- Joined
- Aug 17, 2010
- Messages
- 883
Well, I didn't completely blow it up, but it stopped working. Even though most of you are electrical and mechanical engineers, I thought I would share what I learned this week for the other guys and gal.
A while back I picked up a home-made combo 6X48" belt, 9" disc grinder/sander. I bought it from the son of an aerospace engineer and master welder. I'm told his father designed and built it because he was not satisfied with what was commercially available at the time. I have no doubt this was built 40+ years ago. His dad was a damn good welder and designer.
Powered with a 1/2 HP Craftsman open motor that I could stall, I knew I would need to upgrade to a bigger TEFC at some point. I eventually killed that motor and put on a 3450 2HP Dayton. I've been using that combo unit more and more, especially for the disc. Last week I put a reversing drum switch on it. Worked so good I worked it to death!
On Sunday, after about 6 hours of grinding on the disc, I heard a loud pop and it shut down. Circuit breaker was not tripped and the thermal overload switch on the motor didn't help. The motor was very hot. I turned the wheel by hand and slight smoke came out of the junction box. Crap.
Of the two capacitor covers, I pulled the cover of the one that was hotter. The capacitor had exploded under the cover and released moisture and tar. This was the start capacitor (black phenolic case). The other (run) capacitor (aluminum looking case) was visually fine. I opened the motor and everything in there looked brand new and nothing was burnt.
I went to the internet and found several items of interest, mostly new to me. (My motor is capacitor start/capacitor run. I believe most of you with single phase motors have something similar.)
The start capacitor gives a boost of energy at start up, and then is removed from the circuit by a centrifugal switch within fractions of a second. AC motors without a start capacitor may wind up slowly, like some bench grinders. The centrifugal switch is a weight/spring mechanism that engages a typical switch to remove the start capacitor from the circuit after the motor reaches a certain RPM. You can hear the mechanical switch at least on shut down. Keeping the start capacitor in the circuit more than 5-6 seconds will instantly burn it out from excessive heat. They should be out of the circuit in less than 2 seconds.
Apparently, the most common failure is the start capacitor. Heat kills them, some more than others, and they are only rated for 20 cycles an hour. I probably cycled that motor 30 times an hour for six hours, so I can't complain.
I reassembled the motor, disconnected the start capacitor, powered it up and held the thermal overload switch. 2 to 3 seconds later, the motor started and ran fine, just a bit slower getting to speed. One or two subsequent starts were virtually normal. Sweet, all I need is a new start capacitor.
A few troubleshooting guides on electric motors and the HVAC forums recommend one of two things for premature start capacitor failure. Either, install one with the same MFD rating and increase the VAC, or keep the same VAC and install two capacitors in a series with twice the MFD rating. It's hard for me to consider this premature failure, considering how hard I worked it. But, I'm putting a footswitch on this motor soon and that may be harder yet.
I ordered a new start capacitor with the same MFD rating (270-324) but upped the voltage from 165 to 220-250. $18.50. I think I will mount it remotely to help keep it cool.
Just some ideas to consider. If your motor quits working, you might be able to fix it yourself on the cheap.
If I am incorrect, I appreciate your advice.
Erik
A while back I picked up a home-made combo 6X48" belt, 9" disc grinder/sander. I bought it from the son of an aerospace engineer and master welder. I'm told his father designed and built it because he was not satisfied with what was commercially available at the time. I have no doubt this was built 40+ years ago. His dad was a damn good welder and designer.
Powered with a 1/2 HP Craftsman open motor that I could stall, I knew I would need to upgrade to a bigger TEFC at some point. I eventually killed that motor and put on a 3450 2HP Dayton. I've been using that combo unit more and more, especially for the disc. Last week I put a reversing drum switch on it. Worked so good I worked it to death!
On Sunday, after about 6 hours of grinding on the disc, I heard a loud pop and it shut down. Circuit breaker was not tripped and the thermal overload switch on the motor didn't help. The motor was very hot. I turned the wheel by hand and slight smoke came out of the junction box. Crap.
Of the two capacitor covers, I pulled the cover of the one that was hotter. The capacitor had exploded under the cover and released moisture and tar. This was the start capacitor (black phenolic case). The other (run) capacitor (aluminum looking case) was visually fine. I opened the motor and everything in there looked brand new and nothing was burnt.
I went to the internet and found several items of interest, mostly new to me. (My motor is capacitor start/capacitor run. I believe most of you with single phase motors have something similar.)
The start capacitor gives a boost of energy at start up, and then is removed from the circuit by a centrifugal switch within fractions of a second. AC motors without a start capacitor may wind up slowly, like some bench grinders. The centrifugal switch is a weight/spring mechanism that engages a typical switch to remove the start capacitor from the circuit after the motor reaches a certain RPM. You can hear the mechanical switch at least on shut down. Keeping the start capacitor in the circuit more than 5-6 seconds will instantly burn it out from excessive heat. They should be out of the circuit in less than 2 seconds.
Apparently, the most common failure is the start capacitor. Heat kills them, some more than others, and they are only rated for 20 cycles an hour. I probably cycled that motor 30 times an hour for six hours, so I can't complain.
I reassembled the motor, disconnected the start capacitor, powered it up and held the thermal overload switch. 2 to 3 seconds later, the motor started and ran fine, just a bit slower getting to speed. One or two subsequent starts were virtually normal. Sweet, all I need is a new start capacitor.
A few troubleshooting guides on electric motors and the HVAC forums recommend one of two things for premature start capacitor failure. Either, install one with the same MFD rating and increase the VAC, or keep the same VAC and install two capacitors in a series with twice the MFD rating. It's hard for me to consider this premature failure, considering how hard I worked it. But, I'm putting a footswitch on this motor soon and that may be harder yet.
I ordered a new start capacitor with the same MFD rating (270-324) but upped the voltage from 165 to 220-250. $18.50. I think I will mount it remotely to help keep it cool.
Just some ideas to consider. If your motor quits working, you might be able to fix it yourself on the cheap.
If I am incorrect, I appreciate your advice.
Erik
