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Old July 6th, 2010, 05:22 PM   #1
AmazingTrans
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Smile Reading a DC Motor name plate???

Hello control gurus,

I have a question about reading a DC motor nameplate.

I have a motor below:
200hp
1750/2400 RPM
500 Varm
320 Iarm
150/300 Vfld
8-4.6 / 4-2.4 Ifld

Currently my field on my motor is wired in series F2-F3, f1 to drive, f4 to drive.

What field voltage and current should i program into my drive? Why is there also a 8-4.6 / 4-2.4? (dash?)

My Powerflex DC drive is a 200HP drive, 460VAC.

Any comments?

Hope to hear from someone.
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Old July 6th, 2010, 06:46 PM   #2
jrwb4gbm
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Here is a little help. Series field voltage would be 300 vdc. Using a 300 vdc field supply, it would draw the 4 amps and the motor should turn 1750 RPM. If you lowered the field voltage, say about 30%-40%, the field would draw the 2.4 amps and the motor should turn 2400 RPM. The RPM's are based on having 500 vdc on the Armature. All this information is only approximate so wait for better info from someone else. Of course, the 8-4.6 would apply if the field supply vdc were 150 vdc and the field were wired in parallel.

Last edited by jrwb4gbm; July 6th, 2010 at 06:57 PM. Reason: more info
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Old July 7th, 2010, 07:07 AM   #3
DickDV
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The field on this nameplate is a shunt, not a series field. That simply means that it is wired as a separate circuit from the armature. Probably what jrwb4gbm means is that the field windings are split into two separate sections, each rated 4amps and 150V. You can wire them in parallel to create an 8amp 150VDC field or in series (as the OP indicates has been done) to create a 4amp 300V field.

It is the field current that is important on a DC motor, not so much the voltage. When a field winding is cold, its resistance is lower and it takes less voltage to get the rated current to flow. For this reason, DC drives usually provide a current regulated field supply.

The two field currents (4.0 and 2.4amps) listed in each field configuration are first, for the full rated field which gives you the full rated torque over the speed range of zero to base speed (1750rpm, in this case). If you weaken the field strength by reducing the field current, you reduce the available torque but increase the max speed in the same proportion. In this case, you can weaken the field down to 2.4amps which will cause the speed range to shift from zero to 2400rpm. The torque will be reduced from nominal by the ratio 1750/2400. Further reductions in field current are not advised since the motor can become unstable and may also exceed its maximum rated speed.

It is very important on DC motors to observe the maximum speed limit. DC motors have wound armatures and, compared to AC motors, they are very fragile. On this motor, I would take the 2400rpm to be the maximum safe speed and it should be considered the absolute max limit.

The OP didn't say, but the nameplate should also say something about the way the field is built. The choices are: straight shunt, stab. shunt, and comp shunt. The straight shunt motor only has the shunt field we described above. The other two are compound wound fields having the shunt field plus an additional series field that is part of the armature circuit.

A compound field motor is not symetrical as to direction. It makes more torque forward than reverse. It is essential that the series field be properly wired (the S1 and S2 leads) or the motor will behave very poorly. A straight shunt motor will not have S1 and S2 leads.

Hope this clarifies a few things.
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Old July 8th, 2010, 07:22 AM   #4
Calistodwt
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The data given by you seems to be for a shunt motor. If the motor were to be re-connected as a series motor then the field and armature current would be the same value. The field winding would therefore have to pass a current of about 320 Amps. This does not seem feasible as the field current rating given is less than 10 amps.
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Old July 8th, 2010, 08:55 AM   #5
jimfun71
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Hey DickDV,
I'm confused. This is a 200 HP motor. The armature is listed at 320 amps. What can putting a few amps through the field do? You know the whole Newtons Law "Equal and oppsite thing". Do you know a link for advanced DC motor theroy? Thanks.
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Old July 8th, 2010, 09:31 AM   #6
allscott
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kw = Va * Ia

= 500V * 320A
= 160000W

746 W/HP
= 214.4HP

The extra 14HP calculated are parasitic losses in the motor. A little less or a little more field current won't change the HP of the motor, less will just make it speed up with less torque, more will make it slow down with more torque, the HP remains the same. Make sense?
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Last edited by allscott; July 8th, 2010 at 09:41 AM.
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Old July 8th, 2010, 03:04 PM   #7
DickDV
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jimfun71, the field winding only provides the stationary field that the armature needs to develop torque. Physically, it is many turns of small wire providing high flux with very low current. A field winding designed for a shunt motor must never be wired in series with the armature as the current would be too high or the voltage loss to much to run the armature.
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Old July 9th, 2010, 02:07 PM   #8
The Plc Kid
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@ Dick Dv

Dick your post are always so informative. Do you mind my asking what your educational background is? EE,ME,?

You must have attended a great school. Your drives knowledge is "off the chain"
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Old July 9th, 2010, 03:41 PM   #9
DickDV
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PLC Kid, I'm 64 years old and my formal education ended in 1966. It was a two year program at DeVry Institute of Technology in Chicago and I had a shiny new AAS degree in Electronic Engineering Technology. It became very obselete many years ago.

My Lord has blessed me with a memory that works, a mind that processes information fairly well, and an intense curiosity to discover how things really work. But none of that would have helped much without the willingness of others to share what they know with me. That process is still going on today.

So, thanks for the kind comments but as I'm sure you've heard before---Everything I know I learned from someone else!

These days I get great satisfaction from helping others come to understand mechanical and electrical things. This BBS, one other BBS (Eng-Tips.net), and a part-time job teaching three-day seminars around the country for NTT in Centennial Colorado help me "pass it on".
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Old June 3rd, 2016, 09:36 PM   #10
Albertox
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Question on DC nameplate

Hi all I am a bit new to this worl, I recently graduated. I am working on power quality analysis of a DC motor (800KW).

When I read the armature voltage signal lets say, should I compare its RMS or DC value to the nameplate?

Maybe this is a silly question but I am not sure, can somebody help me?

I can also post all of the plots I get from the armature and field rectified signals.

Thank you!!!!
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Old June 5th, 2016, 05:51 PM   #11
DickDV
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Power quality in the context of DC motors is a bit different than power quality in the AC power world.

In DC motors, it is simply a measure of how smooth (free of ripple) the DC current is. In the case of a very large DC machine, I would contact the motor manufacturer and request the maximum ripple content for both field and armature circuits.

You see, the ripple on top of the DC contains AC energy and the motor has to be designed to absorb it and not overheat due to it.

DC power quality is mostly determined by the type of AC to DC rectifier in the drive feeding the motor. Worst would be single phase half wave rectified and unfiltered. At 800kw, you might have this for the field supply but certainly not for the armature. Best would be three phase AC full wave rectified. This is most likely what feeds your 800kw armature. The field supply is most likely single phase AC full wave rectified. It might be fixed output (rectifiers are all diodes) or if the motor is operated into the field weakened overspeed zone the field supply would have a variable output (rectifiers are SCRs).

The motor manufacturer may simply state the type of rectifier needed for their motor rather than give you power quality numbers. Either would be ok in my opinion.

Of course, in a perfect world, DC would have no ripple but then you would need a huge battery or DC generator source or a huge filter system on rectified power. None of these are likely.

For those that are curious, AC power quality refers to the amount of distortion or harmonics contained in an AC power signal. A perfect sine wave would be zero distortion (unlikely)!

Last edited by DickDV; June 5th, 2016 at 05:54 PM.
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Old June 5th, 2016, 07:54 PM   #12
jdbrandt
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Quote:
Originally Posted by jimfun71 View Post
Hey DickDV,
I'm confused. This is a 200 HP motor. The armature is listed at 320 amps. What can putting a few amps through the field do? You know the whole Newtons Law "Equal and oppsite thing". Do you know a link for advanced DC motor theroy? Thanks.
Oh, boy. Scary.
Well, lets just say that, if while it were running, if that "few amps" of shunt field current went away, you'd figure out, real quick that it was pretty important.
DC motor theory is needed, on this one.
The shunt field develops the flux, and flux develops the torque. T = k F Ia
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Old June 5th, 2016, 09:35 PM   #13
DickDV
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Or, to say it another way, it does no good to energize the armature (or rotor if an AC motor) if you don't first magnetize the motor. A DC motor does this with its field while an AC motor does this with some small part of the incoming AC current.

An exception to this would be a motor which contains permanent magnets thus being magnetized by nature rather than thru some external source.
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Old June 5th, 2016, 10:16 PM   #14
Albertox
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Dear DickDV,

Thank you very much for your reply.

I have attached the signals of F1, F2 and half of power cables of armature (too big for my clamps!).

I do see that the power quality for the fields (F1 and F2 look similar) is not so good. Actually I dont like that the voltage changes polarity. I think we need to investigate the field rectifier.

The armature on the other hand seems to be OK. A bit of filtering would not hurt. Sometimes the voltage goes slightly negative, but I have seen this can happen for inductive loads.

Am I missing something from the signals?

Thank you again so much!
Albert
Attached Files
File Type: pdf Power Quality 800 KW DC Motor.pdf (227.0 KB, 16 views)
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Old June 6th, 2016, 08:28 AM   #15
just the cowboy
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Motor can run away

Quote:
Originally Posted by jimfun71 View Post
Hey DickDV,
I'm confused. This is a 200 HP motor. The armature is listed at 320 amps. What can putting a few amps through the field do? You know the whole Newtons Law "Equal and oppsite thing". Do you know a link for advanced DC motor theroy? Thanks.
I have seen it twice, once just last week on a small drive it lost its field and run at VERY high speed. I also saw the results of a large motor like yours with an old tube drive lose its field. The drives field loss relay did not work and the only thing that stopped the motor was that the 1/2" copper bars that were the armature winding came apart and wedged in the motor.
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