good starting point. Maybe you'll luck out at won't have to go any further.

Guido.
 

[missouri massey man]

I thought start capacitors could be varied by ten percent and only run capacitors could be varied by twenty percent. Start caps typically being high in mfd whereas run capacitors are typically low in mfd. Memory may be a bit fuzzy though. I was thinking this motor is a motor with dual start capacitors, and capacitance being additive in parallel the total capacitance can be varied by ten percent. Let me know if I'm wrong, I don't want to be spreading incorrect info. The voltage induced in the start windings is typically much higher than the applied voltage, which is why you need 370v rating on a 240v motor. Too high on the start winding capacitance will eventually damage the start winding is what I was taught back in the day.

Are the capacitors bad, or do they just look bad. Many digital volt ohmeters have a capacitance scale that can be used to read the mfd ratings on a good capacitor. If one is good and you have the meter to read it, both will be the same size on a dual capacitor start motor.

Hope this helps you and want to be corrected if my memory is wrong.