×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

2 reading of DC Current ; Which one is correct?
3

2 reading of DC Current ; Which one is correct?

2 reading of DC Current ; Which one is correct?

(OP)

When I have measured the DC Current of DC Motor Field Coil . Two reading will be always found different on Positive and Negative Wire . The Positive wire will greater than the Negative. Which is the correct one which should be concerned to meet the rating plate.?

  

RE: 2 reading of DC Current ; Which one is correct?

You then obviously have a "third wire".  Could be a ground fault, could be an actual wire, could be ....

If the ammeter is reasonably accurate, then both readings are correct and you aren't accounting for all current paths.  If the ammeter isn't all that great then neither reading is correct and it is impossible to determine which might be "more correct".

RE: 2 reading of DC Current ; Which one is correct?

(OP)

Thank you davidbeach for your answer; I afraid for the ripple out rectifier may be concerned this account. I think the waveform before and after in and out DC coil should be different. And they may be influence the reading. What do you think ?  

RE: 2 reading of DC Current ; Which one is correct?

I think Davidbeach nailed the original answer.

Quote:

I afraid for the ripple out rectifier may be concerned this account. I think the waveform before and after in and out DC coil should be different. And they may be influence the reading. What do you think ?
If you have ripple, there could be some capacitive component that returns through ground, but should be small.

If you want to check your clamp-on, maybe try wiggling it, tilting it, flipping it in reverse and see if the magnitude of the reading changes.  (maybe stray magnetic field is influencing the measurement).
 

=====================================
(2B)+(2B)'  ?

RE: 2 reading of DC Current ; Which one is correct?

Could the negative conductor be a common return for the field and armature windings?
 
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: 2 reading of DC Current ; Which one is correct?

Kirchhoff clearly states that the current into the coil will be EXACTLY the current out of the coil.  If you measure something different, you aren't measuring exactly 100% of the current into the coil or you aren't measuring exactly 100% of the current out of the coil.  Nothing in the coil can change the current.  Spurious connections, shorts, grounds, connections to other coils, etc. can throw off the results.

RE: 2 reading of DC Current ; Which one is correct?

Most DC clamp meters (if not all) have a directional arrow on the head.. are you following that. If they are used backwards there will be a difference in the reading.

RE: 2 reading of DC Current ; Which one is correct?

(OP)

Thank you all for value answer, No any part draw current out of negative wire . I suspect We may account in limiting of Clamp on performance , DC source come from 3 Phase Power pass through 6 ea.of Diode rectifier and inject to the DC Coil. the insulation of Coil is high reading.The wave form of Dc Voltage before and after Dc coil should be different due to filter action of DC coil . The current in negative wire may shape DC pattern better than positive wire. And that is my opinion which may be cause of 2 different reading. what something which I am misunderstand ? I always found this due to I work in repair shop.

RE: 2 reading of DC Current ; Which one is correct?

The voltage waveform may be different on one side of the inductor compared to the other side but the REAL current will be the same. Instrumentation issues may make it appear to be different.

Please supply us with how much difference you see between the two sides.  Then maybe we can supply a reason.

RE: 2 reading of DC Current ; Which one is correct?

(OP)

Thank you OperaHouse ; Unfortunately I did not record it . But it seem that the different will vary upon the turn of rewound coil .They are different obviously especially Coil brake. After start office hours on Monday morning ; I will measure and post the result again.
 

RE: 2 reading of DC Current ; Which one is correct?

If the reading is made physically close to the motor, then the instrument could be influenced by the magnetic field of the motor. - especially if it is a clipon type meter.

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

RE: 2 reading of DC Current ; Which one is correct?

There are two possible causes;
1> The current is actually different. This would be due to leakage or a third wire. You have ruled that out.
Failing that the current WILL be the same
That leaves.-
2> Some type of metering error.
Try reversing the polarity at the source and take the readings in the same way.

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

RE: 2 reading of DC Current ; Which one is correct?

A simple zero offset error will give that effect. Common mistake by non-electricians.

Some clamps have automatic null. If jaws are not entirely closed when nulling, you get a zero offset.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources