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12-16-2007, 10:12 PM #1
How Do Heaters work in a Magnetic Motor Starter?
In a magnetic motor starter, when too much current comes through the circuit, the heaters are supposed to get hot, and openn the contacts, thereby stopping the motor and preventing it from burning itself up.
Are the heaters reusable after they cool down? Or are they like a fuse, once they've blown, thats it, get new ones.
In particular I am curious about a Furnas magnetic motor stater on a 3 HP early 1970's unisaw. The starter takes Furnas E-52 heaters for my particular motor at 230 volts.
Thanks in advance.
12-16-2007, 10:18 PM #2
No, the heaters are reusable unless burned open by a severe overcurrent (shorted motor winding). The heaters operate either a bimetal switch or melt a "solder pot" filled with a low melting alloy, allowing a spring loaded operating arm to trip the overload.
12-17-2007, 01:55 AM #3
If a bi-metal heater element trips many many times, it builds up what is called a "thermal memory" which means it takes less and less current to make them trip the mechanism. So you get nuisance tripping and it may mean the heaters need to be replaced. That problem doesn't happen with eutectic melting alloy type overload relays, but they are becoming less common.
If your Furnas elements are tripping too quick, you can order replacements from a Siemens distributor in your area.
12-17-2007, 11:24 AM #4
I've come across the "thermal memory" effect you describe a couple times, but the culprit wasn't actually the heaters, but the overload relay itself.
I try to avoid thermal overload relays whenever possible nowadays, going with the heaterless electronic ones whenever available. No problems with incorrect heaters, excessive ambient temps causing false trips, or repeated trips shifting the operating point.
12-29-2007, 09:08 PM #5
and some will not. I know that the common Square D heaters don't - they use a low melting temp alloy and when
it melts, it's melted and you have to buy new heaters. I don't know about Furnas in particular.
12-30-2007, 10:53 AM #6
But should you have any power quality issues, especially harmonics, (pay special attention should you be using VFDs in your shop) the metering circuits of the “Smart” type contactors FAILs to measure current and voltage accurately.
Might not be a concern if you are using fairly inexpensive motors in your equipment, but should you get into the 10hp and above class, the security of traditional heaters far outweighs the risks of programmable contactors.
Believe me I’ve spent the time on the phone to the engineering staff of Cutler Hammer, and GE. The cost to correct the cheap metering circuits puts the option of the programmable contactors out of reach.
12-30-2007, 11:41 PM #7
As to comment from JoeFin on solid state overloads, some cheap ones are indeed not going to be very useful on the output of VFDs, but some are fine; you just need to ask. Almost all VFDs now have built-in overload protection anyway so it's only an issue if you have multiple motors downstream from a single VFD.
I would also say that the more expensive the motor, the MORE I would insist on a solid state programmable OL relay. The issue of accuracy, repeatability and reliability is far far better with SSOLs. Maybe not 20 years ago, but certainly now. Metering is a bonus feature on some of them, but has little or nothing to do with the accuracy of the OL protection algorithm.
01-01-2008, 07:17 AM #8
Not to sure about that Jref
1997, Cutler Hammer, Advantage Series contactors and soft starts, size 00 to NEMA 5s
No Heaters, Small Size
Advantage starters eliminate the need for costly heater elements and their associated installation expense. Standard overload protection functions include phase loss and unbalance protection, selectable trip class, automatic/manual reset and ground current protection
There's a simple law of electronics called Ohm's law that tells us the impedance is the voltage divided by the current. Here is the twist. We need to consider the voltage and current waveforms point by point. The power at any instant in time is the voltage times the current at that instant. Add deviations due to power factor and harmonic distortion and the simple metering circuits become quite useless rather quickly
Figure the instantaneous power (voltage times current) at a bunch of regularly spaced points. This would be hundreds or thousands of points typically. Add up all the instantaneous powers. Divide the sum by the time period the points came from.
The result is the true power!
The optimum phrase would be “Waveform Sampling Rate”. Which minimally should be 120 /cycle if you want to monitor “True Power”
This is what the “Cost effective” heaterless contactors do not do.
Sure you can play around with the dip switches and find a setting that will work, but what happens at 4pm when the utility increases Kvar, or when the 300hp DC motor ¾ mile down the line set turns off. All your trip settings are wrong again.
I’m sure some of the VFD manufactures have addressed this problem as they are a little more of a high dollar item. But yes it would be wise to ask or see if the specify “Sampling Rate”