# Problems with a Surface Grinder



## randyjaco (Oct 29, 2011)

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


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## Hawkeye (Oct 29, 2011)

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.


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## Old Iron (Oct 29, 2011)

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


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## randyjaco (Oct 30, 2011)

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


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## Tony Wells (Oct 30, 2011)

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.


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## randyjaco (Nov 1, 2011)

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


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## Tony Wells (Nov 1, 2011)

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,


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## randyjaco (Nov 1, 2011)

Thanks, but I got one for 20 bucks on Ebay. I figured I couldn't go too wrong for that price 8^)

Randy


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## Hawkeye (Nov 1, 2011)

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.


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## randyjaco (Nov 2, 2011)

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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## Hawkeye (Nov 2, 2011)

Was that grinder also available with a variable-speed DC motor? That could explain the reference to the brushes.


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## Tony Wells (Nov 2, 2011)

Look at all the education you are getting, Randy!


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## randyjaco (Nov 3, 2011)

Yeah, I think I just learned; if it ain't broke don't fix it.  8^)
I don't know if a variable speed was an option. I kind of doubt it; as it was a low end Enco. There may have been one available from the factory though.
My ammeter arrived today. Hopefully I will get to try it out tomorrow.

Randy


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## Tony Wells (Nov 3, 2011)

SOOT (Sort of on topic) user tip for clamp on ammeters:

Since they operate on the principle of induction, they read a current induced in the laminated iron made up by the clamp. These meters typically have a rather high minimum measurable current. It is possible to "trick" the meter into reading lower currents, into the milliamp range. Simply take the wire (remember, only one of the leads of the motor or DUT) and instead of just clamping the meter around it, wind up 10 loops into the clamp of the meter. This increases the account induced in the clamp by a factor of ten. Just remember to divide the reading by 10 to get the actual current.


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## randyjaco (Nov 5, 2011)

Being a total Noob to the clamp on ammeter, that is a great tip. I sure didn't see THAT tip in the instructions.

I am hoping to get to it tomorrow. I had required husbandly duties to perform today 8^(

Tony, do have any ball park idea of what kind of amp range I should be seeing on each wire of a 3 phase 1.1 KW motor?
I am presuming 1/3 of the stated name plate amp draw and relatively equal???

Randy


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## Hawkeye (Nov 5, 2011)

The current rating on the motor plate isn't all three added up. It's what you can expect to see on _each_ wire at _full load._ When you're just running at idle, the current will be somewhat less. You should only see the plate value when it's running at full rated horsepower.


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## Tony Wells (Nov 5, 2011)

Power calculations get pretty involved at times. Your 1.1 KW motor works out to 1.5 HP. You won't see the 8 amps at idle, as Hawkeye points out. In fact, if you calculate back from 8 amps, you have a larger motor. Why? Something called power factor. Look at it as efficiency. If the motor never put out any heat, only rotary motion, it's Pf would be 1 (or 100%). No such motor exists yet. Good motors are 85-90% I believe. Plus, if you stall the motor, you get a whole new set of numbers called FLR amps, for fully locked rotor. This will be quite high, and one you hope to never see. Under no load, you should see less than the 8 amps on the plate. That 8 amp figure is with the Pf considered, and loaded to 1.5 HP. I'd guess (and it's only a guess) that you'd see 3-4 amps on each leg.


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## randyjaco (Nov 6, 2011)

Well I finally got some time to play with the surface grinder. I checked each leg on the motor:
L1- 1.2 amps  W1
L2- 3.3 amps  V1
L3- 3.5 amps   U1

So what do you think? It doesn't say too much to me.

A new mechanical problem has raised its head. The motor and spindle are connected by a coupler. The coupler is 2 cast iron disks each with 2 large tabs. The tabs on the opposing disks interlock with lots of clearance. The disks and tabs are enclosed in a relatively sealed cavity. When disassembling the coupling it seemed to be packed with a substance that looks like a mixture of saw dust and asphalt. I tried to simulate it with silicon sealer. Now that I have it all together; when I start up or shut down the motor I get a considerable amount clacking. I presume it is the tabs slapping against on another. Any ideas on what I could use as a cushioning medium?  

Randy


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## Hawkeye (Nov 6, 2011)

I suspect that the low reading on the W1 wire is due to the RPC. As several members have pointed out, you can only expect about 2/3 of the rated power from an RPC setup. Power is, crudely put, Amps times Volts. Lower amps and/or lower volts = lower power.

The sawdust mixture could be what age does to a rubber insert. Consider replacing the whole coupler. A thick section of silicone sealer can take a week to cure completely. If it's still too soft, it could allow the prongs to prang. Here's a weird idea. Try mixing the silicone with - wait for it - sawdust. The filler may stiffen it enough. Allow plenty of time for it to cure. It does so by reacting with air. The thicker the section, the longer it takes for the reaction.


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## Tony Wells (Nov 6, 2011)

Most of your run-of-the-mill RTV silicones are not going to work there. Get with a power transmission supplier for a coupling. They will need the shaft sizes and horsepower transferred as well as the anticipated RPM.  

Enco might or might not be any help on this. I'd go straight to Applied Technologies or someone like them.


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## randyjaco (Nov 12, 2011)

Well I got it all back together and it is running like a champ. I manufactured a new coupler buffer out of an old tire sidewall. It is running smoother than ever. Hopefully I will never have to tear into that surface grinder again. I have learned Too much 8^)
Thanks for the help and encouragement.

Randy


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## Hawkeye (Nov 12, 2011)

Glad you got it going, Randy. Good work.


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