another field current vs. speed in DC motor issue

[eeinpa]

Greetings!
We located a rebuilt 30hp shunt field DC motor for a customer to put on an air compressor which "kept burning up motors". Hmmmm. Upon installation, they reported it "runs way too fast" compared to its next door neighbor. They wanted to disassemble the motor and move the pole pieces closer to the armature. Yikes! Hold your horses, folks.

Nameplate specs:
Westinghouse, 30hp, 1750/2100 rpm, 104 FLA, 240Vdc, field 2.25A, field 88 ohms

My first thought was that they had too little field current, but upon questioning them, they said they had the field connected directly to the line. Actually, they said the starter (two step resistance type) has a field loss relay, but they had jumpered that out.

The motor shop reports that after rebuilding, the motor was tested at 1825rpm (no load) at 240Vdc with the field connected to line. I calculate 240/88 = 2.73A field current.

The customer reported they were seeing 2200rpm when running the motor on a 255Vdc bus (6% hot, hmmm) and measuring a field current (inline Fluke meter) of 2.0 amps. They reported that the motor was drawing 150 amps (~50% overload!) trying to run at that speed! The field current appears too low, though I can't understand how 255/88 = 2.0 amps field current. They measured the field resistance as 84 ohms outside in the cold, which seems plausible, but still 255/84 != 2.0.

Ultimately, the rebuild shop questioned EASA, who opined that they were initially providing excessive field current which (in combination with the overload on the motor) was causing the field to heat quite rapidly, increasing its resistance and decreasing its current and field, which increased speed, which increased overload and heating, etc. etc.

The last suggestion I made was that they get a rheostat and set the field current to the nameplate value (2.25A) and see what happens. Or put on a smaller pulley

Questions:
1) Does anyone agree with EASA's assessment?
2) Why the 1700/2100rpm spec on the motor? I would think that means rated field current will get you 1700, and reduced field current can get you up to 2100.
3) Is 2.0 amps on a 2.25 rated field enough to get you 20% overspeed? With 6% overvoltage on armature?

Since our customer isn't calling hourly, maybe they decided to take my suggestion and use a smaller drive pulley Still, I'd like to understand what was/is going on. Thank you for thoughtful comments.

 

[edison123]

So, basically they didn't have the overload & overspeed protection ? Serves them right.

 

[eeinpa]

I'm not sure what you mean. The motor is connected to a two-step electromechanical DC shunt field motor controller, probably a Cutler Hammer. The controller would include both an instantaneous and a thermal overload relay which *should* have been sized correctly for the original motor. There are no electronics, so it's pretty hard to have overspeed protection. Believe it or not, there are lots of low tech systems like that still out there, many of which have worked fine for many decades.

It's frustrating for me to deal with these customers, too, but on the other hand I have to understand that not every industry has, or can afford, top notch equipment or personnel. These guys are working in filthy conditions and 10 degree temperatures; I'm trying to help them, not chastise them.

Do you think your remark was helpful to understanding the problem?

 

[edison123]

They "kept burning up motors" and yet can't be bothered (or should I say "chastised" ?) to put in proper overload and overspeed protection ? And who said anything about fancy electronics ?

If you have a problem with the tone of my reply, please read the tone of your op.

 

[PHovnanian]

How long after the motor is started did the customer read their field current of 2.0A? EASA's theory may be correct, but it would be pretty easy to check that out by measuring the initial field current (before overheating).

Your customer jumpering things out might have removed enough resistance from the field circuit to cause what EASA is theorizing.

The high armature voltage/low field currents you mentioned could get you into the speed region you are seeing. I think the nameplate RPM ratings you are seeing are full load/no load figures for the specified field current.

 

[ankervik]

I wonder...1750/2100 rpm. Could this perhaps be a motor that utilizes field weakening to obtain that speed range? But then again it should be stated on the nameplate if that was the case. Would it be possible to test the motor in-situ & supply the shunt field with an external variable voltage? That way you would be able to get an idea of the voltage required to strengthen the field enough to get the speed down to the rpm you need.

 

[roydm]

Does the motor have series fields as well, perhaps if they were backwards.
Could supply ripple be causing the low field current?
Just a couple of thoughts that popped into my head.
Regards
Roy

 

[waross]

An old journeyman gave me a lesson in troubleshooting a several generations ago.
He deliberately set a problem with false information and let me work on it until I finally gave up.
He then said:
"Well, I lied!:
He then gave me the correct information an the solution was immediately obvious.
His last words, and the point of the lesson..
"Remember, BS baffles brains!!"
He liked me. I'm glad he wasn't my enemy.
The point.
Send some calibrated test instruments to the boys.
Point out to the experts that If the voltage is increased THE CURRENT WILL INCREASE!!
The increased current will cause increased temperature which will limit the current below what it would be with no heating, but never below the initial current.
Like 200% voltage may result in less than 200% current due to increased resistance but never less than 100% current.
When I see results for changed voltages and speeds and currents that can not be reconciled, I remember BS baffles brains. The data is highly suspect. Not to attack your field people, the meters they are using have possibly been subject to years of abuse, but please try to get reliable data.
Not to attack the field people, but did I really understand that an :Expert" theorized that the voltage could be raised to the point that the current would decrease? Please tell me that I misread that. Please.

Field current
V = 240 V
R = 88 Ohm
I = 2.25 A
240V/88 Ohm = 2.73 A
2.73 a/2.25A = 1.212
88 Ohms x 1.212 = 107 Ohms hot resistance.

Has anyone checked to see if the motors are burning out because they are just plain overloaded?? The smaller puley may be the solution!!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eeinpa]

Hi, folks. Thanks for the feedback.

First, some further background. These air compressors are located on a HUGE piece of moving machinery, outside, which MUST move at least every 10-20 minutes, around the clock, every day, all day, no matter the weather, no matter what breaks, for years. Yes, years. There is no stopping. Period. It's filthy, the temperatures go from well over 100F to ambient within minutes, there is probably no AC power on the machine, and I'm sure the lighting stinks. They are now owned (and being squeezed) by a corporation in a foreign county which in the past shot large numbers of its own population. I can't imagine it's an easy place to get proper tools, personnel, or management support. It's difficult to even converse with them, because they are on cell phones and squawking radios in a horribly noisy environment full of metal and electrical devices. Oh, did I mention they are union personnel...watch the toes! I am trying to help them out, because we sell them things, and helping them out is therefore my job. I also like to understand what's going on when I see a problem that seems "impossible".

I'll reply in order:

Edison123: Whatever, dude. I don't even know what "tone of my op." is.

PHovnanian: Regretfully, I do not have a clear answer on how long after startup they measured field current; it was a ludicrous conversation. That's why I suggested they put in an adjustable resistance and set it so initial field current is at nameplate spec. I agree: jumpering out any external field resistances *might* be enough to cause EASA's effect. It would seem to me that a nameplate which specifies 2.25A field and 88 ohms REQUIRES some external resistance on 240V, as 240/88=2.73 and 2.73 > 2.25A. It would certainly need some on 255V! Your idea of 1750-2200rpm being a full load/no load rating is certainly logical--anyone know if that is typical of a DC shunt field motor? I don't.

ankervik: Yes, field weakening for the 1750-2200rpm range is what I alluded to in my original post, but I think PHovnanian's idea that it is full load/no load is more logical. Unfortunately (see unfavorable conditions above) it is difficult to set up much in the way of tests. The customer says they have set up the second (identical) motor we sold them with "identical controls" in a shop and that it behaves the same. I am somewhat skeptical of how identical that situation is. They are also using a photo tach one place and a mechanical the other. But they swear the compressor with our motor runs noticeably faster than its neighbor.

roydm: The motors are shunt field (not compound) and the motor shop AND customer claim there are only 4 leads on the motors, so I do not think there is a series field. If it is, it would appear the leads are not brought out. Could the rebuild shop have reversed a series field connected internally? Maybe, but you wouldn't think so. I would hope they wouldn't do the same thing on two motors... Regarding supply ripple: yes, I wondered about that. I asked if an SCR on their rectifier could be bad and putting lots of ripple which would cause IR heating yet be undetected by DC ammeters. Or maybe the converse as you suggest: too little current. I made them swear they tried several True RMS ammeters. They also swore that the (~2000A or so) rectifier is fine. I don't like the fact that it runs at 255Vdc, but that probably deals with the fact that most of the equipment runs off filthy steel collectors rails with considerable resistance.

waross: If push comes to shove, we might buy a couple True RMS meters and ship them out there, but I guarantee we would eat the cost regardless of outcome They do claim they tried different instruments with only nominal variation. Regarding what the EASA "expert(s)" said, I may not have clearly understood OR stated their argument. What I understood was: IF the field is supposed to be at 2.25A with 88 ohms, it needs external resistance even at 240Vdc. At 255Vdc it would REALLY need external resistance. If the motor is running hot and fast on 255V, and the user has NO field resistance, the field would probably be considerably over 2.25A. Since the field winding is fairly small wire, it would heat pretty rapidly when combined with the heating from the motor load. That would push its resistance up and drop the field current enough to increase motor speed. I guess I should look up the tempco for copper and see if that seems possible. Regarding the motor pulley: My very first thought was the compressor is overloaded by design. In that case, though, why does a similar motor next to it run slower than our motor? It's obviously pretty hard for them to do swapouts and A/B comparisons, due to their difficult operating conditions.

QUESTIONS:
Is it typical for a motor like this to be marked 1750-2200rpm and would these be full load/no load values?

Shouldn't a motor marked 240Vdc with a 2.25A/88 ohm field ALWAYS have external resistance in the field? I know lots of motors are applied such that they do NOT.

How much field variation is needed to change rpm from 1825 to 2200? Based on experience with field adjustment in a DC generator, I don't think it would take much.

Thanks for CONSIDERED replies intended to shed light and stimulate discussion. I think you know who you are!

 

[Skogsgurra]

I answer your questions below

QUESTIONS:
"Is it typical for a motor like this to be marked 1750-2200rpm and would these be full load/no load values?"
Answer: Yes, if the motor is intended to be field controlled above its base speed, it is typical to include the information on the nameplate. No, it is not speeds at full load and no load. It is speeds at full load and full exitation and full load and minimum exitation.

"houldn't a motor marked 240Vdc with a 2.25A/88 ohm field ALWAYS have external resistance in the field? I know lots of motors are applied such that they do NOT."
Answer: Why should it? 2.25 amps times 88 ohms is very close to 200 V. A little overexcitation doesn't hurt.

"How much field variation is needed to change rpm from 1825 to 2200? Based on experience with field adjustment in a DC generator, I don't think it would take much."
Answer: I would not be surprised if you need to reduce excitation around 50 % to get the higher speed. This is because most motors operate above the knee in the magnetizing curve, so you have to reduce current more than you think to increase speed. Then, if you think that 50 % is much or not, that is entirely a matter of opinion.


A final comment: If you are trying to "remote control" technical issues in that region (we all think we know where it is), then the experiences you have is the experience you should expect. Better set up an expedition and make sure you have some body-guards with you.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[eeinpa]

Gunnar:

Thank you for your feedback.

1) Ok, so the rpm given is max/min field as I thought. Now, how do I know what _min_ field is? Hmmm.

2) You say 2.25A x 88R is very close to 200V. Where did 200V come from? I'm saying that the field is directly off the line, nominal 240, in this case 255. I think that's why EASA is concerned about field heating.

3) I would consider 50% change in field "a considerable change", more than I expected. Thank you for the knowledge!

4) No, no, no. The plant is in the USA, thankfully! It's just been taken over by a corporation from a country not exactly known for touchy-feely management style

I should mount an expedition there anyway to see what is going on, but I have a tendancy to let sleeping dogs lie. We believe we supplied them with motors rebuilt by a reputable shop. If they supplied insufficient specifications on what they wanted, or their motor controls or load are incompatible, it's really their fault... But that's not really a way to keep customers.

Incidentally, there has only been ONE company I've worked for that consistently did things "right", IMHO. After 55+ years of operation, they were bought by Siemens and have now been basically obliterated. I've found a frightening amount of US heavy industry is being held together by bubblegum and bandaids. Maybe not the industry you work in, but a lot of them.

Regards!

(gosh, wish I had a clever script to paste here)

 

[Skogsgurra]

OK. I left out an assumption about the excitation. If the mains voltage is 240 V, you will get around 200 V after rectification (for those in the know that are going to tell me about peak voltage and 335 V: remember this is done without capacitors). So, a resistor usually isn't needed. Also, the forced cooling (which is always prasent in large DC motors) will take care of possible extra heat. Never had a problem with that.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[eeinpa]

A "mains voltage" of 240V is meaningless in this case.

These machines run off HUGE rectifiers (2000-4000A is typical) which run diverse machine and lighting loads. They are apparently adjusted to meter 255V _DC_. It's not like there's just a bridge on 240Vac running the field. In fact the rectifiers are big enough they are probably powered off medium voltage.

These motors (while continuously rated) do NOT have blower cooling built-on, although I agree that it seems common on DC motors starting about this size. If you were going to speed control these motors, I think you would need the blower. It appears these motors were designed to run continously at ~1800 and be cooled only by their built in fans. These are NOT "modern" DC motors!

Thanks for your comments.

 

[Skogsgurra]

As Jeff says: Ass-u-me nothing! My wrong.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

(gosh, wish I had a clever script to paste here)

.

Don't. See what happened to mine, though I thought it was pretty clever of me.

 

[Marke]

Hello eeinpa

First, I would measure the cold resistance of the field.
It is possible that the field has been rewound with a smaller gauge of wire and therefore has a higher resistance causing a reduced field current. The reduced field current will certainly allow the motor to operate at a higher speed due to field weakening.

I am also concerned about the voltage waveform.
If the voltage comes from an SCR based controller, there will be a lot of ripple on it and some meters are peak reading, so you may have 255 Volts on the crest of the waveform and 180 volts on the trough. The voltage may actually be lower (or higher) than you think.
Similarly, the current measurement may be being affected by the waveform, or the act of including a meter is series could be introducing additional resistance which is dropping the voltage to the field winding.

It is very difficult to comment to much without actually seeing the equipment, but has the voltage at the actual filed terminals been measured? I have found plenty of situations were the voltage on the terminals is not the same as the voltage on the supply for one reason or another.

Best regards,

Mark Empson
L M Photonics Ltd

 

[oftenlost]

It is not unheard of for manufacturers to place cold field resistance on the nameplate but utilize hot current as these are measured at different times. In this case, the 88 ohms resistance makes sense as the field resistance will increase as the fields warm up. An increase of between 20 to 25% is not uncommon so an 88 ohm field would be just about right for a 240 volt, 2.25 amp field. The motor would run slower than 1700 during the warmup and speed up to base speed as the field heats. The motor shop reported that the motor ran at in excess of 1800 RPM in the shop with 240 volts on the field. This is pretty fast unless the fields were run until the temperature stabilized. Sounds like someone rewound the fields by weight instead of counting turns.

 

[waross]

The number of turns does not have that much effect on the field of a DC coil or field. If you cut the number of turns in half, the resistance will be about 1/2. The current will be doubled and the amp turns will stay the same.
Heating changes with a change in the number of turns and the physical center of the coil may change with different numbers of turns, but if the number of turns is off a little bit it shouldn't make that much difference.
Less turns means less resistance and more current and more I[sup]2[/sup]R losses. The greater losses mean higher temperature and increased resistance. The hot amp turns of an under wound coil can be expected to be less than those of a normal coil and the motor will run a little faster when hot.
The point is that the over speed when the motor is hot is temperature driven, not driven by the number of turns in the field coil except indirectly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eeinpa]

The rebuild shop stated they measured the nameplate value (88 ohms) on the field. I suspect that the fields were NOT rewound, but rather reinsulated. Unless there is something seriously wrong with the fields in a motor like this, they probably are not going to be rewound; it's a considerable expense.

The customer reported they measured 84 ohms, which seems plausible since their conditions were almost certainly (hopefully?) substantially colder than the repair shop.

I will probably be at our "regular" rewind shop next week. They have many decades of experience and a proven track record. Just for the heck of it I will ask if they always count turns in field rewinding, also if they have any opinion on this motor.

Thanks for continuing comments. When I eventually talk to our customer (on another topic), I will post an update of what they did. Anyone want to bet "pulley"...?

 

[waross]

All things considered, "Pulley" sounds like a good solution.

Bill
--------------------
"Why not the best?"
Jimmy Carter