DC motor field current

[drbond24]

I have a string of related questions based on a DC motor test I ran a couple of days ago. Fortunately for me, the test procedure makes performing the test fairly easy even though I'm not 100% sure of what I'm doing. The main concern in the test is the oil pressure, not the electrical info on the motor, but I'm still supposed to write all of this down. Now I'm trying to figure out the results I got. Here we go:

1. My test procedure says to measure the 'shunt coil current,' the motor elementary diagram for the motor shows a 'shunt field,' and the motor nameplate has a rating for the 'field current.' Are these all the same thing? I've got college notes and a textbook that I can look up stuff on a 'shunt DC motor,' but I want to make sure I have my terminology straight.
2. If the answer to #1 is a yes, then what would make the field current way too big? The rating on the motor nameplate is 1.2 A and I measured 15.1 A.
3. If the answer to #1 is a no, can you explain the difference(s) to me?

I tried to attach the elementary drawing I'm looking at. Hopefully that worked so we're on the same page. I had a current clamp around the conductor labeled F1 on the left side of the page, which is where I got the 15.1 A from. The motor is a 10 HP DC.

If you need more information or have questions, fire away and I'll do the best I can. I'm obviously rusty on my DC motors. Haven't messed with them since my Electromagnetic Devices college course.

Thanks!

 

[Besoeker]

Dr Bond

Shunt coil and shunt field would generally be taken to mean the same thing.
The shunt field resistance does vary some with temperature.
But not by a factor of over ten to one.
To me, this suggests one of two or three things:

1. You are not measuring the field or maybe not just the field.
2. Decimal point in the wrong place?
3. Incorrectly connected motor circuit? The shunt and series fields swapped?

Note also that, if the shunt field is taking more than ten times it's rated current it will be dissipating more than 100 times its rated power. Few motor fields, yours included, would survive that so it probably isn't happening.

 

[drbond24]

#1 is possible, though it would mean the elementary is wrong or I'm reading it wrong. Both have happened before.

#2 I already thought of that and double checked. I went out and polished the nameplate on the motor to be sure it really said 1.2, and when I was reading the current clamp I turned it every which way to make sure I was seeing the decimal point in the correct place. It definitely said 15.1. I'll look into the possibility that it was reading tenths of an amp or something silly like that.

#3 What would be the effect of that? This motor has been wired this way for years, so if the effects are destructive this is probably not the case. The pump it is running pumps just fine, so the motor runs just fine. It does not run often because it is an emergency backup, but when it runs it seems to do fine.

 

[drbond24]

Something else I just noticed as I was going through the rest of my data: the running current of this motor is only half of what it should be. It was only pulling 16 A when it was running and pumping normally. The nameplate says 37.2 A. Does that help with the diagnosis?

 

[dkarst]

Is there a HP rating on the nameplate and do you have any way to estimate the physical load? It sounds like the motor is lightly loaded and therefore your total current is less than nameplate. It is interesting your measurement of 15 A added to the rated field current of ~ 1A = your total measured load current of 16A. I know you have triple checked but are you certain you are on the correct leads?

 

[drbond24]

All I can say is that it is as loaded as it ever will be with the exception of if the lube oil were very cold. My twisted brain was calling that 'full load' and wondering why it was pulling closer to the 37 A. I have no way of checking how much it is actually loaded other than the running current I measured. The running current was measured using an oscilloscope on a shunt in the breaker cabinet, so it is a much better measurement.

I couldn't be more sure that I'm on the right wire unless I took the whole thing apart, and that isn't an option at this point. I've done this test before on other motors in the plant and recorded readings much more in line with the nameplate data on the motor.

I noticed that current addition thing, but I wrote it off to coincidence. At this point, I'm just going to work on a better way to measure the field current the next time I have to do this test. That won't be for a few months, so I've got some time. I don't like that current clamp I was using.

 

[Besoeker]

#1 is possible, though it would mean the elementary is wrong or I'm reading it wrong. Both have happened before.

#2 I already thought of that and double checked. I went out and polished the nameplate on the motor to be sure it really said 1.2,

I would expect something around there and I wasn't calling that into question.

and when I was reading the current clamp I turned it every which way to make sure I was seeing the decimal point in the correct place. It definitely said 15.1. I'll look into the possibility that it was reading tenths of an amp or something silly like that.

If it really is 15.1A and the motor is working fine then the only plausible explanation I can think of is that your measurement either isn't shunt field or is shunt field plus something else as dkarst has suggested.

Where exactly did you measure the 15.1A and the 16A?
Can you check it at the motor terminal box?
There may be six terminals - two for the armature, two for the series field, and two for the shunt field.
The armature and series field are normally connected in series at the motor so you should have four wires going into the terminal box. If the motor terminals are marked in accordance with your drawing, you should have no difficulty in identifying which conductor is which.
To reiterate what I said before, 15A in a 1.2A field winding just isn't ... well isn't. Maybe the cable or terminal ident is wrong?

I note your point about using a scope on a shunt. I'm not sure why you would conclude that it's a better measurement. The shunt might be some tens of mV (75mV is typical here) on a 250V system. You need to be very careful about where you connect on to the shunt and the quality of the connections. And you'd probably have to isolate the scope from ground or use it in differential mode. In my experience this arrangement is not conducive to making reliable accurate measurements.

I generally use a current clamp which I consider to be quite reliable. I usually check both directions to ensure that there is no zero bias. And all our test equipment carries a calibration sticker referenced back to a calibration log. Apart from it being good housekeeping, our customers require it.

 

[drbond24]

If it really is 15.1A and the motor is working fine then the only plausible explanation I can think of is that your measurement either isn't shunt field or is shunt field plus something else as dkarst has suggested.

That is certainly possible.

Where exactly did you measure the 15.1A and the 16A?

On the elementary drawing I posted, I had a current clamp around the conductor labeled F1. That is where I got the 15.1 A. There is a shunt in series with the Comm and Series fields (I believe it is actually between points A2 and 9 on the drawing). That is where I had my o-scope to measure motor running and inrush current.

Can you check it at the motor terminal box?

I will try to do that next time. I can't think of any reason why I wouldn't be able to do that.

Maybe the cable or terminal ident is wrong?

That is possible, too.

I note your point about using a scope on a shunt. I'm not sure why you would conclude that it's a better measurement. The shunt might be some tens of mV (75mV is typical here) on a 250V system. You need to be very careful about where you connect on to the shunt and the quality of the connections. And you'd probably have to isolate the scope from ground or use it in differential mode. In my experience this arrangement is not conducive to making reliable accurate measurements.

I generally use a current clamp which I consider to be quite reliable. I usually check both directions to ensure that there is no zero bias. And all our test equipment carries a calibration sticker referenced back to a calibration log. Apart from it being good housekeeping, our customers require it.

The shunt I was measuring was 50 mV / 100 A. I just thought the o-scope was a more accurate instrument. Perhaps I need to rethink how I'm approaching this. In any case, I don't know of any other way, with the tools I have, to capture the inrush current and starting time of the motor. I get a nice pretty graph on the o-scope measuring across that shunt which gets me both of those values. I never called their validity into question because they always seemed reasonable before.

 

[Besoeker]

The shunt I was measuring was 50 mV / 100 A. I just thought the o-scope was a more accurate instrument. Perhaps I need to rethink how I'm approaching this. In any case, I don't know of any other way, with the tools I have, to capture the inrush current and starting time of the motor. I get a nice pretty graph on the o-scope measuring across that shunt which gets me both of those values. I never called their validity into question because they always seemed reasonable before.

I agree that if you need timing and current, the scope is the way to go. For the Hall effect clamp on instrument I use, I have a scope attachment so I can make non-contact measurements and display or store them.

Another trick you might be interested in is this. The clamp on meter has a 1000A range. That's fine if you are measuring from a tens of Amps up but lacks resolution at lower currents. For low current measurements I sometimes wrap several turns through the jaws. Put on ten turns and divide the reading by ten for example.