PDA

View Full Version : Problem with Mill motor tripping Circuit Breakers?



Your Old Dog
02-10-2008, 01:48 PM
I have operated this mill on this exact circuit for over a year with no problems with the circuit breaker. As you'll note, the motor tag says it runs at 18 amps continous and has always run just fine on a 20 amp circuit breaker. Well, I put a Kill-a-watt device inline and it says it's drawing 29.8 amps for over about 4-5 seconds? I tried H-3 and H-1 thinking maybe the mill was mechanically bound up but it is not, it turns freely.

Any ideas what might cause this motor to go flooey on me? The voltage is a solid 120 according to the meter. I included the shot of the switch incase someone knows this type to be weak design. Now before someone says the circuit is too light for the load, just understand it has worked just fine up until now.

Thanks to anyone who can offer any suggestions besides the......well........you know..........the circuit is too small bit :D

http://bbs.homeshopmachinist.net/http://inlinethumb58.webshots.com/33081/2669749410102651310S600x600Q85.jpg (http://good-times.webshots.com/photo/2669749410102651310PsMvxN)



http://bbs.homeshopmachinist.net/http://inlinethumb32.webshots.com/9887/2345108800102651310S600x600Q85.jpg (http://good-times.webshots.com/photo/2345108800102651310OISOyy)

http://bbs.homeshopmachinist.net/http://inlinethumb64.webshots.com/34175/2964149950102651310S600x600Q85.jpg (http://good-times.webshots.com/photo/2964149950102651310pACgsv)

Rookie machinist
02-10-2008, 02:03 PM
Have you tried checking the contacts in the switch or motor contactors? You may just have a contact that is burnt, try cleaning the contacts with fine sandpaper. You should also check the internal eletrical connections to make sure nothing is coming loose.

davidh
02-10-2008, 02:04 PM
18 amps is runnning draw, probably pulls 36 amps or nearly that when it starts up. if your breaker is tripping, replace it as a first step. they don;t last forever. . .

fasto
02-10-2008, 02:14 PM
I have operated this mill on this exact circuit for over a year with no problems with the circuit breaker. As you'll note, the motor tag says it runs at 18 amps continous and has always run just fine on a 20 amp circuit breaker. Well, I put a Kill-a-watt device inline and it says it's drawing 29.8 amps for over about 4-5 seconds? I tried H-3 and H-1 thinking maybe the mill was mechanically bound up but it is not, it turns freely.

Any ideas what might cause this motor to go flooey on me? The voltage is a solid 120 according to the meter. I included the shot of the switch incase someone knows this type to be weak design. Now before someone says the circuit is too light for the load, just understand it has worked just fine up until now.

Sound like the start winding is not cutting out, or a bad cap. Does the motor ever come to full speed? I suspect the start winding cutout centrifugal switch.

Oldbrock
02-10-2008, 02:51 PM
try a new breaker, they do trip on a lower amperage sometimes as they age Peter

Your Old Dog
02-10-2008, 03:06 PM
UPDATE:
Thanks guys. Looks like Rookie Machinst might be on to something. I pulled the cover on the switch plate and all the screws were 1/2turn loose. The problem now is, I can only reach half of the screws. When I jiggle the wires on the other screws the connections move. Now to figure out how to get the switch out of the houseing so I can reach the other side. It has to be released from the front switch plate as there are no screws visible from inside the case.

http://inlinethumb25.webshots.com/40920/2226042160102651310S600x600Q85.jpg (http://good-times.webshots.com/photo/2226042160102651310FqOSia)

http://inlinethumb29.webshots.com/31388/2523452580102651310S600x600Q85.jpg (http://good-times.webshots.com/photo/2523452580102651310gtWnaI)

The caps looked pretty good with no burns or swollen bumps anywhere that I can see.

http://inlinethumb42.webshots.com/41321/2782974830102651310S600x600Q85.jpg (http://good-times.webshots.com/photo/2782974830102651310GLPaQr)

rantbot
02-10-2008, 04:59 PM
Not enough information here. You're drawing "29.8 amps for over about 4-5 seconds" when? On startup? After that what does it drop to? How does the device measure this? What is its sampling time? Unless it's intended to make dynamic measurements you don't know that you're reading instantaneous values. In fact digital readout devices don't even try to show instantaneous readings since a human can't read the numbers. A delay will be built into the circuit, basically a sample-and-hold. If you don't know what the hold time is, you can't make a reliable dynamic measurement with it. What does it show when you measure some other load, like, say, an incandescent bulb?

kjbllc
02-10-2008, 06:02 PM
It sounds like you have found the problem. Loose connection means more resistance and more heat/amperage. When a motor starts, it is at a dead stop, as in locked rotor current, which may also be listed on your motor. The startup amps on a motor can vary from 3 to 10 times the running current depending on the torque of the motor. Some high torque motors run startup current for 10 seconds or more.
I am new to this forum and I don't want to make any enemies, but being in the field for 30 years or more, I have only found about 10 bad breakers out of thousands that were "bad" UNLESS they were visually bad, as in overheated and blackened or the clips that connect to the bus bar had the temper taken out of them. Unless you are using it for a switch and turning it off hundreds of times breakers will last for .. well I have seen some from the 40's that are still under load and working great. Again this is just my opinion from what I have seen and I am not saying that others opinions are not correct or the new breaker would not solve the problem. Its just a lot of people automatically change the breaker when it is not really the cause. thanks.

kjbllc
02-10-2008, 06:05 PM
Just for an interesting fact , according to the NEC, a 20 amp circuit that runs for more than 3 hours, (considered continuous use) can only have 16 amps on it. The heat is accumulative. By the way I don't know squat about machining and that is why I am on this forum, I hope that I can add to this forum by what i do know a little something about. thanks.

Your Old Dog
02-10-2008, 06:10 PM
Not enough information here. You're drawing "29.8 amps for over about 4-5 seconds" when? On startup? From the time I start the mill up until the circuit breaker opens.


How does the device measure this? What is its sampling time? Unless it's intended to make dynamic measurements you don't know that you're reading instantaneous values. In fact digital readout devices don't even try to show instantaneous readings since a human can't read the numbers. A delay will be built into the circuit, basically a sample-and-hold. If you don't know what the hold time is, you can't make a reliable dynamic measurement with it. What does it show when you measure some other load, like, say, an incandescent bulb?

This is a device called a Kill-a-watt. It seams pretty reliable to me but it couldn't be considered state of the art equipment. It must sample pretty often because I can watch the amperage increase/decrease when the X-axis drive comes up to speed. I'm thinking Rookie Machinist might have nailed it. I tightened up what I could reach and then powered it up and got a steady state reading of 16.8 amps. All of the screws on the front took anywhere from 3/4 to 1/2 turn just to snug them up and they weren't even tight then.

At the moment it seems to be running fine. Thanks for all your suggestions guys. I might just buy a spare circuit breaker just to have it on hand.

rdfeil
02-10-2008, 06:56 PM
YOD

I have had problems with that type of cam switch. Loose screws (like you found) and internal problems causing them to bind. Anyway, to remove the switch you will first have to remove the front legend plate, most I have worked on are two piece, the outer part snaps off of a mounting base. When you get the front off you will find 2-4 screws that hold the switch and mounting plate on. Remove the screws and remove the switch from the back side. Reverse to reassemble.

Good Luck,
Robin

Oldbrock
02-11-2008, 11:29 PM
Welcome kjbllc. We are machinists or wannabees not electricians and we welcome anything you can submit re your knowledge. Keep looking, there are lots of us who know squat about what drives our toys, so keep watching. We will likely need you sooner than you think. Peter

wierdscience
02-11-2008, 11:32 PM
Is the mill cold when this happens?

Bob Ford
02-12-2008, 12:17 AM
Y O D

The rating plate says 110volt 1100 watts. This equals 10 amps. Watts / Volts = Amps. Then the plate says 18 Amps current??
Using the 1100 Watts as a base you have about 1 1/2 HP. I am guessing that with all the loose connections you are getting a high amp reading. Starting with the switch check all connections including the motor leads. The outlet on the wall is good for 15 amps, but not forever. Does the plug get warm when in use . The connections inside loose gripping power over time and get hot. If so change the outlet. Have you added heat near the panel with the breaker? Breakers work on heat buildup.

The quickest way to ruin contacts is to clean with abrasive. Most are silver plated or have a hard surface. Use contact cleaner and a soft cloth. Thirty five years as a Electrician and I replaced many contacts ruined by abrasives. They usually spark and ark because the springs that hold them are weak.

Bob

Your Old Dog
02-12-2008, 07:08 AM
Y O D

The rating plate says 110volt 1100 watts. This equals 10 amps. Watts / Volts = Amps. Then the plate says 18 Amps current??
Using the 1100 Watts as a base you have about 1 1/2 HP. I am guessing that with all the loose connections you are getting a high amp reading. Starting with the switch check all connections including the motor leads. The outlet on the wall is good for 15 amps, but not forever. Does the plug get warm when in use . The connections inside loose gripping power over time and get hot. If so change the outlet. Have you added heat near the panel with the breaker? Breakers work on heat buildup.

The quickest way to ruin contacts is to clean with abrasive. Most are silver plated or have a hard surface. Use contact cleaner and a soft cloth. Thirty five years as a Electrician and I replaced many contacts ruined by abrasives. They usually spark and ark because the springs that hold them are weak.

Bob

Thanks Bob, I"m getting a steady state of 16.8 amps when it's running but not under load. Before it was 29 amps during steady state.

I'm aware of the contact cleaner problem. A friend who worked in radio service says pink pearl erasers work well on contacts providing the contacts aren't toasted to begin with, in which case a bastard file won't do much more damage!

I have not been able to check the screws on the back of the switch as I can't figure out how to remove it without destroying it. I tried turning the face plate as in a bayonet mount and then slid a thin knife blade back to what looked like little catches but arent'. If the unit fails again I'll have not choice but to trash the unit to get it apart. For now, it's working.

kjbllc
02-12-2008, 08:01 AM
Welcome kjbllc. We are machinists or wannabees not electricians and we welcome anything you can submit re your knowledge. Keep looking, there are lots of us who know squat about what drives our toys, so keep watching. We will likely need you sooner than you think. Peter

thanks peter.

rdfeil
02-13-2008, 01:27 AM
YOD,

Follow this link to some pictures of the type of switch you have. It shows an exploded view of the parts and how it assembles.


http://electram.com/bremas_aci_motor_controls.htm


Robin

ProGunOne
02-13-2008, 02:08 AM
If the diagram provided doesn't work, why not drill a big hole or six the size of your screwdriver in the side plate that bolts to the mill to fit your screw-driver blade in? Beats destroying the switch?

J Tiers
02-13-2008, 08:08 AM
A few things.

1) kjblic is correct about the allowable continuous draw on a "molded case" breaker.

While you won't likely BE using continuous draw without ever turning it off, it still indicates that you are skating the edge of the breaker ratings....... and leaning over.....

2) When you do as per #1, and skate the ratings, you WILL likely eventually cause most "builder's quality" breakers to "drift" their ratings and start to shut off earlier than before.

If you happen to have "Homeline" or Federal Pacific" breakers, that's worse. The Homeline are just cheap, but the F-P may not even be safe. Both will "drift" more than say a Square-D "Q0".

3) the loose connections won't help, they may not allow the full start current to flow, and so prolong the start. it SHOULD start right up and go.

I have a drill press that has very heavy pulleys (it's an 18". ) It draws a heavy current and dims lights, but it starts in a second or so. So should your machine.

If you have an option to go to 220V, you should consider it. (looks like you may not be able to, though).

4) Chinese motors are the very reverse of tough and reliable...... it may have a bad winding that sometimes shorts internally...........

Ryobiguy
02-13-2008, 12:49 PM
I have one of those "Kill-a-watt" devices. Pretty interesting to see how much power anything uses.
I think it updates the display about once a second or maybe every 2 seconds.

However, isn't it only rated up to 15 amps?

J Tiers
02-13-2008, 08:02 PM
Ryobiguy brings up a good point.

With a digital dispay, you often get only the reading at the time the device took the reading....may not be at peak. The real peak may be considerably larger. (Nyquist problem, for the engineering types).

With an analog meter, you may OVER estimate peaks, due to meter needle swing.

Ideally, a digital meter with a fast sample speed, and a peak hold feature is needed.