RF-31 Mill - Start Capacitor

[JZHeyde]

Morning everyone,

I didn't know where to put this so I I thought general discussions was best. I have an older RF-31 mill in my humble garage and I think the start cap has given up the magic smoke. I tested the capacitor with a capacitor tester at work and it is indeed bad. I reviewed the MF rating and purchased a corresponding capacitor from Amazon (150uF).

The replacement capacitor was physically much larger and will not fit in the cover. This brothers me because I do not want something hanging off the machine.

Secondly, the new capacitor did not help at all. The machine is very slow to spool up and if I dont give the spindle a small "push" it will blow the breaker. It is almost as if the machine worked better with the orignal "bad" start capacitor before I put the new one in.

I am running the machine on 110v in my garage.

Can someone point me in the right direction for the correct size and capacity capacitor?

Can someone help me determine if something else is wrong?

I can get pictures tonight of anything that would help trouble shoot this issue.

 

[cgrutt]

Had similar issue with my compressor. Turned out to be bad centrifugal switch within the motor.

 

[larry_g]

agree, you need to check the start (cf) switch inside the motor.

lg
no neat sig line

 

[6PTsocket]

There are many sources of motor start caps, some made with newer chemistry, that are much smaller than the old ones. Check the Mouser catalog or just google motor start capacitor and find a site that gives dimensions before buying. You never gave the capacitance and voltage rating of the original.
Now onto the other problem. The start capacitor needs a means of connecting to the motor on start up and disconnecting as soon as the motor approaches full speed. The most common method is called a centrifugal switch. It is inside the motor, under the end cap. It starts out closed and connects the cap to the motor. As the motor picks up speed centrifugal weights attached to springs spin out and open the switch. In time the contacts on the switch get so burned up that they no longer make contact. They can often be cleaned up with a fine file that will get some more out of them but the plating on the contacts is gone and the eventually need to be replaced. Less often there are mechanical problems with stuck weights or broken springs. Start your motor; give it a push. When it comes up to speed, shut it off. As the motor slows down, just before it stops, listen for a distinct click. That is the switch closing. That does not mean it is good but that there is a switch and it is mechanically functioning. There are other methods of connecting/ disconnecting a start capacitor but they involve an external relay. I think you would have noticed if the capacitor was connected to an external module. If you need a new cenrifugal switch, I suspect you might need a little assistance sourcing and installing it.

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[pilotmotor]

6PTsocket is spot on , start caps are intermittent duty , and are only designed for this so you will damage it if left in the circuit for more than a few seconds. something is not opening ( centrifical sw or start relay) in the start circuit.

 

[JZHeyde]

I never even thought of the centrifugal switch, I am no expert by any means but have replaced some rotors and CS in the past. I appreciate you guys pointing me in the right direction. I think I need to remove the electric motor and dive in. For all you know a replacement motor may be cheaper than you think. I will report back once I am able to get the motor off, it might be this weekend but may need to push it to next.

Thank you again for your replies.

 

[6PTsocket]

6PTsocket is spot on , start caps are intermittent duty , and are only designed for this so you will damage it if left in the circuit for more than a few seconds. something is not opening ( centrifical sw or start relay) in the start circuit.

If they are stuck closed he would most likely blow the cap after what looks like a good start. If the contacts are fried and not making a good connection, he just will get that needs-a-push start. When the contacts open there is arcing at the contacts. Eventually it burns them up and there is no contact. It us time for an ohm meter and a peek inside. The cap is between the AC line and through the CW to the start winding and then to the other side of the AC line. If he unhooks the cap, one lead should read 0 to one side of the line. The other wire should read a few ohms, through the CW and the start winding to the other side of the line. If that second reading is very high or no reading, the switch is bad.

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[pilotmotor]

yea your right , the start circuit is not closing , hence no starting and the "push" needed

 

[6PTsocket]

yea your right , the start circuit is not closing , hence no starting and the "push" needed

I have no idea why I abbreviated centrifugal switch CW instead of CS. Have to stop posting in the middle of the night

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[larry_g]

I have no idea why I abbreviated centrifugal switch CW instead of CS. Have to stop posting in the middle of the night

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It's OK, I called it the cf....

lg
no neat sig line