Sep. 09, 2024
Measurement Instruments
The third thing you need to know is the environment of your project. Where are you going to use your power analyzer? Is it indoors or outdoors, in a laboratory or in a field, in a clean or in a harsh condition? How easy or difficult is it to access and connect your power analyzer to your power system? How safe or risky is it to operate your power analyzer in your project location? These questions will help you choose a power analyzer that is suitable for your project environment. You need to look for a power analyzer that is durable, portable, reliable, and user-friendly.
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(Mechanical)
(OP)
23 Sep 13 13:30Hello,
First be forewarned I'm a mechanical with limited electrical knowledge.
My company is manufacturer of gear-reduced equipment driven by an electric motor. We buy commercial electric motors, typically ranging .25hp to 150hp 4 pole TEFC. We and many of our customers are in the habit of using current measurements and comparing against the FLA to assess the power being drawn by the machine. This is a simplified view of things and can be affected by the actual machine power draw, electrical power factor, line resistance, etc. (I'm sure an EE will know it better than I do.) The wiring and power sources are handled by our customers or their representative - our scope is purely mechanical in design except for choosing an appropriate motor size.
We are trying to solve the problem of how to know when excess current is due to mechanical issues (our problem) vs. electrical support system errors (their problem). Where might I go to begin to learn the parameters that should be concerned, how to measure them, etc to confirm the motor is operating in an acceptable environment from an electrical point of view. FWIW we have a couple of Fluke 345's here to assist.
Ideally this could be a short list that we can run down when things aren't working as expected - but if necessary we could line up a consultant with appropriate experience. (You'd think our motor vendors are the ideal source for this but so far the support we've received is puzzlingly poor).
Thanks in advance,
David
Replies continue below
(Mechanical)
(OP)
23 Sep 13 14:18I should mention that we're not looking for a complete set of tools to diagnose / troubleshoot the electrical issues. I just want to take a reasonable minimum of measurements to understand if the electrical service is within the motor manufacturer's limits. Our field techs have limited time on-site, so it's important that we don't take down redundant measurements from our point of view.
(Industrial)
24 Sep 13 13:34It might help if you describe the problems you're seeing: Overloads tripping on start-up, after running for a while, etc. Are these motors running through VFDs or soft starters? US or elsewhere?
(Mechanical)
(OP)
24 Sep 13 13:58I would like a set of criteria that will catch most issues in most cases - but our negative experiences are typically elevated motor temperatures, elevated current draw, and high noise. We're quite familiar with the mechanical aspects so my focus is to answer the question: is this motor receiving adequate power for long-term warranty and optimal performance?
The unfortunate effect is when electrical systems are not powering the motors very well, the customer complains of elevated current draw and usually the electrical contractor blames it on excess mechanical load. We struggle to identify these kinds of problems in a timely and strategic manner - we can appear to struggle sorting out this problem that actually exists between the electrical supply circuit (our customer) and electrical motor (one of our vendors).
David
(Chemical)
24 Sep 13 14:55Well, it seems tome that if the motors are receiving the correct voltage then any excessive current would be do to excessive mechanical load or a problem in the motor. Relying on operators to watch a current meter to avoid burning out motors does not work well.
(Mechanical)
(OP)
24 Sep 13 15:39Are you sure that "correct voltage" and a good motor alone can lead to the conclusion of a mechanical power issue? That's the assumption that I don't think is working for us. I also say this knowing that we have circumstantial evidence that a mechanical power overdraw is unlikely.
I've been studying this subject a bit and it would appear that a bad power factor, line-to-line voltage imbalance, THD, etc are not uncommon. All of these affect the motor's operating efficiency and current draw.
But I haven't yet found a source that defines measurements and limits for power quality. This seems like a fairly fundamental question that it should be well-defined from a commissioning point of view.
This would not be a task for operators. We have field techs and brand-new Fluke 345's. I'm hoping that is enough to detect significant power quality problems.
David
(Electrical)
24 Sep 13 20:57Power factor is a characteristic of the motor. Don't worry about power factor.Look for voltage unbalances, line to line. A rule of thumb is that the percentage voltage unbalance squared will be equal to current unbalance. It doesn't take much of a combination of a moderate load and a seemingly small voltage unbalance to push the current past the rated current of the motor.Low voltage will cause a motor to run hot.High voltage will cause the motor to run hot for a different reason.Get a temperature gauge of some type. Many people do not realize how hot a motor can run safely.A three phase power analyzer may be useful.I would ask the manufacturer for some indication of how much voltage unbalance and how much low voltage the motor can safely withstand.If the customers power supply is outside those limits it is the customers issue to correct.A Watt meter will be a better indication of the motor load. The Watts drawn by the motor will be a combination of the load on the motor and the losses in the motor. Use the motor efficiency at full load to estimate the motor power and the losses.THD Again ask the manufacturer how much his motors can withstand. If you see levels of THD above the manufacturers limit, again the customer must improve his power quality.I would guess that your problems will be mostly unbalanced and/or low voltage and mechanically overloading.THD will be something to watch for but unless you service an industry that has chronic THD because of the type of equipment they use, it will not be common.Note; THD may be caused by your customer's neighbor on a common transformer and come in on the power lines.
Bill
--------------------
"Why not the best?"
Jimmy Carter
(Electrical)
25 Sep 13 00:53Bill makes a lot of good points, most motor problems I see are due to low supply voltage at the motor terminals or imbalanced supply voltage.
Note I said at the motor terminals, a lot of troubleshooting gets done at the closest point of measurement access. I recently dealt with repeated motor failures on a gas skid that was the result of a wiring problem, most troubleshooting done before was at the disconnect, we removed the pecker head cover and got to the leads to measure there to find the problem. Don't always have to go that far.
I use a Fluke 435 power analyzer to deal with this type of troubleshooting a lot, mainly because I don't get a lot of time to look at it and I get all three phases and power information like Bill pointed out above. And I get a record of what I measured. A bit more expensive but if you have limited time and want to build some history might be worth considering something like that. Especially if you can get known good data from motors leaving your shop before they go on site to use for comparison. Same with temperature measurements, IR thermometers are cheap, taking data on motors during testing at you shop can provide you reference data to use in the field for troubleshooting.
Or find a good electrician with a lot of motor experience and have him help you develop a set of testing and troubleshooting guidelines.
Mike L.
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(Electrical)
25 Sep 13 06:19A note on voltage testing. You may have equal line to neutral voltages but unequal line to line voltages. This is an issue in rural areas with voltage regulators on the distribution circuit. When this happens the phases are no longer displaced 120 degrees and the motor does not like that either. A three phase motor does not see line to neutral voltage. It sees line to line voltage. Don't be mislead by seemingly good line to neutral voltages if the line to line voltages are not equal. This is an issue to watch for on long distribution circuits with a lot of single phase loads.
Bill
--------------------
"Why not the best?"
Jimmy Carter
(Mechanical)
(OP)
25 Sep 13 06:31Thanks for the ideas. This will help.
Two dumb questions about measurements:
1) the Fluke 345's have a 3 phase mode in which current is on one lead, and voltage read from the other two. To detect the kinds of problems, is it necessary to perform this measurement on all 3 phases, or is one per motor a sufficient snapshot to detect trouble?
2) Using this meter I've seen the readings wander up and down as much as 10%. Is this alone a sign of trouble, and what is the best way to take accurate data when the numbers are wandering?
"Or find a good electrician with a lot of motor experience and have him help you develop a set of testing and troubleshooting guidelines."
That is the long term plan. We have training from Fluke on the way, and we are pursuing a consultant to assist us with electrical questions. Testing and troubleshooting plans will be certainly included with that. For the moment, I want to collect the best data we can until those events take place.
David
(Electrical)
25 Sep 13 17:34Simpler.Quit using current alone in addressing the issues, spend a little more on the meter and use kW. Kw is ALWAYS reflective of motor shaft power. That's what your differentiator is; the problem is on YOUR side of the shaft, or not. Period.
"Will work for (the memory of) salami"
(Electrical)
25 Sep 13 19:15The 345 in the three phase mode assumes a balanced load, if you can make a measurement under stable conditions in all combinations of applying the meter you could get usable results. But I think you would have to work at it and be diligent in documenting what you are reading.
What kind of loads are you driving? "gear reduced equipment" covers a lot of territory. Maybe you have another parameter in your process that could be looked at to help compare information?
Mike L.
(Electrical)
25 Sep 13 19:48As noted already, the "power quality" issues that an induction motor cares about will be low voltage or voltage imbalance. A fairly small percentage of voltage imbalance can lead to significant motor heating.
All of this is assuming you are not using a VFD to power the motor. That changes things considerably.
(Mechanical)
(OP)
2 Oct 13 07:43I found a reference in NEMA MG-1 that may apply:
Section: Mechanical Vibration
7.7.3.2: Power Supply: (paraphrased) AC machines shall be run on sinusoidal waveform at rated frequency and voltage. Voltage waveform deviation factor < 10%, Frequency <=+/-0.5% of the value for the test, Voltage unbalance <= 1%, where this is the max voltage deviation from the average voltage.
The scope of this section is defining the limits for a mechanical vibration test. Is this an appropriate set of conditions to apply to a site installation?
David
(Electrical)
2 Oct 13 12:55NEMA MG 1- Part 31 Page 2 (this section is for inverter fed motors) says that greater than 1% unbalance at the motor terminals is an 'unusual service condition' (31.1.3.b.2). Voltage shouldn't vary by more than plus or minus 10% from rated (31.4.3.2).
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