Tips On How To Identify And Correct Electrical System Problems Exhibited By Overheating

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POLY-PHASE MOTORS RUNNING HOT

HOT NEUTRAL ON 120/208 VOLT SYSTEM

POLY-PHASE MOTORS RUNNING HOT

When the author worked for a Fortune 50 electrical manufacturer, customers often called up with the complaint that a motor was running hot. Without further definition, it's pretty hard to determine just what running hot really means. An electric motor will feel hot to the touch if it is significantly loaded. Of course, the word "significantly" in and of itself just begs for a defi- nition, too. The touch factor was at best a very inadequate gauge. For instance, a motor running in a very hot environment will be warmer than one in a colder environ. After all, a motors total temperature will be the ambient plus the rated rise in temperature at rated load. Then, too there is the issue of insulation class. Motors with a higher insulation class are designed to either run hotter, run cooler than the design allowance and hence "live longer", or a combination of the two. In the 1970's for example, old-timers used to U-frame motors thought that the T-frame motors with "better" insulation were way too hot. But let's suppose that you used some sort of technical means of deter- mining that indeed the temperature was hotter than it should be. Maybe you had an infrared technician do a scan of your equipment, and this particular motor was a glowing example of the problem. Or you used one of those fairly inexpensive, albeit limited capability, infrared "guns" you found in some catalog (Hey, we sell them too! They can't be but so bad!). Or you used a "temp stick. In any event, the motor is hot and nothing else I can say will convince you otherwise. It's not unheard of for a motor that is not overloaded to still overheat. A possible cause is a voltage imbalance at the motor terminals. In the worst case, the imbalance is a true single phase condition. But if that was the situation, I'd expect that an overload relay would kick the starter off sooner than later. Voltage imbalance will definitely cause the motor to draw more current in order to perform the required work. So not only do you face having the motor burn out prematurely, but you will be paying for the extra kilowatt hours and maybe even a demand penalty on top of it as well. Sometimes, the solution is to simply redistribute single-phase loads in the area. But in an industrial or even a commercial scenario, that is not always going to work because single phase loads are either insignificant and/or already sufficiently isolated and/or balanced. You can look for other causes like a high impedance in one leg that causes a voltage drop on a phase. That will probably be time consuming and may not bear fruit. If you do find one however, it's usually a cheap fix. Fixing any appreciable voltage imbalance is almost always worth the effort. The result is extended or at least a more normal life for three phase equipment, and a reduced energy bill. What if the voltage seems to be pretty well balanced, but the current is imbalanced? I'd first suspect that there is a problem inside the motor. How else could you explain it? Well, there may other plausible expla- nations. And if so, let me know! I'll gladly publish them right here. Now, what happens if the voltage is balanced, but the voltage is low? I guess you'd find that the motor runs slower. It still takes the same number of kilowatts to rotate the load. The voltage is down, so the current must increase to compensate for it. The current might increase as the motor slows down to find the perfect balance of speed and current. And I'll bet your power factor isn't too pretty, either. Lastly, if the current in all three phases is high, but the voltage and the current seem to be pretty well balanced, and the voltage level is basically normal, maybe the motor is improperly sized for the load. If possible, check the speed, current and power factor against similar loads on similar motors and see how they compare. This all presumes that you have the metering to do the job. In today's world, that means True RMS metering like that manufactured by Power Measurement. 'Nuf said.

HOT NEUTRAL ON 120/208 VOLT SYSTEM

We once again come to the question of how hot is "hot". If you haven't read the first few paragraphs on "overheating poly-phase motor", you may wish to before jumping to conclusions. But, then lets assume you really do suffer one or more hot neutrals ... and they don't really need to be limited to 120/208 volt circuits. Your 277/480 volt industrial systems will do just as well in some instances. But the solution may not be quite the same. This problem is especially common where numerous single-phase loads exist. And potentially, no pun intended, there are multiple causes. Let's first explore voltage imbalance as a potential cause. It's easy enough to confirm with a voltmeter. Of course, I'd suggest that if harmonics are present (or you don't know), you might want to use a True RMS reading instrument like the Tegam 125 Voltman. It is autoranging for both AC and DC. And it also has a Continuity function that the meter automatically switches to if no voltage is present. Just so happens that we sell them. Don't be fooled by meters designed not for low impedance power system troubleshooting and voltage measurement, but high impedance electronics circuitry work. But anyway, presuming that you are rightfully convinced that a voltage imbalance is at least a part of the problem, it's a good place to begin to work at finding the root cause. On the other hand, the voltage imbalance may be a symptom rather than a direct cause. It could be a symptom of a high impedance in one or more legs. It's not likely to be a symptom of a low impedance in a leg. That just almost never happens. Measure the current in the three phases. If they are fairly well balanced, then it's not a significant contributor to high neutral currents, believe me. Unfortunately, I can't offer up a Tegam ammeter for the job, because one doesn't exist. But that leads to why you don't already have a True RMS power meter on circuits of significance. Panel mounted 3-phase digital power meters are very inexpensive these days. Power Measurement, Ltd., whom we represent, sells them for as little as $600 each. That meter, a True RMS device, measures about 200 parameters. Among them are voltage and current imbalance, two of the parameters we just touched on. The meter, a model 7300, can even be configured to tell you what the maximum and minimum imbalance has been for whatever time period since you last reset the device. Maybe, as often happens, you'll find that voltage and current imbalance are negligible. Oh-ohhhh! Now what? Harmonics would be the next suspect. Your typical digital multimeter isn't going to be a lot of help. Fortunately, that 7300 monitors up to the 15th harmonic. That's not bad for a $600 to $800 meter! If you find a significant level of harmonic distortion, bank on there being an associated neutral current. Third order or triplen harmonics are the culprit. In other words, odd harmonics divisible by the number three are the ones to watch out for. The third harmonic, when divided by three is a whole number ... one. The ninth harmonic divided by three yields three. And the fifteenth harmonc yields five. The next one is the twenty-first, and so on. Great, now what do you do about it? You might leave well enough alone. And you might suffer from it as a result. Or you may not. But here's the twist. High neutral currents as a result of anything lend more likelyhood of undesirable neutral-to-ground voltages. It doesn't take much to cause electronic equipment around the office or the shop to start acting in a manner not preferred. Add to that the effects of harmonic induced neutral currents, and the soup gets a bit thicker. One piece of equipment, for instance, can affect the performance of adjacent gear. I've heard tell of one side of a factory causing equipment in the other side of the factory to just go nuts, all over an odd number divisible by three. So what's a manager to do? Consider a zero sequence harmonic filtering transformer. Yeah, you guessed it! We sell those too. The best, of course ... designed and built by Power Quality, International, the folks who invented the device. They are not cheap. And even better, they are not expensive. Most importantly, they tend to pay for themselves. Here's how. The zero sequence harmonics are shunted right back into the load, not up- stream through the conductors to other loads. This mimimizes the likelyhood that the neutral conductor will overheat, destroying not only its insula- tion, but that of the phase conductors. That reduces the chance of a short circuit occuring in some piece of conduit in the middle of the night. Sleeping any better yet. All that current through the neutral is current you are paying for. It is being generated by the load, so to speak, but the load calls for more energy from the phase conductors to create it. There is no free lunch. The Io Filter, a trademark of Power Quality, Int'l., also reduces neutral to ground voltage. I'm not going into a long diser- tation, so you can breathe easier again. But it does, believe me. Now, since there is minimal neutral current being forced back into the system, less phase current needs to be drawn by the load to create it. Less phase current means cooler conductors. But it also means lower kilo- watt consumption, and lower line losses. Are you beginning to see a case for payback here? Now it's been said that payback is hell. But they weren't referring to this kind of payback. It's sort of like an annuity. When the Io Filter is paid for, the payback con- tinues. That's a pretty sweet deal. The deal gets sweeter. There is less thermal stress on the distribution or power transformer supplying electricity to this and other loads. It runs cooler. It lasts longer. It can even power more loads than a general rule of thumb might otherwise provide for ... all because it's not being operated outside its design limits. Is power factor a problem? If you had that 7300 on the circuit, you'd know. It measures PF, too. Switchmode power supplies, the worlds worst offender when it comes to generating harmonics, are non-linear devices. As such, they play havoc with the power factor. The worse the power factor, the more current your load must draw in order to operate. Of course, you could just step the voltage up some to compensate instead. But you just try that and see what happens to the performance and life of the equip- ment. It just so happens that the Io Filter improves the power factor of non-linear loads, so that too is a plus. That doesn't mean that it will also improve the PF contribution of linear loads ... like motors for example. But every little bit contributes to the nice payback you are getting ... long after the Io Filter has been bought and paid for. Accountants like that, even though they don't like to admit that there is ANYTHING they like. We've already mentioned that phase current and phase voltage imbalance are not good. Voltage imbalance causes a three phase device to draw more current than it ordinarily would. So if you can improve balance, that too is a plus. Well, the Io Filter is a transformer. It sets right across the line at or near the load, typically right at the panel feeding several loads. What a bonus! So it improves both voltage and current balance. Equipment draws less current, balance is pretty well maintained and the equipment is less likely to overheat and die an early, perhaps untimely death. You smile all the way to the bank. Just how well does it inhibit imbalance. Well enough that if the system incurs a single phasing condition, the Io Filter will hold up the voltage on the down leg at the panelboard or load. Single phasing is not good for three phase devices of course. But then, too, it is not desirable to have a "phase" that is powering the important work of one third of your operation go belly up on you, is it? That's a true loss of productivity. Sometimes, work is lost and has to be redone because of the power failure. Sure, a UPS unit will resolve some of those problems. But UPS units utilize switchmode power supplies and contribute to the harmonics problem so common on many single phase circuits. Anyway, if you had a Io Filter at the panel feeding these circuits the voltage loss would be minimal. Back upstream, one of your Power Measurement 7300 meters tells you that you have a problem. But the people downstream at the load didn't even notice it. You are a hero and they don't even know! Now that's what power quality remediation is all about.

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