A Milling Machine Electrical Question

[Stillgot10]

I have electrically hooked up a Kearny & Trecker 2CH horizontal/vertical milling machine that I got a heck of a deal on. The thing is in great shape, and works like a marvel, except that any time that I try to run the spindle speed above 500 RPM, it trips the overload relay in the motor controller box.

Frankly, when it comes to things electrical, I’m pretty near to being an idiot. I got onto the internet (the controller manufacturer’s site) to verify that the overload relay and the motor controller are properly rated for the motor size/loads, all that looks good. Someone more knowledgeable than I suggested that the problem might be under-rated ‘overload relay heater element(s)’. Wow, did that one go right over my head!

I have no idea what such an element is, or how to properly rate one. How can I sufficiently educate myself about these heater thingies so that I can start making semi-intelligent phone calls to various vendors to try to get this problem figured out?

If it helps, the motor is a 5HP GE, wired to 60Hz 230V 3 phase, it draws 14 amps per line.

[J. R. Williams]

Check the rating on the overload heaters. The unit may have been wired for 480 volts and the heaters are wasy undersize for 240 volt service.

JRW

[Stillgot10]

Thanks, J.R., that makes a lot of sense because I’m pretty sure that the mill was run at 480V at it’s previous abode. The problem that I’m struggling with now is that I haven’t the foggiest idea how to spec. out an appropriate replacement. Heck, let’s be honest, I don’t have the slightest idea of what the silly things do.

What I’m wondering is how are they rated, i.e. do they size out in ampere capacity, or voltage, or both, or neither? If they do size out in amps, do I just ask the vendor rep (Allen-Bradley) for “an overload heater relay sized for the A-B model XYZ controller, to be used for a 230 volt, 5HP, 3 phase motor that draws 14 amps per line”? Or is there some other magic lingo-whammy that I need to be prepared to sling at the poor factory rep?

[JCHannum]

Different manufacturers use different systems to number their heaters, most of which have no bearing whatsoeverr on their rating.

If you know what the heater is, you can probably find a number or number letter combination that is its size.

There should be a Bulletin Number somewhere on the starter a local supplier can use to identify the range of the heater required, and possibly supply correct sized set of heaters to replace. Hopefully, the starter is not so old, heaters are no longer available.

Some heaters will lose their solder and trip unnecessarily even if they are sized properly. Also, go over all of the connections on the starter, a loose connection will also cause an overload.

[Grady]

Probably the easiest way to would be to get the full load amps off of your motor, the name on the contactor or overload that the motor is connected to, and then find the nearest electrical distributor that handles that brand. Based on the motor current draw and the voltage it is running at, they can size heaters properly for you. The overload heaters are not generic, so a Square D heater element will not fit a GE, etc. If you are running a single phase application you will only need 2 heaters, 3 for a three phase. Depending on the age of the control circuitry, you might get lucky and have a solid state overload that you can dial in the range of trip. I have been in the electrical wholesale business for 20 years. Post back, I will help if I can.

[amateur]

When one refers to a motor"heater" what exactly are they ?

[Stillgot10]

Thanks, Grady. I think that the info that you gave me will be enough to arm me for the purposes of contacting the Allen-Bradley rep on Monday. I now feel that I'm on solid enough ground to give it a good try, at least.

I just wanted to avoid putting him through that "I want a new roundy thing that fits on the uppey-downie part" sort of stuff that those poor fellows always have to put up with. If things go sour communicating with the rep, I'll sure take you up on your kind offer. Thanks a lot.

[Grady]

Amateur

Overload heaters are thermal elements that are designed to fail under a high current load (which creates a thermal issue on the wiring circuit), so that the motor or the wiring does not burn up. Some do not permanently fail, but are just strips of metal that deflect under high heat and cause a bar to trip a normally closed switch that opens the holding coil circuit thus dropping voltage to the motor. This is probably the most common design.