Problems with a Surface Grinder

randyjaco

randyjaco

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I have an Enco model 120-6718 surface grinder. This is an old timer that is no longer carried by Enco. It was manufactured by Hanjiang Machine Tool Works in the PRC back in 1991. It is 220Vac 3 phase run on a RPC. None of the other machines running on the RPC are acting up. I only run one machine at a time.

Generally it runs well and does what it is supposed to do. The problem is that it will shut down on its own, some times while grinding and other times when it is just running free. Basically the motor will just shut off and spin down. Some times I can just hit the start switch and it will restart. Other times, after a few minutes, I will push the reset button and it will restart. This is becoming more and more frequent. According to the "Chinglish" manual the reset is on the "AC contactor" (Seimens 3TB41-17-OA). I am wondering if I should replace the contactor or if there is something else I should do to solve the problem. I am electronically challenged, so I really have no clue as to what to do.
I would appreciate any suggestions.

TIA
Randy
 
Disconnect the power, then try going over the contactor with a screwdriver. A loose connection could give you problems like that.

Another thing to check is the overload heaters, if it has them on the contactor. The heaters cause the contactor to drop out if the motor draws too much current. If it's been heating for a while, it will take a bit longer before it will restart. Again, loose connections will cause excess heating. Or they are the wrong size for the application.
 
I think Hawkeye nailed it, But while your in there blow out the Dust & Dirt. Or you can get a can of contact cleaner & give it a good cleaning.

Paul
 
Hawkeye
Are you talking about the thermal relay? It has one of them too. Or is there a heater in the contactor? I would imagine both could use a good shot of contact cleaner. God knows when they were last serviced.

BTW: I was reading the manual and it said to clean and grease the motor bearings "each half a year". I'll bet those bearings were last serviced in 1991 at the factory. I presumed that they were sealed. Yikes, it looks like I have some servicing to do .

Randy
 
Randy,
if you have an Amprobe type clamp around ammeter, do a bit of checking on the motor draw, unloaded. That will tell you a lot. You didn't say whether it is a TEFC motor, but I assume it would be, on a grinder, so we should be able to rule out excess dust/dirt inside the motor. When doing the ammeter check, let it run until it shuts down and observe the current draw during this process.

The heaters referred to are in the contactor and are simply nichrome coils that slip around a post along side the contactor body. Each leg of the contactor should have one. They are rated to allow a spiked load, while controlling a steady current draw. When they overheat, they kick out the contactor. Rarely should they allow the machine to immediately be restarted, as they don't cool down that quickly. They could be undersized, or marginal, but since this has been the same heaters since new, that doesn't seem likely. I'd definitely start with comparing the actual draw vs the motor plate draw. Should be close.
 
I tightened everything down and sprayed everything electronic with contact cleaner. I ordered a amprobe meter. So while I am waiting for the meter, I am pulling the motor and servicing those bearings. Man, they didn't exactly design this thing for easy servicing.

Randy
 
Randy, I'd have probably sent you an clamp on ammeter.....seems I have 3 or 4 of them. Oh well, now you will have one. They can be very useful in the shop,
 
Thanks, but I got one for 20 bucks on Ebay. I figured I couldn't go too wrong for that price 8^)

Randy
 
Randy,

If it has a thermal relay, I'd assume there aren't any heaters in the contactor. Tony covered the function quite thoroughly. If they have one set, that's enough.
 
Well I got the motor pulled and today I tore it down. I should have realized that when the manual told me to check the motor brushes that I should be skeptical. Well the motor had sealed bearings that didn't need cleaning and regreasing. Of course the brushes on a 3 phase motor are real hard to find. Oh well, when the amp meter comes it will be easier to check the draw. 8^(

Randy
 
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