dc motor humming

Hello, and first off, Thank you for your service to our country.

I assume your motor is connected to a DC drive (controller)

Keeping it simple- verify the motor field is at the rated value 3.7A. Although you can measure the motor field volts (300V) you really need to confirm there are field amps-My guess is there will be.

2nd measure the armature amps and volts when running under no load. Bottom line: if you have substantial armature amps and field amps, and not rotating, and no mechanical load you probably have a bad motor.

Hopefully there are meters (on the cabinet door-or operator's panel) that show the motor amps/volts data. If there are not-be very careful measuring with handheld meters, Also BEWARE that disconnecting the any motor field wires with power on causes a large arc flash-Verify 0 Volts on the field leads before opening connection.

Restating: A DC motor requires rated field amps and rated armature amps to produce the motor's rated torque. By rated I mean nameplate.

Good luck-hope this helps.

thank you for your reply. :) there are meters in the drive, and i know i should have mentioned that it was tested. the arm amps and voltage were with in tolerance. field amps wre at 3.7 (after i set it) voltage was low though about 170volts. (that's what was getting to me) resistance of field was slightly low. nameplate rated 58 ohms. actual was 48ish. we have one that is due to be pulled that is running at low 30's, generally fault out around low 20 ohms.

the breaker feeding the drive has a phase coming in that was arcing between the strands of wire. the lug was lose.

anyway. now, i'm on a new tangent. :) the plant engineer came in and said that it was one of the interpoles. (he found an arc mark, that i could not see). but i was told that it wasn't something that i could have tested and found (no capacity to take motors apart) nice guy. said it's something you just know.

now i need to know/research, if there is a way to test interpoles without "just knowing".

thanks for your answer.

Ken

I presume then, that the interpole connection was loose? As interpoles are connected in series with the main poles, an open one would kill the field.

i have no idea. :) like i said, i have no capability to check. they are sent out to a rewind shop to repair.

You need to get an American book by a guy named Rosenberg, I haven't got it with me so I can't give you the details but it's something like 'Electric Motor Repair', and it must be the gold standard in motor testing and repair. A Seabee friend of mine gave it to me 35 years ago and its never let me down.

Good luck

..."You need to get an American book by a guy named Rosenberg..."

I've posted a link to this book in several other discussions, e.g.:

http://cr4.globalspec.com/comment/87341/Re-Electric-motors

This book was THE "Bible" (mandatory study) for everyone working in an Electrical Apparatus Service Shop that I worked at for a number of years. Self-teaching is great, but it goes a lot faster and a lot further if it is supplemented with GOOD references, and this is the best (for beginners and intermediate, anyway). WELL worth the investment.

Interpole coils on large DC equipment are so low resistance that a low-ohmmeter is necessary for testing (not a standard, or even "good" quality multimeter will do) for shorts ... and a hi-pot tester for grounds (which might be intermittent during running only). Lots of basic education in DCM&G circuit testing will provide the fundamentals necessary to follow the simplest process of elimination for such troubleshooting.

Best of luck to you ~

Ndt-Tom you posted that link before I arrived at this forum, but thanks for the info on the book anyway.

It's an excellent book true enough and well worth the money but it doesn't get any cheaper does it, my copy was $55 in 1973.

All the best...

I am also new to the field of this kind of electricity, and I stand in awe at the thought of such large DC motors. My prior experience is in DC motorpool applications and some gensets. A thought came to mind, I wonder if the DC coming in is clean, free of ripple of some sort. The low field voltage, seems to me to be a failed controller perhaps, as the ohms aren't that far off. To me, a hum says alternating voltage. But I'm just a novice, with no idea of what powers the field. Bad bridge rectifier?? Good luck, Dan

Kandanews

First check mechanical freeness of your motor then follows below.

As per your spec your motor is shount wound dc motor ( to produce this much capacity).

This must have three phase to plane dc dc drive controller.( full bridge type)

Seperate field for exicitation.

first apply field for the field connection and check its dc volts and Amps. as per spec +/- 20% ( mormal condition)

apply armeture supply from zero level to high up to 10 % only with external supply check your amp meter how much consumption is there. if it consum tooooooo much amps. stop there.

dismental motor rotor from motor.

cleen inner surface check all pole are generated while field applied condition check its looseness may be loose hence huming come.

els

check rotot commt, carb brush, spark, short on commt. clean if possible take meger test.

take disigene what fail.

nothings els in motro to fail.

my gaesss rotor fail you need to rewind it ?

thank you all for your tips and comments. especially the link to "the bible" on motor repairs.

I'm not authorized to take motors apart and we had a spare on the shelf, so the machine is up and running. actually under the shelf. :)

the plant engineer worked in a rewind shop for many years. one would think that we would attempt to repair these ourselves, but alas, that is not the case. someday maybe.

thanks again and hope to be chatting with all of you in the years to come as i continue with my degree. :)

In my work I deal with traction motors. The Low Ohm meter (or ductor) for testing low resistance circuits and diagnosing the fault is quite handy. When I check a motor where warranty may be involved GE insist on an AC voltage drop test for the field windings. The reason for this is a turn to turn short will not necessarily show up in a Ductor test because the applied voltage is quite low. The Test involves connecting all the shunt windings in series and bringing out the start and finish points for voltage measurement. 240VAC is then applied to the seriesed up windings, the field coils that would normally draw 400Amps DC will draw around 16amps AC due to their inherent inductance. When measuring the dropped voltages all should be approximately equal, if there are shorted turns the turn to turn voltage is usually high enough to drive current through the short and show it up as a lower voltage across the offending coil and of course higher voltage across one coil means a high resistance.

Interpoles on the other hand don't have enough impedance to test in this manner. To test them once again connect in series and use an AC welder (on small motors don't exceed the full load current) and measure the voltage drops accordingly.

To cross check, using a DC welder inject current through the winding under test, measure both current and voltage then calculate the ohms and compare to values in the manual. Once again take care with small motors not to exceed the current rating.

Insulation resistance is usually checked with a megger as a hipot can create its own faults.

Polarisation. Using a megger test the item for 10 minutes noting the insulation resistance every minute. Assuming the initial result wasn't a short, which is a valid result in its own right, the final value should be more than twice the initial value. Polarisation will tell you if your motor has absorbed moisture. DC is necessary for polarisation.

I'm surprised that no one so far has (unless I missed it) suggested failed rectifiers/diodes. If you hear humming in a DC motor, it means there is a significant AC component. Check the voltage and current for BOTH AC and DC, perhaps looking at the waveform with an oscilloscope. Or, if you have access to the diodes, check for shorted diodes (POWER OFF).

Hmmmmm- so a humming dc motor means a significant ac component- why then does my pwm drive from dc 24v @ 100hz produce hum at the motor(dc pm &brushes)- are you saying that pwm of dc mimics ac?.

Absolutely! True DC is a constant voltage, appearing as a straight horizontal line on a 'scope. PWM is only DC in the sense that it is either zero or positive polarity, never negative. Transformers do not work on DC, but they do on PWM. I have run this computer quite a few hours on a square-wave 120V source in my car. A square wave is just PWM with close to 50% duty cycle.

I'm sorry buddy, but in this example you are wrong.- Shorted power module diodes or SCR's will clear the input fuses or circuit breaker to the drive. A dc drive cannot regulate current when an SCR or diode is shorted.

The poster stated that the drive was regulating armature and field currents, with no trips.

Once the motor contactor is closed, and the first scr fires, that has it's partner diode (S3) or SCR (S6) shorted, there will be fault current, and the drive input short circuit protection will clear.

What you say about an AC component may be true, but it cannot happen using a variable output DC drive.

First off, I have to admit I have zero experience with DC motors anywhere near that large. I was not thinking of a controller, but a simple DC motor running from a simple 4-diode bridge. I have seen diodes fail to open and to short. I think it might be possible for one diode pair to fail open and the other to fail shorted, in which case the motor would be getting AC instead of DC. The impedance would keep the current down, but the AC would keep switching directions, so the motor would not run.

I'm sure you are right in the case of a controller.

I always learn something from these threads.But they often cause one to reassess ones assumptions.

I only come across DC drives infrequently,( often removing them and their brushgear problems,substituting VFDs and squirrel cage motors) .

It had not dawned on me that the power supplies, having no capacitors, actually supplied rectified a.c. to the motors.

Half wave would give a sort of 50hz and full wave 100hz,the current not reversing but being unidirectional form zero to whatever and back to zero.

Does this give a variation in magnetic field or does this get smoothed out somehow?

How does this work with shunt motors? Is the field out of 'phase' with the armature at any point?

Can anyone explain before I have to trot off with my 'scope to see exactly what is being supplied to the field and armature?

Oldeng.

2 common types of DC drives- (6 Pulse or 3 Pulse)

On a 6 Pulse you will have 6 SCR's to provide positive rectified power. 6 additional are required to provide negative power (or regen) 6 scrs will be gated on during each 16.667mS period of a 60Hz 3 Phase supply.

You are basically right regarding the DC current being unidirectional/ returning to 0.

What a scope would see if connected to a DC millivolt shunt in the armature circuit would be 6 humps every 16.667 mSec of sweep. The humps start and return from zero on a lightly loaded motor. As motor load is increased the humps will grow, then the whole wave form rises from 0. Once off of 0 the motor is in continuous conduction and the wave form continues to raise with load.

A picture is worth 1000 words, just wish I had one handy.

No, the armature and shunt fields are never out of phase, because at no point during motoring operation does the current reverse. In fact the inductance helps smooth the wave forms as compared to their corresponding voltage wave forms.

In answer to your performance related questions; some old DC motors running on generator power (purer DC) can have problems using rectified power, especially a 3 pulse drive. Most motors built in US in the last 50 years will have no problem with the rectified power. I believe motor design mods were limited to laminating, insulating and armature banding practices.

Hi,Thanks for the explanation,its whet my appetite to read up a little.I do remember that valve radios had a choke as well as capacitors which implies that inductance has a smoothing effect.We dont have chokes in psu anymore,I wonder why not,I recall they were really heavy.

The dc motors I come across these days are tiddlers,run from small dc controllers with single phase input,bridge rectified,so they are really running on pulsed dc.

The bigger dc motors were traction motors and crane motors,run off diesel generators,and mains dc,so had 'proper' dc input. The magnetic parts including interpoles were not laminated,and I had assumed that the use of laminated material in modern dc motors was just that the best magnetic material was available only in that form,and the fabrication techniques available were those used in ac motors.

I think I now understand the need for minimising losses caused by the pulsating dc and using laminated materials. The hysteresis devils and eddy current demons dont care that its dc,to them it looks just like ac level shifted,so they're going to jiggle around and get hot.

Regards,Oldeng.

This is what happens when they let a gunslinger imitate a snipe!

For a good general reference in continuing your education, try http://www.tpub.com/content/doe/. Integrated Publishing is the company that created most of the rating manuals for the navy. The FC manuals (that I'm sure are near & dear to your heart) are at http://www.tpub.com/content/fc/.

EMC(SS)